Last August, a large shoal of one of the ocean’s most highly prized fish, the Atlantic bluefin tuna (Thunnus thynnus), made a dramatic appearance in the waters of Mount’s Bay, Cornwall, England. Their appearance, reported by a marine wildlife tour boat operating out of nearby Penzance, made national news amidst claims that a fleet of French vessels was hot-footing it to the area in order to exploit this multi-million dollar prize purse, as English boats lacked the quota to do so. Whilst the claims were largely untrue (there were no French boats en route to the scene), the ecological significance of the sighting fits amongst many other cases of distributional changes of marine species over the past two decades, all largely related to climate change. However, the nature and mechanisms of this change are unclear and are currently being explored by an international group of scientists, from the University of Exeter, the Irish Marine Institute, Cefas (Centre for environment, Fisheries and Aquaculture Science), the Tag-a-Giant Foundation of Stanford University and the International Commission for the Conservation of Atlantic Tunas (ICCAT).

Atlantic bluefin tuna are the largest of all the tuna species, growing to lengths of over 3m and weighing more than 700 kg. They are well known for being voracious and highly-mobile predators, and adult fish regularly make trans-Atlantic migrations. They have the unique ability amongst teleosts of endothermy—the ability to maintain their body temperature at a metabolically favourable level, despite external temperature. This physiological trait results in an unusually broad thermal operating range of 0–30°C. By maintaining their internal temperature at an optimal level, bluefin are able to exploit extremely productive, but cold, high-latitude waters, and still return to breed in the warm waters of the Mediterranean and Gulf of Mexico. 

An Atlantic bluefin tuna ‘busting’ clear of the water whilst feeding in Donegal Bay, Ireland. Image: Tom Horton.

There are limits to this ability however, and the likelihood of bluefin occurring in the Atlantic reduces drastically as sea surface temperature (SST) falls below 7–10°C. So their latitudinal extent is thermally limited, and they can only exploit high-latitudes when water temperatures peak in the summer and autumn. In order to feed their high metabolic demand, they must consume 2–10% of their body mass in food daily (based on captive studies). Consequently, whilst adopting a fairly generalist diet, they show a strong preference for high-calorie prey, such as mackerel, sprat, herring and Atlantic saury.

In addition to their unique physiology, Atlantic bluefin tuna exhibit complex movement patterns, which vary with size (larger fish range further) and amongst individual cohorts of fish. There are at least two genetically distinct populations of bluefin in the Atlantic, defined by spawning region: the eastern stock, which spawns in the Mediterranean, and the western stock, which spawns in the Gulf of Mexico. The eastern stock may well comprise other genetically distinct units in the Mediterranean and recent evidence has highlighted a new western spawning ground in the slope sea of the northwestern Atlantic. Bluefin tuna show a high degree of natal fidelity, which maintains the genetic structuring, as research has shown that outside of spawning areas, both stocks mix widely throughout feeding grounds in the open waters of the Atlantic. Fisheries for bluefin in the Atlantic and Mediterranean have existed for a thousand years, and catch records show a high degree of spatial and temporal variability. This has been hypothesized to reflect both long-term environmental change and the influence of large intra-population migratory ‘groups’ arising as a result of strong recruitment years, resulting in short-term colonization (and subsequent disappearance as the new ‘groups’ die or get fished) of ‘new’ regions. The poorly understood interand intra-stock migratory patterns of bluefin have largely precluded effective management of the species, as stock assessments (which currently rely on fisheries-dependent data) still cannot accurately assign catches to the correct stock. Consequentially, documenting and understanding bluefin migration is a research area of high importance.

The reappearance of bluefin tuna in the northeast Atlantic.

Historically, seasonal aggregations of bluefin tuna were present in the waters around the British Isles. During the 1930s there was a burgeoning aristocratic sport fishery for ‘the giant tunny’ off the coast of Scarborough, and an English fisherman, Edward Peel, even held the world record for the species in 1932 (362 kg, beating the standing record of 344 kg; Nova Scotia). However, due to the rapid development and industrialization of the commercial herring fleet operating in the North Sea, the local herring stock collapsed and the giant tunny had all but disappeared by the early 1950s. Similarly, the waters off Ireland played host to the ronnoch mor (giant mackerel in Gaelic), and since the 1970s commercial pair-trawlers have caught them off County Donegal as bycatch. A dedicated fishery emerged in 1999, only to cease in 2006 due to a lack of fish. This was seen as symptomatic of the state of the stock at the time.

Over the past three years bluefin tuna have been recorded with increasing regularity, and often in appreciable numbers, in places where they haven’t been seen for as long as 50 years: southwest England; the Outer Hebrides, Scotland; and the hole of the west coast of Ireland.

