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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123. No. 4. December 2011 
1999, Bertellotti and Yorio 2000b, Gonzalez- 
Zevallos and Yorio 2006). The regular presence of 
Kelp Gulls in this and other fisheries in the region 
is the result of its generalist and opportunistic 
feeding habits, which are mainly characterized by 
coastal feeding and regular use of human-derived 
food sources (Bertellotti and Yorio 1999, Yorio 
et al. 2005). 
Black-browed Albatrosses are also regularly 
present at trawl fisheries in the southwest Atlantic 
(Thompson 1992, Thompson and Riddy 1995, 
Yorio and C’aille 1999, Gonzalez-Zevallos and 
Yorio 2006, Bugoni et al. 2011, Favero et al. 
2011) and in other regions of the Southern 
Hemisphere (Abrams 1983, Ryan and Moloney 
1988, Petyl 1995). Mean abundance in the Isla 
Escondida area was relatively lower than ob¬ 
served at coastal trawlers in Golfo San Matfas and 
Golfo San Jorge, Patagonia (Yorio and Caille 
1999). In contrast, both abundance and frequency 
of occurrence of Black-browed Albatross al hake 
and shrimp high-sea trawlers operating in Golfo 
San Jorge were larger, on occasions reaching 
numbers per haul three times greater than 
recorded at Isla Escondida (Gonzalez-Zevallos el 
al. 2007. Gonzalez-Zevallos et al. 2011). This 
may be the result of larger amounts of fishery 
waste discarded overboard and/or the greater 
distances offshore at which the high-seas trawlers 
operate (Gonzalez-Zevallos et al. 2011). 
Only the Kelp Gull and Black-browed Alba¬ 
tross of the most abundant seabird species 
attending vessels had significantly higher abun¬ 
dances during discarding activities. Differences 
among species may relate to their feeding 
behaviors. Kelp Gulls and Black-browed Alba¬ 
trosses obtain food mainly through surface 
feeding, while Imperial Shags and White-chinned 
Petrels do so by diving (Gonzdlez Zevallos and 
Yorio 2006). Thus, diving species start obtaining 
prey directly from the net or capturing those that 
fall off the net as it reaches the surface during 
haulback, while gulls and albatrosses increase in 
numbers as a result of their attraction when prey 
start to be available during discarding. Birds were 
often observed moving among vessels as in other 
fisheries in Patagonia (Bertellotti and Yorio 
2000b. Gonzdlez Zevallos and Yorio 2006). 
Argentine hake was the main discard compo¬ 
nent, as it was one of the most frequently caught 
species and was totally retained on board in only a 
few cases. Previous studies in Patagonia have 
■shown that Argentine hake is one of the most 
taken species by adult and young Kelp Gulls 
feeding on discards (Bertellotti and Yorio 2000b, 
Gonzalez-Zevallos and Yorio 2006. Gonzalez- 
Zevallos and Yorio 2011). The Argentine hake is 
a demersal fish not normally available to seabirds 
that surface-feed, such as the Kelp Gull, and it 
constitutes an important supplementary food 
resource. The use of this food may be advanta¬ 
geous for Kelp Gull breeding success and 
survival, as several studies have shown that fish 
are important for both egg formation and chick 
growth of gulls (Pierotti and Annett 1991. Bolton 
et al. 1992) and may favor long-term breeding 
performance (Annett and Pierotti 1999). 
The fishing season in the Isla Escondida area 
coincides with the Kelp Gull’s breeding season at 
Punta Tombo and Punla Clara. However, discards 
were not uniformly available within the known 
foraging range of this species during the breeding 
season. Several studies have shown the relative 
contribution of supplementary food in gull diets is 
related to accessibility of these resources (Oro 
1995, Bertellotti and Yorio 1999, Pedrocchi et al. 
2003, Duhem et al. 2005, Ramos et al. 2009). 
Thus, the extent of discard use and its potential 
effect on breeding success in a given locality 
would depend on the spatio-temporal pattern of 
vessel distribution. Discard availability could be 
particularly beneficial for young Kelp Gulls 
which, in contrast to breeders, are not spatially 
constrained to the nest during foraging activities. 
Young Kelp Gulls were recorded in most hauls 
during this study. Young individuals are in 
general less efficient than adults in obtaining 
food (Burger 1987. Hockey and Steele 1990. 
Bertellotti and Yorio 2000a). Thus, discard use 
may have an important influence on survival, 
particularly during February and March when 
young gulls start to become independent from 
their parents. 
Seabird attraction to vessels to make use of 
discards may lead to an increase in mortality 
resulting from interaction with fishing gear 
(Gonzdlez-Zevallos and Yorio 2006. Sullivan et 
al. 2006. Watkins et al. 2008). Incidental mortality 
in nets was associated with diving species such as 
Magellanic Penguin and Imperial Shag, most 
likely because these species dive to take prey 
directly from the net during haulback increasing 
their chances ol becoming entangled. Studies at 
Punta Tombo show that breeding Magellanic 
Penguins during December and January forage 
in relatively coastal areas at mean distances ol 
