Predation of northern pike (Esox lucius L.) eggs: A possible cause of regionally poor recruitment in the Baltic Sea

Northern pike ( Esox lucius) spawning habitat and egg mortality were studied in three spawning areas in 2001 along the Swedish coast of the Baltic proper: the Blekinge Archipelago, Kalmar Sound and coastal freshwater streams. Spawning peaked during the last week of April in streams, at temperatures ranging from 7.7 to 8.9 degrees C and during the first week of May in brackish waters, at temperatures ranging from 8.9 to 13.8 degrees C. Spawning occurred in shallow waters, at depths between 0.2-1.5 m, but generally most of the spawn was found in the shallowest areas. In streams, eggs were mainly attached to emersed vegetation, while in brackish sites pike eggs were well scattered among flooded emersed plants, submersed plants and filamentous algae. Mean egg density varied between 469-1829 m(2) with the lowest density observed in Kalmar Sound. The calculated egg loss occurring from approximately one day after spawning to one day before hatching ranged from 41 +/- 7% in coastal streams to 67 +/- 6% in the Blekinge Archipelago and 100% in Kalmar Sound. The significant removal of eggs from spawning sites in Kalmar Sound and Blekinge was most likely due to predation from several fish species. In situ observations and stomach analyses suggested that many pike eggs in Kalmar Sound were lost to the three-spined stickleback (Gasterosteus aculeatus), a species that presently dominate the littoral small fish community. This study therefore suggests that egg predation by sticklebacks and other fishes may be a possible cause of the reported poor recruitment of coastal pike populations in the Kalmar Sound region.