Hypoxia reduces predation on the invasive comb jelly <i>Mnemiopsis leidyi</i>

ABSTRACT Coastal marine ecosystems are increasingly affected by rising temperatures, declining oxygen and other human-driven stressors, which can alter predator–prey interactions. In the Baltic Sea, native fish and invasive comb jellies interact within a changing physical environment, with outcomes likely impacted by their different hypoxia tolerances. We investigated whether the abundant, native three-spined stickleback (Gasterosteus aculeatus) feeds on larvae (~400 μm) of the invasive comb jelly Mnemiopsis leidyi. Experiments were conducted under normoxic (100% O2) and moderate hypoxic (30% O2) conditions. We here present the first evidence of fish predation on M. leidyi larvae. Oxygen conditions had a significant impact on feeding rates (P &amp;lt; 0.001) and hypoxia led to a 68% and 69% reduction in G. aculeatus feeding on M. leidyi and the copepod Acartia tonsa, respectively. Consequently, while sticklebacks can ingest M. leidyi larvae, even moderate hypoxia will reduce its predation pressure on M. leidyi larvae and moreover reduce its potential to compete with larger M. leidyi for the common copepod prey A. tonsa.