Discovery of a spawning ground reveals diverse migration strategies in Atlantic bluefin tuna (Thunnus thynnus)

Fish ectoparasite detection, collection and curation

Fish parasitology is a dynamic and internationally important discipline with numerous biological, ecological and practical applications. We reviewed optimal fish and parasite sampling methods for key ectoparasite phyla (i.e. Ciliophora, Platyhelminthes, Annelida and Arthropoda) as well as recent advances in molecular detection of ectoparasites in aquatic environments. Ideally, fish capture and anaesthesia as well as parasite recovery methods should be validated to eliminate potential sampling bias and inaccuracy in determining ectoparasite population parameters. There are considerable advantages to working with fresh samples and live parasites, when combined with appropriate fixation methods, as sampling using dead or decaying materials can lead to rapid decomposition of soft-bodied parasites and subsequent challenges for identification. Sampling methods differ between target phyla, and sometimes genera, with optimum techniques largely associated with identification of parasite microhabitat and the method of attachment. International advances in fish parasitology can be achieved through the accession of whole specimens and/or molecular voucher specimens (i.e. hologenophores) in curated collections for further study. This approach is now critical for data quality because of the increased application of environmental DNA (eDNA) for the detection and surveillance of parasites in aquatic environments where the whole organism may be unavailable. Optimal fish parasite sampling methods are emphasised to aid repeatability and reliability of parasitological studies that require accurate biodiversity and impact assessments, as well as precise surveillance and diagnostics.

Unidirectional trans-Atlantic gene flow and a mixed spawning area shape the genetic connectivity of Atlantic bluefin tuna

The commercially important Atlantic bluefin tuna (Thunnus thynnus), a large migratory fish, has experienced notable recovery aided by accurate resource assessment and effective fisheries management efforts. Traditionally, this species has been perceived as consisting of eastern and western populations, spawning respectively in the Mediterranean Sea and the Gulf of Mexico, with mixing occurring throughout the Atlantic. However, recent studies have challenged this assumption by revealing weak genetic differentiation and identifying a previously unknown spawning ground in the Slope Sea used by Atlantic bluefin tuna of uncertain origin. To further understand the current and past population structure and connectivity of Atlantic bluefin tuna, we have assembled a unique dataset including thousands of genome-wide single-nucleotide polymorphisms (SNPs) from 500 larvae, young of the year and spawning adult samples covering the three spawning grounds and including individuals of other Thunnus species. Our analyses support two weakly differentiated but demographically connected ancestral populations that interbreed in the Slope Sea. Moreover, we also identified signatures of introgression from albacore (Thunnus alalunga) into the Atlantic bluefin tuna genome, exhibiting varied frequencies across spawning areas, indicating strong gene flow from the Mediterranean Sea towards the Slope Sea. We hypothesize that the observed genetic differentiation may be attributed to increased gene flow caused by a recent intensification of westward migration by the eastern population, which could have implications for the genetic diversity and conservation of western populations. Future conservation efforts should consider these findings to address potential genetic homogenization in the species.

Thermal sensitivity of field metabolic rate predicts differential futures for bluefin tuna juveniles across the Atlantic Ocean

Changing environmental temperatures impact the physiological performance of fishes, and consequently their distributions. A mechanistic understanding of the linkages between experienced temperature and the physiological response expressed within complex natural environments is often lacking, hampering efforts to project impacts especially when future conditions exceed previous experience. In this study, we use natural chemical tracers to determine the individual experienced temperatures and expressed field metabolic rates of Atlantic bluefin tuna (Thunnus thynnus) during their first year of life. Our findings reveal that the tuna exhibit a preference for temperatures 2-4 °C lower than those that maximise field metabolic rates, thereby avoiding temperatures warm enough to limit metabolic performance. Based on current IPCC projections, our results indicate that historically-important spawning and nursery grounds for bluefin tuna will become thermally limiting due to warming within the next 50 years. However, limiting global warming to below 2 °C would preserve habitat conditions in the Mediterranean Sea for this species. Our approach, which is based on field observations, provides predictions of animal performance and behaviour that are not constrained by laboratory conditions, and can be extended to any marine teleost species for which otoliths are available.

