Timing and selectivity of mortality in reared Atlantic cod revealed by otolith analysis

Abstract

In juvenile fish production, large samples of known-aged material can be sampled at pre-determined time intervals from the same population. This enables an accurate determination of size-selective mortality by means of repeated samplings of fish and comparison of otolith size-at-age based on samples from different dates. An example is provided from an experiment with larval and juvenile Atlantic cod, Gadus morhua, where groups fed smaller sized enriched rotifers did not reveal any size-selective mortality during weaning to formulated feed, while those groups that were fed larger live natural zooplankton lost a significant fraction of the smaller-sized individuals during the same time period. This was contrary to the overall mortality which was higher among the rotifer fed groups during weaning. Part of this difference may be attributed to size differences between the groups, where larger zooplankton fed larvae were more prone to engage in cannibalistic behaviour.