Variability in growth rates of larval haddock in the northern North Sea

The large-scale distribution of haddock (Melanogrammus aeglefinus) larvae in the northern North Sea was mapped in a grid survey carried out in late April 1996. A drifting buoy was deployed in the centre of one of the areas of concentration of larvae located off the east coast of the Shetland Isles, where intensive sampling was carried out for approximate to 10 days. Daily larval haddock growth variability, estimated from otolith microstructure analysis, was independent of the measured variability of the physical and biological environment of the larvae. The survey coincided with the onset of the spring plankton production bloom, and a likely explanation for the absence of environmental effects on larval growth was high food availability and larval feeding rates. Nevertheless, differences in growth were observed between cohorts, with larvae hatched later in the spring displaying higher growth at age than those hatched earlier. Particle-tracking modelling suggested that differences in temperature history between cohorts, on their own or compounded by a potential interaction between temperature and the development of plankton production, may explain the higher growth rate of the larvae hatched later in the season.