Buckley, Larry
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Abstract In northern waters intra-annual trends in the abundance of fish larvae, their prey and predators are associated with strong seasonal trends in solar radiation, water temperature, water-column structure and circulation. With very few exceptions, any corresponding seasonal trends in the vital rates of fish larvae are poorly known, largely due to the scale of the sampling effort required. As part of the US GLOBEC: NW Atlantic Georges Bank Program, we were able to compile extensive data on weight-specific growth (G) and instantaneous mortality (M) rates of cod and haddock larvae each winter and spring over the period 1995 to 1999, along with data on the physical and biotic environment. M of young larvae was lowest in March, corresponding to the annual minimum in water temperature. G increased with larval size and Julian Day (photoperiod) between March and May. The M/G ratio, an index of stage-specific mortality, was lowest in March. The dependence of G on larval size resulted in an expanding temporal window of opportunity for fast growing cohorts when M/G < 1. Cohorts that grew rapidly remained within this window, while slow growing cohorts fell behind and rapidly lost biomass. Both cod and haddock exploit a brief period early in the seasonal cycle when, in some years, there is sufficient prey to support good larval growth, while the abundance and activity of their predators are still at low levels. Annual and climate induced changes in minimum water temperature, the rate of warming and cooling, the timing and intensity of the production cycle and the migration of mobile predators will affect recruitment through either an expansion or contraction of this temporal window of opportunity. |