Publication Abstract

Movements of cod (Gadus morhua)tracked in the southern North Sea

G.P. Arnold, M. Greer-Walker, L.S. Emerson and B.H. Holford

Twenty-four cod fitted with 300 kHz transponding acoustic tags were tracked bysector- scanning sonar in the southern North Sea for periods up to 52 h and over distancesup to 72 km. Three fish were released at the surface; the others were released on the seabed at depths of 24 to 73 m after a period of enforced pressure adaptation in a smallcage. Tidal currents, which were measured with moored current meters during five tracks,strongly influenced all movements. The three fish released at the surface were transportedto and fro over the ground by successive tides, as were five fish that moved net distancesof 15 to 20 km off shore across the tidal stream axis when released close to the EastAnglian coast. Three cod released east of the Norfolk Banks moved net distances of 40 to70 km to the north by selective tidal stream transport. Another released north of theBanks swam to the east along the tidal stream axis. Most of these cod turned to headagainst the prevailing tide when they went to the seabed and one that showed tidal streamtransport in an area of moderate tidal currents made ground against the opposing tide whenit was on the bottom. Several fish maintained a heading in midwater for a number of hoursand one deviated by no more than ±45° from its mean heading during an 8-h period. Threefish made slow sweeping turns in midwater to adopt a downtide heading after previouslyswimming across the tide. Average swimming speeds in midwater were 0.3-0.9 L s ^-1and similar speeds were estimated for fish in the bottom boundary layer: ground speeds,which reflected the speed of the tidal current, were proportionately greater thanthrough-water speeds. Several fish made pronounced vertical movements to or from theseabed at or near sunrise or sunset and one fish showed a diel pattern of verticalmigration. The results are discussed in relation to the sensory cues and clues that mightbe used for orientation and migration, swimming performance, and the role of selectivetidal stream transport in the spawning migrations of cod in the southern North Sea.

Reference:

G.P. Arnold, M. Greer-Walker, L.S. Emerson and B.H. Holford, 1994. Movements of cod (Gadusmorhua) tracked in the southern North Sea. ICES Journal of Marine Science, 51(2): 207-232.