The persistence of oil in the ecosystem and seabird chronic exposures for years after the spill was evident in the yellow-legged gulls breeding in northwest of Spain. An spatial study on seven colonies found that blood samples from yellow-legged gulls breeding in colonies that were in the trajectory of the spill doubled in their total PAH concentrations when compared to samples from unoiled colonies (Perez et al. in press). Since blood cells are continuously being produced and have a lifespan of several weeks, the presence of PAHs in blood cells probably indicates a recent incorporation. The presence of PAHs in chicks indicates that these pollutants were incorporated into the food chain because nestlings would have been only exposed to contaminated organisms in the diet (Alonso-Alvarez et al. 2007a). Contrary to common thinking, yellow-legged gulls feed mainly on marine animals (>80% in 2004, including fishing discards and benthic or intertidal organisms) around the breeding colonies, thus, these studies revealed that seabirds were chronically exposed to oil incorporated in the food web.
    An experiment showed that the Prestige oil spill is toxic to seabirds (Alonso-Alvarez et al. 2007b) and, indeed yellow legged gulls breeding in oiled colonies suffered health damages of sublethal nature, 17 months after the Prestige oil spill (Alonso-Alvarez et al. 2007a). Gull sampled in oiled colonies were suffering damages on vital organs (i.e. liver and kidney). Overall, these studies suggested different sub-lethal effects on seabirds associated to a long-term exposure to fuel oil. Sublethal effects could even have a stronger impact on population dynamics than direct mortality (Peterson et al. 2003).