A joint project between the Irish fishing industry and scientists from the Marine Institute in Galway to track stocks of cod in the Celtic Sea is starting to yield interesting results.
Funded with monies administered under the Bord Iascaigh Mhara (BIM - the Irish Sea Fisheries Board) National Development Plan over the last two years, 4,063 cod have been tagged and released in the Celtic Sea by Institute scientists working aboard commercial fishing vessels from Dunmore East. The project is designed to study the growth and migration of both the inshore juvenile component and the offshore adult spawning component of the stock.
“To date around 10% of the tagged fish have been returned by a combination of fishermen, anglers and processors from Ireland, UK, Spain and France,” said Macdara Ó Cuaig, a scientist with the Fisheries Science Services team of the Marine Institute. “What makes this project such a success is that every fish that is recaptured and reported adds its own piece of information to the jigsaw and helps us get a better understanding of the stock. This start-stop data is helpful in understanding the behaviour of the fish but does not provide information on where the fish has been at in between. To inform us on the fish activity in-between release and recapture a number of large fish (>50cm) were fitted with an electronic tag.”
These electronic Data Storage Tags (DTS tags), are surgically inserted into the gut cavity of the fish. The tags record time, temperature and depth of the environment surrounding the fish while it is at sea. Each DST tag deployed to date has been set to measure temperature and depth every four minutes for a period of up to two years. Once retrieved, they can yield valuable information about the behaviour of the fish over time by comparing the data recorded by the DST with known temperature and depth data for the area. This allows the scientists to calculate where the fish has been between the release and recapture position, which in turn builds an accurate picture of the migration patterns and associated growth of the stock.
“While the recapture of DST tagged cod to date has provided some interesting data, the amount of information can be limited if the fish is recaptured shortly after release,” explains Macdara. “However, a fish reported to us last month with a conventional tag not only confirmed the rapid growth associated with Celtic Sea cod, but also has the unique distinction of being caught three times and released twice. This fish was originally released in Waterford Estuary on the 6th May 2007 when he was 23 centimetres long with an estimated weight of 120g. He was then caught and released again around St Patrick’s Day this year by our tagging vessel up river above Dunmore East. At that time the fish was 47 centimetres long and weighed over a kilo. He was finally caught and reported by a local on the 26th August at a length of 56 centimetres, weighing a hefty 1.9 kilos, sixty miles south of Hook Head.”
The fact that this fish had grown 33 cm in fifteen months highlights the fact that Celtic Sea cod have high growth rates—in this case an almost nine-fold increase in weight in the first ten months since its initial release, followed by a slower but still significant increase over the next five months to 1.9 kilos.
“This fish had a total of a sixteen-fold increase in weight in just fifteen months!” said Macdara. “This demonstrates the potential yield possible from the Celtic Sea cod stock.”
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