Search

Limited food availability and altered thermal regimes (e.g. cold water releases from dams) are two common stressors threatening the persistence of fishes inhabiting anthropogenically disturbed freshwater systems. Yet, the combined effects of these stressors remain poorly characterised. To remedy this, we examined the isolated and combined effects of low temperature exposure and food restriction on specific growth rate (SGR, % body mass/day) and upper thermal tolerance (critical thermal maxima, CTMax) in larval white sturgeon (Acipenser transmontanus [Acipenseridae], 32 days post-hatch, body mass: 0.25 ± 0.03 g, mean ± standard deviation). A 2 × 2 factorial design was implemented with fish exposed to one of two ecologically-relevant acclimation temperatures (cold exposure: 11°C or a control temperature: 18°C) and one of two food restriction treatments designed to emulate observed declines in food availability (100% or 40% optimal feed rate) for 6 weeks (N: 3 replicate tanks/treatment, 50 fish/tank). Specific growth rate was affected by both low temperature exposure and food restriction in isolation; low temperature exposure reduced SGR by 56.5% and food restriction reduced SGR by 30.6%. Simultaneous exposure to low temperature and food restriction resulted in a greater but less than additive reduction in SGR (80.6%), indicating that the stressors interacted antagonistically. Critical thermal maxima were c. 2°C higher in 18°C-acclimated fish (CTMax = 30.7 ± 0.4°C, mean ± standard error) compared to 11°C-acclimated fish (CTMax = 28.6 ± 0.2°C, mean ± standard error); however, CTMax was independent of food restriction in both 11°C- and 18°C-acclimated fish. These data highlight the unpredictability of stressor interactions and may guide holistic conservation strategies, which target co-occurring stressors in freshwater habitats.

Reversing global declines in the abundance and diversity of fishes is dependent on science-based conservation solutions. A wealth of data exist on the ecophysiological constraints of many fishes, but much of this information is underutilized in recovery plans due to a lack of synthesis. Here, we used the imperiled green sturgeon (Acipenser medirostris) as an example of how a quantitative synthesis of physiological data can inform conservation plans, identify knowledge gaps and direct future research actions. We reviewed and extracted metadata from peer-reviewed papers on green sturgeon. A total of 105 publications were identified, spanning multiple disciplines, with the primary focus being conservation physiology (23.8%). A meta-analytical approach was chosen to summarize the mean effects of prominent stressors (elevated temperatures, salinity, low food availability and contaminants) on several physiological traits (growth, thermal tolerance, swimming performance and heat shock protein expression). All examined stressors significantly impaired green sturgeon growth, and additional stressor-specific costs were documented. These findings were then used to suggest several management actions, such as mitigating salt intrusion in nursery habitats and maintaining water temperatures within optimal ranges during peak spawning periods. Key data gaps were also identified; research efforts have been biased towards juvenile (38.1%) and adult (35.2%) life-history stages, and less data are available for early life-history stages (embryonic, 11.4%; yolk-sac larvae, 12.4%; and post yolk-sac larvae, 16.2%). Similarly, most data were collected from single-stressor studies (91.4%) and there is an urgent need to understand interactions among stressors as anthropogenic change is multi-variate and dynamic. Collectively, these findings provide an example of how meta-analytic reviews are a powerful tool to inform management actions, with the end goal of maximizing conservation gains from research efforts.

