Diel differences in avoidance of three vertical profile sampling gears by herring larvae

Abstract

Three plankton sampling systems [pump, Longhurst-Hardy Plankton Recorder (LHPR) and MOCNESS] were used to obtain vertical profiles of herring larvae off the north-east coast of England in October 1986 during the International Herring Larval Survey (IHLS). A factorial sampling design allowed a comparison of the number of herring larvae caught by depth (four levels), time of day (three levels) and gear (three levels). The size composition of herring larvae taken in the different gears has also been examined. Catches taken at night were greater than those taken during the day throughout the water column, particularly with larger larvae. This indicates that visual avoidance of all gears is an important factor, which increases as the larvae grow bigger. LHPR night catches were greater than in the MOCNESS and far greater than in the pump, which indicates that avoidance is also due to senses other than vision. Examination of the 1981–1986 International Herring Larvae Survey catch data showed that night-time catches were similar to daytime catches for herring larvae <10 mm long, but night time catches caught progressively more as the length of the larvae increased to 24 mm.