Evidence for impacts by jellyfish on North Sea herring recruitment

Jellyfish (Scyphozoa) prey on and consume many of the same food items as do larvae of herring Clupea harengus and could therefore have a detrimental impact on larval survival. A reduction in the spawning stock biomass of herring may release jellyfish from competition for prey with herring and exacerbate any impact by jellyfish on herring survival. Both jellyfish abundance and the spawning success of herring fluctuate from year to year and increase under similar environmental conditions. The abundance of medusae (diameter = 1 to 47 cm, June to August) at herring spawning areas in the North Sea was correlated positively with the abundance of recently hatched herring larvae (< 1 cm, sampled 1 to 15 September 1971 to 1986), and this concurrence heightens the potential for competition. A significant negative relationship (p < 0.05) between the survival of herring to Age 0 and the abundance of the scyphozoan Aurelia aurita implies that this jellyfish might adversely impact the North Sea herring population. Furthermore, regression analyses suggest that climate variability may mediate the possible impact by A. aurita on herring survival and 0-group recruitment through changes in the North Atlantic Oscillation Index and the abundance of the scyphozoan Cyanea capillata (r(2) > 0.60, p < 0.05). Management of North Sea herring may benefit from the inclusion of predation and competition effects of medusae into forecasting models of herring recruitment.