Introduced piscivore scope for predation can limit the success of native fish reintroduction efforts

Objective Rivers worldwide are heavily altered by human activities. The interactive effects of habitat alteration and introduced piscivores can jeopardize the persistence of native fish species in rivers. Along with leading to increased encounter frequencies and range overlap of introduced and native fish, alterations to the channel topography or hydrodynamic regime of rivers can benefit warmwater piscivores that may prey on juveniles of native coolwater species. Bioenergetics modeling approaches can be readily used to investigate how predation by introduced species may limit the success of river restoration and native fish reintroduction programs because bioenergetics modeling integrates the thermal effects of habitat alteration to provide the “scope for predation,” the predation capacity of the introduced predator population. The rivers of California’s Central Valley are sites of intense management efforts and expenditures to maintain and restore anadromous fish species, especially Chinook Salmon Oncorhynchus tshawytscha, such as via the San Joaquin River Restoration Program. Predation on this species by introduced piscivores has become the subject of concern across water management sectors in California due to the need to balance water supplies. The objective of our study was to provide estimates of the scope for predation of the introduced piscivore guild on juvenile Chinook Salmon to contextualize the potential scale of this stressor in light of the program’s restoration goals. Methods We applied bioenergetics modeling in a scenario-based approach to data on piscivore populations in gravel mine pits in the San Joaquin River of central California, focusing on the numerically dominant introduced piscivore, the Largemouth Bass Micropterus nigricans, as a proxy for the piscivore guild. We specified diet scenarios to mimic observed consumption and potential consumption of Chinook Salmon juveniles under conditions of a successful restoration program using Largemouth Bass diet data to generate annual estimates of scope for predation of Largemouth Bass on juvenile Chinook Salmon. Results Our modeling approach showed that introduced piscivores, as represented by Largemouth Bass, had high monthly and annual values of scope for predation on juvenile Chinook Salmon across different years and scenarios, but the levels of consumption varied notably among months and between diet scenarios. Conclusions The levels of scope for predation found in our simulation modeling results urge the consideration of predation as an important stressor for reintroduced Chinook Salmon populations and effective strategies to ameliorate predation and improve habitat. Due to increasing worldwide effects of water development and hydropower, these methods may be applied to restoration and translocation efforts in modified and invaded river systems worldwide.