Oocyte dynamics for Atlantic bluefin tuna (Thunnus thynnus) in the southern Gulf of Mexico: A comparative assessment of Mediterranean spawners using two stereological methods

Although most research focused on the northern Gulf of Mexico for western Atlantic bluefin tuna, the histological records of reproductive activity of this species in the southern Gulf of Mexico (Mexican waters) have been presented for the first time. This work is the first to study oocyte dynamics in Atlantic bluefin tuna caught in the southern Gulf of Mexico by assessing and comparing them with Mediterranean stock (BFT-E) through stereology using two different methods. Regardless of Atlantic bluefin tuna females returning to their respective spawning grounds at different months in the southern Gulf of Mexico and the Mediterranean, both stocks arrived reproductively inactive and remained in these zones during periods of similar length; they were reproductively active until March for the southern Gulf of Mexico and May for the Mediterranean females. The comparison of the size structure between the two stocks examined using kernel density estimators demonstrated a quite remarkable difference in mean fork lengths between stocks. The ovarian oocyte density, that is, the number of oocytes per gram of ovary, for each gonad stage predicted using the Weibel and Gomez and oocyte packing density (OPD) methods did not significantly differ between stocks and showed that advanced vitellogenic oocytes from spawning-capable females are an appropriate indicator to estimate potential fecundity, presenting values of c. 1273 and ~1355 eggs per gram for the southern Gulf of Mexico and Mediterranean females, respectively. Females caught in Mexican waters (southern Gulf of Mexico) were larger than those caught in the Mediterranean; however, it was demonstrated that the length and weight of females did not affect ovarian oocyte density production. In addition, densities estimated for each gonad stage using W&G and OPD methods did not differ between stocks and presented equal patterns in their oocyte dynamics. These findings contribute to a better understanding of the reproductive biology of Atlantic bluefin tuna, especially in the southern Gulf of Mexico, due to the lack of information regarding this zone, and may allow to support strategies for proper assessment, management, and conservation.

Evidence of isotopic maternal transmission influence on bluefin tuna (Thunnus thynnus) larval growth

Pre-flexion stages of Atlantic bluefin tuna (Thunnus thynnus) larvae were collected in 2014 during the peak of spawning in the two main spawning areas: Gulf of Mexico (GOM) and Mediterranean Sea (MED). We examined daily growth, otolith biometry, and stable isotopes and found that the GOM grew at a faster rate, had larger otoliths, wider daily increments, and significantly lower values of δ15N when compared to the MED. In addition, an intra-population comparative analysis between slow- and fast-growing individuals (deficient vs. optimal growth groups, respectively) showed that optimal growth groups had significantly lower δ15N within each spawning area, implying a direct relationship between growth potential, development, and maternal transmission of isotopic signatures. A third pre-flexion larval group that was aquaculture-reared also exhibited the same pattern to the wild larval groups. In addition, for the first time, we estimated the maternal trophic niches using models developed with field-captured pre-flexion larvae. The estimated maternal trophic niches for the GOM were narrower than the MED, implying differences in the maternal trophodynamics from each nursery area. Overall, the inter-population (GOM vs. MED) and intra-population growth groups (deficient vs. optimal) grew faster and had narrower maternal niches. This study shows the advantages that larval SIA research can aid in the understanding of the trophodynamics of their breeders by examining the trophic relationship of a spawning stock jointly with the development of growth potential in offspring within the same breeding season.

Context drives movement patterns in a mobile marine predator

Intra-specific variability in movement behaviour occurs in all major taxonomic groups. Despite its common occurrence and ecological consequences, individual variability is often overlooked. As a result, there is a persistent gap in knowledge about drivers of intra-specific variability in movement and its role in fulfilling life history requirements. We apply a context-focused approach to bull sharks (Carcharhinus leucas), a highly mobile marine predator, incorporating intra-specific variability to understand how variable movement patterns arise and how they might be altered under future change scenarios. Spatial analysis of sharks, acoustically tagged both at their distributional limit and the centre of distribution in southern Africa, was combined with spatial analysis of acoustically tagged teleost prey and remote-sensing of environmental variables. The objective was to test the hypothesis that varying resource availability and magnitude of seasonal environmental change in different locations interact to produce variable yet predictable movement behaviours across a species' distribution. Sharks from both locations showed high seasonal overlap with predictable prey aggregations. Patterns were variable in the centre of distribution, where residency, small- and large-scale movements were all recorded. In contrast, all animals from the distributional limit performed 'leap-frog migrations', making long-distance migrations bypassing conspecifics in the centre of distribution. By combining multiple variables related to life history requirements for animals in different environments we identified combinations of key drivers that explain the occurrence of differing movement behaviours across different contexts and delineated the effects of environmental factors and prey dynamics on predator movement. Comparisons with other taxa show striking similarities in patterns of intra-specific variability across terrestrial and marine species, suggesting common drivers.

