Photo: Tagging bigeye tuna in Kiribati, Gilbert Islands EEZ, 2009 (Credit: Bruno Leroy, Copyright: Secretariat of the Pacific Community).
Thursday 16 October 2014, Secretariat of the Pacific Community (SPC) headquarters, Noumea, New Caledonia – Predominantly west-to-east movements and geographical concentration – the Equatorial Pacific bigeye tuna’s behaviour is gradually revealing its secrets. This and other findings have been reported in a new scientific paper authored by scientists from SPC and the Inter-American Tropical Tuna Commission, published recently in the journal Fisheries Research.
Bigeye tuna: better management through better understanding
Bigeye tuna (Thunnus obesus) live in temperate-to-tropical waters of the Pacific Ocean. In the 1950s, this species began to be targeted by longline fleets; fishing then intensified in the 1990s as purse seine fishing developed, leading to overfishing in the 2010s.
Since 1977, Pacific Island countries and their development partners have realised that tagging is essential in order to gather information on the state of stocks, growth and tuna movements. Tagging with conventional tags has been providing these data for more than 60 years, while over the past 15 years or so, electronic tags have produced additional information, especially on fish behaviour.
Large-scale tagging campaigns
As the first glimmer of light appears on the horizon, bigeye tuna are being constantly reeled in by the trolling lines of the longliner ‘Pacific Sunrise’. One after the other, these tuna are placed on the tagging tables, where they are measured by the scientists, who insert a tag behind the second dorsal fin and return them to the water, all in under 10 seconds.
From 2008 to 2014, 10 tagging cruises were conducted in the central equatorial Pacific. Chartered fishing vessels were fitted with trolling lines to catch the bigeye tuna associated with seamounts or FAD’s (Fish Aggregation Devices). These cruises tagged more than 35,000 bigeye tuna, more than 10,000 of which have been recaptured, mostly by the industrial purse seine fishing fleet.
A major information campaign is in constant progress on board fishing vessels and in ports around the Pacific to inform fishing crew and industry about the tagging programme. The SPC’s contact details are given and ‘Reward’ is printed on the tags for the benefit of fishing crew.
Tuna carrying conventional tags are measured and the date and geographical coordinates of the catch location sent to the SPC. The electronic tags follow the same process but also record extra data such as light intensity (allowing daily position estimation), temperature and depth.
Pacific-wide fishery management
Bigeye tuna living in the equatorial zone are apparently relatively isolated from those inhabiting higher latitudes. Very few (less than 1% of recaptures) tagged fish have been caught beyond latitudes 10N and 10S. One possible explanation for this geographical concentration could lie in the equatorial current system, producing an upwelling conducive to major tuna prey aggregations. This availability of forage associated with a physical environment favourable to reproduction induces geographical concentration in the region’s bigeye tuna.
With reference to the longitudinal movements of tuna tagged in the central Pacific, they predominantly feature easterly migrations and may be influenced by the powerful (>1m/s) Equatorial Undercurrent (EUC), forming a sub-surface layer some 200 m thick and moving in an easterly direction. Further research on these movements also shows greater mobility in the central Pacific (180 – 120°W) as compared to fish tagged to the east and west of this zone. This central zone can be considered as a mixing or exchange area between the eastern and western parts of the Pacific.
These results demonstrate the need for Pacific-wide fishery management approaches. Bigeye tuna management measures should in future be well coordinated between the Western and Central Pacific Fisheries Commission and the IATTC (in the eastern Pacific) so that this Pacific-wide resource can be conserved and sustainably fished into the future.