Larval Fish/Plankton Interactions on Georges Bank

Cod and haddock spawn during the early spring and growth of their larvae appears linked closely to the seasonal development of their prey populations (Sherman et al., 1984; Figures 9b, 11). Energetic demands of a fish larva increase as the larva grows so that the highest rations are required at the end of the larval stage. Both cod and haddock spawn prior to the peak in zooplankton biomass on Georges Bank (Figures 9b, 11), suggesting that food limitation during the larval stage may be important in regulating their population cycles. The timing of spawning such that the late larval stages coincide with the peak in zooplankton biomass has been suggested recently as a mechanism that enhances recruitment success; this is an extension of Cushing's (1972, 1975) match-mismatch hypothesis (Bollens et al., in press). The decline of the zooplankton mass in July-August coincides with the settling of larval cod and haddock and development of their benthic feeding habits.

In addition to the coincidence of larval fish food demand and the peak in total zooplankton mass, the size distribution of prey found in the stomachs of larval cod and haddock matches closely the size distribution of the developing zooplankton assemblage. In late winter/early spring (March-April) when the fish larvae are of small body size (2-8 mm - Stage 2), the zooplankton is dominated numerically by the small copepod Pseudocalanus spp. (1 mm prosome length), whereas later in spring (May-June) when the larvae have higher energetic demands and are capable of ingesting larger prey, most of the biomass consists of the larger Calanus finmarchicus (2.5 mm prosome length) (Davis, 1987b). By early summer (July), the larger crustaceans, including mysids, amphipods, and euphausiids, reach peak abundance coinciding with the mean prey size ingested by the late larvae and pelagic juveniles. By mid- summer (August), the zooplankton biomass has declined due to increases in predatory gelatinous species in the water column, and the juveniles seek refuge on the bottom.