Dependence of survival on growth in larval pollock Pollachius virens and haddock Melanogrammus aeglefinus: A field study based on individual hatchdates

The hatchdate frequency distributions (HFD) of pollock and haddock larvae sampled at monthly intervals west of Sable Island (Scotian Shelf, northwest Atlantic) in 1992 and 1993 were reconstructed for different age intervals (0-20, 21-40, 41-80 d) and corrected for aliasing due to sampling discontinuity and mortality-dispersion. The ratio of the HFD at a given age to the HFD at an earlier age was used as an index of the relative survival of larvae grouped into 5 d hatchdate cohorts. Pollock hatched from November to March and haddock from February to June. In pollock, seasonal variations in relative survival of the cohorts over the 21-40 and 41-80 d age intervals were correlated to strong variations in growth. In haddock, growth varied little over the hatching season and there was no significant link with survival. For pollock, slow growth invariably resulted in low survival but fast growth resulted in either low or high survival, indicating that fast growth is a necessary but not sufficient condition for survival. Increased predation pressure late in the hatching season of both species could explain the decoupling of growth and survival in cohorts hatched in spring and early summer.