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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 123. No. 4. December 2011 
TABLE I. Frequency of occurrence (%F) and abundance (mean ± SD; range in parentheses) per haul of seabirds 
attending coastal shrimp trawlers in the Isla Escondida area. Argentina, during fishing seasons of 2006-2007 (n = 20 hauls! 
and 2(X)7-2008 (n = 20 hauls). (*): Species thai breed in Chubut. 
2006-2007 
2007-2008 
Species 
%F 
Abundance 
Abundance 
Mann-Wtimev 
Kelp Gull* (Lams dominicantts) 
100 
112.3 ± 68.4 (30-270) 
100 
263.4 ± 384.3 (32-1.700) 
P = 0.09 
Black-browed Albatross 
(Thalassarche melatiophris ) 
85 
17.6 ± 18.0 (0-60) 
90 
32.4 ± 47.3 (0-210) 
P = 0.39 
White-chinned Petrel ( Procellaria 
aec/uinocilalis) 
50 
5.9 ± 9.5 (0-30) 
90 
9.9 ±11.7 (0-55) 
P = 0.20 
Imperial Shag* ( Leiuocarbo 
atriceps) 
75 
7.3 ± 15.3 (0-70) 
90 
17.8 ± 29.5 (0-100) 
P = 0.14 
Magellanic Penguin* (Spheniscux 
magellanicus) 
30 
0.8 ± 1.3 (0-4) 
55 
13.7 ± 24.9 (0-80) 
P = 0.04 
Southern Giant Petrel* 
(Mucronectcx giganteus ) 
45 
1.1 ± 1.8 (0-6) 
25 
0.9 ± 2.7 (0-12) 
P = 0.03 
Great Shearwater (Puffinus gravis ) 
5 
0.3 ± 1.1 (0-5) 
45 
1.4 2.2 (0-8) 
P = 0.006 
Sooty Shearwater (P. griseus) 
30 
1.6 ± 3.2 (0-11) 
0 
0 
P = 0.009 
categories, based on their percent representation 
in the catch: (1) dominant (Do): >50% of the 
catch, its presence gives the general appearance to 
the catch: (2) abundant (Ab): between 25 and 50% 
of the catch, its presence is easily detected; (3) 
common (Co): between 5 and 25% of the catch 
its presence is detected when paying attention and 
searching through the catch: and (4) rare (Ra): 
<5% of the catch, only a few individuals. 
Similarly, the fate of species caught in each haul 
was assigned to one of three categories: (1) totally 
discarded (TDi). (2) totally retained (TRe), and 
(3) partially retained (PRe). We defined the 
frequency of occurrence for each caught species 
as the percentage of hauls in which each species in 
the catch was observed. 
RESULTS 
Species and Abundance of Seabirds Attending 
Vessels. Eight seabird species were recorded 
foraging upon discards from the coastal shrimp 
fishery in the Isla Escondida area (Table 1). The 
most frequent and abundant seabird was the Kelp 
Gull, which was present at all hauls evaluated in 
both years, followed by Black-browed Albatross 
(Thalassarche melannphris) (Table I). Flock size 
of both species varied throughout the fishing 
season, reaching 270 and 1.700 Kelp Gulls, and 
60 and 210 Black-browed Albatrosses in the two 
study seasons, respectively (Fig. 2). Young Kelp 
Gulls during the 2007-2008 fishing season were 
recorded at 85% of hauls, bu, in significantly 
lower numbers than adults (x ± SD. 44.2 ± 88.0.; 
range = 0-400; n = 20 vs. 219.3 ± 300.8; range 
= 25—1,300; n = 20, respectively; Wilcoxontcst 
W = 0.002, P < 0.0001). 
Frequencies of occurrence of White-chinned 
Petrels ( Procellaria aequinoctialis) and Imperial 
Shags, particularly during the 2007-2008 fishing 
season, were relatively high, although abundances 
were relatively low (Table 1). The highest number 
of White-chinned Petrels was recorded during the 
2007-2008 fishing season (55 individuals). When 
present. Imperial Shag numbers varied between 1 
and 100 individuals (Fig. 2). Magellanic Penguins 
and Great Shearwaters (Puffituts gravis) during 
2007-2008 had frequencies of occurrence <50 r L 
but their abundances were low (Table 1). South¬ 
ern Giant Petrels (Macronectes giganteus) and 
Sooty Shearwaters (P. griseus ) had low frequen¬ 
cies of occurrence and only a few individuals per 
haul. 
The mean numbers of Magellanic Penguins. 
Southern Giant Petrels. Great Shearwaters, and 
Sooty Shearwaters were significantly higher 
during 2007-2008 than in 2006-2007 (Table !)• 
Only the Kelp Gull and Black-browed Albatross 
of the four most frequent and abundant species 
attending vessels had significantly higher number 
during discarding than haulback (Table 2). Num¬ 
bers of Kelp Gulls and Black-browed Albatrosses 
attending a vessel were significantly higher when 
there were only one or two vessels operating 
simultaneously than when there were three or 
more (Kelp Gulls: 569.5 ± 623.1 vs. 132.2 ± 
69.9. Mann-Whitney U = 82, P = 0.03; Black- 
