What the Science Says: Can Fish Eat Their Way Out of Climate Change?

 

By Dr. Rob Lusardi
CalTrout - UC Davis Wild and Coldwater Fish Scientist


Can fish eat their way out of climate change?

About two years ago, I wrote an article on the importance of food for foraging salmon, particularly during their early life history stages.  In that article, I suggested that food rich habitats such as spring-fed rivers, floodplains, estuaries, and lagoons would become disproportionally important for coldwater fishes under a rapidly changing climate.  

The argument largely relies on bioenergetic theory: as water temperatures increase with the onset of global change, the metabolic activity of coldwater fishes will also increase.  If those increases in metabolic activity are not compensated for by either moving to cold water or increasing food consumption, salmon may experience reductions in growth, increases in stress, and higher rates of disease prevalence.  

Increasing water temperatures, as a result of climate change, are already reducing thermal habitat for coldwater fishes throughout California and elsewhere.  For instance, Wenger et al. (2011) projected nearly a 50% decline in thermal habitat for all trout species by 2080. Ultimately, this suggests that salmonids find themselves in an increasingly precarious predicament, particularly those species at the southern end of their range where the impacts of global warming may be greater.  

While conserving and enhancing coldwater habitat has received ample attention in the literature and is likely the single best strategy to mitigate the effects of climate change, few have focused on food rich habitats as possible refuges for coldwater fishes.

 

Cover Photo: Fish enclosures in the Shasta River by Rob Lusardi

 

 

 

 

 

Photo: Fish enclosures by Mt. Shasta by Rob Lusardi

 

 

Testing our theory

A few years back, we decided to test a simple question.  Could fish compensate for increases in water temperature if high densities of prey were available under natural stream conditions?  The focus on “natural conditions” (e.g., a stream or river) was key to our scientific pursuit.

Numerous physiological studies have shown that salmonids can compensate for increases in water temperature when enough food is available.  These studies, however, are almost always conducted in the laboratory, where fish are fed to satiation in tanks (an uncommon if not entirely rare phenomenon in the wild). 

 

Photo: Fish on the left were reared near coldwater springs with limited prey availability, while fish on the right experienced relatively warmer water conditions, but with high prey availability.By Rob Lusardi

 

 

Results show abundant food can offset increased temperature

60 days later we pulled the fish and tabulated the results. We found that juvenile coho were able to compensate for increases in temperature when sufficient food was present.  Coho growth rates peaked at a mean temperature of 16.6 ºC (~62 ºF) and a maximum temperature of 21.1 ºC (~70 ºF).  The growth rates of these fish were six-fold greater than those reared in the coldest study reach (mean temperature: 13.0 ºC (55.4 ºF); maximum: 16.0 ºC (~61 ºF)).  Further, the statistics specifically pointed to food as being the primary mechanism affecting observed differences in fish growth.   

There are a couple caveats to this study.  First, the findings should not be misconstrued as an endorsement of warmer water temperatures in lieu of functioning stream habitat. Elevated water temperatures have scientifically been shown to negatively affect coldwater species in numerous ways.  Further, the fish in this study were unable to move to colder water during the experiment because they were enclosed in cages, thus forcing them to experience a range of conditions throughout the experiment. 

 

Photo: Fish enclosures in the Shasta River by Rob Lusardi

 

 

So, can fish eat their way out of climate change? 

Probably not.  However, even though food has received relatively little attention as a key component to fish habitat, it may just be one of the most important habitat components to consider under a changing climate.

 

Dr. Robert Lusardi is the California Trout-UC Davis Wild and Coldwater Fish Scientist.

The scientific publication referenced above appeared in the Canadian Journal of Fisheries and Aquatic Sciences:

Lusardi, R.A. Hammock, B.G., Jeffres, C.A., Dahlgren, R.A., and Kiernan, J.D. 2020. Oversummer growth and survival of coho salmon (Oncorhynchus kisutch) across a natural gradient of temperature and prey availability: an in situ enclosure experiment. Canadian Journal of Fisheries and Aquatic Sciences 77(2): 413-424.

 

Photo: Salmon Spawning in Shasta River by Carson Jeffres

 

 

 

Other literature cited:

Bisson, P. A., Nielsen, J. L., & Ward, J. W. (1988). Summer production of coho salmon stocked in Mount St. Helens streams 3-6 years after the 1980 eruption. Transactions of the American Fisheries Society, 117(4), 322-335.

Corline, N. J., Sommer, T., Jeffres, C. A., & Katz, J. (2017). Zooplankton ecology and trophic resources for rearing native fish on an agricultural floodplain in the Yolo Bypass California, USA. Wetlands Ecology and Management, 25(5), 533-545.

Katz, J. V. E., Jeffres, C., Conrad, J. L., Sommer, T. R., Martinez, J., Brumbaugh, S., . . . Moyle, P. B. (2017). Floodplain farm fields provide novel rearing habitat for Chinook salmon. Plos One, 12(6), e0177409.

Osterback, A.-M. K., Kern, C. H., Kanawi, E. A., Perez, J. M., & Kiernan, J. D. (2018). The effects of early sandbar formation on the abundance and ecology of coho salmon (Oncorhynchus kisutch) and steelhead trout (Oncorhynchus mykiss) in a central California coastal lagoon. Canadian Journal of Fisheries and Aquatic Sciences.

Wenger, S.J., et al.  2011. Flow regime, temperature, and biotic interactions drive differential declines of trout species under climate change.  Proceedings of the National Academy of Sciences 108: 14175-14180.

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