Interactive effects of introduced Pacific salmon and brown trout on native brook trout: an experimental and modeling approach

Abstract

Pacific salmon (Oncoryhnchus spp.) and brown trout (Salmo trutta) are introduced species stocked in the Laurentian Great Lakes. In their native range, salmon deliver material that enhances growth, alters isotopic ratios, and increases contaminant burdens of resident fish. However, whether salmon subsidies mediate interactions between competing species is unknown. Here, we employed a mesocosm experiment and a simulation model to determine if salmon tissue consumption influences brook trout (Salvelinus fontinalis) growth, isotopic ratios, and mercury concentrations and whether these were modified by brown trout. Our results indicate that brook trout growth did not increase with provision of salmon tissue and was not reduced by brown trout. However, brook trout exhibited isotopic enrichment and increased mercury concentrations, suggesting dietary intake of salmon tissue. Because salmon eggs have a higher energy density and lower mercury concentration compared with salmon tissue, our simulation model suggests that consumption of salmon eggs rather than tissue can increase growth while reducing mercury accumulation. Overall, our results suggest that the role of introduced Pacific salmon is dependent on both food quantity and quality along with diet contaminant concentrations.

Environmental context and contaminant biotransport by Pacific salmon interact to mediate the bioaccumulation of contaminants by stream‐resident fish

“…() observed that increased egg consumption was negatively correlated to resident fish Hg concentrations in SE Alaska. Prior studies have hypothesized that consumption of carcass tissue is a primary route for energy acquisition (Schindler et al., ) and contaminant exposure to stream‐resident fish (Gerig et al., ). However, if resident fish were consuming large quantities of carcass material in our study, we should observe a concomitant increase in both PCB and Hg concentrations due to the high PCB and Hg concentrations in salmon tissue (Christensen, MacDuffee, MacDonald, Whiticar, & Ross, ; Gerig et al., ).…”

“…Prior studies have hypothesized that consumption of carcass tissue is a primary route for energy acquisition (Schindler et al., ) and contaminant exposure to stream‐resident fish (Gerig et al., ). However, if resident fish were consuming large quantities of carcass material in our study, we should observe a concomitant increase in both PCB and Hg concentrations due to the high PCB and Hg concentrations in salmon tissue (Christensen, MacDuffee, MacDonald, Whiticar, & Ross, ; Gerig et al., ). Overall, our findings suggest that differential accumulation of contaminants between salmon tissue types coupled with a dietary choice could be a primary determinant of the magnitude of bioaccumulation of biotransported contaminants by stream‐resident fish.…”

“…For analysis, 0.02 g of homogenized sample was weighed into ashed nickel boats, placed into the DMA‐80, and analysed via fixed wavelength atomic absorption spectrophotometry (cf. Gerig, Weber, Chaloner, McGill, & Lamberti, ). The DMA‐80 was calibrated using standard reference materials (National Research Council of Canada, DORM‐4, Ottawa, ON).…”

Contaminant Biotransport by Pacific Salmon in the Great Lakes

“…Moreover, Hg in stream-resident fish decline as Hg supplied by salmon and stream-resident fish size increases (Gerig et al, 2018). In contrast, a mesocosm experiment that measured the effect of carcass consumption on Hg bioaccumulation determined that brook and brown trout Hg increased with increased salmon consumption (Gerig et al, 2017). However, similar to field studies from the Great Lakes, Dolly Varden char (Salvelinus malma) in Alaska have lower Hg concentrations as the abundance of spawning salmon increases (Cyr et al, 2016).…”

Increasing connectivity of Great Lakes tributaries: Interspecific and intraspecific effects on resident brook trout and brown trout populations

“…For example, by providing access to additional spawning habitat, increasing the connectivity of tributaries to North America's Great Lakes (hereafter Great Lakes) may result in greater reproduction for invasive species such as sea lamprey Petromyzon marinus and round goby Neogobius melanostomus, a negative effect, but also increased reproduction of highly valued introduced Pacific salmonids currently stocked into the Great Lakes, namely Chinook salmon Oncorhynchus tshawytscha, coho salmon Oncorhynchus kisutch, and adfluvial rainbow trout or steelhead Oncorhynchus mykiss. Eggs and carcasses of migratory Pacific salmonids can provide an additional food resource for resident trout populations (Ivan, Rutherford, & Johengen, 2011), though the degree to which they produce a measurable increase in growth is less certain (Gerig, Weber, Chaloner, McGill, & Lamberti, 2018). Regardless of the geographic location, understanding how resident fish populations will respond to dam removals or increased fish passage is needed to inform management decisions on dam removals and river connectivity, and for informing anglers on how fish populations might respond to river connectivity changes.…”