Fish Migration by Selective Tidal Stream Transport: First Results with a Computer Simulation Model for the European Continental Shelf

Abstract

The demonstration of selective tidal stream transport in species as diverse as plaice, sole, cod, dogfish and eels suggests that the phenomenon may be of widespread significance in the migration of fishes on the European continental shelf. A computer simulation model has been written which interpolates the speed and direction of the tidal stream from the data given for the tidal diamonds on the appropriate British Admiralty Charts. The model allows for the cyclical change of current speed between neap and spring tides and includes three behavioural options for the fish: 1) semidiurnal vertical migration initiated at either slack water with a variable period of transport in midwater; 2) diurnal vertical migration with a variable ratio of time spent in midwater to time spent on the bottom; and 3) continuous transport (i.e. drift) in midwater. The aims of the model are to generate simulated tracks which: 1) taken individually, may lead to predictions about the behaviour of individual fish, which can be tested by further tracking experiments; and 2) taken collectively, may lead to predictions about the resulting distributions of whole populations, which can be tested by conventional tagging experiments. The first simulations have been concerned with the prespawning and postspawning migrations of plaice in the Southern Bight and English Channel and with the spawning migrations of silver European eels crossing the European shelf on their return to the Sargasso Sea. Some simulations have also been made of the tracks of plaice exhibiting diurnal vertical migrations on various feeding, spawning and staging grounds in the southern North Sea and English Channel.

Details are given of a series of semidiurnal tracks running from the southern North Sea through the Strait of Dover into the English Channel, and it is shown that the four centres of plaice spawning off Flamborough Head, in the Southern Bight and in the English Channel lie along a common streampath. A second series of semidiurnal tracks is used to predict the distributions of spent plaice migrating out of the Southern Bight into the central North Sea. The distributions are plotted on a lunar time base, and it is shown that fish contained within the area of peak spawning will use the western migration route. The diurnal tracks show that a fish adopting the diurnal mode of vertical migration will remain within a relatively confined locality even in areas of fast and directional tidal streams.

The simulated silver eel tracks show that a fish leaving the River Severn, England, should be able to reach the edge of the continental shelf to be the southwest of the British Isles in about 40 days by selective tidal stream transport. A fish from the River Humber, England, should similarly be able to reach the edge of the shelf by the northern North Sea in about 100 days. But there is a problem for eels using tidal stream transport from the River Elbe, West Germany. They would appear to become permanently trapped in the system of east-west tidal streampaths in the central North Sea and might fail to reach the edge of the continental shelf in sufficient time to complete the subsequent oceanic phase of the migration. An analogy is drawn between this apparent “tidal gyre” in the North Sea and the areas of the central North Pacific from which Pacific salmon fail to return to North America. An eel from Texel, The Netherlands, relying on tidal stream transport would similarly reverse in the English Channel in the area of dividing tidal streampaths to the west of the Cherbourg peninsula, France, and return to the North Sea. Reversal is also relevant to the migrations of the plaice and appears to be limited to tracks passing through the Straight of Dover within 20 km of Cap Gris Nez, France.

Future work will be concerned with extending the simulations to the distributions of other plaice stocks on the European shelf—and to other species such as cod and haddock—and to predictions aimed at furthering our understanding of the mechanism of selective tidal stream transport itself. The phenomenon of reversal, for example, suggests the possibility of conducting a sea-going experiment to determine whether the underlying mechanism is, as we suspect, based on a succession of temporal cues or instead involves an absolute sense of direction on the part of the fish.