Movements of thornback rays in the Thames estuary in relation to special protected areas and possible management options
Life history traits of rays, in particular their low fecundity, late age and large size at maturity, make them particularly vulnerable to exploitation. Catch statistics show the catches of rays and skates around the UK, between 1955-1995, decreasing at a rate of about 2 thousand tonnes per decade. Consequently conservation measures need to be considered.
The movements and migrations of rays are poorly understood. Conventional tagging experiments, involving the thornback ray (Raja clavata) in the Thames Estuary, indicate that the majority of fish were recaptured within a limited area around the release sites suggesting that these rays form local sub-populations and exhibit only small-scale movements. As a result, ICES (International Council for the Exploration of the sea) Advisory Committee on Fishery Management (ACFM) has suggested closed area management as a possible conservation measure. However, conventional tagging data are compromised by the level and distribution of fishing activity. Recent studies, using electronic data storage tags on plaice in the North Sea, show that individual fish can move up to ten times further than the net distance between release and recapture position.
We have been using depth and temperature sensing data storage tags (DST's) successfully on plaice in the North Sea to obtain behavioural and environmental data, see the plaice website. Vertical patterns of behaviour (depth) are recorded by changes in pressure. Spatial movements are interpreted from hydrostatic data (times of high and low water), recorded when a fish is stationary on the seabed, and from temperature records.
In order to describe the extent of their movements we are now applying the methods developed during our plaice studies to the thornback ray population in the Thames Estuary. In 1999 we tagged 97 thornback rays with depth, temperature and light sensing DST's in the Thames Estuary in order to describe their behaviour and geographical movements, a further 100 rays were tagged in 2000.
So far 82 (42%) DST's have been returned, yielding about 11500 days of data. Initial results indicate that these fish are not restricted to the Thames Estuary but move more widely in the Southern Bight of the North Sea. There also is an apparent seasonal pattern to this movement, with the fish spending more time outside the Thames Estuary between November and March.
