NOTE Koen Alonso et al.: Diet of Lagenorhynchus obscurus in waters off Patagonia 
369 
about the location of collection of two dolphins (LO07 
and L026). The entire sample was biased toward 
females and it was composed of immature and ma- 
ture dolphins (Table 2) (Dans et al., 1997). 
Some dolphins in the sample may have been caught 
in the same haul because fishermen reported that 
occasionally more than one dolphin are caught. How- 
ever, when fishermen capture several dolphins in a 
fishing trip (between 30 and 60 days), they may dis- 
card some of them. In these cases it is uncertain 
whether those dolphins caught during the same fish- 
ing trip (same date of death, see Table 2) were caught 
together in the same haul. 
With the exception of one stomach (L022) which 
contained fish eye lenses and a few beaks, all others 
were half full or full. On average (± standard devia- 
tion), 3.40 ± 1. 19 prey species per stomach were found, 
but only one or two of them were important by weight 
in the stomach (Table 2). The mean number of prey 
items per stomach was 148.08 ±137.91, and the mean 
regression-estimated ingested biomass was 2,675.63 
±1,848.44 g per stomach. 
A total of 3,702 prey items belonging to eight spe- 
cies were found in the 25 stomach contents analyzed, 
including four fish and four cephalopod species. The 
fish species were Argentine anchovy, Engraulis 
anchoita; Argentine hake, Merluccius hubbsi; 
“pampanito, ” Stromateus brasiliensis, and the south- 
ern cod Notothenia sp. The cephalopods were Argen- 
tine shortfin squid, Illex argentinus; Patagonian 
squid, Loligo gahi, the sepiolid Semirossia tenera; 
and common octopus, Octopus tehuelchus. 
Argentine anchovy was the most important prey 
species, representing 39% of prey by number, 46% 
by weight (Table 3). However, the most frequent prey 
was the Patagonian squid which was present in 84% 
of stomachs (Table 3). According to the IRI, the sec- 
ond species in importance was the Argentine shortfin 
squid, followed by the Patagonian squid and the Ar- 
gentine hake, whereas by weight the second species 
was hake, followed by Argentine shortfin and 
Patagonian squid (Table 3). The “pampanito,” octo- 
pus, and southern cod were of little importance in 
dusky dolphin diet (Table 3). 
The Argentine anchovy had a unimodal length-fre- 
quency distribution with a mode at 16 cm (Fig. 1). 
Individual anchovies eaten by dusky dolphins were 
mostly of mature sizes (Hansen, 1994). 
Both Argentine shortfin and Patagonian squids 
were consumed at small sizes. Patagonian squids had 
a modal DML of 3 cm (Fig. 2), and most of them were 
smaller than 8 cm, a size that corresponded to that 
of immature squid (Hatfield et al., 1990). The DML 
frequency distribution of shortfin squid showed two 
important peaks at 1-2 cm, and 7-8 cm of DML (Fig. 
3) which corresponded to juvenile sizes ( Brunetti and 
Ivanovic, 1992). 
Argentine hake showed the widest size range (Fig. 4). 
Its TL frequency distribution showed three peaks at 
8 cm, 22-24 cm, and 36 cm in order of importance 
(Fig. 4). These sizes corresponded to age-0 group, age 
1-2 group, and age 2-5 group, respectively (Gaggiotti 
and Renzi, 1990). 
Average individual length of fish eaten by dusky 
dolphins ranged from 7.54 cm (southern cod) to 19.81 
cm (Argentine hake). The DML range for cephalo- 
pod preys was estimated to be between 2 cm (ob- 
served DML of S', tenera) and 6.35 cm (shortfin squid). 
Relative 
Table 3 
importance of prey species of dusky dolphins in 
the Patagonian continental shelf. 
Percent total 
number 
(%N) 
Percent total 
wet weight 
(%W) 
Percent frequency 
of occurrence 
(%FO) 
Index of relative 
importance 
(IRI) 
Cephalopods 
Illex argentinus 
30.71 
21.05 
68.00 
3,520.11 
Loligo gahi 
20.64 
4.58 
84.00 
2,118.09 
Semirossia tenera 
3.40 
0.57 
36.00 
142.87 
Octopus tehuelchus 
0.05 
0.00 
4.00 
0.22 
Fishes 
Engraulis anchoita 
39.25 
46.36 
80.00 
6,848.60 
Merluccius hubbsi 
5.19 
26.46 
48.00 
1,518.82 
Stromateus brasiliensis 
0.14 
0.29 
12.00 
5.05 
Notothenia sp. 
0.03 
0.00 
4.00 
0.11 
Unidentified fishes 
0.59 
0.70 
4.00 
5.18 
Total 
100.00 
100.01 
