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The effects of temperature acclimation and heating rate on the thermal tolerance of juvenile white sturgeon (Acipenser transmontanus) Penman, Rachael


Freshwater fish such as white sturgeon (Acipenser transmontanus) are particularly vulnerable to the effects of anthropogenic global warming; however, little is known about how acclimation to higher temperatures or rate of temperature increase affects their thermal tolerance. The Kenney dam on the Nechako River is home to the northern-most population of white sturgeon and is mandated to maintain water temperatures below 20°C for migrating sockeye salmon, but it remains unclear whether 20°C is an appropriate threshold for developing white sturgeon. To address this, 37-51 days post hatch (dph) and 66-80 dph juvenile white sturgeon were acclimated to one of four ecologically relevant temperatures (15°C, 18°C, 21°C, and 24°C) for two weeks, following which thermal tolerance (CTmax), size, condition factor, and survival were assessed. White sturgeon displayed highly plastic CTmax in response to acclimation, illustrated by a positive relationship between acclimation temperature and CTmax and large acclimation response ratios compared to other fish species. Acclimation to temperatures above 18°C was found to negatively affect condition factor, which suggests the presence of a sub-lethal threshold between 18°C and 21°C. Their highly plastic response to temperature was further demonstrated when the effect of heating rate (0.3°C/min, 0.03°C/min, 0.003°C/min) on thermal tolerance, somatic indices, and Hsp mRNA expression was assessed. White sturgeon CTmax was highest in the slowest heating rate, contrary to what has been observed in most other fish species. Hepatosomatic index decreased in all heating rates relative to control fish, indicative of the metabolic costs of thermal stress. Expression of Hsp70 mRNA was increased in all heating rates relative to controls, whereas expression of Hsp90a and Hsp90b mRNA only increased in the two slower trials. Together these data indicate that while white sturgeon have a very plastic thermal response, acclimation to temperatures above 18°C may negatively affect overall health, indicated by lower condition factor. As such, in the best interest of white sturgeon conservation, the operators of the Kenney dam may want to reconsider whether the 20°C threshold is appropriate.

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