Predatory Threat of Introduced Yellow Perch in a Salmonid‐Dominated Reservoir Food Web

Abstract

Unauthorized or unanticipated introductions of sport fish can threaten native species and expose existing fisheries to unnecessary risk. Although introductions of apex predators draw more attention, introductions of coolwater mesopredators like Yellow Perch Perca flavescens could also be problematic yet are poorly studied. Here, we quantified the seasonal and size‐dependent diet and distribution of introduced Yellow Perch and evaluated their direct effects on age‐0 kokanee Oncorhynchus nerka in a 3,793‐ha mesotrophic reservoir during 2016, 15 years after the initial discovery of Yellow Perch. We compared these effects to those of Brown Trout Salmo trutta, which have been present in the reservoir since it was formed. Spatial overlap between kokanee and Yellow Perch occurred for less than 7 d during April and was confined to the inlet region as fry stocked upstream entered the reservoir. Predation from Brown Trout was 39‐fold greater than predation by Yellow Perch during this time. After fry funneled through the inlet, minimal spatial overlap was observed seasonally between Yellow Perch and age‐0 kokanee, particularly during summer, when a thermal refuge formed. Furthermore, age‐0 kokanee outgrew 58% of piscivorous‐sized Yellow Perch by June and outgrew 95% by October. Although age‐0 kokanee were more morphologically vulnerable to Brown Trout throughout the year, Brown Trout occupied nearshore areas away from pelagic kokanee. Yellow Perch exhibited a similar trophic role as Brown Trout by consuming mostly invertebrates (95.8% and 88.1%, respectively). The diet of each species contained over 25% Daphnia, but we saw no evidence that planktivory by Yellow Perch affected the abundance of Daphnia available to kokanee. Peak Daphnia densities in June 2016 (23.9–39.0 individuals/L) were the same as or higher than those recorded during 1993–2000, before Yellow Perch were discovered. Our study represents one of few in‐depth evaluations of introduced Yellow Perch in a montane reservoir. More empirical evaluations of the effects of introduced mesopredators are needed for refining risk assessments and guiding appropriate management actions.

Size‐Dependent Retention of Pelagic‐Oriented Kokanee in Multimesh Gill Nets

“…; Hansen et al. ), emerging parasite–host dynamics involving Salmincola (Hargis et al. ), recreational harvest dynamics (Askey and Johnston ), and predicting spawner escapement for informing wild‐spawn and hatchery operations (Baldwin and McLellan ) require minimally biased information on size and age structure.…”

“…Densities of daphnia Daphnia spp. typically peak in June at~10-30 individuals/L in surface waters (Hansen et al 2019). Overall, feeding and growth conditions in the reservoir support high kokanee growth rates (Stockwell and Johnson 1997).…”

“…Because of this, size structure is fundamental to quantifying and managing (e.g., through harvest regulations, predator suppression, or adjusting stocking practices) the often complex interactions among kokanee, their predators (native or introduced), and anglers. For example, quantifying kokanee population dynamics and productivity (Hansen et al 2010;Pate et al 2014;Corsi et al, in press), the susceptibility of kokanee to different introduced predators (Schoen et al 2012;Hansen et al 2019), emerging parasite-host dynamics involving Salmincola (Hargis et al 2014), recreational harvest dynamics (Askey and Johnston 2013), and predicting spawner escapement for informing wild-spawn and hatchery operations (Baldwin and McLellan 2008) require minimally biased information on size and age structure. Although retention models alone likely do not account for all processes influencing the size selectivity of experimental gill nets, they remove some bias and strengthen evaluations like those listed above.…”

Simulated effects of imperfect sterile sport fish stocking on persistence of fertile fish in new exploited populations

“…We tailored the modeling analysis to reflect fisheries management and ecological constraints facing reservoirs within the upper Colorado River basin as this region is at the forefront of balancing multiple interests (Clarkson et al 2005; Hansen et al 2019). The upper Colorado River basin contains critical habitat for the endemic Colorado Pikeminnow Ptychocheilus lucius , Humpback Chub Gila cypha , Bonytail G. elegans , and Razorback Sucker Xyrauchen texanus .…”

Rapid proliferation of the parasitic copepod, Salmincola californiensis (Dana), on kokanee salmon, Oncorhynchus nerka (Walbaum), in a large Colorado reservoir

“…The reservoir thermally stratifies, generally between May and October, with peak surface temperatures reaching 20–22°C. Densities of Daphnia spp and typically peak in June at ~10–30 individuals/L in surface waters (Hansen et al., 2019). Kokanee salmon are stocked annually in Blue Mesa Reservoir, and overall, feeding and growth conditions in the reservoir support high kokanee salmon growth rates (Stockwell & Johnson, 1997).…”