Numbers of juvenile Atlantic tuna at a major spawning site in the Gulf of Mexico probably fell by at least a fifth this year as a result of the BP oil spill, the European Space Agency (ESA) said Monday.

The assessment comes from satellite images and data of the Gulf from April 20 to August 29, it said in a press release.

The Atlantic tuna is a valuable commercial species that is in alarming decline, especially in the western part of the ocean, where stocks have plummeted by 82 percent over the last 30 years.

Western Atlantic tuna migrate to the Gulf from January to June each year to reproduce, spawning in two important sites in April and May.

In the northeastern site, the number of bluefin fry fell by more than 20 percent as the suspected result of surface oil that was tracked by radar from the Earth-sensing satellite Envisat, ESA said.

The fish were especially vulnerable as they spawn in surface waters, which means the floating oil could harm eggs, larvae and even adult tuna, the agency said.

The other site, in the northwestern part of the Gulf was apparently unaffected.

The surveillance project was launched on behalf of a marine biology group, the Ocean Foundation.

It brought together images from Envisat and two NASA satellites of the oil spill, the ocean's temperature and sea-surface heights with data from electronically tagged tuna.

An estimated 4.9 million barrels of oil spewed from the shattered Deepwater Horizon oil rig during the 87-day crisis.