Hematological indicators of stress in longline-captured sharks

Metal(loid) contamination in tiger sharks (Galeocerdo cuvier) from a remote oceanic island in the Equatorial Atlantic Ocean and potential impacts on physiological parameters

We investigated the relationship between blood metal(loid) concentrations and plasma levels of glucose, proteins, triglycerides, cholesterol, lactate, urea, and polyunsaturated fatty acids in tiger sharks (Galeocerdo cuvier) sampled off the Fernando de Noronha Archipelago, a remote oceanic marine protected area in the Equatorial Atlantic Ocean. Results revealed that Al, As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Zn were detected in the whole blood of tiger sharks and no sexual differences in blood metal(loid) concentrations were observed. Females had higher concentrations of plasma proteins and docosahexaenoic acid. In females, all analyzed elements were positively correlated with each other, except As. Only As was positively correlated with triglycerides, suggesting a potential impact of As exposure on the nutritional condition of this species. The results presented herein reinforce that sharks, even from remote sites, are exposed to metal(loid)s and that such exposure might elicit physiological responses.

The physiological stress response of juvenile nurse sharks (Ginglymostoma cirratum) to catch-and-release recreational angling

Nurse sharks (Ginglymostoma cirratum), especially juveniles, are often encountered by near-shore and shore-based recreational anglers and are suggested to exhibit minimal behavioral and physiological responses to capture, largely based on studies of adults using commercial or scientific fishing methods. To quantify the sub-lethal effects of recreational angling on juvenile nurse sharks, 27 individuals (across 31 angling events) were caught using hook-and-line fishing methods. Over a 30-min period, 4 blood samples were taken with variable time intervals between sampling (i.e., randomized ordering of an interval of 5, 10, and 15 min between each sampling event). Lactate increased by 611% (6.7 ± 2.17 mmol/L) on average over the 30-min fight, and significant relationships were identified between lactate and blood draw number, fight time, and temperature, with large effect sizes. Significant relationships were also detected between blood draw number, glucose, and hematocrit, while osmolality was only affected by fishing site. These results suggest juvenile nurse sharks may exhibit a greater physiological stress response when exposed to recreational angling than adults captured with other fishing methods.

Comparison of Physiological Stress Indices in Anesthetized and Manually Restrained Leopard Sharks, Triakis semifasciata

BACKGROUND

Leopard sharks (Triakis semifasciata) are abundant, coastal, eastern Pacific, mesopredatory sharks and are frequently managed in aquariums and zoos. Medical examinations are a routine part of good husbandry practices, but the handling protocols vary greatly between facilities. In this study, we compared the physiological stress responses of T. semifasciata associated with manual restraint and sedated handling under parallel holding and handling procedures in a 10 min interval.

METHODS

Blood was collected and analyzed for the hematocrit (Hct), lactate, glucose, beta-hydroxybutyrate (β-HB), pH, blood gasses, 1α-hydroxycorticosterone (1α-OHB), and corticosterone (B).

RESULTS

Overall, a minimal physiological stress response was observed in both groups. However, statistically significant increases in the PCO2, β-HB and lactate occurred, while the pHTC, SO2, and BE decreased over time in the manual-restraint group. In the sedated group, the B, PCO2, and lactate increased while the pHTC, SO2, and base excess (BE) decreased over time. When comparing treatments, the sedated group had a higher B and PCO2 TC and a lower pHTC, BE, and SO2 than the manual-restraint group at the second time point.

CONCLUSIONS

This study indicates that manual restraint is comparable to the described sedation in T. semifasciata for minimally invasive procedures. The overall results show that this species had low physiological stress responses to both methods.

Maximum storage time of refrigerated blood and frozen plasma samples from tambaqui, Colossoma macropomum

Blood analysis is an important tool for monitoring the health status of fish, but the time between collection and analysis can affect the outcome of the analysis. This study sought to determine the maximum time refrigerated blood and frozen plasma samples of the tambaqui, Colossoma macropomum, can be stored without affecting analysis. Samples from 12 fish were obtained, stored under refrigeration at 4 °C and evaluated after 0, 24, 48, 72, and 96 h, while samples from 14 fish were centrifuged, and the resulting plasma was frozen at -20 °C and then evaluated after 0, 8, 12, 16 and 20 weeks. The parameters analyzed were hematocrit (Ht), hemoglobin content (Hb), total erythrocytes (RBC), total (WBC) and differential leukocytes, total thrombocytes (TC), glucose content (Glc), total protein (TP), triglyceride content (TG), total cholesterol (CoT), and the activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). For refrigerated whole blood samples, mean corpuscular hemoglobin content (MCHC) showed a transient decline in 24 h, and there were decreases in WBC, TC, Glc and TG that persisted until the 72 h sample point (for Glc and TG) or persisted until the 96 h sample point (for WBC and TC). A decrease in RBC was noted from 48 h on, while ALT was significantly higher in the 96 h sample. Significant decreases in lymphocytes, monocytes, neutrophils and eosinophils were noted from 48 h of storage on, while a significant decline in basophil counts were noted over the last two sampled timepoints. The coefficient of variation was greatest at the 96 h timepoint, indicating increased variability in measured parameters after 4 d of refrigeration. Plasma samples frozen at -20 °C showed a significant variation in ALT after 8 weeks, and increases in TP and TG after 20 weeks. Therefore, it is recommended that refrigerated tambaqui whole blood samples be analyzed within 24 h and frozen tambaqui plasma samples analyzed within 8 weeks.

A review of elasmobranch catch-and-release science: synthesis of current knowledge, implications for best practice and future research directions

Until relatively recently commercial fisheries have been considered the main driving factor for elasmobranch population declines. However, this belief has begun to shift with the realization that recreational elasmobranch catches may equal or exceed commercial catches in some regions. Many recreational angling fisheries for elasmobranchs involve high participation in catch-and-release angling practices. However, high release rates may not necessarily equate to high survival rates. Therefore, to assist accurate assessment of the potential impact of recreational angling on elasmobranchs, we attempted to summarize and integrate currently available information on specific risk factors associated with recreational angling, alongside associated mortality rates, as well as information on angler behaviour as it relates to identified risk factors. We categorized the major angling-related effects into two groups: injury-induced effects; and biochemical disruption-induced effects; providing a summary of each group and outlining the main lethal and sub-lethal outcomes stemming from these. These outcomes include immediate and delayed post-release mortality, behavioural recovery periods (which may in-turn confer increased predation risks), chronic health impacts and capture-induced parturition and abortion. Additionally, we detailed a range of angling practices and equipment, including hook-type, hook removal and emersion (i.e. air exposure), as well as inter- and intra-specific factors, including aerobic scope, respiratory mode, body size and species-specific behaviours, which are likely to influence injury and/or mortality rates and should therefore be considered when assessing angling-related impacts. We then utilized these data to provide a range of actionable recommendations for both anglers and policymakers which would serve to reduce the population-level impact of recreational angling on these enigmatic animals.

