The main cause of the decline of the Atlantic bluefin tuna in the 1960s was due to intensive fishing of immature specimens rather than environmental factors as previously thought, reveals a work of the Spanish Institute of Oceanography (IEO) that analyzes the catches of Atlantic fisheries of adults and juveniles of this species between 1914 and 2010.
In addition, the study concludes that only from the recovery plan adopted by the International Commission for the Conservation of Atlantic Tunas (ICCAT) in 2006 and currently in force, the overall situation of overfishing that has suffered this species has begun to reverse.
Last September, researchers from the IEO published a study showing how high fishing mortality exerted on the young population was the determining factor in the decline of spawning bluefin tuna fisheries that took place from 1963.
The study, published in the journal Reviews in Fisheries Science and Aquaculture, contains an extensive review of data on catches of Atlantic fisheries of adult and juvenile bluefin tuna between 1914 and 2010 and more than 200 references on scientific publications related to biology and fishing of the species.
Between 1949-1962 approximately seven million juvenile bluefin tuna were caught. “A capture of such magnitude could be the main cause of limited recruitment of youth to adult ages, which left future generations of spawning specimens severely depleted,” said Jose Luis Cort, researcher at IEO’s Oceanographic Centre of Santander and co-author together Pablo Abaunza.
Until shortly before the mid-twentieth century, the most important catches of bluefin tuna in the eastern Atlantic Ocean were carried out mainly with traps that captured adults between 40 and 500 kilograms. However, from the second half of the century other fishing methods such as purse seine, longline and live bait cane began to be used.
The introduction of live bait in the Bay of Biscay and the purse-seine fishing on the Atlantic coast of Morocco started an immature fish catch, which had not occur until that date. Just over ten years after these events, fisheries in northern Europe collapsed and traps suffered such a sharp decline that caused the disappearance of most of them. Those that remain active since then have not returned to the levels of production that had been registered during the first half of last century.
Numerous theories have tried to explain what happened to the spawning bluefin tuna from the 1960s in the eastern Atlantic, and what made this species disappear in Northern Europe fisheries. In general, it has been attributed to environmental factors, to prey (herring and mackerel) shortage, or the change in the migratory behaviour of bluefin tuna.
However, so far no one had demonstrated the relationship these events had with the expansion of juvenile catches in the eastern Atlantic region.
When these events occurred, there was no international organization dedicated to the control and management of tuna fisheries in the Atlantic and Mediterranean, as the ICCAT was established in 1966. It is precisely the management of this organization that has enabled the species upturn, when in 2006, the annual recovery plan was adopted, which is still in force today, almost eliminating juvenile bluefin tuna catches, along with other measures such as reducing Total Allowable Catch (TAC), favouring the restoration of spawning stocks.
“If this scenario continues, it would be impossible that something like what happened in the 1960s occurred again,” Cort said. “Indeed, all indicators the evaluation group of bluefin tuna in the ICCAT Scientific Committee manage point to a spectacular increase in the spawning population in recent years,” he added.
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