Seasonal and inter-annual trends in the zooplankton prey and growth rate of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae on Georges Bank

Abstract

Abstract The combination of two data sets compiled as part of the US GLOBEC NW Atlantic Georges Bank Program afforded us the opportunity to examine prey abundance and larval growth rates over extended spatial and temporal scales. These data demonstrate a strong correlation between larval growth rates and estimated prey biomass concentration. The dominant copepod taxa on Georges Bank were monitored between January and June of 1995 through 1999, using a pump sampler and a 1-m MOCNESS. Between March and May of these years over 10,000 cod and haddock larvae were collected and recent growth rates of individual larvae estimated using RNA/DNA ratio analysis. The zooplankton sampling was completed on monthly broadscale cruises covering much of the Bank, while most of the larval collections were made on process cruises concentrated on the southern flank. Indices of prey abundance on the southern flank were estimated for different size-classes of larvae. Prey counts (# m−3) and prey biomass concentration (μg dry weight m−3) increased between February and May of each year. Following a similar trend, larval growth rates increased between March and May. Particularly strong correlations were observed between the biomass of Pseudocalanus spp. and larval growth rates. These correlations persisted when the seasonal trend was removed from the estimates of prey biomass concentration, and the seasonal and ontogenetic trends were removed from the estimated growth rates. The spring of 1995 and to a lesser extent 1996 appeared to be periods of low prey availability and poor growth of cod and haddock larvae on Georges Bank. Although no large year class of cod was produced during the study period, the 1998 year-class of haddock was among the largest produced over the previous 20 years on Georges Bank. The data suggest that rapid larval growth maybe a necessary but not sufficient requirement for production of a strong year class. The strong correlations observed between prey biomass concentration and larval growth rates provide additional support for the “growth–mortality” hypothesis that links favorable feeding conditions to rapid growth, reduced stage duration and enhanced survival and recruitment.