Can young‐of‐the‐year invasive fish keep up with young‐of‐the‐year native fish? A comparison of feeding rates between invasive sticklebacks and whitefish

Invasion of non‐native species might alter food web structure and the strength of top‐down control within lake ecosystems. As top‐down control exerted by fish populations is often dominated by young of the year fish, the impact of new fish species might depend on the feeding rates of the juvenile fish. Here we provide comparative analyses of feeding rates of juvenile whitefish (Coregonus wartmanni) – a native and specialised planktivore and an invasive generalist (sticklebacks, Gasterosteus aculeatus). We studied feedings rates of whitefish and sticklebacks in aquaria experiments using 2 cm to 8 cm fish feeding on seven zooplankton species common to Lake Constance. As whitefish hatch several months earlier than sticklebacks, 0+ whitefish are larger than 0+ sticklebacks throughout the year and hence are predicted to have higher feeding rates on especially large zooplankton species. We show that sticklebacks as small as 2 cm were able to feed on the largest zooplankton species of Lake Constance. Further, stickleback feeding rates were similar to both the same size 0+ whitefish and the larger 0+ whitefish co‐occurring with smaller 0+ sticklebacks. Hence, 0+ sticklebacks will compete with 0+ whitefish for the same zooplankton species, therefore the invasion of sticklebacks is unlikely to change the relative feeding pressure by individual 0+ fish on zooplankton species. The juvenile period of fish is crucial for their establishment in a new habitat as well as for the grazing impact of fish on prey communities. Here, we show that juveniles of an invasive species, sticklebacks, do have similar feeding rates on various zooplankton species compared to native whitefish despite the latter are specialized planktivores and advanced sticklebacks in growth during the seasonal course. Our results suggest that the invasion of sticklebacks and partial replacement of whitefish should not result in changing relative feeding pressures on the various zooplankton species.