Mechanisms of cardiac collapse at high temperature in a marine teleost (Girella nigrians)

Heat-induced mortality in ectotherms may be attributed to impaired cardiac performance, specifically a collapse in maximum heart rate (fHmax), although the physiological mechanisms driving this phenomenon are still unknown. Here, we tested two proposed factors which may restrict cardiac upper thermal limits: noxious venous blood conditions and oxygen limitation. We hypothesized elevated blood [K+] (hyperkalemia) and low oxygen (hypoxia) would reduce cardiac upper thermal limits in a marine teleost (Girella nigricans), while high oxygen (hyperoxia) would increase thermal limits. We also hypothesized higher acclimation temperatures would exacerbate the harmful effects of an oxygen limitation. Using the Arrhenius breakpoint temperature test, we measured fHmax in acutely warmed fish under control (saline injected) and hyperkalemic conditions (elevated plasma [K+]) while exposed to hyperoxia (200% air saturation), normoxia (100% air saturation), or hypoxia (20% air saturation). We also measured ventricle lactate content and venous blood oxygen partial pressure (PO2) to determine if there were universal thresholds in either metric driving cardiac collapse. Elevated [K+] was not significantly correlated with any cardiac thermal tolerance metric. Hypoxia significantly reduced cardiac upper thermal limits (Arrhenius breakpoint temperature [TAB], peak fHmax, temperature of peak heart rate [TPeak], and temperature at arrhythmia [TARR]). Hyperoxia did not alter cardiac thermal limits compared to normoxia. There was no evidence of a species-wide threshold in ventricular [lactate] or venous PO2. Here, we demonstrate that oxygen limits cardiac thermal tolerance only in instances of hypoxia, but that other physiological processes are responsible for causing temperature-induced heart failure when oxygen is not limited.

First report on the serum chemistry and haematology of free-ranging dusky (Carcharhinus obscurus) and sandbar (Carcharhinus plumbeus) sharks in the eastern Mediterranean Sea

Shark assessments in the Mediterranean Sea are still scarce, and serum chemistry and haematological data have yet to be reported for wild dusky (Carcharhinus obscurus) or sandbar (Carcharhinus plumbeus) shark populations in the Mediterranean Sea. Herein, blood samples were obtained from adult dusky (n = 23) and sandbar (n = 14) sharks from an aggregation site near the Hadera power and desalination plants in Israel in the winters of 2016-20. Several serum chemistry analytes were characterized with relation to stress, body size and environmental conditions. Glucose concentrations were higher, while total cholesterol concentrations were lower in dusky sharks than in sandbar sharks, potentially due to distinct metabolic pathways utilized during the capture-related activity by both species. However, differences in sex and size are noted and should be considered. The blood cell morphology of both species was consistent with previous findings for sandbar sharks. Atypical monocytes were noted in one dusky shark. Preliminary and exploratory reference intervals for female dusky sharks were calculated for glucose, triglycerides, total cholesterol, total protein and creatine kinase. These data must be viewed with caution due to the potential influence of capture-related stress on analyte concentrations and activities and the fact that only females were employed in the calculations. Moreover, the sampling site is adjacent to coastal power and desalination plants, which may significantly affect shark physiology. Although limited, this novel database on dusky and sandbar shark serum chemistry and haematology aspects is essential as a first attempt to obtain data on these species in the eastern Mediterranean Sea and for future conservation and long-term biomonitoring efforts.

Gradients of Variation in the At-Vessel Mortality Rate between Twelve Species of Sharks and Skates Sampled through a Fishery-Independent Trawl Survey in the Asinara Gulf (NW Mediterranean Sea)

Elasmobranchs are priority species for conservation due to their rapid decline determined by the unbalanced struggle between a fragile bio-ecology and strong anthropogenic impacts, such as bycatch from professional fishing. In this context, measuring species resistance to catch of poorly selective gear is of paramount importance. During June–October 2022, five experimental fishing campaigns were carried out in the Asinara Gulf (northern Sardinia) through 35 geographically and bathymetrically representative hauls of an area between 30 and 600 m in depth. Skates prevailed over sharks in the number of species, with seven and five species, respectively. We first evaluated the status of each individual with respect to stress due to the trawl’s catch using a three-graded scale. We also recorded individual biometrics (total and disk length, weight and sex, and maturity for males) on board by implementing the best practices in manipulating individuals for physiological recovery and release at sea. After capture, skates resulted in generally better conditions than sharks, although deepwater species of both groups exhibited a worse state than coastal species. The estimated vitality rates also depended on the size of the individuals. This work provides standardized data on the intermingled effect of size, species type, and inhabited depth on the resistance response of some elasmobranch species against capture by trawl fishery activities.

Variable post-release mortality in common shark species captured in Texas shore-based recreational fisheries

The practice of catch and release fishing is common among anglers but has been shown to cause unintended mortalities in some species. Current post-release mortality estimates used in coastal shark stock assessments are typically derived from boat-based shark fisheries, which differ from shore-based operations that expose sharks to potentially more stressful environmental and handling conditions. Recreational post-release mortality rates in shore-based fisheries must be quantified to improve stock assessment models and to create guidelines that protect species from overexploitation. Here, we partnered with experienced anglers acting as citizen scientists to deploy pop-up satellite archival transmitting tags (PSAT, n = 22) and acceleration data loggers (ADLs, n = 22). on four commonly caught sharks including the blacktip shark (Carcharhinus limbatus, n = 11), bull shark (Carcharhinus leucas, n = 14), tiger shark (Galeocerdo cuvier, n = 6), and great hammerheads (Sphyrna mokarran, n = 2). Mortality occurred within minutes to hours post-release. If evidence of mortality occurred after normal diving behavior had been re-established for 10 days, then the mortality was considered natural and not related to the catch-and-release process. Post-release mortality estimates ranged from 0% for bull and tiger sharks to 45.5% for blacktip sharks. Of the two great hammerheads, one died within 30 minutes post-release while the other exhibited mortality characteristics 14 days after release. Moribund blacktip sharks experienced on average 3.4-4.9°C warmer water compared with survivors. Recovery periods were estimated for survivors of each species and were highly variable, differing based on duration of tag deployment. High variability in responses to capture and release between species demonstrates the need for species-specific assessments of post-release mortality in shore-based recreational fisheries.

