Publication Abstract

Behavioural mechanisms of migration inthe sea: how fish take advantage of their environment

GP Arnold

Water is short in oxygen and much moreresistant to movement than air. As a result, fish swim slowly, except when feeding oravoiding predators, and currents have a significant affect on their movements. Eggs andnewly hatched larvae drift downstream from spawning ground to nursery ground but currentsare often important for adult fish, too, and there is a prima facie case that migrationcircuits fish follow the direction of oceanic gyres. Several species of Pacific salmon,for example, spend several years in the Alaskan Gyre. Albacore tuna appear to follow theclockwise movements of the Kuroshio, California and North Equatorial Currents that make upthe subtropical gyre in the North Pacific. Two bluefin tuna, which travelled from theBahamas to Bergen in 118 days, probably followed the Gulf Stream and its extension, theNorth Atlantic Drift. There are, however, as yet too few observations of the detailedmovements of highly migratory fish to know to whether they routinely follow oceancurrents, or only take advantage of them when it is energetically favourable to do so.

Rather more is known about migration in shelf seas, wheremany species of demersal fish exhibit a 12-h pattern of vertical movement known as selectivetidal stream transport. Fish leave the seabed at one slackwater and spend about sixhours in midwater. They return to the seabed at the next slackwater and remain there forthe ensuing tide, not moving significantly. Fish select opposing tidal streams before andafter spawning. Migration speed is determined by a number of factors, including the speedof the tidal stream (commonly between 1-2 m s^-1), the size of the fish andwhether it uses all available tides. Plaice migrating by tidal stream transport saveenergy by heading downtide within ±60⁰ of the tidal stream axis and swimmingthrough the water at approximately 0.6 fish lengths s'. For a 35-cm female plaice thesaving on a typical migration circuit of 560 km is equivalent to 30% of the energy contentof the eggs she spawns each year. Recent results indicate that plaice do not use tidalstream transport in areas of the North Sea where the speed of the tidal stream is too lowto save energy but resort to other patterns of movement that are independent of the tidalstreams. It can be hypothesised therefore that whilst tidal currents are a source oftransport they are probably not a source of directional information; the same principlemay well apply to currents in the oceans.

Reference:

G.P.Arnold, 2000. Behavioural mechanisms of migration in the sea: how fish take advantageof their environment. Comparative Biochemistry and Physiology, 126B(Suppl. 1): S6.