UC San Diego

The impact of predators on population growth of Pseudocalanus sp. was investigated in Dabob Bay, Washington. Mortality of Pseudocalanus sp. was determined from stage-specific survivorship, from seasonal changes in mortality rates of adult males and females and from incidence of injuries to adult copepods. The principal predators of adult Pseudocalanus were identified as the predatory copepod Euchaeta elongata, the omnivorous euphausiid Euphausia pacifica and the chaetognath Sagirta elegans. Predator attack rates - and prey mortality rates - are highly density-dependent and thus sensitive to prey dispersion in the water column, particularly to layering in the vertical plane. Predation rates by the three principal predators exceeded 100% of the recruitment rate to adult Pseudocalanus sp. beginning in early summer, thus restricting population growth. Planktivorous fish predation (by adult three-spine stickleback, Gasterosteus aculeatus, and juvenile chum salmon, Oncorhynchus keta) on Pseudocalanus sp. adults was estimated to be two orders of magnitude lower than consumption rates by predatory zooplankton, at a deep water station in July. Analysis of seasonal changes in prey ingested by Sagitta elegans revealed that Pseudocalanus sp. was the major prey item of S. elegans in April (61.0% of prey) and in June (67.0% of prey), thereafter declining seasonally in importance. Predation by S. elegans varied seasonally with changes in chaetognath stage structure, vertical distribution and diapause, not size structure alone. Although chaetognath recruitment and population growth appear to be directly coupled to the abundance of Pseudocalanus sp., predation by S. elegans has little reciprocal impact on Pseudocalanus sp. population growth; hence asymmetries may occur in the in teraction of planktonic prey and predators. © 1986 IRL Press Limited.