Stress related blood values in Scyliorhinus canicula as live-indicators of physiological status after bottom trawling capture activity

The quantification of capture-related physiological stress is an important factor when assessing the potential for post-release survival in sharks that are incidentally captured. In the absence of these biological data and when the post-release fate is unknown, effective management plans cannot be formulated and may lead to highly susceptible shark populations being overfished. Here, we measured the levels of lactate, glucose, alanine amino transferase (ALT), aspartate amino transferase (AST), Ca2+, Na+ K+,Cl - Mg 2+ and Pi in the plasma of mature and immature lesser spotted dogfish (Scyliorhinus canicula, herein dogfish) which were incidentally captured at two depths (shallow: 50-200 m, and deep: 201-500 m) by bottom trawl off the coast of southern Sicily. These values were used as biomarkers and physiological indicators of the secondary stress response associated with capture. This study found that dogfish captured in deeper waters (below 200 m) had elevated levels of glucose, Na+, Ca2+ and K+ compared to those inhabiting depths less than <200 m. We hypothesize that the elevated levels of physiological stress in dogfish captured at greater depths may be related to the prolonged duration of the interactions with the fishing gear in the area off southern Sicily. Our findings provide new data on the capture-related stress in dogfish and increase the understanding of the potential for post-release survival in sharks captured at two depths by bottom trawl, information that is important for improving the general management plans for the fishery. However, our PC Analysis results revealed that Maturity have a positive contribution from the sample weight, sample length, ALT, AST and a negative contribution from Pi.

Eugenol and Lippia alba essential oils as effective anesthetics for the Amazonian freshwater stingray Potamotrygon wallacei (Chondrichthyes, Potamotrygonidae)

This study assessed the potential of eugenol and the essential oil of Lippia alba (EOLA) in providing suitable anesthetic induction and recovery times, and their consequent effects on the blood and respiratory physiology, as well as the gill architecture of an Amazonian freshwater stingray, Potamotrygon wallacei, shortly after reaching the recovery and 48 h later. Juveniles of P. wallacei were exposed to increasing concentrations of eugenol (75, 100, 125, and 150 µL L^-1) and EOLA (150, 175, 200, and 225 µL L^-1) in an immersion bath. Anesthetic induction was found to be faster with the use of eugenol compared to EOLA. On the other hand, the stingrays anesthetized with eugenol displayed a longer recovery time than those exposed to EOLA. The highest concentrations of eugenol caused moderate to severe histological changes in the gills. No significant changes were found for hematocrit and plasma energy metabolites in the stingrays anesthetized with all concentrations of both eugenol and EOLA shortly after reaching the recovery from the ansthesia, when compared to those recovered after 48 h. Investigations regarding the potential use of these natural anesthetics are unprecedented for freshwater stingray species, and 200 μL L^-1 EOLA is recommended as the most suitable anesthetic for use in juveniles of P. wallacei.

Connecting post-release mortality to the physiological stress response of large coastal sharks in a commercial longline fishery

Bycatch mortality is a major factor contributing to shark population declines. Post-release mortality (PRM) is particularly difficult to quantify, limiting the accuracy of stock assessments. We paired blood-stress physiology with animal-borne accelerometers to quantify PRM rates of sharks caught in a commercial bottom longline fishery. Blood was sampled from the same individuals that were tagged, providing direct correlation between stress physiology and animal fate for sandbar (Carcharhinus plumbeus, N = 130), blacktip (C. limbatus, N = 105), tiger (Galeocerdo cuvier, N = 52), spinner (C. brevipinna, N = 14), and bull sharks (C. leucas, N = 14). PRM rates ranged from 2% and 3% PRM in tiger and sandbar sharks to 42% and 71% PRM in blacktip and spinner sharks, respectively. Decision trees based on blood values predicted mortality with >67% accuracy in blacktip and spinner sharks, and >99% accuracy in sandbar sharks. Ninety percent of PRM occurred within 5 h after release and 59% within 2 h. Blood physiology indicated that PRM was primarily associated with acidosis and increases in plasma potassium levels. Total fishing mortality reached 62% for blacktip and 89% for spinner sharks, which may be under-estimates given that some soak times were shortened to focus on PRM. Our findings suggest that no-take regulations may be beneficial for sandbar, tiger, and bull sharks, but less effective for more susceptible species such as blacktip and spinner sharks.

Physiological response to capture stress in endemic Southern African catsharks (family Scyliorhinidae)

Fishing is the major threat to marine fish populations, particularly to higher trophic-level predators such as sharks. Many sharks, and other fish, are caught as commercial by-catch or for recreational purposes and then released; therefore, it is important to understand the effects of capture stress on their physiology and subsequent survival. Nonetheless, although important data have been collected for some sharks, there can be substantial interspecific differences, and the consequences of capture stress are still poorly understood for most species. In this study, the authors quantified the physiological effect of capture on four catshark species endemic to Southern Africa, which are regularly discarded as by-catch and targeted by recreational fisheries. Fifteen pyjama sharks, nine leopard sharks and nine shysharks were captured, and a blood sample was collected to measure their physiological response to capture stress. Stressed blood biochemistry was compared to samples obtained after the sharks recovered for 24 h in an underwater pen. Levels of pH and K⁺ were significantly lower, and lactate levels were significantly higher, in sharks immediately after capture stress compared to after the 24 h recovery period. Although the species showed a similar response to capture stress, they differed significantly in pH, K⁺ and lactate levels, and there was some evidence of size affecting the strength of the response to capture stress. The substantial physiological response elicited by even the relatively quick capture event in this study suggests that common fishing practices will have a stronger impact on catshark homeostasis because of longer hooking times and more disruptive fishing gear. Although the relationship between survival and physiological changes elicited by capture needs further investigation, the results provide further evidence that minimizing stress would be beneficial to maximize the survival of sharks and other fish following capture-and-release fishing practices.

