Cod tagging in the Celtic Sea

Wednesday 29th April 2009
Cod tag near dorsal fin

Fisheries scientists from the Marine Institute in Galway, Ireland are asking fishermen and fish merchants to keep an eye out of any of the 6,500 specially tagged cod that have been released into the Celtic Sea as part of a successful joint Industry-Science Partnership Survey that is now into its second year. Local fishermen, keen not to let the current economic difficulties disrupt the programme, have contributed greatly to ensure the maximum return was achieved with the limited funds available, tagging and releasing 2,875 cod during the latest 12 day survey.

This year, for the first time, some 350 live juvenile cod have been brought ashore and transported to the laboratory of the Martin Ryan Institute of NUI Galway at Carna, Conamara. Studies to date indicate that Celtic Sea cod are the fastest growing cod around the Irish coast and work at Carna aims to investigate the possible reasons why, by comparing growth under controlled conditions to that of fish recaptured later from the wild.

A number of factors will be studied, including the influence of genetics, temperature and food availability. These juvenile fish will add an extra dimension to the work already underway at the MRI on larval growth from Celtic Sea cod eggs collected and fertilised during the survey.

“As we now have over 6,500 tagged cod released in the Celtic Sea anyone handling fish, especially cod, should keep an eye out for the standard external tags fitted to the first dorsal fin,” said Macdara Ó Cuaig of the Institute’s Fisheries Services team (left).

“It is important to check the fish prior to gutting as some fish have been recaptured and the DST has been lost during the gutting process. On finding a tagged fish one should record the length, weight, recapture position & date and contact the Marine Institute to arrange fish pick up.”

Over 80% of the cod released this year were juveniles captured in the Waterford Estuary during dedicated one hour fishing tows in the channel from Dunmore East up to Cheek Point and Bell View.

“We’ve seen over the last few years that juvenile one year old cod reside in this area during the spring and provide us with a great opportunity to investigate this component of the stock,” says Macdara. “Our tag recaptures to date show that these cod can be subsequently caught off shore where the mature fish feed. Some of this year’s juvenile cod have already been recaptured and reported by local anglers fishing the estuary. These angler returns coupled with the data from the offshore fishermen will help us track the movements of the cod over the seasons.”

In the offshore sector, twenty to sixty miles off, 328 mature cod were tagged and released. Over 40% of these mature fish were fitted with a Data Storage Tag (DST) along with dorsal tags. Each DST tag is capable of recording temperature and pressure and on retrieving can be used to map the likely route traversed by the cod between release and recapture. This current work builds on last year’s work where 100 DST carrying fish were released.

“Some of these fish have subsequently been recaptured and returned to us with the associated DST and data,” said Macdara. “A Dunmore East based fishing vessel has recently spotted and returned a DST carrying cod that has been in the water for a year and a day. This cod has returned a massive dataset that gives us the temperature and depth the cod was at, every four minutes for 366 days.

Initial analysis of the DST shows that the fish stays in the temperature range 8-12oC, starts off the spring at around 80m depth and spends August to January at around 110m before returning to around the 80m depth contour again for the following spring. Once the mapping of the dataset is complete we will be able to tell where this cod spent every day of the last year. As with all data associated with the tagging programme every piece of information is adding more to our understanding of how the fish behave.”

The Institute are asking fishermen, fish merchants and anglers to keep the fish whole if possible but, if tag is spotted after gutting, to still  please contact them directly as any information in relation to a tag and its’ associated fish will help increase their. knowledge of the stock.

“We would like to take this opportunity to thank all fishermen, anglers and processors who to date have reported tagged fish recaptures to us,” said Macdara. “Your enthusiasm and knowledge makes a vital contribution to these projects. As per usual we will keep you all updated on progress and new information as the seasons go on.”

Marine Instute, Ireland
Martin Ryan Institute, NUI Galway

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