Patterns in tidal migration of fish in a Brazilian mangrove channel as revealed by a split-beam echosounder

A 200-kHz circular split-beam echosounder (BioSonics, DT6000) was placed in the center of a macrotidal mangrove channel in north Brazil to study the movement patterns of tidal migrating fish. Vertical beaming gave high signal-to-noise ratios (21 dB) during neap tides in the dry season 2002. Despite low Secchi depths, a diel change in the vertical target distribution was apparent when fish inhabited the water column only during the night flooding. Moreover, responses in vertical distribution occurred at dusk and dawn. The multispecies population simultaneously caught with a tidal trap consisted of juveniles and adults of small species, and juveniles of larger species (mean total length of fish: 14 cm), being reflected in target strength values ranging from -60 to -40 dB. Nekton organisms usually traveled with the tide. At low water fish concentrated in the subtidal channel sections, swain slowly and meandered in different directions. At first flood rise-the strongest ambient change-the entire fish population 'rode the tide' to immigrate into the intertidal zone, indicated by fast, linear, upstream tracks throughout the water column. At slack high water fish milled around (likely Cetengraulis edentulus and Anchovia clupeoides at night and epibenthic fish in daytime). When sampling the spatiotemporal distribution of fish in intertidal environments, the 3D spatial heterogeneity versus time should be considered. A net upstream longitudinal current together with a regular first flood rise likely promoted retention of fish in this mangrove nursery from one tide to the next. In well-mixed shallow-water environments with a dominant epibenthic fish population, horizontal and vertical beaming should be combined since deeper sections may serve as a refuge for the fish. (C) 2004 Elsevier B.V. All right, reserved.