Human-induced thermal variability can disrupt energy balance and performance in ectotherms; however, phenotypic plasticity may play a pivotal protective role. Ectotherm performance can be maintained in thermally heterogeneous habitats by reducing the thermal sensitivity of physiological processes and concomitant performance. We examined the capacity of juvenile green sturgeon (Acipenser medirostris) to respond to daily thermal variation. Juveniles (47 days post-hatch) were exposed to either stable (15 ± 0.5 °C) or variable (narrowly variable: 13-17 °C day-1 or widely variable 11-21 °C day-1) thermoperiod treatments, with equivalent mean temperatures (15 ± 0.5 °C), for 21 days. Growth (relative growth rate, % body mass gain), upper thermal tolerance (critical thermal maxima, CTMax) and the thermal sensitivity of swimming performance (critical swimming speed, Ucrit) were assessed in fish from all treatments. Accelerated growth was observed in fish maintained under widely variable temperatures compared to narrowly variable and stable temperatures. No significant variation in CTMax was observed among thermoperiod treatments, suggesting all treatment groups acclimated to the mean temperature rather than daily maximums. The widely variable treatment induced a plastic response in swimming performance, where Ucrit was insensitive to temperature and performance was maintained across a widened thermal breadth. Maximum Ucrit attained was similar among thermoperiod treatments, but performance was maximised at different test temperatures (stable: 4.62 ± 0.44 BL s-1 at 15 °C; narrowly variable: 4.52 ± 0.23 BL s-1 at 21 °C; widely variable: 3.90 ± 0.24 BL s-1 at 11 °C, mean ± s.e.m.). In combination, these findings suggest juvenile A. medirostris are resilient to daily fluctuations in temperature, within the temperature range tested here.

Exercise training may be an effective tool for promoting growth and production efficiency in finfish aquaculture. Yet, changes in growth and feed conversion ratios (FCR) following periods of exercise can be varied and context dependent. Variation in species and training protocols (duration, intensity and regime) can misalign interpretations, and individual studies hold limited statistical power. To remedy this, we compiled data from 69 studies spanning 31 species to quantify the effects of exercise training on growth rates and FCRs. Using meta‐analytic tools, we show that exercise training significantly increases growth rates across a range of taxonomically diverse finfish. Optimal training regimes were identified; effects were strongest when fish were exercised continuously rather than intermittently, at target swimming speeds ≤ 2 body lengths s−1, for longer durations of time. These results are most applicable to juvenile fish (mean length of fish in the analyses = 16 cm). Exercise training was also shown to decrease variability in growth rates, leading to fish that are more uniform in size. FCRs were unaffected by exercise, showing that exercised‐ and control‐fish were equally efficient at converting food into body mass. Overall, our analyses show that exercise is a powerful management tool in promoting rapid growth, where fish can reach marketable sizes faster. [ABSTRACT FROM AUTHOR]

Ectotherms are predicted to show a reduction in absolute aerobic scope (AAS = maximum − standard metabolic rates) if habitat temperatures surpass optima. However, thermal phenotypic plasticity may play a protective role in the maintenance of AAS. In fishes, resting physiological rates ("physiological floors," e.g., standard metabolic rates [SMR]) are typically thermally phenotypically plastic whilst maximum physiological rates ("physiological ceilings," e.g., maximum metabolic rate [MMR]) are typically fixed. This observation led to the "plastic floors and concrete ceilings" hypothesis. The applicability of this hypothesis to nonavian reptiles remains untested, despite this group being at risk of climate warming‐induced extinction. We tested this hypothesis in juvenile estuarine crocodiles (Crocodylus porosus) by maintaining animals at a water temperature indicative of current summer conditions (28°C) or at a water temperature reflecting a high magnitude of warming (34°C; i.e., thermal acclimation treatments) for 6 months. Metabolic traits (SMR, MMR, and AAS) were subsequently quantified between 28–36°C. A twofold increase in SMR was observed between 28°C and 36°C in both thermal acclimation treatments (pooled Q10 = 3.2). MMR was thermally insensitive between 28°C and 36°C in 28°C‐acclimated crocodiles but doubled between 28°C and 36°C in 34°C‐acclimated crocodiles. These findings demonstrate thermal phenotypic plasticity in a "physiological ceiling" (MMR) and rigidity in a "physiological floor" (SMR), showing the opposite pattern to many fishes. Overall, crocodiles displayed impressive aerobic capacity at temperatures reflecting climate warming scenarios. AAS remained unchanged across an 8°C temperature range in 28°C‐acclimated animals and doubled in 34°C‐acclimated animals. HIGHLIGHTS: Crocodylus porosus showed thermal phenotypic plasticity in maximum metabolic rates but not standard metabolic rates.Quantification of only standard or maximal metabolic rates likely provide incorrect estimates of species' climate change vulnerability. [ABSTRACT FROM AUTHOR]