A global, historical database of tuna, billfish, and saury larval distributions

Knowing the distribution of fish larvae can inform fisheries science and resource management in several ways, by: 1) providing information on spawning areas; 2) identifying key areas to manage and conserve; and 3) helping to understand how fish populations are affected by anthropogenic pressures, such as overfishing and climate change. With the expansion of industrial fishing activity after 1945, there was increased sampling of fish larvae to help better understand variation in fish stocks. However, large-scale larval records are rare and often unavailable. Here we digitize data from Nishikawa et al. (1985), which were collected from 1956-1981 and are near-global (50°N-50°S), seasonal distribution maps of fish larvae of 18 mainly commercial pelagic taxa of the families Scombridae, Xiphiidae, Istiophoridae, Scombrolabracidae, and Scomberesocidae. Data were collected from the Pacific, Atlantic, and Indian Oceans. We present four seasonal 1° × 1° resolution maps per taxa representing larval abundance per grid cell and highlight some of the main patterns. Data are made available as delimited text, raster, and vector files.

First tagging data on large Atlantic bluefin tuna returning to Nordic waters suggest repeated behaviour and skipped spawning

Atlantic bluefin tuna (Thunnus thynnus; ABFT) is one of the most iconic fish species in the world. Recently, after being very rare for more than half a century, large bluefin tunas have returned to Nordic waters in late summer and autumn, marking the return of the largest predatory fish in Nordic waters. By tagging 18 bluefin tunas with electronic tags (pop-up satellite archival tags), we show that bluefin tuna observed in Nordic waters undertake different migration routes, with individuals migrating into the western Atlantic Ocean, while others stay exclusively in the eastern Atlantic and enter the Mediterranean Sea to spawn. We additionally present evidence of possible skipped spawning inferred from behavioural analyses. In Nordic waters, ABFT are primarily using the upper water column, likely reflecting feeding activity. The results support the hypothesis that ABFT migrating to Nordic waters return to the same general feeding area within the region on an annual basis. These observations may have important implications for management because (1) tunas that come into Nordic waters might represent only a few year classes (as evidenced by a narrow size range), and thus may be particularly vulnerable to area-specific exploitation, and (2) challenge the assumption of consecutive spawning in adult Atlantic bluefin tuna, as used in current stock assessment models. Without careful management and limited exploitation of this part of the ABFT population, the species’ return to Nordic waters could be short-lived.

Recent and historical data show no evidence of Pacific bluefin tuna reproduction in the southern California Current system

Despite their broad distribution across the North Pacific Ocean, the only known spawning grounds for Pacific Bluefin Tuna (Thunnus orientalis) are around coastal Japan and the East China Sea. However, an increase in the prevalence of large bluefin tuna up to 10 years old in the California Current System during exceptionally warm ocean conditions has led to speculation that they may be spawning in this region. To investigate this possibility, we collected samples from 36 females (estimated 3–8 years old) between 2015 and 2019. Histological analyses revealed no signs of imminent, active, or recent spawning. Further examination of historical ichthyoplankton collections showed no records of larval bluefin tuna, but confirmed the presence of the larvae of other tuna species in waters > 24°C. Fishery-dependent records showed that bluefin tuna are rarely recorded in purse seine catches where surface temperatures exceed 23°C. Our study, therefore, provided no evidence of bluefin tuna reproduction in the CCS. However, more comprehensive sampling, in particular off southern Baja California, may be required to confirm the absence of spawning.

Ancient DNA SNP-panel data suggests stability in bluefin tuna genetic diversity despite centuries of fluctuating catches in the eastern Atlantic and Mediterranean

Atlantic bluefin tuna (Thunnus thynnus; BFT) abundance was depleted in the late 20th and early 21st century due to overfishing. Historical catch records further indicate that the abundance of BFT in the Mediterranean has been fluctuating since at least the 16th century. Here we build upon previous work on ancient DNA of BFT in the Mediterranean by comparing contemporary (2009–2012) specimens with archival (1911–1926) and archaeological (2nd century BCE–15th century CE) specimens that represent population states prior to these two major periods of exploitation, respectively. We successfully genotyped and analysed 259 contemporary and 123 historical (91 archival and 32 archaeological) specimens at 92 SNP loci that were selected for their ability to differentiate contemporary populations or their association with core biological functions. We found no evidence of genetic bottlenecks, inbreeding or population restructuring between temporal sample groups that might explain what has driven catch fluctuations since the 16th century. We also detected a putative adaptive response, involving the cytoskeletal protein synemin which may be related to muscle stress. However, these results require further investigation with more extensive genome-wide data to rule out demographic changes due to overfishing, and other natural and anthropogenic factors, in addition to elucidating the adaptive drivers related to these.

