Information on the movements of fish can be gained from marking and tagging experiments (Jakobsson, 1970; Jones, 1977; Earll and Fowler, 1994). In addition to indicating the extent of migrations and interchanges of population between different areas, tagging also provides data on growth rates and for calculations of fishing intensity, stock size and mortality.

Tagging involves the attachment to the fish of some form of label, usually a small metal or plastic disc of which there are various types. For example, in plaice, a tag consisting of two plastic discs engraved with numbers for identification is commonly used. One disc is threaded on a short length of wire which is pushed through the muscles at the base of the dorsal fin at its midpoint and attached to the other disc. This can be done without drawing blood. A small reward is offered for the return of a disc with details of the circumstances of recapture of the fish. In Europe, tagging experiments have been performed on most species of commercial importance, including plaice, lemon sole, halibut, cod, haddock, hake and herring.

Tagging studies of sharks and rays are now providing particularly important information in the light of increasing, and often uncontrolled, fisheries for these groups (Earll and Fowler, 1994). In the USA, a cooperative shark tagging programme, run by the National Marine Fisheries Service, was started in 1962. It is still running and involves recreational and commercial fishermen, scientists and fisheries observers. When a shark is caught on a line, a dart tag with a message inside a capsule is harpooned into the muscle at the base of the dorsal fin with a pole. The shark is then released.

In Europe, an international system has been set up for the exchange of release information and recaptured fish data. Countries that carry out tagging studies circulate a summarized release list to all the countries likely to have these fish recaptured in its fisheries. In most countries, an agreed amount is paid to the finder for each tag returned, irrespective of the tag's origin. The tags and recapture data are then sent to the country of origin, identified by a prefix code on the tags. In the UK, the scheme is coordinated by the Ministry of Agriculture, Fisheries and Food (MAFF).

Recent developments in the use of sonic tags are providing new information on migrations of large species such as basking sharks in the UK and blue sharks in the USA. The tags can record water temperature, depth, speed and position and relay the information via satellites.

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