The invasive ctenophore Mnemiopsis leidyi poses no direct threat to Baltic cod eggs and larvae

Since its invasion in to the Baltic Sea in 2006, the ctenophore Mnemiopsis leidyi has been suspected of seriouspredation on the early life stages of Baltic cod (Gadus morhua callarias L.) due to a temporal and spatial overlap inthe most important cod spawning ground, the Bornholm Basin. We conducted laboratory incubation experimentsand video observations to quantify feeding rates on Baltic cod eggs and larvae. Ingestion rates increased with codlarvae concentrations up to 8 prey L21, beyond which ingestion remained constant. Neither Mnemiopsis size noregg concentration (1–16 prey L21) affected feeding rates on cod eggs. Observed feeding rates pooled from allexperiments conducted at nonsaturating prey concentrations were low, with the highest volume-specific clearanceon , 4.5-d-old yolk-sac larvae (0.05 6 0.02 L (mL Mnemiopsis)21 h21), and lower rates on 4.5–8-d-old larvae(0.02 6 0.02 L (mL Mnemiopsis)21 h21) and eggs (0.02 6 0.03 L (mL Mnemiopsis)21 h21). When offered Artemiasalina and cod eggs simultaneously, Mnemiopsis passively selected against cod eggs. Video recordings showed thateggs did not trigger the capture response that Mnemiopsis shows toward motile prey, and ingested eggs were oftenejected (88%, n 5 8). Applying our clearance rates to in situ abundances of cod eggs, larvae, and Mnemiopsis forthe peak of the spawning season, we demonstrate that the predation pressure of the invasive ctenophore isnegligible. We conclude that Mnemiopsis constitutes no direct threat to the Baltic cod population