Body size dictates physiological and behavioural responses to hypoxia and elevated water temperatures in Murray cod (Maccullochella peelii)

Hyperoxia disproportionally benefits the aerobic performance of large fish at elevated temperature

Increasing evidence shows that larger fish are more vulnerable to acute warming than smaller individuals of the same species. This size-dependency of thermal tolerance has been ascribed to differences in aerobic performance, largely owing to a decline in oxygen supply relative to demand. To shed light on these ideas, we examined metabolic allometry in 130 rainbow trout ranging from 12 to 358 g under control conditions (17°C) and in response to acute heating (to 25°C), with and without supplemental oxygen (100% versus 150% air saturation). Under normoxia, high temperature caused an average 17% reduction in aerobic scope compared with 17°C. Aerobic performance disproportionally deteriorated in bigger fish as the scaling exponent (b) for aerobic scope declined from b=0.87 at 17°C to b=0.74 at 25°C. Hyperoxia increased maximum metabolic rate and aerobic scope at both temperatures and disproportionally benefited larger fish at 25°C as the scaling exponent for aerobic scope was reestablished to the same level as at 17°C (b=0.86). This suggests that hyperoxia may provide metabolic refuge for larger individuals, allowing them to sustain aerobic activities when facing acute warming. Notably, the elevated aerobic capacity afforded by hyperoxia did not appear to improve thermal resilience, as mortality in 25°C hyperoxia (13.8%, n=4) was similar to that in normoxia (12.1%, n=4), although we caution that this topic warrants more targeted research. We highlight the need for mechanistic investigations of the oxygen transport system to determine the consequences of differential metabolic scaling across temperature in a climate warming context.

Upper thermal limits are 'hard-wired' across body mass but not populations of an estuarine fish

Climate warming is seeing temperatures breach exceptional thresholds as the frequency and intensity of heat waves increase. Efforts to forecast species vulnerability to climate warming often focus on upper thermal limits threatening survival, overlooking the role of intraspecific variation in determining vulnerability. Using an estuarine fish (black bream, Acanthopagrus butcheri) as a model, we explore how intraspecific variation in body mass and among populations affects upper thermal tolerance. Upper thermal limits were quantified using critical thermal maxima (CTmax) of wild fish. We used a ∼500 g (mean = 52.4 g, range = 0.57-541 g) mass range to test the relationship between body mass and thermal tolerance. Four distinct black bream populations were chosen along a 5° latitudinal cline to explore population differences in thermal limits. Contrary to expectations, there was no effect of body mass on upper thermal limits. However, significant population differences in thermal tolerance were observed that correlate with mean habitat temperatures. Specifically, the southern population had a significantly lower CTmax (35.57 ± 0.43 °C) compared to northern (36.32 ± 0.70 °C) and mid-latitude (36.36 ± 1.15 °C) populations. These data underscore the importance of observing intraspecific variation in thermal limits to reveal the capabilities of individuals within a species to cope with climate warming and improve the management of at-risk life stages and populations.

Physiologically-informed predictions of climate warming effects on native and non-native populations of blue catfish

Blue catfish Ictalurus furcatus has been widely introduced throughout the United States to enhance recreational fisheries. Its success in both its native and non-native range, especially in the context of climate change, will be influenced by its thermal performance. We conducted a laboratory experiment to investigate the responses of wild-captured, subadult blue catfish to temperatures ranging from 7 °C to 38 °C. Blue catfish had relatively low standard metabolic rates, indicating low energetic demands, and hence an ability to survive well even during low-food conditions. Metabolic scope and food consumption rate increased with temperature, with metabolic scope peaking at 29.1 °C, and consumption rate peaking at 32 °C. Body condition remained high up to 32 °C, but decreased drastically thereafter, suggesting limitations in maintaining metabolism through food consumption at temperatures >32 °C; blue catfish cannot survive in such habitats indefinitely. Yet, many fish were able to survive temperatures as high as 38 °C for 5 days, suggesting that acute and occasionally chronic heat waves will not limit this species. Using these results, we also predicted the performance of blue catfish under prevailing conditions and under climate warming at seven locations throughout their current range in the U.S. We found that some blue catfish populations in southern and southeastern areas will likely experience temperatures above the optimal temperature for extended periods due to climate change, thus limiting potential habitat availability for this species. But, many non-native populations, especially those in northern areas such as Idaho, North Dakota, and northern California, may benefit from the expected warmer temperatures during spring and fall.

Integrated behavioural and physiological responses of sand smelt larvae to the effects of warming and hypoxia as combined stressors

Forecasts indicate that rising temperatures towards the future and the expansion of dead zones will change environmental suitability for fish early stages. Therefore, we assessed the chronic effects of warming (26 °C), hypoxia (<2-2.5 mg L-1) or their combination on mortality rate, growth, behaviour, energy metabolism and oxidative stress using Atherina presbyter larvae as a model species. There were no differences between the treatments in terms of mortality rate. The combination of warming and hypoxia induced faster loss of body mass (+22.7%). Warming, hypoxia or their combination enhanced boldness (+14.7-25.4%), but decreased exploration (-95%-121%), increased the time in frozen state (+60.6-80.5%) and depleted swimming speed (-45.6-50.5%). Moreover, routine metabolic rate was depleted under hypoxia or under the combination of warming and hypoxia (-56.6 and 57.2%, respectively). Under hypoxia, increased catalase activity (+56.3%) indicates some level of antioxidant defence capacity, although increased DNA damage (+25.2%) has also been observed. Larvae also exhibited a great capacity to maintain the anaerobic metabolism stable in all situations, but the aerobic metabolism is enhanced (+19.3%) when exposed to the combination of both stressors. The integrative approach showed that changes in most target responses can be explained physiologically by oxidative stress responses. Increased oxidative damages (lipid peroxidation and DNA damage) and increased interaction between antioxidant enzymes (superoxide dismutase and catalase) are associated to increased time in frozen state and decreased swimming activity, growth rates and boldness. Under all stressful situations, larvae reduced energy-consuming behaviours (e.g. depleted exploration and swimming activity) likely to stabilize or compensate for the aerobic and anaerobic metabolisms. Despite being an active small pelagic fish, we concluded that the sensitive larval phase exhibited complex coping strategies to physiologically acclimate under thermal and hypoxic stress via behavioural responses.

Pairing lab and field studies to predict thermal performance of wild fish

In thermally variable ecosystems, temperatures can change extensively on hourly and seasonal timescales requiring ectotherms to possess a broad thermal tolerance (critical thermal minima [CTmin] and maxima [CTmax]). However, whether fish acclimate in the laboratory similarly as they acclimatize in the field under comparable thermal variation is unclear. We used temperature data from a tidal salt marsh to design 21-day lab-acclimation treatments (static: 12, 17, 22, 27 °C; daily variation with mean 22 °C: i) range 17-27 °C, ii) range 17-27 °C with irregular extremes within 12-32 °C). We compared thermal limits in lab-acclimated and field-acclimatized eurythermal arrow goby (Clevelandia ios). Variable temperature-acclimated and acclimatized fish had similar CTmin and CTmax. Notably, arrow gobies showed rapid plasticity in their absolute thermal tolerance within one tidal cycle. The daily mean and max temperatures experienced were positively related to CTmax and CTmin, respectively. This study demonstrates that ecologically informed lab acclimation treatments can yield tolerance results that are applicable to wild fish.