Feeding and vertical distribution of cod larvae in relation to availability of prey organisms

Abstract

The year-class strength of the Arcto-Norwegian cod stock varies on an annual basis by a factor of about 20:1 and is determined at the 0-group stage (about 1/2 year old cod). Since the number of eggs spawned far exceeds the number of 0-group cod, there must be an enormous mortality during the first half year. The aim of the present project is to test Hjort's hypothesis and to increase out knowledge about the growth and survival of cod larvae. The main area of investigation is Vestfjorden in northwestern Norway. Several aspects of the larvae were examined, such as standard length, functional jaw, yolk sac stages, development of swim-bladder, gut content, etc. The results of bioassay experiments showed that first feeding cod larvae were able to capture nauplii even at very low prey densities. However, feeding incidence never exceeded 80% at the highest prey densities (50-60 nauplii/l) even in the most developed larvae. The size range of nauplii eaten by cod larvae is seemingly uncorrelated to larval length at first-feeding. The larvae seemed to avoid the smallest sized particles and were possibly unable to eat the largest particles. First-feeding cod larvae seem to select nauplii within the size of 120-130 mu m (carapax length). The majority of first-feeding cod larvae in Lofoten were distributed in the upper 20 m.