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

The mesopelagic fishes Müller's pearlside (Maurolicus muelleri) and northern lantern-fish (Benthosema glaciale) were studied in near-surface water during their diel vertical migrations. The fish apparently alternated between swimming predominantly vertically and horizontally, although many fish were descending slowly (∼1 cm s ^-1) during horizontal swimming. Vertical 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, others displaying pronounced smooth or sharp turns. Horizontal swimming speeds appeared to be in the same range as the vertical swimming speeds during ascents 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 are discussed in light of these observations.