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Results from the tagging of Blue Shark in Irish Waters
Dr. Paddy Fitzmaurice & Peter Green
The Central Fisheries Board's Marine Sportfish Tagging Programme was initiated in 1970 by the Inland Fisheries Trust, who had responsibility for developing and promoting recreational sea angling. By the late 1960s, results from sea angling competitions were showing a decline in the capture of some of the most important species such as Blue Shark, Tope, Monkfish, Skates and Rays. At that time, virtually all fish caught by anglers were killed and taken ashore for weighing and photographic opportunities. As little was known at the time about the biology and migration patterns of these species, the Inland Fisheries Trust decided to introduce a tagging programme with the explicit purpose of introducing conservation measures and to learn more about the migratory patterns of sea angling species. Sea angling tourism revenue is worth about £25 million to the Irish economy.
The skippers of angling charter boats were becoming conscious of the need for conservation and, in discussions with the Inland Fisheries Trust's staff, offered their assistance in carrying out a tagging programme. This was to be a voluntary operation.
Different types of tags were tested, including Petersen Discs, Spaghetti
type tags, and jumbo Rototags which were manufactured in Ireland and used
for tagging cattle. The jumbo Rototag was chosen because of ease of supply,
ease of application, it is easily seen and carries a clear message stating
that a reward was offered for its return to the address on the tag. The
tag was applied to the posterior edge of the dorsal fin by means of an
applicator. A few enthusiastic skippers were selected to initiate the
tagging operation. The feedback from the participating skippers and from
the anglers was most encouraging. Today there are in excess of 70 charter
skippers taking part in the programme around the Irish coast.
Blue Shark is one of the largest of our marine sport fishes. They are found in Irish waters from Malin Head in Donegal, around the west and south coasts as far as Hook Head in Wexford. They occur mainly during the months of June to October and are readily caught on rod and line during settled weather conditions, and especially when the sea water temperatures exceed 140°C. They are captured by anglers by laying a trail of chopped up mackerel and fish oil, usually at a distance of 5 to 10 miles offshore.
Tag Returns
Up the end of 1998 15,037 Blue Shark were tagged around the Irish coast.
Recaptures have amounted to 490 fish representing a minimum return of
3.25%. From the pattern of returns it is clear that the Irish stocks of
Blue Shark extend across the North Atlantic. The migratory pattern appears
to follow the continental shelf of Europe and West Africa across the Atlantic
ocean, northward along the north-eastern
coast of South America past the West Indies and upwards towards Newfoundland, in a clockwise direction. The largest number of recaptures were in the vicinity of the Azores Islands, where 350 recaptures have been made (see illustration). The majority of these are taken by Spanish fishing vessels using longlines. In the vicinity of the Canary and Cape Verde Islands, 80 tagged fish have been recaptured mainly by Korean and Japanese longlines.
The other two major groups of recaptures have been taken by anglers off
Long Island, Montauk and Nantucket in the USA, and tags have been returned
by American and Canadian longliners from the fishing grounds south of
Newfoundland, Canada. So far, there have not been any recaptures of Blue
Shark south of the equator, although a number of recaptures have been
reported within 150 miles of the equator. As with all tag and recapture
programmes, the exact details as to the recapture location is vital but,
despite everyone's great endeavour, anomalies can arise. Some fishermen
hold on to the tags as souvenirs, some report them immediately and some
hold on to them for a considerable length of time before reporting or
returning them. They may also have tags from other programmes and it is
possible that they can be mixed up (e.g. the Board has had one of its
tags returned stating that it was taken from a turtle off the Azores),
Other returns have given locations up to 200 miles from the sea: one tag
was found in a magpie's nest in Holland and a tag was found by an English
tourist, on the beach, in the Gulf of Mexico.
The longest distance travelled by a recaptured Blue Shark was 4,250 miles. This fish was tagged off Loop Head, Co. Clare, and was recaptured off Venezuela. The longest number of days at liberty is 1,623 days (4.5 years) although tagged fish in Irish waters can be caught again between 2-15 days after initial capture. We are confident that the retention of tags is good because we have had returns from other shark species after 15 years at liberty. Blue Shark are a pelagic species that must continually swim for water to pass over their gills for the abstraction of oxygen. The returns have indicated that the minimum continuous swimming speed is at least one mile per hour based on direct routes between tagging locations and recapture locations.
A lot of valuable information has been collected during this voluntary tagging programme. The charter skippers take details of the tagged fish and also note the number of anglers on board, the other species of fish caught as well as the nationalities of the anglers. On the 74 charter boats participating last year(1999), the total number of rod angling days was 32,800. Almost 33,000 fish were tagged to the end of December, 1999.
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