Interaction of Norwegian spring-spawning herring larvae (Clupea harengus) and Barents Sea capelin larvae (Mallotus villosus) in a mesocosm study

Growth and survival of herring ( Clupea harengus ) larvae released into a 4400-m 3 basin were measured for four months. Schooling was observed at an age of 50 days (32 mm long), and metamorphosis took place at an age of about 60 days (34 mm long). About 3000 yolk-sac capelin ( Mallotus villosus ) larvae were released into the basin when the herring were 34 days old (20 mm long), and another group of about 50000 when the herring were 46 days old (25 mm long). Both capelin groups met with excellent feeding conditions and showed rapid growth. However, they expired at an age of 30 and 22 days respectively, despite the apparently suitable feeding conditions. The newly schooling herring population had a mean density of 1 to 2 fry/m 3 when the capelin disappeared. Laboratory studies on the ability of juvenile herring (25–35 mm) to prey on food items 6–7 mm in size showed that the herring could prey upon capelin larvae of this size. Separate survival and growth studies were carried out on another group of capelin larvae which were released into a 2000-m 3 basin, where they met rather marginal feeding conditions. About 2·8% of these larvae survived to an age of 120 days. Capelin larvae from both groups were transferred to bags suspended in the 4400-m 3 basin containing herring. From 0·5 to 5·5 % of group 1 survived to age 40 days, while survival rates for group 2 to age 31 days were higher, ranging from 10·5 to 30·5 %. These results are discussed in relation to the distribution of juvenile herring on the spawning grounds of capelin in northern Norway. The sharp decline in Norwegian spring-spawning herring in the mid-1960s coincided with a huge increase in the capelin population in the Barents Sea. Recently the herring population has recovered, and the potential influence of herring predation on the capelin population is assessed.