MEPS 232:281-290 (2002)  -  doi:10.3354/meps232281

ABSTRACT: We determined mercury levels in internal tissues and feathers from corpses of Audouin¹s Larus audouinii and yellow-legged gulls L. cachinnans michaellis, common terns Sterna hirundo and European shags Phalacrocorax aristotelis desmarestii, as well as from fish representative of trawler discards, collected at the Ebro Delta (NW Mediterranean) between March and July (seabird¹s breeding season) in 1997 to 1999. The levels of mercury were significantly lower in epipelagic (Clupeiforms) than in demersal fish. When representation of each species in the discards is taken into account, the mean mercury concentration from this resource is more than double that of epipelagic fish (the main natural prey for most seabirds in the area). The shag was the only species with direct access to benthic fish, as it can dive to the seabed, and shags presented high levels of mercury even though they do not feed on discards. The other seabirds showed mercury levels in accordance with their seasonal use of discards. Audouin¹s gull, which exploits discards extensively during the breeding season, had the highest levels in those tissues reflecting mercury intake during the breeding season (liver and 1st primary feathers). In contrast, the common tern makes little use of discards and presented the lowest levels of mercury. For those samples reflecting the intake of mercury during the winter (mantle feathers), when only the yellow-legged gull exploits discards extensively, this species presented the highest values. Audouin¹s gull and the common tern showed similarly low concentrations of mercury for this period. We conclude that consumption of discarded demersal fish strongly influenced mercury contamination of surface-feeding seabirds.

KEY WORDS: Biomonitoring · Diet · Heavy metals · Mediterranean · Seabird-fisheries interactions