The movements of plaice (Pleuronectes platessa L.) tracked in the open sea

Abstract

Plaice fitted with 300 kHz acoustic transponding tags were tracked by sector scanning sonar off the East Anglian coast for periods up to 54 h and over distances up to 61 km. Using horizontal (plan) and vertical (elevation) scanning, the tracking technique enabled a fish to be positioned to within ±2·5 m in range and 1° in bearing from the research ship, and to within ±1 m in depth. The position of the research vessel was determined by reference to the Decca Navigator system with a maximum variable error of ±45 m by day and ±90 m by night. After release a fish was kept under continual surveillance and a complete record of its movements in the sea was reconstructed from the sonar data recorded during the track. Water current measurements were made during some tracks. Plaice which moved more than 15 km usually came off the bottom at slack water, moved downstream with the tide in midwater and returned to the bottom at the next slack water. When on the bottom the fish showed little or no movement during the opposing tide. The semidiurnal (12 h period) vertical movements were clearly related to the tidal cycle, ascents being more closely linked to slack water than descents. The regular pattern of behaviour - here called selective tidal stream transport - could provide a quick and economical means of movement for fish on migration through areas with strong tides. The observations are discussed in relation to the results obtained when midwater trawling for plaice along the line of their migration route in the Southern Bight of the North Sea. Other aspects of plaice behaviour observed during the tracking are discussed in relation to ship's noise, weather, bottom topography, the energy cost of migration, sensory clues initiating and controlling migration, orientation in midwater, and the development of telemetering acoustic tags.