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

Abstract

Since its invasion in to the Baltic Sea in 2006, the ctenophore Mnemiopsis leidyi has been suspected of serious predation on the early life stages of Baltic cod (Gadus morhua callarias L.) due to a temporal and spatial overlap in the most important cod spawning ground, the Bornholm Basin. We conducted laboratory incubation experiments and video observations to quantify feeding rates on Baltic cod eggs and larvae. Ingestion rates increased with cod larvae concentrations up to 8 prey L−1, beyond which ingestion remained constant. Neither Mnemiopsis size nor egg concentration (1–16 prey L−1) affected feeding rates on cod eggs. Observed feeding rates pooled from all experiments conducted at nonsaturating prey concentrations were low, with the highest volume‐specific clearance on < 4.5‐d‐old yolk‐sac larvae (0.05 ± 0.02 L (mL Mnemiopsis)−1 h−1), and lower rates on 4.5‐8‐d‐old larvae (0.02 ± 0.02 L (mL Mnemiopsis)−1 h−1) and eggs (0.02 ± 0.03 L (mL Mnemiopsis)−1 h−1). When offered Artemia salina and cod eggs simultaneously, Mnemiopsis passively selected against cod eggs. Video recordings showed that eggs did not trigger the capture response that Mnemiopsis shows toward motile prey, and ingested eggs were often ejected (88%, n = 8). Applying our clearance rates to in situ abundances of cod eggs, larvae, and Mnemiopsis for the peak of the spawning season, we demonstrate that the predation pressure of the invasive ctenophore is negligible. We conclude that Mnemiopsis constitutes no direct threat to the Baltic cod population.