Blood biochemical status of deep-sea sharks following longline capture in the Gulf of Mexico

Prior to the Deepwater Horizon (DWH) oil spill, little research effort was focused on studying deep-sea sharks in the Gulf of Mexico (GoM). While the biology of these fishes remains virtually unknown, they are routinely captured in commercial fisheries as bycatch. In the absence of basic biological data, and with the probability of post-release survival unknown for most species, effective management plans cannot be formulated, making populations highly susceptible to overfishing. Any potential detrimental effects of the DWH oil spill, which occurred at 1500 m deep, are also unknown. Following longline capture, we characterized the physiological blood biochemical parameters related to secondary stress and compared them among seven shark species occurring on the continental shelf edge and slope in the GoM at depths ranging from 200 to 2000 m. We also investigated the relationship between blood parameters and depth as well as proximity to the oil spill site. The deep-sea sharks examined here exhibited variability in blood chemistry associated with the secondary stress response, with values falling within published records for previously studied elasmobranchs. Results suggested that there is greater relative physiological stress in shallower-dwelling sharks as well as smaller-bodied sharks. Further, the rate of core temperature warming was fastest in smaller bodied sharks, which likely contributes to greater physiological stress. The core temperatures of the larger-bodied, deeper-dwelling species were not altered as drastically as the smaller-bodied sharks after being hauled to the surface. Any chronic physiological effects of the oil spill were not detectable as there were no relevant correlations between blood chemistry metrics and proximity to the DWH oil spill site.

The effects of elevated potassium, acidosis, reduced oxygen levels, and temperature on the functional properties of isolated myocardium from three elasmobranch fishes: clearnose skate (Rostroraja eglanteria), smooth dogfish (Mustelus canis), and sandbar shark (Carcharhinus plumbeus)

Elevated plasma potassium levels (hyperkalemia), reduced plasma pH (acidosis), reduced blood oxygen content, and elevated temperatures are associated with species-specific rates of at-vessel and post-release mortality in elasmobranch fishes. The mechanism linking these physiological disturbances to mortality remains undetermined however, and we hypothesize that the proximate cause is reduced myocardial function. We measured changes in the functional properties of isolated ventricular myocardial strips from clearnose skate (Rostroraja eglanteria), smooth dogfish (Mustelus canis), and sandbar shark (Carcharhinus plumbeus) when subjected to the following stressors (both in isolation and in combination): hyperkalemia (7.4 mM K⁺), acidosis (from 7.9 to 7.1), and reduced oxygen (to 31% O₂ saturation) applied at temperatures 5 °C above and below holding temperatures. We selected these species based on phylogenetic distance, diverse routine activity levels, and their tolerance to capture and transport. Stressors had a few significant species-specific detrimental impacts on myocardial function (e.g., a 33-45% decrease in net force under acidosis + low O₂). Net force production of myocardial strips from clearnose skate and smooth dogfish approximately doubled following exposure to isoproterenol, demonstrating that these species possess beta-adrenergic receptors and that their stimulation could provide a mechanism for preservation of cardiac function during stress. Our results suggest that disruption of physiological homeostasis associated with capture may fatally impair cardiac function in some elasmobranch species, although research with more severe stressors is needed.

Measurement of 1α hydroxycorticosterone in the Japanese banded houndshark, Triakis scyllium, following exposure to a series of stressors

An endocrine glucocorticoid response following exposure to a stressor has been well described for many vertebrates. However, despite demonstration of secondary stress responses in a number of elasmobranchs, our understanding of the endocrine control of these responses is lacking. This is largely due to the unusual structure of the dominant corticosteroid in elasmobranch fish, 1α-hydroxycorticosterone (1α-OH-B). Here we describe plasma extraction and HPLC separation procedures that allowed for the measurement of 1α-OH-B and corticosterone from plasma samples in the cannulated, conscious free-swimming Japanese banded houndshark, Triakis scyllium. While patterns of concentration in the plasma for 1α-OH-B and corticosterone were found to be similar in all experiments conducted, circulating levels of 1α-OH-B were consistently 100-fold greater than circulating levels of corticosterone. Immediately following cannulation surgery, circulating levels of 1α-OH-B increased 7-fold compared to pre-surgery levels, while the levels were 11-fold higher than pre-stress levels 30 min post a repeated handling/air-exposure stress. A three week period of fasting resulted in a 22-fold increase in circulating levels of 1α-OH-B in the banded houndshark. This is the first report of direct measurement of changes in circulating levels of the primary corticosteroid in elasmobranch fish, 1α-OH-B, following exposure to a stressor such as handling/air-exposure. Data indicate the steroid may respond similarly to the classic glucocorticoid response, such as cortisol in teleosts.

Capture stress and post-release mortality of blacktip sharks in recreational charter fisheries of the Gulf of Mexico

Understanding the stress responses of sharks to recreational catch and release fishing has important management and conservation implications. The blacktip shark Carcharhinus limbatus is a popular recreational species targeted throughout the western, central and eastern Gulf of Mexico (Gulf) yet it is unclear what levels of physiological stress result from catch-release fishing practices with hook and line gear and if the stress levels result in post-release mortality. This study correlates physiological response to stress through blood chemistry analysis and examines post-release behaviour of adult blacktip sharks caught to determine post-release mortality rates. Release behaviour was determined by pop-up satellite archival transmitting (PSAT) tags that record temperature, depth and light level data. To quantify physiological stress levels, blood samples were collected from 52 blacktip sharks and a suite of metabolic and osmotic markers were measured. Thirty-six of those blacktip sharks were also outfitted with a PSAT tag yielding time-at-large from 3 to 180 days. Of the 36 tags, 22 (61%) provided sufficient data to confirm post-release fate and 11 (31%) were recovered providing high-resolution data. Tag data suggests a post-release morality rate of 22.7% (95% confidence interval 7.8-45.4%), with mortality occurring within minutes (immediate mortality) to over 12 h post-release (delayed mortality). Compared to survivors, immediate mortalities exhibited significantly higher lactate (median 2.8 mmol/L_survivor vs 5.9 mmol/L_{immediate mortality}) and significantly lower hematocrit (median 24.4% _survivor vs 14% _{immediate mortality}) levels, but no difference was detected between survivors and delayed mortalities. Higher mortality in the western (30%) compared to the central (20%) Gulf may be due to shark handling. All PSATs from mortalities (N = 5) were recovered, and archived data revealed evidence of tag ingestion by predators. Results suggest reduced fight time, decreased handling time and limited air exposure provide blacktip sharks the best survival chances after release by recreational anglers.