Why is this happening?

As is the case with any enigmatic species, the relatively sudden appearance of bluefin tuna has garnered a high degree of interest. The explanation is nuanced and involves multiple factors likely acting in concert; there are at least three mechanisms that may have contributed to the bluefin having repatriated our coastal waters:
i.Thermal Habitat. In terms of the species’ environmental niche, there is no reason why bluefin shouldn’t be in British waters, as is well evidenced by historical catches. At 62–65° N, the leading edge of bluefin tuna distribution is much further north than the British Isles. Historically, only large bluefin tuna (>200 cm CFL (curved fork length)) made long-distance migrations into cooler high-latitude waters, which included British waters. This is due to the inefficiency of heat retention in small fish, which have a larger surface-area to volume ratio. However, as the waters around the British Isles warm they will become suitable thermal habitat for a greater range of bluefin from different life stages and for longer, which may result in elevated catches and sightings. This is corroborated by recent sightings and catches of smaller bluefin, which were historically absent.

ii. Prey. Migrations by bluefin into the North Atlantic are driven by a constant need to forage, ideally on high-energy food. In turn, their fine scale distribution has been shown to be closely related to that of their preferred prey. The only recorded stomach content for a bluefin in the western English Channel, showed that the fish had been feeding solely on mackerel. The phenology, path and magnitude of the mackerel migration has changed considerably over the past decade, as have he spatial dynamics of other important prey such as herring and sprat. These changes will also likely play a structuring role in the fine-scale distribution of bluefin.

iii. Stock ‘health’. Demand for bluefin has reached a record high over the past two decades as a direct result of the growing sushi sashimi market in Japan, in which bluefin is most highly prized. The majority of bluefin caught are flash frozen and shipped to Japan for auction, where individual fish regularly sell for tens of thousands of dollars . Bluefin tuna is infamous for being over-exploited due to challenging management conditions and a combination of high market demand and consistent and extreme illegal fishing (e.g. the reported catch in 2006 was 31,000 t, but after including Japanese import records, it was found to be in excess of 50,000 t). In 2008, the eastern and western Atlantic stocks were estimated to be at 33% and 17% of 1950s spawning stock biomass respectively. However, the initiation of a recovery program by ICCAT in 2006, coupled with favourable recruitment appears to have resulted in a stock recovery; the 2014 ICCAT stock assessment showed a three-fold increase in eastern spawning-stock biomass in the period 2008-2014. So it could simply be the case that there are more fish to see, not just in British waters, but throughout the Atlantic.

In short, there is no simple answer, and there is no guarantee that bluefin will once again become a regular fixture on the ‘resident species’ list for our coastal waters. Our work aims to put these recent sightings into a historical context, and by using a multi-disciplinary approach, investigate the influence of climate and prey availability on bluefin tuna distribution. One thing that is abundantly clear is that the mechanisms of this change have far reaching consequences. Understanding how apex predator distribution may alter as the ocean climate changes is vital, in order to understand and maintain ecosystem function, and to bolster effective fisheries management.

Bluefin in the UK – a timeline

• 1930: The rise of ‘giant tunny’ fishing off Scarborough
• 1933: Lorenzo Mitchell Henry sets the British bluefin record – 386kg caught off Scarborough
• 1950s: Extirpation of ‘giant tunny’ off the northeast English coast
• 1970s: First records of bluefin from commercial fishermen in Ireland
• 24th September 2000: Alan Glanville lands the first rod-and-line caught bluefin tuna in Irish waters, in Donegal Bay
• 5th October 2001: Adrian Molloy catches a fish weighing 439 kg and sets a new Irish record
• 2011: An 11 kg bluefin tuna was caught by a spearfisherman off the Dorset coast
• 2013-15: Numerous bluefin tuna caught by rod-and-line off the Outer Hebrides, Scotland, including a 234 kg specimen in 2013 and three electronically tagged in 2014
• 2013-15: Bluefin records begin again from Ireland in 2013, and there were more records in 2015 than ever before, from around the whole coast
• 2015: Large shoal of BFT spotted multiple days in August, and multiple times in October by Marine Discovery, a wildlife tour operator in Cornwall
• 2015: Welsh shark fishermen catch two adult bluefin on the Celtic Deeps

Tom Horton is a postgraduate student at the University of Exeter, UK supervised by Dr Matthew WittDr Lucy Hawkes and Dr Barbara Block, investigating the spatial ecology of Atlantic bluefin tuna in the northeast Atlantic, with the Marine Institute in Ireland. Follow them and their work on bluefin tuna on twitter @t__horton, @mjwitt1, @DrLucyHawkes and feel free to contact with any queries.


Tom Horton