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Bluefin tuna population takes big hit from Gulf oil spill

Bluefin tuna spawning took a hit from the Gulf oil disaster.

Satellite study shows key spawning area was inundated by oil during critical time of year

By Summit Voice

SUMMIT COUNTY — Oil spewing from BP’s failed Deepwater Horizon drilling rig may have reduced the humber of juvenile bluefin tuna by 20 percent in one of two critical spawning areas in the Gulf of Mexico, according to the European Space Agency (ESA).

Careful study of satellite images showing the extent of the spill compared against a habitat index showed that the oil spread into the key spawning area in the northeastern Gulf at the most critical time, when young tuna are feeding near the surface.  That means the presence of oil there is likely fatal for such tiny organisms, according to a press release from the ESA.

Atlantic bluefin tuna can grow the size of a Volkswagen Beetle. They come to the Gulf yearly from January to June, with peak spawning time in April and May – just when some 10 million liters of oil a day was pouring into the water following the explosion of the Deepwater Horizon rig on 20 April.

The commercially valuable fish spawn in surface waters, with females releasing eggs and males following behind to fertilize them. The presence of surface oil could harm eggs, larvae and even adults.

The western Atlantic tuna population’s spawning stock has declined by about 80 percent over the last 30 years, leading the National Marine Fisheries Service to consider an endangered species listing for the fish. The ESA data may figure into the fisheries service listing decision, as the agency said it would consider oil spill impacts when it accepted the listing petition. Read more about the proposed listing here.

The International Union for Conservation of Nature has listed western Atlantic bluefin tuna as critically endangered with an extremely high risk of extinction in the wild in the immediate future. According to IUCN, the population meets the critically endangered criteria of having declined more than 80 percent during the last 10 years, or 3 generations.

The ESA tried to track the oil spill’s impact by creating weekly maps showing the location, shape and size of the spill. In the Gulf, biologists used electronically tagged tuna and an ocean model based on measured ocean temperatures, sea-surface heights, as well as ocean color information, which can signify the presence of plankton for the tuna to feed on.

By overlaying the oil spill extent maps and the ‘spawning habitat index’, it was possible to see where and how often the oil spill and spawning habitats had overlapped between 20 April and 29 August.
Fortunately, the spawning hotspot in the west was apparently unaffected by the pollution, as observed from satellite images.

“This analysis will help us and our colleagues elevate our understanding of these impacts to another level and guide the development of strong policy recommendations,” said Dr David Guggenheim of the Ocean Foundation. “In addition, this analysis and its approach represents a next-generation tool that can help equip researchers and decision-makers should we face another similar environmental disaster in the future.”

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