Movements of cod (Gadus morhua L.) in relation to the tidal streams in the southern North Sea

• Published in: ICES Journal of Marine Science
• Main Authors: Arnold, G. P., Walker, M. Greer, Emerson, L. S., Holford, B. H.
• Format: Text
• Language: English
• Published: Oxford University Press 1994
• Subjects: Articles; Gadus morhua
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/51/2/207; https://doi.org/10.1006/jmsc.1994.1021

Twenty-four cod fitted with 300 kHz transponding acoustic tags were tracked by sector-scanning sonar in the southern North Sea for periods up to 52 h and over distances up to 72 km. Three fish were released at the surface; the others were released on the sea bed at depths of 24 to 73 m after a period of enforced pressure adaptation in a small cage. Tidal currents, which were measured with moored current meters during five tracks, strongly influenced all movements. The three fish released at the surface were transported to and fro over the ground by successive tides, as were five fish that moved net distances of 15 to 20 km off-shore across the tidal stream axis when released close to the East Anglian coast. Three cod released east of the Norfolk Banks moved net distances of 40 to 70 km to the north by selective tidal stream transport. Another released north of the Banks swam to the east along the tidal stream axis. Most of these cod turned to head against the prevailing tide when they went to the seabed and one that showed tidal stream transport in an area of moderate tidal currents made ground against the opposing tide when it was on the bottom. Several fish maintained a heading in midwater for a number of hours and one deviated by no more than ± 45° from its mean heading during an 8-h period. Three fish made slow sweeping turns in midwater to adopt a downtide heading after previously swimming across the tide. Average swimming speeds in midwater were 0.3-0.9 L s−1 and similar speeds were estimated for fish in the bottom boundary layer; ground speeds, which reflected the speed of the tidal current, were proportionately greater than through-water speeds. Several fish made pronounced vertical movements to or from the seabed at or near sunrise or sunset and one fish showed a diel pattern of vertical migration. The results are discussed in relation to the sensory cues and clues that might be used for orientation and migration, swimming performance, and the role of selective tidal stream transport in the spawning ...