Age and growth of early-life-stage Atlantic tarpon (Megalops atlanticus) from the northcentral Gulf of Mexico

Age and growth of early-life-stage Atlantic tarpon Megalops atlanticus collected from Mississippi coastal waters in the northcentral Gulf of Mexico (GOM) are described using otolith microstructure analysis. Tarpon leptocephali (n = 95, 16.0-27.8 mm standard length, L_S ) collected from June throughOctober 2013-2018, ranged in age from 22 to 43 days (mean = 30.9 ± 0.5 days). Leptocephalus somatic growth rates ranged 0.46-1.24 mm day^-1 (mean = 0.76 ± 0.02 mm day^-1 ), and leptocephalus otolith growth rates ranged 1.78-3.97 μm day^-1 (mean = 2.58 ± 0.04 μm day^-1 ). Growth rates were inversely correlated to leptocephalus age, indicating the shrinkage phase associated with leptocephalus metamorphosis. Juvenile tarpon (n = 358, 50-359 mm fork length, L_F ) were collected from August through December 2007-2018. Juveniles exhibited a positive allometric relationship (adjusted R²  = 0.99, P < 0.001) between length and mass. The age of 100 juveniles (71-277 mm L_F ) ranged from 76 to 174 days. Juvenile growth rate was estimated as 1.56 ± 0.11 mm day^-1 . Significant (P < 0.001) linear relationships were found between juvenile age and otolith metrics, including otolith mass (R²  = 0.81) and radius (R²  = 0.68). Evaluation of the backcalculated hatch dates suggests that specimens in the collection hatched from late May through mid-September with slight peaks during July and August. A Rao's Spacing Test of Uniformity indicates the presence of significant lunar periodicity in leptocephalus hatch dates (n = 95, U = 250.1, P < 0.05), with 50% of the leptocephali hatched within 5 days (before or after) of the full moon. This study fills critical gaps in the scientific knowledge of tarpon and provides estimates of early-life-history metrics for an iconic game fish at the northernmost extent of its GOM range.

Bluefin tuna reveal global patterns of mercury pollution and bioavailability in the world's oceans

Bluefin tuna (BFT) is an apex predatory, long-lived, migratory pelagic fish that is widely distributed throughout the world's oceans. These fish have very high concentrations of neurotoxic methylmercury (MeHg) in their tissues, which increase with age. Our study shows that Hg accumulation rates (MARs) in BFT as a global pollution index can reveal global patterns of Hg pollution and bioavailability in the oceans and further reflect both natural and anthropogenic emissions and regional environmental features. Overall, MARs provide a means to compare Hg bioavailability among geographically distinct populations of upper trophic level marine fish across ocean subbasins, to investigate trophic dynamics of Hg in marine food webs, and furthermore, to improve public health risk assessments of Hg exposure from seafood.

Bluefin tuna (BFT), highly prized among consumers, accumulate high levels of mercury (Hg) as neurotoxic methylmercury (MeHg). However, how Hg bioaccumulation varies among globally distributed BFT populations is not understood. Here, we show mercury accumulation rates (MARs) in BFT are highest in the Mediterranean Sea and decrease as North Pacific Ocean > Indian Ocean > North Atlantic Ocean. Moreover, MARs increase in proportion to the concentrations of MeHg in regional seawater and zooplankton, linking MeHg accumulation in BFT to MeHg bioavailability at the base of each subbasin's food web. Observed global patterns correspond to levels of Hg in each ocean subbasin; the Mediterranean, North Pacific, and Indian Oceans are subject to geogenic enrichment and anthropogenic contamination, while the North Atlantic Ocean is less so. MAR in BFT as a global pollution index reflects natural and human sources and global thermohaline circulation.

Evidence of Atlantic bluefin tuna spawning in the Bay of Biscay, north-eastern Atlantic

The spawning grounds of the Atlantic bluefin tuna (Thunnus thynnus) are traditionally considered to be the Gulf of Mexico (Gulf of Mexico) and the Mediterranean Sea (Mediterranean Sea). However, for the western Atlantic, unequivocal evidence of bluefin spawning outside the Gulf of Mexico has been shown. In this study we present the first records of genetically confirmed bluefin larvae in the southern Bay of Biscay (eastern Atlantic). These findings provide evidence of bluefin spawning activity outside the Mediterranean Sea, in the north-eastern Atlantic. However, our results suggest that the bluefin spawning in the Bay of Biscay is a sporadic phenomenon.

