Atlantic bluefin tuna, among the world's largest fish, come to the gulf yearly from January to June with their peak spawning period in April and May, just when about 2 million gallons of oil a day was pouring into the ocean following the explosion of BP's Deepwater Horizon rig on 20 April.
The Ocean Foundation, a non-profit group dedicated to protecting ocean environments and species, turned to the European Space Agency and its satellites to assess the impact on the two main spawning grounds in the gulf, an ESA release said.
Radar data from ESA's Envisat and other satellites were used to create weekly maps showing the location, shape and size of the spill.
By overlaying the oil spill maps on known spawning habitat areas, it was possible to see where and how often the spill and the habitats had overlapped between April 20 and Aug. 29.
In the northeast gulf, the spill area and the most favored spawning grounds coincided at the end of the breeding season, and researchers estimate the spill reduced the number of juvenile bluefin by 20 percent.
The satellite scans showed the other favored spawning spot in the northwest of the gulf escaped the pollution, researchers said.