Growth patterns of three cohorts of Atlantic cod larvae (Gadus morhua L.) studied in a macrocosm

More than 40 000 cod larvae from three cohorts were sampled from a macrocosm (enclosed pond) in Austevoll, Norway. Specific growth rates (dry weight) to day 25 averaged 12.7, 13.1, and 12.4% per day for cohort 1, 2, and 3, respectively. Larvae within the same cohort had comparable specific growth rates in the macrocosm, and no signs of starvation among the larvae were evident from the cumulative distributions of dry weights at different sampling dates. Incorporating information on size variability at the different sampling dates gave an improved understanding of the growth dynamics in the pond compared to an analysis using only average values. The gut-filling and growth rate of the cod larvae reflected the food availability in the pond. The increase in size variation of cohort 1 after metamorphosis may be linked to the ability to eat copepodite and adult stages of Calanus finmarchicus. In this period a 20-day younger cohort (cohort 3) disappeared in spite of normal gut filling and high growth rate. The disappearance of the youngest cohort also coincided with a decline in food availability for the two older cohorts. It is concluded that the youngest cohort was eaten by the older cohorts. The lack of suitable C. finmarchicus copepodite stages was most likely to be responsible for the reduction in growth rate of cohort 2 in the latter half of the larval stage. Survival of cohort 2 was also lower than for cohort 1, and an age difference of 10 days or more between cohort may favour the oldest cohort in periods of food limitation and co-occurence.