Influence of trawling activity on the breeding ecology of a threatened seabird, Audouin's gull Larus audouinii

The influence of a trawling moratorium on the breeding ecology of Audouin's gulls (Larus audouinii Payraudeau) was investigated during 1992 and 1993 in the largest colony of the species in the world (70 % of total population), at the Ebro Delta, NE Spain, western Mediterranean. In this colony, Audouin's gull behaves partly as a scavenger following the trawling fleet and exploiting fishing discards. Each year, the trawling moratorium affected different breeding stages of the species: in 1992 it overlapped with pre-laying and laying stages, while in 1993 it affected most of the chick-rearing stage. We examined differences in laying dates, laying interval, clutch and egg sizes, nest desertion during laying, hatching success, hatchling mass and condition, growth rates, chick survival and breeding success. The moratorium greatly affected egg production in 1992, delaying the start of laying and dramatically decreasing clutch and egg sizes. Intraclutch egg size variation be tween the 2 years suggests that egg size profiles are outcomes of the interaction between physiological and environmental factors rather than the result of ultimate factors. Nest desertion was much higher in 1992, probably because some females were under a critical body condition threshold and they deserted after laying the first egg. Hatching success also decreased significantly during 1992, probably related to changes in the size and the quality of the eggs. Chick growth and survivorship was higher in 1992, when trawler discards were normally available, than in 1993. Trawler discard availability during chick rearing was more important than egg size in determining chick survival. However, differences were not always significant, since in 1993 the moratorium did not start until the chicks were 1 wk old, and, further, chicks had higher skeletal size and body condition (in order of importance) than in 1992. In fact, chicks had similar fledgling condition in both years at the end of the rearing stage. In 1993, the overall breeding success decreased by 48%. Nevertheless, gulls' breeding performance for the 2 years was much lower than before the establishment of the first moratorium in 1991, and the situation suggests that clupeoids, the prey on which Audouin's gull feed, are scarce. It seems that gulls were not able to compensate for the lack of trawler discards during egg production, since the availability of secondary feeding resources was much lower in March and April (pre-laying and laying stages) than in June (chick-rearing stage). Although in long-lived species a decrease in food supply should affect fecundity before affecting adult survival, results confirm the importance of the trawler fleet activity for the conservation of this threatened species.