Aerobic response to thermal stress across ontogeny and habitats in a teleost fish.

Near-future climate change projections predict an increase in sea surface temperature that is expected to have significant and rapid effects on marine ectotherms, potentially affecting a number of critical life processes. Some habitats also undergo more thermal variability than others, and the inhabitants therefore must be more tolerant to acute periods of extreme temperatures. Mitigation of these outcomes may occur through acclimation, plasticity or adaptation, although the rate and extent of a species' ability to adjust to warmer temperatures is largely unknown, specifically as it pertains to effects on various performance metrics in fishes that inhabit multiple habitats throughout ontogenetic stages. Here, the thermal tolerance and aerobic performance of schoolmaster snapper (Lutjanus apodus Walbaum, 1792) collected from two different habitats were experimentally assessed under different warming scenarios (temperature treatments = 30, 33, 35, 36°C) to assess vulnerability to an imminently changing thermal habitat. Larger subadult and adult fish collected from a 12 m deep coral reef exhibited a lower critical thermal maximum (CTmax ) compared to smaller juvenile fish collected from a 1 m deep mangrove creek. However, the CTmax of the creek-sampled fish was only 2°C above the maximum water temperature measured in the habitat from which they were collected, compared to a CTmax that was 8°C higher in the reef-sampled fish, resulting in a wider thermal safety margin at the reef site. A generalized linear model showed a marginally significant effect of temperature treatment on resting metabolic rate (RMR), but there were no effects of any of the tested factors on maximum metabolic rate or absolute aerobic scope. Post hoc tests revealed that RMR was significantly higher for creek-collected fish at the 36°C treatment and significantly higher for reef-collected fish at 35°C. Swimming performance [measured by critical swimming speed] was significantly lower at the highest temperature treatment for creek-collected fish and trended down with each successive increase in temperature treatment for reef-collected fish. These results show that metabolic rate and swimming performance responses to thermal challenges are somewhat consistent across collection habitats, and this species may be susceptible to unique types of thermal risk depending on its habitat. We show the importance of intraspecific studies that couple habitat profiles and performance metrics to better understand possible outcomes under thermal stress.

Trophodynamics and mercury bioaccumulation in reef and open-ocean fishes from The Bahamas with a focus on two teleost predators.

Identifying prey resource pools supporting fish biomass can elucidate trophic pathways of pollutant bioaccumulation. We used multiple chemical tracers (carbon [δ13C] and nitrogen [δ15N] stable isotopes and total mercury [THg]) to identify trophic pathways and measure contaminant loading in upper trophic level fishes residing at a reef and open-ocean interface near Eleuthera in the Exuma Sound, The Bahamas. We focused predominantly on the trophic pathways of mercury bioaccumulation in dolphinfish Coryphaena hippurus and wahoo Acanthocybium solandri, 2 commonly consumed pelagic sportfish in the region. Despite residing within close proximity to productive and extensive coral reefs, both dolphinfish and wahoo relied almost exclusively on open-ocean prey over both short and long temporal durations. A larger isotopic niche of dolphinfish suggested a broader diet and some potential prey differentiation between the 2 species. THg concentrations in dolphinfish (0.2 ± 0.1 ppm) and wahoo (0.3 ± 0.3 ppm) were mostly below recommended guidelines for humans (US Environmental Protection Agency (EPA) = 0.3 ppm, US Food and Drug Administration (FDA)= 1.0 ppm) and were within ranges previously reported for these species. However, high THg concentrations were observed in muscle and liver tissue of commonly consumed reef-associated fishes, identifying a previously unrecognized route of potentially toxic Hg exposure for human consumers on Eleuthera and neighboring islands.

Seasonal blood chemistry response of sub-tropical nearshore fishes to climate change.

Climate change due to anthropogenic activity will continue to alter the chemistry of the oceans. Future climate scenarios indicate that sub-tropical oceans will become more acidic, and the temperature and salinity will increase relative to current conditions. A large portion of previous work has focused on how future climate scenarios may impact shell-forming organisms and coral reef fish, with little attention given to fish that inhabit nearshore habitats; few studies have examined multiple challenges concurrently. The purpose of this study was to quantify the blood-based physiological response of nearshore fishes to a suite of seawater conditions associated with future climate change. Fish were exposed to an acute (30 min) increase in salinity (50 ppt), acidity (decrease in pH by 0.5 units) or temperature (7-10°C), or temperature and acidity combined, and held in these conditions for 6 h. Their physiological responses were compared across seasons (i.e. summer vs. winter). Bonefish (Albula vulpes) exposed to environmental challenges in the summer experienced a suite of blood-based osmotic and ionic disturbances relative to fish held in ambient conditions, with thermal challenges (particularly in the summer) being the most challenging. Conversely, no significant treatment effects were observed for yellowfin mojarra (Gerres cinereus) or checkered puffer (Sphoeroides testudineus) in either season. Together, results from this study demonstrate that acute climate-induced changes to thermal habitat will be the most challenging for sub-tropical fishes (particularly in the summer) relative to salinity and pH stressors, but significant variation across species exists.