ABSTRACT
Female-biased mortality has been repeatedly reported in Pacific salmon during their upriver migration in both field studies and laboratory holding experiments, especially in the presence of multiple environmental stressors, including thermal stress. Here, we used coho salmon (Oncorhynchus kisutch) to test whether females exposed to elevated water temperatures (18°C) (i) suppress circulating sex hormones (testosterone, 11-ketotestosterone and estradiol), owing to elevated cortisol levels, (ii) have higher activities of enzymes supporting anaerobic metabolism (e.g. lactate dehydrogenase, LDH), (iii) have lower activities of enzymes driving oxidative metabolism (e.g. citrate synthase, CS) in skeletal and cardiac muscle, and (iv) have more oxidative stress damage and reduced capacity for antioxidant defense [lower catalase (CAT) activity]. We found no evidence that a higher susceptibility to oxidative stress contributes to female-biased mortality at warm temperatures. We did, however, find that females had significantly lower cardiac LDH and that 18°C significantly reduced plasma levels of testosterone and estradiol, especially in females. We also found that relative gonad size was significantly lower in the 18°C treatment regardless of sex, whereas relative liver size was significantly lower in females held at 18°C. Further, relative spleen size was significantly elevated in the 18°C treatments across both sexes, with larger warm-induced increases in females. Our results suggest that males may better tolerate bouts of cardiac hypoxia at high temperature, and that thermal stress may also disrupt testosterone- and estradiol-mediated protein catabolism, and the immune response (larger spleens), in migratory female salmon.
Footnotes
Competing interests
The authors declare no competing or financial interests.
Funding
This work was supported by a University of California Santa Barbara Faculty Research Award to E.J.E. A.P.F. and S.J.C. are funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chair program. Funding was also provided by NSERC Discovery and Strategic Grants to S.G.H., S.J.C. and A.P.F. Funded in part by DFO's Aquatic Climate Change Adaptation Services Program to D.A.P.
Author contributions
Conceptualization: A.P.F., S.J.C., D.A.P., S.G.H., E.J.E.; Methodology: A.G.L., E.H., K.K., T.D., T.S.P., B.H., E.J.E.; Validation: E.J.E.; Formal analysis: A.G.L., K.K., E.J.E.; Investigation: A.G.L., E.H., K.K., T.D., T.S.P., B.H., E.J.E.; Resources: J.N.P., A.P.F., S.J.C., D.A.P., S.G.H., E.J.E.; Data curation: E.H.; Writing - original draft: A.G.L., E.J.E.; Writing - review & editing: A.G.L., E.H., K.K., T.D., T.S.P., B.H., J.N.P., A.P.F., S.J.C., D.A.P., S.G.H.; Supervision: A.G.L., J.N.P., A.P.F., S.J.C., D.A.P., S.G.H., E.J.E.; Project administration: E.J.E.; Funding acquisition: J.N.P., A.P.F., S.J.C., D.A.P., S.G.H., E.J.E.
- Received September 16, 2019.
- Accepted June 10, 2020.
- © 2020. Published by The Company of Biologists Ltd
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