Availability of Alternative Prey Influences Avian Predation on Salmonids
2022
Good, Thomas P. | Weitkamp, Laurie A. | Lyons, Donald E. | Roby, Daniel D. | Andrews, Kelly S. | Bentley, Paul J.
The diet of two piscivorous bird species nesting in the lower Columbia River was compared with purse seine samples of the fish community to explore the influence of alternative prey availability on the consumption of juvenile salmonids in the Columbia River Estuary. The composition of Caspian tern (CATE) and double-crested cormorant (DCCO) diets broadly overlapped with each other and with independent purse seine sampling. However, consistent seasonal differences were observed in the mass of salmonid and marine forage fish prey in net sampling and the diets of piscivorous birds; Caspian terns had a higher proportion of salmonids in their diets than did cormorants or net sampling across seasons in all years. Caspian terns responded nonlinearly to the availability of marine forage fish, with the proportion of juvenile salmonids in their diet significantly declining when the proportion of marine prey species available exceeded a threshold of 48%. In contrast, cormorants showed a weak, linear response in this relationship. These results demonstrate how temporal variation in one prey type (marine forage fish that enter the estuary later in summer) can indirectly reduce consumption of another prey type (juvenile salmonids) by a common predator (piscivorous birds)—an apparent mutualism. This work shows the importance of understanding the broader food web. Developing management strategies that account for avian predator response to variation in the prey fish community will be important for increasing the survival of threatened and endangered salmonids in a changing environment.
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