Stressor interactions in freshwater habitats: Effects of cold water exposure and food limitation on early-life growth and upper thermal tolerance in white sturgeon, Acipenser transmontanus

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 x 2 factorial design was implemented with fish exposed to one of two ecologically-relevant acclimation temperatures (cold exposure: 11 degrees C or a control temperature: 18 degrees 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 degrees C higher in 18 degrees C-acclimated fish (CTMax = 30.7 +/- 0.4 degrees C, mean +/- standard error) compared to 11 degrees C-acclimated fish (CTMax = 28.6 +/- 0.2 degrees C, mean +/- standard error); however, CTMax was independent of food restriction in both 11 degrees C- and 18 degrees 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.