VERTICAL-DISTRIBUTION OF COD AND HADDOCK EGGS AND LARVAE, FEEDING ANDCONDITION IN STRATIFIED AND MIXED WATERS ON SOUTHERN GEORGES BANK, MAY 1992

In late May 1992, cod and haddock eggs and larvae were sampled at disc rete depths using MOCNESS on the southern flank of Georges Bank when s easonal stratification of the water column was developing. A deeper, s tratified site (80-m bottom) was sampled over 7 days in relation to th e temporal evolution of vertical structure and compared to a nearby sh oal, mixed site (50 m). Sampling also was conducted to the west along two transects in the Great South Channel. Larval biochemical compositi on was measured and nutritional condition inferred from RNA:DNA ratios . During the period 22-24 May 1992, surface warming resulted in a temp erature gradient from 6 to 10 degrees C (0.6 sigma(t) units) in the up per 20 m at the stratified site. Gadid eggs (93% haddock) were most ab undant (6-14 per 100 m(3), on average) in the surface 20 m, with maxim um density just above the base of the thermocline at 20-10 m. The few recently-hatched larvae (5-6 mm) caught were broadly distributed in th e water column, with maximum average densities (1-3 per 100 m(3)) deep er at 10-40 m. The naupliar and copepodite stages of Pseudocalanus spp . were principal prey of these larvae, and also were more abundant (3- 7 prey l(-1)) in the surface 20 m, with a maximum density at 20-10 m. A moderate storm on 25 May mixed the upper part of the water column to 5-6 degrees C. The day after the storm, gadid eggs and copepods had h ighest densities in the surface 10 m. At the shoal (mixed site) egg de nsities were low (1-3 per 100 m(3)) and distributed evenly through the water column. Cod and haddock larvae were larger (7-8 mm modal length ) and more abundant than at the stratified site. At night, they were e venly distributed at 4-6 per 100 m(3) on average, but by day, maximum density increased to 616 per 100 m(3) in the deepest strata sampled (4 0-30 m). The larger larvae at this site preyed predominantly on copepo dite stages of Pseudocalanus spp., which were uniformly distributed in the water column. Of all the larvae sampled for biochemical analysis, 10% of haddock and 1.5% of cod had ratios below 4.1. Based on laborat ory studies, fish larvae with RNA:DNA ratios below 4.1 are considered to have been in poor condition. There were no larvae in poor condition at the Great South Channel Transect, where their mean size was greate st; no larvae were collected at the stratified site, for comparison. A t the mixed site, cod larvae sampled before and after the storm had id entical ratios, while haddock larvae had significantly higher ratios a fter the storm. Larvae from different sampling depths had significantl y different ratios, inferring that they remained together long enough to acquire a unique signature. Copyright (C) 1996 Elsevier Science Ltd