Oxygen consumption and haematology of juvenile shortnose sturgeon Acipenser brevirostrum during an acute 24 h saltwater challenge

Atlantic sturgeon and shortnose sturgeon exhibit highly divergent transcriptomic responses to acute heat stress

In comparison to most modern teleost fishes, sturgeons generally display muted stress responses. While a muted stress response appears to be ubiquitous across sturgeon species, the mechanisms unpinning this muted response have not been fully described. The objective of this study was to determine the patterns of hematological and transcriptomic change in muscle tissue following an acute high temperature stress (critical thermal maxima; CT_max) in two locally co-occurring but evolutionarily distant sturgeon species (Atlantic and shortnose sturgeon). The most striking pattern found was that Atlantic sturgeon launched a vigorous transcriptomic response at CT_max, whereas shortnose sturgeon did not. In contrast, shortnose sturgeon have significantly higher cortisol than Atlantics at CT_max, reconfirming that shortnose have a less muted cortisol stress response. Atlantic sturgeon downregulated a number of processes, included RNA creation/processing, methylation and immune processes. Furthermore, a number of genes related to heat shock proteins were differentially expressed at CT_max in Atlantic sturgeon but none of these genes were significantly changed in shortnose sturgeon. We also note that the majority of differentially expressed genes of both species are undescribed and have no known orthologues. These results suggest that, while sturgeons as a whole may show muted stress responses, individual sturgeon species likely use different inducible strategies to cope with acute high temperature stress.

Lack of change in swimming capacity (Ucrit) following acute salinity exposure in juvenile shortnose sturgeon (Acipenser brevirostrum)

The Saint John River (SJR) is home to the only Canadian population of shortnose sturgeon, Acipenser brevirostrum. Adult shortnose sturgeon routinely enter saltwater to forage, yet less is known about how juveniles cope with the associated osmoregulatory pressures. Recently, it has been shown that short-term (24 h) exposure to saltwater causes significant changes to ion and water levels in juvenile shortnose. In some species of fish, notably salmonids, it has been shown that shifts in fluid and ion levels following saltwater challenges reduce the swimming capacity. The relationship between ion concentration and swimming capacity is not well understood for sturgeon species. Our research aimed to determine whether short-term salt exposure affects swimming ability in juvenile shortnose sturgeon. Juvenile, SJR, hatchery-raised shortnose sturgeon (< 1 year old) were exposed to salinities of 0 (control), 16, or 24‰ for 24 h and then subjected to a critical swimming speed test (U_crit) to quantify swimming ability. Following the test, the fish were weighed and blood samples were drawn to be analyzed for plasma ion and cortisol levels. While ion levels and weight loss were significantly higher in salt exposed fish, there were no significant differences in critical swimming speed or cortisol concentrations. This is in contrast to what has been observed in salmonids and Adriatic sturgeon. This suggests the hydromineral imbalance caused by moderate salt exposure is not sufficient to affect the swimming performance of shortnose sturgeon. Shortnose sturgeon are not thought to enter the saline stretches of the SJR until roughly 8 years of age, yet this research shows that much younger juveniles withstand moderate salinity for short periods, with little whole-animal ramifications.

The effects of repeat acute thermal stress on the critical thermal maximum (CTmax) and physiology of juvenile shortnose sturgeon (Acipenser brevirostrum)

The shortnose sturgeon (Acipenser brevirostrum Lesueur, 1818) is a species of special concern in Canada, but little is known about their thermal biology. Information on the upper thermal tolerance of shortnose sturgeon becomes valuable for predicting future survival particularly with climate change and improving species management. Using a modified critical thermal maximum (CTmax) methodology, the objective is to determine whether previous thermal stress affects the thermal tolerance of juvenile shortnose sturgeon when exposed to a second thermal stress event. Prior exposure to thermal stress (CTmax1) did not affect the thermal tolerance (CTmax2) of juvenile shortnose sturgeon when a 24 h recovery period was allotted between tests. However, a significant increase in thermal tolerance occurred when the recovery time between the two thermal challenges was 1 h. Plasma glucose, lactate, and osmolality were all significantly affected by thermal stress, but values returned to control levels within 24 h. Hematocrit and plasma chloride concentrations were not significantly affected by thermal stress. All fish survived the CTmax testing. The data indicate that the thermal tolerance of juvenile shortnose sturgeon is modified when multiple thermal stresses occur closer together (1 h) but not if separated by a longer time period (24 h).