Genetic connectivity between Atlantic bluefin tuna larvae spawned in the Gulf of Mexico and in the Mediterranean Sea

The highly migratory Atlantic bluefin tuna (ABFT) is currently managed as two distinct stocks, in accordance with natal homing behavior and population structuring despite the absence of barriers to gene flow. Larval fish are valuable biological material for tuna molecular ecology. However, they have hardly been used to decipher the ABFT population structure, although providing the genetic signal from successful breeders. For the first time, cooperative field collection of tuna larvae during 2014 in the main spawning area for each stock, the Gulf of Mexico (GOM) and the Mediterranean Sea (MED), enabled us to assess the ABFT genetic structure in a precise temporal and spatial frame exclusively through larvae. Partitioning of genetic diversity at nuclear microsatellite loci and in the mitochondrial control region in larvae spawned contemporarily resulted in low significant fixation indices supporting connectivity between spawners in the main reproduction area for each population. No structuring was detected within the GOM after segregating nuclear diversity in larvae spawned in two hydrographically distinct regions, the eastern GOM (eGOM) and the western GOM (wGOM), with the larvae from eGOM being more similar to those collected in the MED than the larvae from wGOM. We performed clustering of genetically characterized ABFT larvae through Bayesian analysis and by Discriminant Analysis of Principal Components (DAPC) supporting the existence of favorable areas for mixing of ABFT spawners from Western and Eastern stocks, leading to gene flow and apparent connectivity between weakly structured populations. Our findings suggest that the eastern GOM is more prone for the mixing of breeders from the two ABFT populations. Conservation of this valuable resource exploited for centuries calls for intensification of tuna ichthyoplankton research and standardization of genetic tools for monitoring population dynamics.

Development stage and season influence in the metal content of small pelagic fish in the North-West Africa

Marine organisms are exposed to great changes caused by man due to discharges into the oceans, so the study of marine pollution is of great value for each ecosystem. For this study, 963 specimens of pelagic fish have been collected, of which 345 are Scomber colias, 294 are Trachurus picturatus and 324 are Sardina pilchardus, it was wanted to study the development and ontogeny in the three species observing if there were variations in their metallic content. The study covered a total of 2 years, from June 2016 to May 2018.16 samples of each species were studied each month, during the 2 years of sampling, from the samplings carried out by the Canary Islands Oceanographic Center of the Spanish Institute Oceanography (COC-IEO) in the markets of the Island of Tenerife. The concentration of 11 anthropic metals was determined in each sample using the Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) technique. Statistical studies were carried out with factors such as the oceanographic season, and the level of maturity of the gonads. S. colias was the species that presented the highest concentration in a greater number of metals. S. pilchardus presents higher concentration in Cu, Li and Zn, in the warm season. These are metals of a very anthropic character on coasts; This species tends to get closer to the coasts compared to the other two and is the cause of its higher concentration in these metals due to the influence of pollution linked to the coast. The immature specimens of the three species had a higher concentration in more metals than the mature specimens.

Feeding ecology of Atlantic bluefin tuna (Thunnus thynnus) in the Gulf of Saint Lawrence, Canada

Feeding ecology of Atlantic bluefin tuna (ABFT, Thunnus thynnus), from the Gulf of Saint Lawrence, Canada, was assessed by stomach content and stable isotope analyses. Stomachs were collected from 161 individuals averaging 249 ± 20 cm in curved fork length and 248 ± 64 in round weight. The fish were caught by commercial fishing boats using rod and reel gear during fall 2015-2017. Liver and muscle samples were taken from 133 fish for stable isotope analysis. Atlantic mackerel (Scomber scombrus) was the most abundant prey item in 2015 (%Alimentary index, %AI = 88.94), whereas in 2016 and 2017 the ABFT diet was mainly composed of Atlantic herring (Clupea harengus) (%AI = 56.73 and %AI = 77.23, respectively). PERMANOVA analysis revealed significant differences in the dietary composition between years. δ¹⁵N values varied between 12.3 and 15.4‰ for muscle and between 11.1 and 14.6‰ for liver. δ¹³C values ranged from -19.9 to -16.5‰ for muscle and from -20.7 to -17.3‰ for liver. PERMANOVA analysis also detected inter-annual isotopic differences in both muscle and liver samples (p < 0.05). Daily rations (DR) were similar in 2015 and 2016 (DR = 1.02 ± 0.32 and DR = 1.10 ± 0.21, respectively), but higher in 2017 (DR = 1.73 ± 0.54).

