Researchers believe that numbers of the bluefin tuna are increasing in British waters because of warmer sea surface temperatures due to a long-term ocean current
Photo by Vlad Karpinskiy (Flickr)
by Claudia Santos Gonçalves
The topic of climate change is slowly but surely becoming a consistent part of our everyday life as, for instance, the extensively illustrated tragic fate of too many endangered species exemplify the inevitable consequences of human errors. The Atlantic bluefin tuna, “one of the world’s most commercially exploited marine fishes”, was once the unfortunate protagonist of the spectacular collapse of the Nordic fishery in the 1960’s, caused mainly by overfishing, leading to that species’ disappearance off the British coast. However, in recent years some specimens have been sighted in the Nordic region, inciting a contagious wave of enthusiasm among amateur fishers, hoping the government will allow the catching of this endangered species anew. Indeed, the Angling Trust has encouraged the establishment of a “catch and release” licensed fishery, supporting their argument with the promise of significant economic benefits for certain British areas, such as Cornwall. Nevertheless, the recurrent observations of the bluefin tuna in the North Atlantic have also caught the attention of cautious scientists who have investigated the variability of the distribution of this IUCN red list species, including the assessment of the potential influence of vast, hydroclimatic variability possibly affected by climate change. Their results have led to the conclusion that the long-overlooked Atlantic Multidecadal Oscillation (AMO), a naturally occurring climate cycle that affects the sea surface temperature of the North Atlantic Ocean, is key to the understanding of the distribution of the bluefin tuna in the Atlantic over the past 200 years. In fact, positive AMO phases, where typically warmer than average sea surface temperatures are measured across most of the Northern Atlantic with lower temperatures in the south, were shown to be related to higher values of abundance and conversely, negative AMO phases to lower values. However, despite the increasing number of sightings in the UK, scientists warn about falsely optimistic conclusions, since the current positive AMO may move the bluefin tuna further north while having an adverse effect on its recruitment in the Mediterranean, which is currently thought to be the most important spawning ground and therefore could negatively affect future abundance. In addition, experts believe that global warming may amplify the impact of the AMO with more extreme weather patterns affecting the north and south of the Atlantic region, facilitating hurricanes and droughts, for instance. Meanwhile, while some optimists might find comfort in the recent increase of the bluefin tuna in cooler waters around the UK and in the Nordic seas, for others this enrichment might taste bittersweet.
