Interannual variability in a predator–prey interaction: climate, chaetognaths and copepods in the southeastern Bering Sea

The interaction of the chaetognath Sagitta elegans with the copepod community of the southeast Bering Sea middle shelf was examined in relation to environmental conditions during 1995-1999. Predation impact was estimated for 2 years, 1995 and 1997, using gut content analysis, experimentally derived digestion time (DT) and abundances of chaetognaths and prey. Pseudoca- lanus concentrations correlated with water temperature and Calanus marshallae with sea ice extent. Sagitta elegans were less abundant but individuals were larger in 1995, when C. marshallae predominated, compared to 1997, when Pseudocalanus and Acartia were the primary prey. Predation by S. elegans removed <1% standing stock day 1 of Pseudocalanus or C. marshallae in 1995 and 1.7 to 2.3% of Pseudocalanus in 1997. The percent of the copepod community biomass required by chaetognaths was estimated to be <1% in 1995 compared with 8-12% in 1997. Calanus marshallae may be more vulnerable than Pseudocalanus to cumulative predation effects because of its reproductive strategy. The effect of chaetognath predation on the copepod community depends on which copepod species is predominant and its susceptibility to cumulative predation effects, as well as on daily predation impact, both of which varied between years with different climatic conditions.