Circulating adrenocorticotropic hormone levels, lactate levels, hematocrit and osmolality in relation to capture stress in Atlantic sharpnose sharks, Rhizoprionodon terraenovae

Incidental capture of sharks during commercial and recreational fishing is of major conservation concern because of the potential effects it can have on physiological stress responses and survival. Endocrine aspects of the stress response are, however, poorly understood in elasmobranchs because of difficulties in measuring the primary glucocorticoid (1α-hydroxycorticosterone). Here, we combined measures of plasma adrenocorticotropic hormone (ACTH), the highly conserved pituitary hormone responsible for stimulating the release of adrenal/interrenal glucocorticoids, with measures of plasma lactate, osmolality, hematocrit, and behavior to gain a greater understanding of the capture stress response in Atlantic Sharpnose sharks, Rhizoprionodon terraenovae. Individuals were subject to a non-repeated measures blood sampling protocol in which blood samples were obtained following exposure to capture stress for <3 min (designated baseline), and 15, 30, 45 and 60 min, after which behavior was categorized during release. Results revealed that ACTH was significantly higher at 15, 30, 45, and 60 min than at baseline. Lactate levels were highest at 45 and 60 min whereas osmolality and hematocrit did not differ significantly among the sampling periods. Lactate was the only variable to significantly predict the shark's behavior upon release with higher lactate levels correlating with sluggish behavior. Measurements of stress indicators are important in understanding the effects of capture on shark populations, which has been implicated in population declines.

Analysing tropical elasmobranch blood samples in the field: blood stability during storage and validation of the HemoCue® haemoglobin analyser

Blood samples collected from wild-caught fishes can provide important information regarding the effects of capture (and thus post-release survival) as well as other stressors. Unfortunately, blood samples often cannot be analysed immediately upon sampling, and blood parameters (e.g. blood oxygen levels and acid-base parameters) are known to change with storage duration due to the metabolic activity of the red blood cells. We obtained blood samples from both untreated and stressed individuals of both blacktip reef shark (Carcharhinus melanopterus) and sicklefin lemon shark (Negaprion acutidens) to determine the effects of storage duration on blood pH, haematocrit and haemoglobin concentration ([Hb]). We found no significant effects after storage on ice for up to 180 minutes. Moreover, to validate the usability of a HemoCue haemoglobin analyser (a point-of-care device), we compared data from this device to [Hb] determined using the cyanomethaemoglobin method with blood samples from 10 individuals from each of the aforementioned species as well as epaulette shark (Hemiscyllium ocellatum). Values from the HemoCue consistently overestimated [Hb], and we therefore developed the necessary correction equations. The correction equations were not statistically different among the three elasmobranch species within the biologically relevant range but did differ from published corrections developed using blood from temperate teleost fishes. Although the HemoCue is useful in field situations, development of species-specific calibration equations may be necessary to ensure the reliability of inter-species comparisons of blood [Hb]. Together, these data should increase confidence in haematological stress indicators in elasmobranch fishes, measurements of which are critical for understanding the impact of anthropogenic stressors on these ecologically important species.

Legacy environmental polychlorinated biphenyl contamination attenuates the acute stress response in a cartilaginous fish, the Round Stingray

In a population of Round Stingrays (Urobatis halleri) sampled from mainland California (polychlorinated biphenyl [PCB] exposed site, n = 46), relative to a nearby offshore island (reference site, n = 34), we tested the hypothesis that stingrays from the PCB-exposed site would have a compromised stress response. Adult male and pregnant female (pregnancy = matrotrophic histotrophy), stingrays were captured via hook and line at both locations over a breeding season and plasma was sampled either immediately (Baseline, males = 10, females = 31), or after ∼5 min of struggle on the line followed by a 15 min confinement stressor (Stressed, males = 16, females = 23). Biomarkers of the primary stress response (1α-OH-corticosterone) and the secondary response (energy mobilization; glucose, glycogen, and lactate in liver and muscle) were assessed. Females from both sites demonstrated the expected primary stress response of 1α-OH-corticosterone elevation, but the contaminant-exposed males did not. PCB-exposed stingrays, regardless of sex, failed to produce a plasma glucose increase in response to the applied stress, even though the stressor increased liver glucose as expected. This suggests a dysfunction in glucose transport due to PCB exposure. The Round Stingray accumulates lower PCB loads than other, predatory elasmobranchs, and by extension, the stress axis effects could be more severely impacted in those species as well. Lay summary Legacy polychlorinated biphenyl (PCB) contamination continues to adversely affect marine life. We show that PCB-exposure interferes with the ability of pregnant female and adult male stingrays to mobilize the energy necessary to respond appropriately to an acute stress like capture. Other cartilaginous fish species, such as sharks, accumulate considerably more PCB as a result of their predatory diet, and are likely to be more adversely impacted.

The acute physiological status of white sharks (Carcharodon carcharias) exhibits minimal variation after capture on SMART drumlines

Drumlines incorporating SMART (Shark-Management-Alert-in-Real-Time) technology are a new tool used in several bather protection programmes globally. In New South Wales (NSW), Australia, the white shark (Carcharodon carcharias) is a target species for SMART drumlines because they are often involved in attacks on humans. To understand white shark sensitivity to capture and to establish protocols around acceptable timeframes for responding to alerts, 47 juvenile and subadult white sharks were caught on SMART drumlines at five locations off the east coast of Australia. There was no at-vessel mortality during the sampling period. After capture, blood was sampled from each shark to assess its acute physiological status. Of the 18 metabolites investigated, only lactate and aspartate aminotransferase exhibited significant positive relationships with the capture duration on SMART drumlines. These results indicate that the capture process is relatively benign and that the current response times used here are appropriate to minimize long-term negative impacts on released white sharks. Where white sharks are likely to interact negatively with beachgoers, SMART drumlines can therefore be a useful addition to bather protection programmes that also aim to minimize harm to captured animals. Other shark species captured on SMART drumlines should also be investigated to gain broader understanding of potential physiological consequences of using this new technology.

