Detailed studies of the migratory movements of marine animals are still hindered by the difficulty in obtaining repeated and frequent estimates of their geographical location over appropriate seasonal time-scales. Many marine animals do not surface with sufficient frequency to be tracked using satellite-based telemetry, and are often too small to bear satellite tags. Furthermore, the use of ambient light records to estimate geoposition from data loggers is not possible for many marine fish, particularly in relatively turbid shelf seas like the North Sea. However, the unsustainable levels of exploitation currently being experienced by fish stocks makes an understanding of their behavior paramount to the implementation of effective conservation measures.

Towards this end, we have successfully used small electronic data storage tags (DST), which measure ambient depth (from water pressure) and sea-water temperature, on North Sea plaice, cod and rays. These tags can record and store data for many months, providing information over seasonal time-scales appropriate to the understanding of annual migratory movements. Tidal data (times of high water and tidal ranges), recorded when fish remain motionless on the sea-bed, have been used to estimate geoposition with a degree of accuracy unattainable using light-based geolocation. Our results have provided detailed information on fish spatial dynamics, encompassing individual vertical activity patterns to population sub-structures (see following abstract).

In terms of fish stock conservation, our data provide a fundamental method by which accurate predictions can be made regarding the occupancy and transition times of fish in defined areas. With fisheries exclusion zones increasingly being employed as a measure to regulate fishing intensity, the incorporation of fish behavior data into biologically-based predictive models of spatial dynamics has an important potential role to play in the formulation of stock recovery plans.

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