Zooplankton feeding ecology

Abstract

Hjort proposed that fishery year-class fluctuations are due mainly to variable larval mortality, and that most mortality is due to early starvation. Some larvae die because they do not find enough zooplankton to eat, but others may die because zooplankton cat them. We examined predation upon eggs, yolk-sac, and/or first-feeding larvae of Atlantic menhaden (Brevoortia tyrannus), gulf menhaden (B. patronus) and spot (Leiostomus xanthurus) by adults of larger (Anomalocera ornata) and smaller (Centropages typicus) copepods. B. tyrannus eggs were too large for either copepod to grasp or ingest. A. ornata could grasp and apparently kill, but not ingest, the smaller L. xanthurus eggs, but C. typicus could not. Both yolk-sac and first-feeding B. tyrannus larvae and first feeding B. patronus larvae were grasped and completely consumed in<4 min by A. ornata. C. typicus ingested yolk-sac larvae of both fish, but not first-feeding larvae of either species. Ingestion rates by A. ornata were significantly related to prey density (ANOVA; p<0.001). Ingestion rates by C. typicus (<2 larvae copepod d-1) were much lower than those of the larger A. ornata (up to 14 larvae copepod d-1) at food concentrations of 10 to 50 larvae l-1. However, expressed as % copepod body carbon ingested copepod d-1, ration by the smaller copepod equalled or exceeded that of the larger. Since copepods and fish larvae can become concentrated together in surface windrows, copepod predation may represent a substantial source of mortality of fish larvae.