Combining genetic markers with stable isotopes in otoliths reveals complexity in the stock structure of Atlantic bluefin tuna (Thunnus thynnus)

Atlantic bluefin tuna (Thunnus thynnus) from the two main spawning populations in the Mediterranean and Gulf of Mexico occur together in the western, central and eastern Atlantic. Stock composition of catches from mixing areas is uncertain, presenting a major challenge to the sustainable management of the fisheries. This study combines genetic and chemical markers to develop an integrated method of population assignment. Stable isotope signatures (δ¹³C and δ¹⁸O) in the otolith core of adults from the two main spawning populations (adult baselines) showed less overlap than those of yearlings (12-18 months old) from western and eastern nursery areas suggesting that some exchange occurs towards the end of the yearling phase. The integrated model combined δ¹⁸O with four genetic markers (SNPs) to distinguish the adult baselines with greater accuracy than chemical or genetic markers alone. When used to assign individuals from the mixing areas to their population of origin, the integrated model resolved some (but not all) discrepancies between the chemistry and genetic methods. Some individuals in the mixing area had otolith δ¹⁸O values and genetic profiles which when taken together, were not representative of either population. These fish may originate from another Atlantic spawning area or may represent population contingents that move away from the main spawning areas during the first year of life. This complexity in stock structure is not captured by the current two-stock model.

Prevalence, intensity and pathology of the nasal parasite Nasicola hogansi in Atlantic bluefin tuna (Thunnus thynnus)

Ectoparasitic flatworms of Nasicola (Monogenoidea: Capsalidae), which infect nasal epithelium of true tunas (Thunnus spp.), are not well studied, nor have their impacts on the host's olfactory organ been evaluated. Infections of Nasicola hogansi on Atlantic bluefin tuna, Thunnus thynnus, were investigated with emphasis on the relationship between infection prevalence, abundance and mean intensity with bluefin tuna size, sex, body condition and capture month, as well as histopathological effects. Commercially caught Atlantic bluefin tuna (n = 161, 185-305 cm curved fork length) from the Gulf of Maine were sampled during June through August 2009 for infections by N. hogansi. A total of 247 specimens of N. hogansi were collected, with a prevalence of 45.3%, mean abundance of 1.57 (CI: 1.21-2.03) and mean intensity of 3.45 (CI: 2.91-4.22). Neither fish sex nor landing month had a significant effect on parasite parameters. Larger and better-conditioned Atlantic bluefin tuna had a higher mean intensity of infection. Pathology associated with infection by N. hogansi included extensive necrosis, sloughing of the nasal epithelium and associated inflammation of underlying connective tissues. Further epidemiological and pathological study of this host-parasite system is warranted since impaired olfaction, if present, could adversely affect spawning and migration of this top ocean predator.

Inferring trophic groups of fish in the central-east Atlantic from eco-toxicological characterization

The marine organisms are exposed to great human-induced alterations due to the indiscriminate discharges into the sea, which is why the study of marine pollution is of great value for each ecosystem. Each organism bioaccumulates distantly the heavy metals and trace elements in its organism. Because of this it is possible to classify different groups of fish according to their feeding with the content of these metals. Ten fish species were grouped considering their trophic level and habitat ecology (benthic predators, herbivores, omnivores, pelagic predators and superpredator) and analyzed for its metal content. Statistically significant differences were found among all the fish groups, with the Superpredator group containing the highest concentrations in all metals, mainly Fe (103.751 ± 92.151 mg/kg) and Al (28.908 ± 21.221 mg/kg). Therefore, this study highlights that the selection of the species taking into account feeding and habitat partitioning must be carefully considered being crucial to identify fish groups as biological indicators of marine pollution.

