368 
Fishery Bulletin 96(2), 1998 
chovy average individual weight was used to esti- 
mate the W of these unidentified fishes. This last 
estimation was made to evaluate the relative contri- 
bution by weight of these unidentified fishes. 
The “percent frequency of occurrence” (%FO), the 
“percent total number” (%N), and the “percent total 
wet weight” (%W) were calculated (Hyslop, 1980; 
Castley et al., 1991; McKinnon, 1994). The relative 
importance of each prey species was evaluated with 
the index of relative importance (IRI) (Pinkas et al., 
197 1 ), where the volumetric percentage was replaced 
by the %W (. IRI=(%N+%W)%FO ) (Castley et al., 
1991). 
Results 
Dolphins used in this study (Table 2) were caught 
mostly at night in midwater hauls for the shrimp 
Pleoticus muelleri, with the exception of LO10 and 
L023 dolphins that were caught in diurnal bottom 
hauls for shrimp, L022 dolphin that was captured 
in a diurnal bottom haul for hake, and L026 dolphin 
that was caught by an unknown fishing vessel. The 
locations of the catches were principally in the Golfo 
San Jorge (approximately between 45 and 46°30'S), 
except L022 dolphin that was captured in the vicin- 
ity of Isla Escondida (43°S). There is no information 
Table 2 
Biological information (sex, size, age, and reproductive status), number of prey species eaten, and most important prey species by 
weight in the stomach of each dusky dolphin considered in this study. The biological information of females (F) were taken from 
Dans et al. (1997) and those of males (M) were provided by S.L. Dans. 1 The date of death is approximate and indicates which 
dolphins were caught in the same fishing trip. 
Field 
number 
Date of 
death 
Sex 
Length 
(cm) 
Age 
(years) 
Reproductive 
status 
No. of 
prey eaten 
Important prey species 
in the stomach 
LOOl 
19 Nov 89 
F 
158 
5 
immature 
3 
Argentine anchovy and 
Argentine shortfin squid 
LO02 
19 Nov 89 
F 
157 
4 
immature 
4 
Argentine shortfin squid and 
Argentine anchovy 
LO04 
27 Apr 90 
F 
159 
6 
immature 
3 
Argentine shortfin squid and 
Argentine anchovy 
LO05 
01 Jun 90 
F 
166.5 
6 
mature (lactating) 
1 
Argentine anchovy 
LO06 
01 Jun 90 
F 
174 
8 
mature (resting) 
4 
Argentine anchovy 
LO07 
Jul 90 
M 
169 
6 
immature 
5 
Argentine anchovy 
LO08 
Apr 92 
F 
161 
7 
mature (pregnant) 
2 
Argentine anchovy 
LO09 
Apr 92 
F 
174 
11 
mature (pregnant) 
4 
Argentine anchovy 
LO10 
08 Sep 92 
F 
172 
3+ 
immature 
4 
Argentine shortfin squid and 
Argentine hake 
LOll 
Sep 92 
F 
172 
7+ 
mature (resting) 
4 
Argentine anchovy 
L012 
Sep 92 
F 
162 
7+ 
mature (pregnant) 
6 
Argentine hake 
L013 
Sep 92 
F 
170 
8+ 
mature (resting) 
5 
Argentine anchovy and hake 
L014 
Sep 92 
F 
167 
7+ 
immature 
3 
Argentine hake 
L015 
11 Mar 93 
F 
166 
5 
immature 
2 
Argentine anchovy 
L016 
11 Mar 93 
F 
174 
6 
mature (pregnant) 
3 
Argentine anchovy 
L017 
16 Mar 93 
F 
164 
4 
immature 
2 
Argentine anchovy 
L018 
06 Apr 94 
F 
171 
5 
immature 
4 
Argentine anchovy 
L019 
11 May 93 
M 
164 
8 
mature 
3 
Argentine anchovy 
LO20 
Sep 93 
M 
161 
9 
mature 
2 
Argentine hake 
L021 
02 Sep 93 
F 
158 
3 
immature 
4 
Argentine hake 
L022 
30 Oct 93 
F 
164 
3 
immature 
2 
Unidentified fishes 
L023 
15 Oct 93 
M 
169 
9 
mature 
3 
Argentine shortfin squid 
L024 
Mar 94 
M 
173 
10 
mature 
4 
Argentine anchovy 
L025 
Mar 94 
M 
162 
8+ 
unknown 
3 
Argentine anchovy 
L026 
94 
F 
161 
8 
mature 
5 
Argentine anchovy and hake 
1 Dans, S. L. Laboratorio de Mamiferos Marinos, CENPAT (CONICET), Boulevard 3600, (9120) Puerto Madryn, Chubut, Argentina 
