In situ swimming behaviour of individual mesopelagic fish studied by split-beam echo target tracking

The mesopelagic fishes Muller's pearlside (Maurolicus muelleri) and norther n lantern-fish (Benthosema glaciale) were studied in near-surface watts dur ing their diel vertical migrations. The fish apparently alternated between swimming predominantly vertically and horizontally, although many fish were descending slowly (similar to1 cm s(-1)) during horizontal swimming. Verti cal swimming speeds during rapid ascents and descents were typically 10-30 cm s(-1). corresponding to several body lengths s(-1). Horizontal swimming paths varied extensively, some fish following a rather straight track, othe rs displaying pronounced smooth or sharp turns. Horizontal swimming speeds appeared to be in the same range: as the vertical swimming speeds during as cents and descents. but were less reliably estimated as this component was more affected by noise than the vertical component. Both the advantages and limitations of the split-beam tracking technique in behavioural studies ar e discussed in light of these observations. (C) 2001 International Council for the Exploration of the Sea.