The invasive ctenophore Mnemiopsis leidyi: a threat to fish recruitment in the North Sea?

The invasive ctenophore Mnemiopsis leidyi has a strong reputation as a threat to fish stocks. Apart from competitive relationships between M. leidyi and fish larvae, direct predation by the ctenophore on both eggs and larvae is considered an important factor linking ctenophore and fish populations. We therefore estimated abundances of both the ctenophore and its potential prey in the spring of 2008. No significant correlations were detected between ctenophore numbers and the abundance of fish eggs. We further carried out stable isotope analyses to investigate the trophic position of M. leidyi. Carbon and nitrogen stable isotope signatures of three potential prey groups (fish eggs, small plankton and larger plankton) showed that M. leidyi primarily feeds on plankton, while fish eggs are of minor importance. Mnemiopsis leidyi was located roughly one trophic level above the indigenous ctenophore Bolinopsis infundibulum, whereas its trophic position was more similar to another native ctenophore, Pleurobrachia pileus. A feeding selection experiment, with fish eggs and copepods offered in the same proportion, corroborated these findings. Mnemiopsis leidyi ingested significantly more copepods; feeding on fish eggs was not significantly different from zero. Based on these experiments, we conclude that in the North Sea, M. leidyi has no serious potential as a direct predator of fish eggs, but individuals of this species might compete for food with larval fish as well as with the native ctenophore P. pileus.