Premature schooling of larval herring in the presence of more advanced conspecifics

Abstract

The availability of food to fish larvae influences the mean and variance of the growth rate in a cohort. At higher food densities, slower developing herring,Clupea harengus, larvae in the pre-metamorphic stage may co-occur with more developed, post-metamorphic individuals. The development of schooling in herring is closely associated with metamorphosis. To assess whether the presence of schooling conspecifics encouraged the premature development of schooling in larvae, laboratory reared larvae (length 29-31mm) were exposed to wild-caught schooling juveniles (length 55mm) for the duration of the experiment. Observations of larvae from the experimental group were carried out for 37 days and the behaviour of the fish was recorded on video for subsequent analysis. These larvae were observed in the presence and absence of schooling juveniles with no food in the observation tank, and a few minutes and 1h after food was added. The behaviour of these two groups (juveniles present and juveniles absent) was compared with that of two additional groups (different rearing densities), where the larvae had not been exposed to schooling herring. The exposure to schooling conspecifics did not affect the degree of interaction among herring larvae unless the juveniles were present in the observation tank. Consequently, in the presence of a school the larvae seemed to opt for the increased protection of the school, regardless of associated disadvantages, such as greater competition for food. These findings are discussed in relation to larval vulnerability to predators.