Effects of introduced round goby (Neogobius melanostomus) on diet composition and growth of zander (Sander lucioperca), a main predator in European brackish waters

We inspected diet and growth of zander Sander lucioperca, a main predator in North European brackish waters, from the Kiel Canal, northern Germany, to analyse effects of the introduced round goby Neogobius melanostomus as possible prey. Round goby represented the most important fish prey taxon in the zander diet in 2011-2013 with 21.8% of relative importance index (RI) over all inspected specimens. In particular zander of total length >= 200 mm fed heavily on round goby, at least in the warmer quarters (2nd and 3rd) of the years under study. RI for these quarters was ranging from 10.4% to 52.1% for predator specimens of 200-399 mm and from 45.6% to 79.7% of RI for specimens of = 400 mm. In the colder quarters of the inspected years (1st and 4th), zander >= 400 mm showed a high amount of cannibalism, with RI of young-of-the-year zander as prey ranging from 23.9% to 38.9%. The asymptotic total length (L-infinity) of the zander, calculated by the von Bertalanffy growth equation, was 937 mm. In comparison with data from 1995/1996, before the round goby invasion in 2006, zander from our study showed considerable differences in diet composition. RI of the prey organisms herring and Pomatoschistus spp. decreased from 26% to 13.4% and 14.2% to 4.4%, respectively. RI of young-of-the-year zander as prey for larger zander increased from 1.2% to 7.7% and round goby occurred as newly established prey taxon. Zander also showed faster growth and a better condition factor. We assume that this development has the potential to enhance the situation of the local fisheries that are strongly related to the catch of zander. In addition, abundance control of round goby seems to result from the high predator abundance.