Exercise intensity while hooked is associated with physiological status of longline-captured sharks

Some shark populations face declines owing to targeted capture and by-catch in longline fisheries. Exercise intensity during longline capture and physiological status may be associated, which could inform management strategies aimed at reducing the impacts of longline capture on sharks. The purpose of this study was to characterize relationships between exercise intensity and physiological status of longline-captured nurse sharks (Ginglymostoma cirratum) and Caribbean reef sharks (Carcharhinus perezi). Exercise intensity of longline-captured sharks was quantified with digital cameras and accelerometers, which was paired with blood-based physiological metrics from samples obtained immediately post-capture. Exercise intensity was associated with physiological status following longline capture. For nurse sharks, blood pH increased with capture duration and the proportion of time exhibiting low-intensity exercise. Nurse sharks also had higher blood glucose and plasma potassium concentrations at higher sea surface temperatures. Associations between exercise intensity and physiological status for Caribbean reef sharks were equivocal; capture duration had a positive relation with blood lactate concentrations and a negative relationship with plasma chloride concentrations. Because Caribbean reef sharks did not appear able to influence blood pH through exercise intensity, this species was considered more vulnerable to physiological impairment. While both species appear quite resilient to longline capture, it remains to be determined if exercise intensity during capture is a useful tool for predicting mortality or tertiary sub-lethal consequences. Fisheries management should consider exercise during capture for sharks when developing techniques to avoid by-catch or reduce physiological stress associated with capture.

Review of Current Conservation Genetic Analyses of Northeast Pacific Sharks

Conservation genetics is an applied science that utilizes molecular tools to help solve problems in species conservation and management. It is an interdisciplinary specialty in which scientists apply the study of genetics in conjunction with traditional ecological fieldwork and other techniques to explore molecular variation, population boundaries, and evolutionary relationships with the goal of enabling resource managers to better protect biodiversity and identify unique populations. Several shark species in the northeast Pacific (NEP) have been studied using conservation genetics techniques, which are discussed here. The primary methods employed to study population genetics of sharks have historically been nuclear microsatellites and mitochondrial (mt) DNA. These markers have been used to assess genetic diversity, mating systems, parentage, relatedness, and genetically distinct populations to inform management decisions. Novel approaches in conservation genetics, including next-generation DNA and RNA sequencing, environmental DNA (eDNA), and epigenetics are just beginning to be applied to elasmobranch evolution, physiology, and ecology. Here, we review the methods and results of past studies, explore future directions for shark conservation genetics, and discuss the implications of molecular research and techniques for the long-term management of shark populations in the NEP.

A review of capture and post-release mortality of elasmobranchs

There is a need to better understand the survivorship of discarded fishes, both for commercial stocks and species of conservation concern. Within European waters, the landing obligations that are currently being phased in as part of the European Union's reformed common fisheries policy means that an increasing number of fish stocks, with certain exceptions, should not be discarded unless it can be demonstrated that there is a high probability of survival. This study reviews the various approaches that have been used to examine the discard survival of elasmobranchs, both in terms of at-vessel mortality (AVM) and post-release mortality (PRM), with relevant findings summarized for both the main types of fishing gear used and by taxonomic group. Discard survival varies with a range of biological attributes (species, size, sex and mode of gill ventilation) as well as the range of factors associated with capture (e.g. gear type, soak time, catch mass and composition, handling practices and the degree of exposure to air and any associated change in ambient temperature). In general, demersal species with buccal-pump ventilation have a higher survival than obligate ram ventilators. Several studies have indicated that females may have a higher survival than males. Certain taxa (including hammerhead sharks Sphyrna spp. and thresher sharks Alopias spp.) may be particularly prone to higher rates of mortality when caught.

Talking to the dead: using Post-mortem data in the assessment of stress in tiger sharks (Galeocerdo cuvier) (Péron and Lesueur, 1822)

Sharks are very sensitive to stress and prone to a high mortality rate after capture. Since approximately 50 million of sharks are caught as bycatch every year, and current recommendations to reduce the impact of commercial fishing strongly support immediate release, it is imperative to better understand post-release mortality caused by the stress of capture and handling. Blood samples allow the assessment of stress levels which are valuable tools to reduce mortality in commercial, recreational and scientific fishing, being essential for the improvement in those conservation measures. Biochemical analyses are widely used for sharks as stress indicators, with secondary plasma parameters (lactate, glucose and ions) being the most often employed assays. However, it is virtually impossible to determine baseline plasma parameters in free-ranging sharks, since blood withdrawal involves animal capture and restrain, which are stressful procedures. This study aims at analyzing secondary parameters of five healthy tiger sharks captured with circular hooks and handlines in Fernando de Noronha (Northeastern Brazil) and comparing them with secondary parameters of three dead tiger sharks caught off Recife (also Northeastern Brazil). The results showed that the analysis of some plasma constituents in dead animals may be an efficient tool to assess stress and lethality. However, traditional parameters such as glucose and calcium, need to be used with caution. The results also demonstrated the extreme importance of urea and phosphorus for assessing stress response and mortality in tiger sharks, both parameters frequently neglected and of utmost importance for shark's homeostasis.

Characterization of the functional and anatomical differences in the atrial and ventricular myocardium from three species of elasmobranch fishes: smooth dogfish (Mustelus canis), sandbar shark (Carcharhinus plumbeus), and clearnose skate (Raja eglanteria)

We assessed the functional properties in atrial and ventricular myocardium (using isolated cardiac strips) of smooth dogfish (Mustelus canis), clearnose skate (Raja eglanteria), and sandbar shark (Carcharhinus plumbeus) by blocking Ca²⁺ release from the sarcoplasmic reticulum (SR) with ryanodine and thapsigargin and measuring the resultant changes in contraction-relaxation parameters and the force-frequency relationship at 20 °C and 30 °C. We also examined ultrastructural differences with electron microscopy. In tissues from smooth dogfish, net force (per cross-sectional area) and measures of the speeds of contraction and relaxation were all higher in atrial than ventricular myocardium at both temperatures. Atrial-ventricular differences were evident in the other two species primarily in measures of the rates of contraction and relaxation. Ryanodine-thapsigargin treatment reduced net force and its maximum positive first derivative (i.e., contractility), and increased time to 50 % relaxation in atrial tissue from smooth dogfish at 30 °C. It also increased times to peak force and half relaxation in clearnose skate atrial and ventricular tissue at both temperatures, but only in atrial tissue from sandbar shark at 30 °C; indicating that SR involvement in excitation-contraction (EC) coupling is species- and temperature-specific in elasmobranch fishes, as it is in teleost fishes. Atrial and ventricular myocardium from all three species displayed a negative force-frequency relationship, but there was no evidence that SR involvement in EC coupling was influenced by heart rate. SR was evident in electron micrographs, generally located in proximity to mitochondria and intercalated discs, and to a lesser extent between the myofibrils; with mitochondria being more numerous in ventricular than atrial myocardium in all three species.

