BIM bluefin tuna tagging programme

2004

YEAR BOOK

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Bord Iascaigh Mhara � The Irish Sea Fisheries Board

Ronan Cosgrove

BIM bluefin tuna tagging programme

Figure 1. A tagged bluefin tuna is released off Donegal (photo copyright Shay Fennelly)

Bluefin tuna (Thunnus thynnus) have been observed in Irish waters as a by-catch of commercial fishing vessels since the late 1970s. The first ever rod and line caught bluefin tuna in Irish waters was taken in September 2000. Since then, a number of angling operators have expanded their business to cater for big game anglers who travel from far afield in search of the thrill of hooking one of these leviathans � which can reach in excess of 250 kg. This provides a welcome economic boost to charter vessel owners and the local tourist industry from September to November each year.

Recognising the fact that fishing of a novel species should not be allowed to expand faster than the acquisition of information necessary to provide a basis for effective management of the resource, BIM started a tagging programme for bluefin tuna in 2003. Carried out in collaboration with the 'Tag a Giant' team from Stanford University, California, and the Irish Game Fish Association, the programme aims to provide biological information, such as migratory paths, depth preferences and behaviour of bluefin tuna in Irish waters. This information will contribute to the knowledge and international management of this highly migratory species. A broader benefit of the programme is the creation of a culture of capture and release amongst Irish charter skippers and anglers, which will assist in conservation of this magnificent species.

The tags used are called 'pop-up archival transmitting' (PAT) tags, which are designed to track the large scale movements and behaviour of pelagic fish and other animals. Summaries of depth, temperature and light-level data are archived in the tag, while being towed by the animal. Neither the fish nor the tag needs to be retrieved in order to obtain these data. At a user-specified date and time, the PAT tag actively corrodes a pin to which a tether is attached, thus releasing the tag from the animal. It then floats to the surface and transmits the summarised information via the Argos satellite system. Daily longitude of the migration track is calculated onboard the PAT tag using geo-location by light level techniques. Daily latitude can be calculated from transmitted light level curves using software provided by the tag manufacturer. The results provide the migration path and depth and temperature preferences of the study animal, as well as oceanographic data in the form of depth-temperature profiles. Stanford University has reported a 90% data retrieval rate for bluefin tuna tagged with PAT tags, indicating a very high survival rate for captured and released bluefin tuna.

Figure 2. Chart tracking the location of two bluefin tuna which were tagged and released off Donegal in September 2003

A total of three bluefin tuna, two off Donegal and one off Mayo in the west of Ireland, were tagged and released in 2003. To date, data from two tags from the fish that were tagged off Donegal have been retrieved. Initial analysis from the pop-up satellite tag endpoints indicates that one fish was located in the West Atlantic six months post-tagging, while the tag on the second fish popped up in the East Atlantic close to the European coastline after seven months at liberty (Figure 2). The fish that crossed to the West Atlantic first travelled to the west of Ireland and the Celtic Sea in November and December, before moving to the West Atlantic in January. This bluefin tuna eventually crossed the Atlantic, with the tag popping off in a known spawning area north of the Bahamas in late March. The second fish moved to the mid-Atlantic ridge area, where it remained throughout December and January before returning to the east. This tag popped up west of Portugal in mid April.

The data from the pop-ups which have occurred to date suggest that Ireland may be a feeding location for fish of eastern and western Atlantic origin. The tagging work continued in 2004.

Contact: Ronan Cosgrove, BIM, New Docks, Galway City, Co. Galway; E-mail: [email protected]