Fish predation on newly stocked juvenile predatory fish in a biomanipulated reservoir

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Jůza T, Blabolil P, Čech M, Draštík V, Kubečka J, Muškaa M, Prchalováa M, Sajdlováa Z, Tušera M, Vašeka M, Vejříka L, Peterkaa J. 2024. Fish predation on newly stocked juvenile predatory fish in a biomanipulated reservoir. Lake Reserv Manag. 40:325–337. Juveniles of pikeperch, asp, and northern pike were stocked into the Římov Reservoir. Subsequently, potential fish predators were monitored by gillnets at the stocking sites and their diet was analyzed for presence of stocked fish. Asp and European perch were predominant predators, while pikeperch and northern pike were less abundant in gillnet catches. Digestive tract analysis revealed the highest percentage of ingested fish in northern pike, followed by pikeperch, asp, and European perch. The amount of stocked fish did not correlate with the number of predatory fish caught in gillnets. When asp was stocked, this species was strongly preferred by northern pike and partly by pikeperch and 0.3 individual of stocked asp was eaten per 1 kg of predatory fish captured by gillnets. During the northern pike stocking, this species was mainly preyed upon by asp and occasionally by northern pike, and 0.8 individual of stocked northern pike was eaten per 1 kg of predatory fish. When pikeperch was stocked, it was strongly preferred by pikeperch, northern pike, and European perch, it was almost absent in the diet of asp, and 1.1 individual of stocked pikeperch was eaten per 1 kg of predators. Our results indicate that pikeperch juveniles had the highest mortality rate due to predation after stocking, mainly due to sedentary behavior and the high proportion of cannibalism. In northern pike, cannibalism was found in only one case, and was never found in asp, which was under the lowest predation pressure.