Publication Abstract

Time and tide wait for no plaice

J.D. Metcalfe and G.P. Arnold

Unless you are a mariner, or a hapless summer bather, it is not immediately apparent that the seas around Britain are ripped by powerful tidal streams. Yet these streams can reach speeds of up to 1^.5 metres a second (a brisk walking pace) with average speeds of about 0^.5 metres a second. They do not flow continuously in one direction, but first one way as the tide floods and then back the ether as it ebbs, with a small residual drift in one direction. These tidal streams are superimposed on the overall circulation produced by currents from the open ocean flowing onto the continental shelf. Fisheries scientists have known for many years that the residual drift plays an important part in the dispersal of fish eggs and larvae, carrying them away from the spawning grounds towards nursery areas near the coast. What was not so obvious was the role of tidal streams. We now know that fish make use of these too, but in an altogether more sophisticated way.

The idea that fish might move around on tidal streams emerged only 45 years ago. This might seem curious to the ightermen of the Port of London, who for centuries let the tides move their cargoes about. But it is extraordinarily difficult to follow the movements of fish in the open sea. So another 15 years passed before the Dutch zoologist Frederick Creutzberg showed that young eels use tidal streams to migrate into the Wadden Sea on their way to fresh water. Even so, the problems of following the movements of wild dish remained.

A group of scientists at the Ministry of Agriculture's Fisheries Laboratory in Lowestoft, on the Suffolk coast, has been trying to solve these problems. We adopted equipment originally developed for the Royal Navy, and the microelectronic technology behind pocket-sized televisions and digital watches. From the Admiralty Research Laboratory at Teddington, we borrowed a high-resolution, sector-scanning sonar that sweeps a sector of 30^° two to four times a second with an acoustic beam. Objects in the path of the beam reflect some of the acoustic waves, which are converted to a signal on a sonar screen. Although the device was developed to detect underwater objects, such as wrecks or mines, in dark and murky waters, we installed it on the ministry's research ship Clione to watch how shoals of fish respond to commercial fishing gear. We also hoped that the sonar would be useful for watching the behaviour of fish. But although it gave excellent pictures of trawls, wrecks and even fish, an individual fish could not be followed for long because when it came close to the sea floor, its echo merged with those from the seabed.

Electronic engineers at the laboratory overcame this problem by designing a small acoustic "transponder", which could be tied to an ordinary identification tag and attached to the fish. The acoustic tag "listens" for the sonar signal and responds by transmitting a pulse of sound at the same frequency. This allows scientists aboard a research ship to identify and follow a single fish and so test the idea that adult fish migrate on tidal streams.

Reference:

J.D. Metcalfe and G.P. Arnold, 1990. Time and tide wait for no plaice. New Scientist, 125(1698): 52-55.