The impact of acute salinity exposure and temperature on the survival, osmoregulation, and hematology of juvenile shortnose sturgeon (Acipenser brevirostrum)

Juvenile shortnose sturgeon (Acipenser brevirostrum Lesueur, 1818) were exposed to seawater and freshwater for 24 h to evaluate the osmoregulatory capabilities over a seasonal temperature gradient (5, 10, 15, 20 °C). Additionally, juveniles were exposed to 5 °C seawater and freshwater over 72 h to evaluate survival and osmoregulatory capacity under cold water conditions. Osmoregulatory capability was evaluated using standard metrics: survival rate, mass loss, plasma chloride ion (Cl–) concentrations, osmolality, oxygen-carrying variables, and energy metabolites. Three mortalities occurred following 24 h exposure to 20 °C seawater (73% survival) and one mortality occurred within 72 h in 5 °C seawater (89% survival). Plasma Cl– concentrations and osmolality were elevated in seawater-exposed juveniles at every exposure time, regardless of temperature. The least mass was lost in juveniles exposed to 5 and 10 °C seawater, versus 15 and 20 °C seawater. Low mass loss is likely due to a lower metabolic rate and lower ventilation, which would slow the rate by which osmotic stress would occur under cold conditions.

The effect of temperature on the resting and post-exercise metabolic rates and aerobic metabolic scope in shortnose sturgeon Acipenser brevirostrum

The effects of acclimation temperature (15, 20, 25 °C) on routine oxygen consumption and post-exercise maximal oxygen consumption rates (MO₂) were measured in juvenile shortnose sturgeon (Acipenser brevirostrum LeSueur, 1818). The routine MO₂ of shortnose sturgeon increased significantly from 126.75 mg O₂ h^-1 kg^-1 at 15 °C to 253.13 mg O₂ h^-1 kg^-1 at 25 °C. The temperature coefficient (Q ₁₀) values of the routine metabolic rates ranged between 1.61 and 2.46, with the largest Q ₁₀ values occurring between 15 and 20 °C. The average post-exercise MO₂ of all temperature groups increased to a peak value immediately following the exercise, with levels increasing about 2-fold among all temperature groups. The Q ₁₀ values for post-exercise MO₂ ranged from 1.21 to 2.12, with the highest difference occurring between 15 and 20 °C. Post-exercise MO₂ values of shortnose sturgeon in different temperature groups all decreased exponentially and statistically returned to pre-exercise (resting) levels by 30 min at 15 and 20 °C and by 60 min at 25 °C. The aerobic metabolic scope (post-exercise maximal MO₂-routine MO₂) increased to a maximum value ∼156 mg O₂ h^-1 kg^-1 at intermediate experimental temperatures (i.e., 20 °C) and then decreased as the temperature increased to 25 °C. However, this trend was not significant. The results suggest that juvenile shortnose sturgeon show flexibility in their ability to adapt to various temperature environments and in their responses to exhaustive exercise.

The physiology of juvenile shortnose sturgeon (Acipenser brevirostrum) during an acute saltwater challenge

One-year-old shortnose sturgeons (Acipenser brevirostrum LeSueur, 1818) were exposed to seawater (32 ppt) and brackish water (16 and 24 ppt) for 12, 24, 48, and 72 h and % body mass loss, plasma ions, energy metabolites, and oxygen-carrying variables were measured to evaluate survival and the physiological response to the acute salinity challenges. Survival in seawater was poor and plasma ion concentration was elevated in seawater. In sturgeons exposed to 24 ppt brackish water, ion concentrations remained elevated for 48 h, but began to decrease by 72 h. Fish exposed to 16 ppt brackish water did not show significant changes in ion concentrations over the 72 h period. Exposure to 32 ppt seawater resulted in significant and large decreases in body mass (about 20%–25%), whereas body mass loss was significantly less in fish exposed to 16 and 24 ppt brackish water. Overall, these findings suggest that juvenile sturgeons are able to tolerate lower salinities well but are less capable to endure higher salinities even on the short term (>48 h). These results suggest that juvenile A. brevirostrum could inhabit brackish environments earlier than previously expected, and that there appears to be a threshold at which salinity becomes a significant stress to these animals.