Saturday, 28 November 2015

Bluefin tuna: a new perspective in the NE Atlantic

 In 2013, Angus Campbell caught a 515lb Atlantic bluefin tuna with a rod and reel off the Isle of Harris, the Outer Hebrides. Although this wasn’t out of the realms of possibility, this had never been done before in Scotland. In 2014, Dr. Francis Neat (Marine Scotland Science) initiated a scientific program on bluefin tuna in Scotland with the aim of finding out three things: 1) how long bluefin resided in Scottish waters, 2) where they went, when they left, and 3) what stock the fish belonged to. That year we successfully tagged three bluefin with miniPAT tags from Wildlife Computers. Although the project was a success, the results from our work were far from conclusive. We’re now in the process of starting a collaborative study with Stanford University, to contribute knowledge on this enigmatic species to the bigger picture of bluefin in the northeast Atlantic.

Atlantic bluefin tuna post-release off the Isle of Harris, Scotland exhibiting miniPAT

Bluefin tuna are commercially important, highly migratory apex predators, split into three geographically distinct species: Atlantic, Southern and Pacific bluefin tuna. Demand for these fish has skyrocketed over the last few decades in line with the rise of the Japanese sushi-sashimi market, in which bluefin is the most highly prized delicacy. The majority of bluefin caught is flash frozen and shipped to Japan for auction, with single fish fetching exorbitant amounts of money: in 2013 a fish weighting 489lbs sold for $1.76m. Although this figure was especially high, fish regularly sell for tens of thousands of dollars.

Graph showing price of inaugural bluefin tuna sold at the Tsuiji fish market, Japan

Atlantic bluefin tuna are comprised of at least two genetically distinct stocks, designated by their spawning region: the eastern stock in the Mediterranean and the western stock in the Gulf of Mexico. Fish from both stocks make seasonal migrations from warm low-latitude waters to highly productive foraging grounds at higher latitudes [1]; although not all fish do this, and whether bluefin migrate or not is related to maturity and body size, with larger fish ranging further. Despite being genetically distinct, migratory fish mix extensively outside of their spawning areas and fish from the western stock can be found in the eastern Atlantic and vice versa [2]. As a result of prolonged overfishing on both sides of the Atlantic, the western and eastern stocks were reduced to 17% and 33% respectively, of 1950’s spawning stock biomass by 2008 [3]. This caused the bluefin regulatory body, ICCAT (the International Commission for the Conservation of Atlantic Tunas) to introduce stock rebuilding programs, ultimately resulting in slashed catch quotas.

Atlantic bluefin tuna, south Donegal, Ireland 2015
The extent of bluefin distribution is limited by temperature, despite their advanced thermoregulatory capacity. Recent reports of bluefin in the Greenland strait (2010), the establishment of small-scale fisheries off Iceland and Norway (2014), increased sightings off Ireland and Scotland (2012-13-14), fish caught off Wales (2015) and even sightings off Cornwall, England (2015) suggests bluefin have repatriated highly productive northern latitudes in significant numbers in recent years. This simple fact would lead us to believe that something has changed; what that ‘something’ is, is cryptic.

Map showing UK and Ireland Atlantic bluefin tuna sightings 2013-15

We are exploring three possible causes for these recent changes; 1) a warming ocean climate, allowing tuna to exploit areas previously too cold, 2) the forage prey are now ranging further north and in greater abundance than previously believed, e.g. mackerel, or 3) a recovery of the eastern bluefin stock, as has been heralded by ICCAT; this would result in a more significant cohort of larger, more migratory fish. These hypotheses are not mutually exclusive and may all be acting in concert. Consequently, this is just the beginning of tuna research in the UK and Ireland.

Atlantic bluefin tuna feeding on sprat off Donegal, Ireland 2015

Our work would certainly not be possible without the efforts of a number of recreational fishermen, acting responsibly on a catch-and-release basis. This form of fishing represents a hugely sustainable way of gaining revenue from bluefin tuna. A report looking into further developing the existing bluefin recreational fishery in Canada’s Atlantic provinces, estimated that 1 tonne of bluefin quota allocated to a live-release fishery could yield up to $100,000; 6 times that of a capture fishery, whilst having minimal effect on the stock [4]. If the eastern Atlantic bluefin stock has bounced back, such fisheries may have a place in the UK and Ireland, and one of the key project aims of our work in Scotland is to advise as to whether or not this would be a possibility. Well managed catch-and-release fisheries represent a way of providing much needed revenue to often remote coastal communities, whilst also supporting vital scientific research on apex predators and maintaining good fish stocks.

1: Walli, A. et al. PLOS One, 4(7): e6151. (2009) doi: 10.1371/journal.pone.0006151
2: Block, B. et al. Nature, 434: 1121-1127. (2005) doi: 10.1038/nature03463
3: Taylor, N. et al. PLOS One, 6(12): e27693. (2011) doi: 10.1371/journal.pone.0027693

Tom Horton is a marine biologist, wildlife guide and photographer specialising in the spatial ecology of marine megavertebrates around the UK & Ireland. His current work involves basking sharks, ocean sunfish and Atlantic bluefin tuna. You can follow Tom and check out his work and pictures on Twitter @profhorts.