The species had suddenly disappeared from the Channel and North Sea waters in the 1960s before reappearing in recent years. The sign that the species has regained strength? Not necessarily answer marine biologists …

Four hundred kilograms on average in adulthood for three meters long … Difficult for an Atlantic bluefin tuna to go unnoticed, especially since the fish has a habit of moving on a bench… So when the latter points again the end of a gill in the Channel and the North Sea, 50 years after having left this zone, inevitably the news spreads like wildfire (or plankton to stay in the marine register).


It is not unusual to see it in the Iroise Sea, not far from Brest, reported at the end of August the local newspaper Telegram. He was also spotted in the North Sea, off the coast of Scandinavia, DutchNews enthused in September 2017, hoping to see him soon arrive in Dutch waters.


An invitation to increase fishing quotas?

Many see in this return the success of the emergency recovery plan adopted from 2007 in the Mediterranean and Atlantic, after the observation of a collapse of bluefin tuna populations until the 2000s after years of intensive fishing. There has been no moratorium, but a drastic reduction in fishing quotas [catch levels over a year] quickly doubled with increased controls in boats and markets to deter illegal fishing.


Since the bluefin tuna has regained strength, the International Commission for the Conservation of Atlantic Tunas (Cicta), the intergovernmental body that fixes each November the quotas for Atlantic bluefin tuna and Mediterranean bluefin tuna for the year following, has marked the passage of a recovery plan to a management plan of the species, which will result in a gradual increase in quotas to reach 36,000 tonnes in 2020 [against 23,655 in 2017 and 12,000 tonnes in 2011].


Just a new geographical distribution?

A good idea? An international team of scientists, led by marine biologist Robin Faillettaz, then a postdoctoral fellow at the University of Lille, calls for more caution in a study published this month in the scientific journal Science Advances. “We are not questioning the finding of an increase in the abundance of bluefin tuna today,” says Robin Failletaz. But its presence in Northern Europe does not translate into an increase in the total abundance of Atlantic bluefin tuna. In other words, this reappearance of the bluefin tuna in the English Channel and the North Sea would only translate a new geographical distribution of the species, returning to waters which are again favorable to it.


This is the original question of the authors of the study who, in order to answer it, have peeled the intensity of catches of bluefin tuna in the Northeast Atlantic Ocean for several centuries and the changes in the spatial distribution of the species… This is not the first time that bluefin tuna suddenly desert an area. “At the beginning of the 1960s, the North Sea fisheries had almost collapsed from one year to the next as a result of a sudden decrease in bluefin tuna in the area,” says Robin Faillettaz. The stock of fish was probably already overexploited, but this factor alone could not explain this sudden disappearance of the North Sea. To tell the truth, no one was able to explain it clearly. ”


The multidecadal Atlantic oscillation to take into account

The authors of the study then looked at another parameter, totally independent of the man. The Multidecial Atlantic Oscillation (AMO). This is a natural oscillation of ocean surface temperatures observed in the North Atlantic. This oscillation alternates warm phases and cold phases that can spread over periods of 40 to 80 years. “Sometimes, the transition is brutal, sometimes it is more progressive, complete Robin Faillettaz. And the temperature differences between a hot phase and a cold phase of the AMO are slightly lower than 1 ° C. ”


A small degree that can still cause changes. This multi-decadal Atlantic oscillation could thus influence the intensity and frequency of hurricanes [we were talking about it here], the intensity and direction of ocean currents, precipitation events, but also have consequences for biological systems. “Like the primary production of plankton, which is very sensitive to water temperature, or the abundance of certain small pelagic fish, such as anchovies and sardines,” adds Robin Failletaz.


“A compromise between the abundance of food and energy expenditure”

It would also have an effect on the distribution of bluefin tuna, a migratory species capable of traveling thousands of kilometers to find food. “It’s a compromise he has to find between energy dependence and foraging,” says the marine biologist. The ideal water temperature for the species is around 21 ° C, but it can withstand colder waters, up to 4 or 5 ° C. But in these cold areas, the energy expenditure of bluefin tuna is greater. If they can not compensate with abundant food, they will look elsewhere. ”


This is what would have happened in the early 1960s when bluefin tuna almost disappeared year after year from the waters of the North Sea. “This is precisely the fastest and most intense transition ever seen in the AMO phases,” says Robin Failletaz. In just two years, the AMO has gone from its warmest intensity to its strongest cold intensity. ”


In a hot phase since 1995

The study published in Science Advances then establishes a sort of general framework for the geographical distribution of bluefin tuna in the North Atlantic. In a positive (warm) phase of AMO, bluefin tuna travels to Greenland, Iceland, and Norway in search of food and becomes scarce in southern and central Atlantic North. In the negative (cold) phase, the species is more likely to explore the tropics (West, Central, and South Atlantic) and seldom cross the 45 ° N latitude. ”


Since 1995, the multi-decadal Atlantic Oscillation is again in the positive phase of its cycle, which would explain the gradual reappearance of Channel and North Sea bluefin tuna. Until when? “It is impossible to say when the AMO will switch back to a cold phase as the overall increase in temperatures could counterbalance the AMO’s next cold phase,” says Robin Faillettaz. This natural phenomenon is still relatively little studied by the scientific community. The first publications that examined the impact of AMO on biological systems date back to the mid-2000s alone. This oscillation of temperatures could very well also affect the geographical distribution of other migratory fish species. Other species of tuna, marlin, shark species, dolphinfish … ”


Be that as it may, the authors of the study urge not to hastily accede to the increase in bluefin tuna quotas simply because the species would have returned to an area it had neglected.