Seasonal trends in mortality and growth of cod and haddock larvae result in an optimal window for survival

Changing climate and global depletion of fish stocks have added urgency to the century-old quest to understand the factors controlling fish production. The leading hypotheses advanced to explain the large (orders of magnitude) inter-annual variability in recruitment of young fish emphasize rapid growth and a match to prey production in the first weeks after hatching, although avoiding predators may be of equal or greater importance. Here we show for 2 important North Atlantic groundfish (Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus) that seasonal patterns in growth (G) and mortality (M) rates of young larvae combine in some years to yield a window of opportunity when M falls below G within days after hatching and cohort biomass increases rapidly possibly leading to high recruitment. Contrary to expectations, this window occurs early in the seasonal production cycle (February to March) when temperatures are near their annual minimum, fish larvae and their prey are relatively scarce, and G is low. In most cases examined, later, faster-growing cohorts appear to be rapidly lost to a suite of vertebrate and invertebrate predators whose abundance, metabolism, and consumption increase through the spring with increasing water temperature. In this seasonally varying environment, size on year day may be critical to minimizing M/G. Our data demonstrate the importance of rapid growth throughout the season cycle, but particularly early in the season when temperatures, potential prey, and predators are at seasonal low levels. These findings reinforce the importance of management practices protecting larger, older females that begin spawning earlier in the season and produce larger, more viable eggs and larvae.