A possible new spawning area for Atlantic bluefin tuna (Thunnus thynnus): the first histologic evidence of reproductive activity in the southern Gulf of Mexico

The number of studies of reproductive biology for Atlantic bluefin tuna carried out in the Gulf of Mexico is significantly lower than those undertaken in the Mediterranean Sea. Four spawning areas have been found for the eastern Atlantic bluefin tuna stock in the Mediterranean Sea, so it is not implausible that there is more than one spawning area in the Gulf of Mexico for the western Atlantic bluefin tuna stock. The individuals used in this study were caught as bycatch by the Mexican surface longline fleet between January and April 2015. A total of 63 individuals ranging between 192 and 293 cm L_F (mean = 238 ± 22.52 cm) were measured. Gonads from 46 fish (31 females and 15 males) were collected for histological examination. All the individuals were classified as mature; 25 were reproductively active (in spawning capable and spawning stages). The histological analysis indicates spawning activity in Mexican waters (the southern Gulf of Mexico). Spawning occurred in March and April, when the sea surface temperature was 25.57 °C ± 0.69 in March and 27.03 °C ± 0.69 in April. Information on the location of the spawning areas is necessary for a correct management of species. The present study provides the first histological evidence of reproductive activity in Mexican waters, and indicates a wider spawning area, beyond just the northern zone, potentially encompassing the entire Gulf of Mexico.

Atlantic bluefin tuna spawn at suboptimal temperatures for their offspring

Life-history traits such as spawning migrations and timing of reproduction are adaptations to specific environmental constraints and seasonal cycles in many organisms' annual routines. In this study we analyse how offspring fitness constrains spawning phenology in a large migratory apex predator, the Atlantic bluefin tuna. The reproductive schedule of Atlantic bluefin tuna varies between spawning sites, suggesting plasticity to local environmental conditions. Generally, temperature is considered to be the main constraint on tuna spawning phenology. We combine evidence from long-term field data, temperature-controlled rearing experiments on eggs and larvae, and a model of egg fitness, and show that Atlantic bluefin tuna do not spawn to optimize egg and larval temperature exposure. The timing of spawning leads to temperature exposure considerably lower than optimal at all spawning grounds across the Atlantic Ocean. The early spawning is constrained by thermal inhibition of egg hatching and larval growth rates, but some other factors must prevent later spawning. Matching offspring with ocean productivity and the prey peak might be an important driver for bluefin tuna spawning phenology. This finding is important for predictions of reproductive timing in future climate warming scenarios for bluefin tuna.

Atlantic Multidecadal Oscillations drive the basin-scale distribution of Atlantic bluefin tuna

The Atlantic bluefin tuna (hereafter referred to as "bluefin tuna"), one of the world's most valuable and exploited fish species, has been declining in abundance throughout the Atlantic from the 1960s until the mid-2000s. Following the establishment of drastic management measures, the stock has started to recover recently and, as a result, stakeholders have raised catch quotas by 50% for the period 2017-2020. However, stock assessments still omit the natural, long-term variability in the species distribution. Here, we explore the century-scale fluctuations in bluefin tuna abundance and distribution to demonstrate a prevailing influence of the Atlantic Multidecadal Oscillation (AMO) to provide new insights into both the collapse of the Nordic bluefin tuna fishery circa 1963 and the recent increase in bluefin tuna abundance in the Northeast Atlantic. Our results demonstrate how climatic variability can modulate the distribution of a large migrating species to generate rapid changes in its regional abundance, and we argue that climatic variability must not be overlooked in stock management plans for effective conservation.

Potential medium-term impacts of climate change on tuna and billfish in the Gulf of Mexico: A qualitative framework for management and conservation

A systematic review of scientific papers on the potential impacts of climate-driven environmental changes on tuna and billfish in the Gulf of Mexico (GOM) was conducted to identify the climate-driven pressures and their associated potential impacts on the reproductive success and survival of tuna and billfish, and which of those impacts may have more relevance for their management and conservation in the GOM by 2050. An Impact Screening Analysis (ISA) was developed to evaluate the potential climate impacts discovered in the literature synthesis by assessing each impact against four criteria, and assigning it a ranking based on likelihood of occurrence (High, Medium, or Low). Results show three types of climate-driven pressures within the High ranking: increased water temperature; changes in ocean circulation and eddy kinetic energy; and changes in storm and wind patterns. Our findings provide valuable information to advance our understanding of key climate-driven physico-chemical processes that can impact the biology of tuna and billfish in the GOM, and enhance conservation and management of these species.

