The invasive ctenophore Mnemiopsis leidyi in northern European waters and its potential impact on fisheries

The recent invasion by Mnemiopsis in northern European waters has lead to concerns for fishery interests especially in the central Baltic Sea, where it overlaps with commercially important cod recruits on their spawning grounds. We present laboratory feeding rate experiments along with video recordings, in situ reproduction rates and distribution data to investigate the direct and indirect effect on the Baltic cod population. Feeding rates on cod larvae were low and negligible for eggs under the hydrographic conditions characteristic for the spawning grounds. Further, Mnemiopsis passively selected against cod eggs. Application of our clearance rates to in situ abundances confirmed that Mnemiopsis has a negligible direct predation impact on cod offspring. Further, due to drastically reduced reproduction rates at low salinities, occurrence of Mnemiopsis in the central Baltic appears to be dependent on advection, and is unlikely to reach large population sizes. Hence, Mnemiopsis constitutes neither a direct nor a potential indirect threat to the cod population in the central Baltic. However, its large reproduction potential in high saline areas with 11,500 eggs ind-1 d-1 and observed high abundances in parts of northern Europe make Mnemiopsis a severe potential food competitor with fish in these higher saline system