Physiological stress and post-release mortality of white marlin (Kajikia albida) caught in the United States recreational fishery

White marlin, a highly migratory pelagic marine fish, support important commercial and recreational fisheries throughout their range in the tropical and subtropical Atlantic Ocean. More than 10 000 individuals can be caught annually in the United States recreational fishery, of which the vast majority are captured on circle hooks and released alive. The probability of post-release mortality of white marlin released from circle hooks has been documented to be <0.02, but the associated physiological stress resulting from capture and handling techniques has not been characterized despite its importance for understanding the health of released fish. We examined the physiological response of 68 white marlin caught on circle hooks in the recreational fishery and followed the fate of 22 of these fish with pop-up satellite archival tags programmed to release after 30 days. Measures of plasma sodium, chloride, glucose and lactate concentrations taken from fish that were briefly and consistently (mean = 120 s, standard deviation = 40 s) removed from the water increased with angling time, but post-release mortality was inversely related to angling time. The probability of post-release mortality was predicted by elevated plasma potassium concentrations and was more than 10 times greater than has been previously reported for white marlin caught on circle hooks that were not removed from the water. This disparity in estimates of post-release mortality suggests that removal of fish from the water for physiological sampling greatly heightens stress, disrupts homeostasis and thus increases the probability of post-release mortality. Our results demonstrate that elevated concentrations of plasma potassium predict mortality in white marlin and that the probability of post-release mortality is highly dependent on post-capture handling procedures.

Fisheries conservation on the high seas: linking conservation physiology and fisheries ecology for the management of large pelagic fishes

Populations of tunas, billfishes and pelagic sharks are fished at or over capacity in many regions of the world. They are captured by directed commercial and recreational fisheries (the latter of which often promote catch and release) or as incidental catch or bycatch in commercial fisheries. Population assessments of pelagic fishes typically incorporate catch-per-unit-effort time-series data from commercial and recreational fisheries; however, there have been notable changes in target species, areas fished and depth-specific gear deployments over the years that may have affected catchability. Some regional fisheries management organizations take into account the effects of time- and area-specific changes in the behaviours of fish and fishers, as well as fishing gear, to standardize catch-per-unit-effort indices and refine population estimates. However, estimates of changes in stock size over time may be very sensitive to underlying assumptions of the effects of oceanographic conditions and prey distribution on the horizontal and vertical movement patterns and distribution of pelagic fishes. Effective management and successful conservation of pelagic fishes requires a mechanistic understanding of their physiological and behavioural responses to environmental variability, potential for interaction with commercial and recreational fishing gear, and the capture process. The interdisciplinary field of conservation physiology can provide insights into pelagic fish demography and ecology (including environmental relationships and interspecific interactions) by uniting the complementary expertise and skills of fish physiologists and fisheries scientists. The iterative testing by one discipline of hypotheses generated by the other can span the fundamental-applied science continuum, leading to the development of robust insights supporting informed management. The resulting species-specific understanding of physiological abilities and tolerances can help to improve stock assessments, develop effective bycatch-reduction strategies, predict rates of post-release mortality, and forecast the population effects of environmental change. In this synthesis, we review several examples of these interdisciplinary collaborations that currently benefit pelagic fisheries management.

The adenylate energy charge as a new and useful indicator of capture stress in chondrichthyans

Quantifying the physiological stress response of chondrichthyans to capture has assisted the development of fishing practices conducive to their survival. However, currently used indicators of stress show significant interspecific and intraspecific variation in species' physiological responses and tolerances to capture. To improve our understanding of chondrichthyan stress physiology and potentially reduce variation when quantifying the stress response, we investigated the use of the adenylate energy charge (AEC); a measure of available metabolic energy. To determine tissues sensitive to metabolic stress, we extracted samples of the brain, heart, liver, white muscle and blood from gummy sharks (Mustelus antarcticus) immediately following gillnet capture and after 3 h recovery under laboratory conditions. Capture caused significant declines in liver, white muscle and blood AEC, whereas no decline was detected in the heart and brain AEC. Following 3 h of recovery from capture, the AEC of the liver and blood returned to "unstressed" levels (control values) whereas white muscle AEC was not significantly different to that immediately after capture. Our results show that the liver is most sensitive to metabolic stress and white muscle offers a practical method to sample animals non-lethally for determination of the AEC. The AEC is a highly informative indicator of stress and unlike current indicators, it can directly measure the change in available energy and thus the metabolic stress experienced by a given tissue. Cellular metabolism is highly conserved across organisms and, therefore, we think the AEC can also provide a standardised form of measuring capture stress in many chondrichthyan species.

High survivorship after catch-and-release fishing suggests physiological resilience in the endothermic shortfin mako shark (Isurus oxyrinchus)

The shortfin mako shark (Isurus oxyrinchus) is a species commonly targeted by commercial and recreational anglers in many parts of the developed world. In Australia, the species is targeted by recreational anglers only, under the assumption that most of the sharks are released and populations remain minimally impacted. If released sharks do not survive, the current management strategy will need to be revised. Shortfin mako sharks are commonly subjected to lengthy angling events; however, their endothermic physiology may provide an advantage over ectothermic fishes when recovering from exercise. This study assessed the post-release survival of recreationally caught shortfin mako sharks using Survivorship Pop-up Archival Transmitting (sPAT) tags and examined physiological indicators of capture stress from blood samples as well as any injuries that may be caused by hook selection. Survival estimates were based on 30 shortfin mako sharks captured off the south-eastern coast of Australia. Three mortalities were observed over the duration of the study, yielding an overall survival rate of 90%. All mortalities occurred in sharks angled for <30 min. Sharks experienced increasing plasma lactate with longer fight times and higher sea surface temperatures (SSTs), increased plasma glucose at higher SSTs and depressed expression of heat shock protein 70 and β-hydroxybutyrate at higher SSTs. Long fight times did not impact survival. Circle hooks significantly reduced foul hooking when compared with J hooks. Under the conditions of this study, we found that physical injury associated with hook choice is likely to have contributed to an increased likelihood of mortality, whereas the high aerobic scope associated with the species' endothermy probably enabled it to cope with long fight times and the associated physiological responses to capture.

