The responses of subtropical nearshore fishes to climate change

Global climate change is expected to alter seawater conditions and marine ecosystems. The Intergovernmental Panel on Climate Change (IPCC) predicts that sea surface pH will decrease by 0.06 - 0.32 pH units and temperature will increase by 0.6-2.0°C over the next 100 years, and extreme weather events, such as major storms (e.g., tropical cyclones), floods, heat waves, and cold spells, are expected to increase in intensity and frequency. Nearshore ecosystems serve as critical habitat for juvenile fishes (e.g., schoolmaster snapper, yellowfin mojarra) and function as foraging grounds for adults (e.g., bonefish, checkered puffer), yet it is unclear how nearshore fishes will respond to forecasted increases in temperature and pCO2. To fill this knowledge gap, I assessed the physiology, behavior, and thermal plasticity of nearshore species challenged by climate change stressors, and determined how the presence of a predator will influence habitat choice under forecasted conditions. Climate change research on marine organisms has primarily focused on species that inhabit the open ocean or coral reef environments. Elevated temperature, salinity, and acidity (reduced pH) have all been shown to result in varying degrees of physiological disturbance in organisms across ecosystems, yet little effort has been devoted to understanding the magnitude of these disturbances in nearshore fishes across seasons. Moreover, few studies have investigated the synergistic (or canceling) effects of combined climate-related challenges. Based on this background, the purpose of this aspect of the study was to assess the relative impacts of climate-related challenges across seasons on fishes that inhabit the nearshore ecosystem. To do this, blood-based physiological responses to elevated temperature, salinity, acidity, and temperature + acidity of adult bonefish, adult checkered puffer, and juvenile yellowfin mojarra were compared across seasons (i.e., summer and winter). Bonefish exposed to climate-related challenges experienced elevated Cl-, Ca2+, lactate, and glucose levels in the summer relative to winter, and elevated temperature resulted in glucose and ionic disturbances in both seasons. Similarly, independent of season, checkered puffer exhibited elevated hematocrit, glucose, and cortisol levels when exposed to elevated temperature relative to ambient conditions. Yellowfin mojarra experienced minimal physiological disturbance to the climate-related challenges, but independent of season, elevated temperature increased hematocrit values relative to ambient conditions. Interestingly, the combined stressor, temperature + acidity, did not result in a synergistic or additive physiological disturbance in nearshore fishes. Results indicate that adult bonefish exposed to climate-related challenges experience the greatest physiological disturbances in the summer relative to the winter, with temperature being the most challenging stressor. Collectively, temperature change will be the most challenging climate-related stressor for nearshore fishes, particularly in the summer, and may result in more energy being expended to maintain homeostasis, which could translate into altered behavior, performance, and/or habitat choice under forecasted conditions. Global climate change scenarios forecasted by the Intergovernmental Panel on Climate Change (IPCC) are expected to alter the performance of marine fishes and potentially disrupt ecosystems. Indeed, climate-related stressors have been shown to reduce metabolic performance in fish from coral reef environments, which may have consequences for growth, reproduction, and distribution of these fishes. Unfortunately, very few studies focus on more than one species or expose fish to several scenarios, which hinders our ability to predict how fish communities and ecosystems will respond to future climate change. The purpose of this aspect of the study was to define the performance of subtropical nearshore fishes to projected climate change scenarios and relate these findings to the ecology of nearshore ecosystem in the future. Adult bonefish, adult checkered puffer, juvenile yellowfin mojarra and juvenile schoolmaster snapper, were acclimated to ambient conditions, along with a range of elevated pCO2, salinity, and temperatures that coincide with, or exceed, the worst-case scenario predicted by the IPCC in 100 years. Post acclimation, fish performance was quantified by measuring standard metabolic rate (SMR) with intermittent-flow, static respirometry, and by chasing the fish to exhaustion (i.e., swimming performance (SP)). Bonefish and yellowfin mojarra experienced an increase in SMR of more than 65% when acclimated to seawater at 33ºC, and all species experienced an increase in SMR and a