Growth pattern and growth dependent mortality of larval and pelagic juvenile North Sea cod Gadus morhua

We investigated growth patterns and evidence of growth dependent survival for a population of Atlantic cod Gadus morhua using analysis of their otolith microstructure. Central concentrations of a population of cod larvae and juveniles in the north-eastern North Sea were sampled twice (in April and May 2001), and larval/juvenile growth history from each of the sampling sequences was outlined.

Growth rate was estimated by fitting a Laird-Gompertz equation to lengths-at-age, and we found the mean specific growth rate in length at age 20 d was 3.2% d(-1), declining to 1.9% d(-1) at an age of 90 d. Otolith radius and larval standard length were highly correlated, and otolith growth was used as a measure of larval somatic growth.

The larvae were divided into 3 groups dependent on their hatch-date, and for each hatch group, the same period of past growth was compared between fish sampled in April and May. A 2-way repeated-measurement ANOVA revealed a significant higher past growth of fish sampled in May in 2 of the 3 hatch-groups, implying a higher mortality of the slow growing larvae.

Additionally, otolith size at age differed significantly between the April and May sampling of the oldest larvae. Using the obtained information we illustrate the consequences of being larger at age by nonparametric spline functions. These show up to 10-fold differences in the relative survival probabilities between cod of smallest or largest size at age, suggesting that mortality can be strongly dependent on growth.