Population trends in Vermivora warblers are linked to strong migratory connectivity

Migratory species can experience limiting factors at different locations and during different periods of their annual cycle. In migratory birds, these factors may even occur in different hemispheres. Therefore, identifying the distribution of populations throughout their annual cycle (i.e., migratory connectivity) can reveal the complex ecological and evolutionary relationships that link species and ecosystems across the globe and illuminate where and how limiting factors influence population trends. A growing body of literature continues to identify species that exhibit weak connectivity wherein individuals from distinct breeding areas co-occur during the nonbreeding period. A detailed account of a broadly distributed species exhibiting strong migratory connectivity in which nonbreeding isolation of populations is associated with differential population trends remains undescribed. Here, we present a range-wide assessment of the nonbreeding distribution and migratory connectivity of two broadly dispersed Nearctic-Neotropical migratory songbirds. We used geolocators to track the movements of 70 Vermivora warblers from sites spanning their breeding distribution in eastern North America and identified links between breeding populations and nonbreeding areas. Unlike blue-winged warblers (Vermivora cyanoptera), breeding populations of golden-winged warblers (Vermivora chrysoptera) exhibited strong migratory connectivity, which was associated with historical trends in breeding populations: stable for populations that winter in Central America and declining for those that winter in northern South America.

Spatial dynamics and mixing of bluefin tuna in the Atlantic Ocean and Mediterranean Sea revealed using next-generation sequencing

The Atlantic bluefin tuna is a highly migratory species emblematic of the challenges associated with shared fisheries management. In an effort to resolve the species' stock dynamics, a genomewide search for spatially informative single nucleotide polymorphisms (SNPs) was undertaken, by way of sequencing reduced representation libraries. An allele frequency approach to SNP discovery was used, combining the data of 555 larvae and young-of-the-year (LYOY) into pools representing major geographical areas and mapping against a newly assembled genomic reference. From a set of 184,895 candidate loci, 384 were selected for validation using 167 LYOY. A highly discriminatory genotyping panel of 95 SNPs was ultimately developed by selecting loci with the most pronounced differences between western Atlantic and Mediterranean Sea LYOY. The panel was evaluated by genotyping a different set of LYOY (n = 326), and from these, 77.8% and 82.1% were correctly assigned to western Atlantic and Mediterranean Sea origins, respectively. The panel revealed temporally persistent differentiation among LYOY from the western Atlantic and Mediterranean Sea (F_ST  = 0.008, p = .034). The composition of six mixed feeding aggregations in the Atlantic Ocean and Mediterranean Sea was characterized using genotypes from medium (n = 184) and large (n = 48) adults, applying population assignment and mixture analyses. The results provide evidence of persistent population structuring across broad geographic areas and extensive mixing in the Atlantic Ocean, particularly in the mid-Atlantic Bight and Gulf of St. Lawrence. The genomic reference and genotyping tools presented here constitute novel resources useful for future research and conservation efforts.

Long-term aggregation of larval fish siblings during dispersal along an open coast

Pelagic dispersal of most benthic marine organisms is a fundamental driver of population distribution and persistence and is thought to lead to highly mixed populations. However, the mechanisms driving dispersal pathways of larvae along open coastlines are largely unknown. To examine the degree to which early stages can remain spatially coherent during dispersal, we measured genetic relatedness within a large pulse of newly recruited splitnose rockfish (Sebastes diploproa), a live-bearing fish whose offspring settle along the US Pacific Northwest coast after spending up to a year in the pelagic environment. A total of 11.6% of the recruits in a single recruitment pulse were siblings, providing the first evidence for persistent aggregation throughout a long dispersal period. Such protracted aggregation has profound implications for our understanding of larval dispersal, population connectivity, and gene flow within demersal marine populations.

Quantifying overlap between the Deepwater Horizon oil spill and predicted bluefin tuna spawning habitat in the Gulf of Mexico

Atlantic bluefin tuna (Thunnus thynnus) are distributed throughout the North Atlantic and are both economically valuable and heavily exploited. The fishery is currently managed as two spawning populations, with the GOM population being severely depleted for over 20 years. In April-August of 2010, the Deepwater Horizon oil spill released approximately 4 million barrels of oil into the GOM, with severe ecosystem and economic impacts. Acute oil exposure results in mortality of bluefin eggs and larvae, while chronic effects on spawning adults are less well understood. Here we used 16 years of electronic tagging data for 66 bluefin tuna to identify spawning events, to quantify habitat preferences, and to predict habitat use and oil exposure within Gulf of Mexico spawning grounds. More than 54,000 km² (5%) of predicted spawning habitat within the US EEZ was oiled during the week of peak oil dispersal, with potentially lethal effects on eggs and larvae. Although the oil spill overlapped with a relatively small portion of predicted spawning habitat, the cumulative impact from oil, ocean warming and bycatch mortality on GOM spawning grounds may result in significant effects for a population that shows little evidence of rebuilding.