decrease in SP at 34ºC. Nearshore fishes acclimated to elevated pCO2 and salinity experienced minimal disturbances in SMR or SP. Results indicate that elevated temperatures that coincide with (33ºC), or exceed (34ºC), the worst-case scenario predicted by the IPCC increased standard metabolic rates and reduced swimming performance for a number of nearshore fishes, with elevated pCO2 and salinity resulting in minimal disturbances in performance. To avoid costs associated with elevated temperatures, nearshore fishes may choose to migrate to more thermally stable environments, allocate energy differently, or increase feeding rates to meet energetic demands. The capacity of fishes to offset these costs will determine ecological “winners” and “losers” in the future and potentially result in a slow restructuring of the nearshore ecosystem. The proximity of fishes to their thermal limits, coupled with their potential to acclimatize to future environmental conditions, will be additional factors influencing the structure of marine ecosystems as the climate changes. Species that have relatively high thermal maxima are expected to have a limited capacity to acclimatize to new conditions, but this assumption as not been tested on fishes in variable thermal environments in subtropical nearshore ecosystems. The goal of this aspect of the study was to assess the vulnerability of nearshore fishes to climate change and to evaluate the ability of these fishes to adjust their physiological limits across seasons (i.e., phenotypic plasticity). To do this, the critical thermal maximum (CTMax) and minimum (CTMin) of adult bonefish, adult checkered puffer, juvenile yellowfin mojarra, and juvenile schoolmaster snapper were determined across seasons. Acclimatization response ratios (AZRR; ΔCTMax ΔT-1 and ΔCTMin ΔT-1) were typically greater than 0.60 for all species, a value greater than most previously reported for fish species from variable thermal environments. Present day maximum and minimum temperatures in the nearshore environment are approximately equal to or exceed the thermal tolerance limits of the fish in this study, making thermal safety margins (TSM; i.e., the difference between thermal tolerance limit and extreme environmental temperature) very small or even negative for nearshore fishes (TSM upper = -4.9-0.5; lower = -0.2-0.4). The thermal landscape in the nearshore ecosystem in the future will likely benefit species with positive thermal safety margins that are capable of acclimatizing (e.g., schoolmaster snapper) while relatively intolerant species (e.g., bonefish) may select habitats in this ecosystem less frequently or will be absent from this ecosystem in the future. Habitat selection in fish is typically governed by the tradeoff between the benefit and cost of acquiring food. Nearshore fishes reside in shallow environments to reduce the threat of predation and/or take advantage of feeding opportunities and routinely experience challenging abiotic conditions. Little is known about the tradeoff between accepting the physiological costs of maintaining homeostasis in novel environments (e.g., elevated temperature associated with climate change) or risking predation by moving to new, less physiologically demanding habitats. The purpose of this aspect of the study was to define the relative cost of habitat selection by measuring temperature and pCO2 avoidance thresholds of nearshore fishes under altered abiotic conditions in the presence or absence of a predator, (i.e., a juvenile lemon shark). To do this, common subtropical nearshore fishes (i.e., juvenile schoolmaster snapper, juvenile yellowfin mojarra, adult bonefish, and adult checkered puffer) were acclimated to a behavioral choice arena (i.e., two chambers connected by a central corridor). Temperature or CO2 were manipulated in one chamber while the other chamber was maintained at ambient conditions. Results show that elevated temperatures and pCO2 alter habitat choice in all nearshore fishes in this study, and that temperature and pCO2 avoidance thresholds increase in the presence of a predator. Collectively, elevated temperatures and pCO2 may alter habitat use and distribution of fishes in nearshore ecosystems, with community structure in predator-rich environments looking very different from environments with low predator burdens. The outcomes of this research indicate that nearshore fishes exposed to elevated temperatures experience physiological disturbances in the summer, decreased swimming and metabolic performance, some plasticity in thermal tolerance limits, and altered habitat choice in the presence of a predator. Collectively, temperatures that coincide with and exceed future predictions will have species-specific impacts, potentially resulting in shifting community structure in the nearshore ecosystem