Chaperone roles for TMAO and HSP70 during hyposmotic stress in the spiny dogfish shark (Squalus acanthias)

Salinity decreases are experienced by many marine elasmobranchs. To understand how these fishes cope with hyposmotic stress on a cellular level, we used the spiny dogfish shark (Squalus acanthias) as a model to test whether a reciprocal relationship exists between the cell's two primary protein protection mechanisms, the chemical (e.g., trimethylamine oxide, TMAO) and molecular (e.g., heat shock protein 70, HSP70) chaperone systems. This relationship is interesting given that many elasmobranchs are expected to gain water and lose osmolytes, chemical chaperones, and ions as they osmoconform to new, lowered salinity. Dogfish were cannulated for repeated blood sampling and exposed to 70% seawater (SW) for 48 h. These hyposmotic conditions had no effect on red blood cell (RBC) and white muscle TMAO concentrations, and did not result in HSP70 induction or signs of protein damage (i.e., increased ubiquitin), suggesting that TMAO levels were sufficiently protective in these tissues. However, in the gill, we observed a significant decrease in TMAO concentration and a significant induction of HSP70 as well as signs of protein damage. In the face of this cellular stress response, gill Na(+)/K(+)-ATPase (NKA) activity significantly increased during hyposmotic conditions, as expected. We suggest that this functional preservation in the gill is partly the result of HSP70 induction with lowered salinity. We conclude a reciprocal relationship between TMAO and HSP70 in the gills of dogfish as a result of in vivo hyposmotic stress. When osmotically induced protein damage surpasses the protective capacity of remaining TMAO, HSP70 is induced to preserve tissue and organismal function.

The epigonal organ and mature pole of the testis in the recreationally fished blue shark (Prionace glauca): histochemico-functional correlates

The exact role of the immune system in normal spermatogenesis is poorly understood. The attachment, however, of the lymphomyeloid epigonal organ specifically to the testis's mature pole in many shark species is a curious finding. Unlike the histology of the lymphomyeloid tissues of many other elasmobranchs, the epigonal organ leukocytes of wild-caught blue shark (Prionace glauca), besides exhibiting extensive nuclear heterogeneity, contain some of the largest known granules ever seen in vertebrate white blood cells. It was previously shown that the blue shark epigonal organ remains unremarkable and functionally unchanged despite cestode parasites embedded into its surface, suggesting that it might have other functions in addition to microbial defense. We show here that Prionace epigonal leukocytes shed their granule-laden cytoplasm into the cyst resorption zone (RZ) of the testis, i.e. the region separating the spermatogenic tissue from the epigonal organ, as they begin to migrate into the RZ. Using the immunoreactivity of the conserved transcription factor (proliferating cell nuclear antigen) as marker, it is shown that the granule-lacking leukocytes exclusively infiltrated spermatozoal cysts leftover after the wave of wide-spread multinuclear cell death in summer-breeding males in a seasonally dependent manner. By contrast, Prionace caught 2 months later showed fully recovered testes containing numerous completely intact spermatozoal cysts. Conversely, degenerating immature spermatids were gradually phagocytized by their accompanying Sertoli cells, and leukocytes did not infiltrate such cysts. The autoimmune response described here resembles in every aspect the testicular autoimmune response induced experimentally in a teleost fish. These observations suggest functional adaptation of shark leukocytes in response to specific changes in the testicular microenvironment.

Impacts of crowding, trawl duration and air exposure on the physiology of stingarees (family: Urolophidae)

Research on physiological stress and post-capture mortality of threatened species caught as bycatch is critical for the management of fisheries. The present study used laboratory simulations to examine the physiological stress response of sparsely spotted stingarees (Urolophus paucimaculatus) subjected to one of four different trawl treatments, including two different trawl durations as well as ancillary stressors of either air exposure or crowding. Physiological indicators (plasma lactate, urea, potassium and glucose) and changes in white blood cell counts were measured from blood samples taken throughout a 48 h recovery period. Mortality was low throughout this study (15% overall) and occurred only after >48 h following air exposure, crowding and 3 h trawl simulations. Plasma lactate, glucose and urea concentrations were identified as potential indicators of physiological stress, while plasma potassium and white blood cell counts were too variable to identify changes that would be expected to have biological consequences for stingarees. The characterization of the temporal profiles of physiological indicators facilitates a more accurate assessment of secondary stressors by identifying the best timing to sample stingaree blood when investigating post-capture stress physiology. High levels of lactate, increasing glucose and depressed urea were all recorded in response to air exposure following trawling, indicating that this is the primary source of stress in stingarees caught in trawling operations. These findings highlight the importance of improving bycatch sorting procedures to reduce the time out of the water for trawl-caught stingarees.

Reproductive endocrinology in chondrichthyans: the present and the future

The class Chondrichthyes, that includes Elasmobranchii and Holocephali, is a diverse group of fish occupying a key position at the base of vertebrate evolution. Their evolutionary success is greatly attributed to their wide range of reproductive strategies controlled by different endocrine mechanics. As in other vertebrates, hormonal control of reproduction in chondrichthyans is mediated by the neuropeptide gonadotropin-releasing hormone (GnRH) that regulates the brain control of gonadal activity via a hypothalamus-pituitary-gonadal (HPG) axis. Chondrichthyans lack of a direct vascular supply from the hypothalamus to the zone of the pituitary where the gonadotropic activity resides, thus transport between these two zones likely occurs via the general circulation. In the brain of elasmobranchs, two groups of GnRH, GnRH-I and GnRH-II were identified, and the presence of two immunoreactive gonadotropins similar to the luteinising (LH) and follicle stimulating (FSH) hormones was identified in the pituitary. In holocephalans, only GnRH-II has been confirmed, and while gonadotropin activity has been found in the buccal pituitary lobe, the presence of gonadotropin receptors in the gonads remains unknowns. The diversity of reproductive strategies display by chondrichthyans makes it difficult to generalize the control of gametogenesis and steroidogenesis; however, some general patterns emerge. In both sexes, androgens and estrogens are the main steroids during gonadal growth; while progestins have maturational activity. Androgens also form the precursors for estrogen steroid production. Estrogens stimulate the hepatic synthesis of yolk and stimulate the development of different part of the reproductive tract in females. The role of other gonadal steroids may play in chondrichthyan reproduction remains largely unknown. Future work should concentrate in filling the gaps into the current knowledge of the HPG axis regulation, and the use of reproductive endocrinology as a non-lethal technique for management of chondrichthyan populations.