PATCHINESS OF LONGITUDINAL FISH DISTRIBUTIONS IN A RIVER AS REVEALED BY A CONTINUOUS HYDROACOUSTIC SURVEY

During July and September 1993, the longitudinal distributions of fish communities in five reaches of the River Thames were obtained by mobi le acoustic surveys at night, using a BioSonics dual-beam echosounder (420 kHz) with a transducer beaming horizontally across the river. Alt hough acoustic ranges were small (10-20 m), the total sampled volume w as large. Continuous records of absolute fish densities in the water c olumn were obtained by echo integration at 20 m or 40 m intervals alon g the river. The striking feature of the acoustic data is the evidence they offer for different scales of patchiness in continuous longitudi nal distributions of fish targets. The scale of patches varied in dens ity and size. Some obvious larger patches could be associated with riv er features (sewage outfalls) or particular events (mass emergence of insects at dawn). For other larger patches, there were no obvious caus es but they were persistent in replicated runs during one night or occ upied the same locality in both months. Patchiness at smaller scales w as also detectable all along the river course between the larger patch es. (C) 1996 International Council for the Exploration of the Sea