VERTICAL MOVEMENTS OF COD (GADUS-MORHUA L) IN THE OPEN SEA AND THE HYDROSTATIC FUNCTION OF THE SWIMBLADDER

The vertical movements of 24 individual cod fitted with 300 kHz transponding acoustic tags were described using a sector scanning sonar operating in vertical mode. The fish, which were 50-76 cm in length, were tracked continuously for periods of up to 52 h in the southern North Sea. Three fish adapted to atmospheric pressure were released at the surface; the others were released on the sea bed at depths of 24 to 73 m, after a period of confinement in a small cage, during which they had the opportunity to adapt, or partially adapt, to the ambient pressure. Most fish spent long periods in midwater and a third of them increased their depth slowly and steadily, suggesting that they were maintaining neutral buoyancy and at the same time secreting swimbladder gas. The rates of descent, which ranged from 0.3 to 0.8 mh-1, were consistent with predictions made from measurements of gas secretion rates of small cod in the laboratory. Similar predictions indicated that about half the caged fish had achieved neutral buoyancy at the end of the adaptation period: on release these fish moved into midwater but did not swim above the upper limit of the free vertical range for the predicted depth of neutral buoyancy. A series of rapid ascents allowed us to test and reject the hypothesis that cod are neutrally buoyant at all depths and conclude that most fish are negatively buoyant on the sea bed and only in neutral buoyancy at the top of their vertical range. These ascents, which occurred on release and at the beginning of each major excursion into midwater, involved vertical movements of 20-30 m, speeds of up to 3 m min-1, and pressure reductions of 30-70%, sufficient in two cases to have burst the swimbladder had it initially been fully inflated. The results are discussed in relation to the known vertical migrations of cod populations in the North Atlantic and tilt angles and target strengths of individual fish in relation to acoustic surveys. © 1992 International Council for the Exploration of the Sea.