Competition between the yellow-legged gull Larus cachinnans and Audouin's gull Larus audouinii associated with commercial fishing vessels: the influence of season and fishing fleet

Abstract.

Competition between the yellow-legged gull Larus cachinnans and Audouin's gull Larus audouinii while foraging at commercial fishing vessels off the Ebro Delta (NW Mediterranean) was assessed in 1997 and 1998. Observations were performed on board two kinds of fishing vessels with different timetables: bottom trawlers (diurnal activity) and purse seiners (nocturnal activity). Three situations were distinguished with respect to the season and the fishing regime: (1) breeding season, both fleets operating; (2) breeding season, only purse seiners operating due to a trawling moratorium; (3) non-breeding season, both fleets operating. Overall, the yellow-legged gull behaved as an opportunist species and exerted pressure over Audouin's gull through kleptoparasitism and agonistic interactions (i.e. contest competition). Despite this, Audouin's gull was more efficient at capturing discards through scramble competition and was able to take profit from fishing vessels when the capture of fish required high skill, in accordance with its higher specialisation. Competition varied in intensity according to the fishing fleet and the season. Indeed, Audouin's and the yellow-legged gulls only interfered at trawlers, since only Audouin's gull attended purse seiners. During the breeding season competition at trawlers was not severe and Audouin's gull preferentially attended these vessels. Purse seiners acted as a secondary food resource and only attracted important numbers of Audouin's gulls during trawling moratoriums. Out of the breeding season the number of Audouin's gulls strongly declined in the area. Furthermore, the intensity of kleptoparasitism increased at trawlers, and the average size of the fish discarded was larger and less suitable. In parallel with these changes, Audouin's gull shifted to attend purse seiners preferentially, thus avoiding the high levels of competition at the trawlers. The lower representation of Audouin's gull in other breeding areas in the Mediterranean, as well as the less important fishing fleets in these areas, would probably reduce the attractiveness of trawlers for this species, even during the breeding season. Moreover, changes in fishing policies aimed to reduce discarding practices would lead to a globally less favourable situation for Audouin's gull.