260 



Fishery Bulletin 98(2) 



TL are not caught by either sea Hons or the 

 fishery because of the pelagic behavior offish 

 in this size range (Angelescu and Prenski, 

 1987). 



The commercial squid species found in the 

 diet of sea lions (Argentine shortfin and Pata- 

 gonian squid) form schools of restricted size 

 range ( Brunetti and Ivanovic, 1992; Hatfield, 

 1996). Therefore, it is difficult to determine 

 if the consumed sizes represent the environ- 

 mental availability of these prey species. The 

 commercial size of shortfin squid is approxi- 

 mately 15 cm DML, and this species was con- 

 sumed by sea lions at commercial sizes (Fig. 

 4). Patagonian squid was consumed mostly 

 at noncommercial sizes (less than 10 cm 

 DML) because that most of the squid catches 

 in the Falkland fishery were between 10 and 

 15 cm DML (Hatfield, 1996). 



These results indicate some overlap be- 

 tween the South American sea lion diet and 

 fishery catches but not enough to conclude 

 that competition exists with the fishery. The 

 population of South American sea lions in northern 

 Patagonia has been increasing during recent years 

 at a rate of increase greater than 39( (Crespo and 

 Pedraza, 1991; Dans et al.'). There is no indication 

 that fishery catches affect the availability of food for 

 sea lions at the present time. More detailed estimates 

 of food consumption by South American sea lions are 

 needed. Also, estimates of the catch and bycatch of 

 the fishery are needed to evaluate conclusively the 

 existence of ecological competition. 



Differences in the diet between sexes are probably 

 associated with different utilization of common and 

 frequent food resources, suggesting some kind of dif- 

 ferential feeding behavior between the sexes. The 

 South American sea lion is a dimorphic and poly- 

 gamous species. Therefore each sex must have dif- 

 ferent ecological constraints. Adult female feeding 

 trips last for about three days during reproductive 

 (Cappozzo et al., 1991) and nonreproductive (Reyes 

 and Crespo^) seasons. Nursing pups may limit the 

 distance that females can travel to feeding ground. 

 Males are not restricted by nursing pups and their 

 feeding trips seem to be less constant (Reyes and 

 Crespo^). There is also some evidence obtained from 

 sightings from fishing vessels that males move far- 

 ther offshore than do females which remain closer 



50 100 150 200 250 



Standard length of sea lions(cm) 



300 



• Maks o Females 



Figure 6 



Scatterplot of total weight of gastroliths versus standard length for 

 South American sea lions, by se.xes. 



* Reyes, L. M,, and E. A. Crespo. 1993. Variaciones diarias y 

 lunares y viajes de alimentacion en el lobo marino del sur Otarin 

 ftavescens en el norte de Patagonia. Abstracts of the Jornadas 

 Nacionales de Ciencias del Mar "93," 19-25 September 1993, 

 Puerto Madryn, Argentina, 156 p. 



to the coast (Crespo et al., 1997). Thus, differences 

 in the diet could be associated with different feeding 

 grounds or different home ranges between the sexes. 



The prey of female South American sea lions were 

 more evenly distributed within ecological groups 

 than the prey of males (Fig. 7). The prey of females 

 were mostly benthic and demersal-pelagic species. 

 The mean dive depth recorded by lactating females 

 in Patagonia was 60.9 m, and GdVc of the dives were 

 flat-bottomed and U-shaped ( Werner and Campagna, 

 1995) — data that agree with the bottom and coastal 

 feeding behavior suggested by the stomach contents. 

 On the other hand, the most important prey of males 

 were demersal-pelagic species (Fig. 7). 



The Patagonian squid spawns in shallow waters, 

 and the new generation migrates offshore to feed, 

 grow, and mature (Hatfield, 1996). This migration 

 pattern implies that small Patagonian squid must 

 be more abundant in shallow, coastal waters than 

 in deeper, offshore areas, but their size range in the 

 coastal area must be broader than that in offshore 

 areas because mature (and large) squids return to 

 shallow waters to spawn. The consumption of larger 

 Patagonian squid by male sea lions and the broader 

 range size of Patagonian squid eaten by female sea 

 lions agree with the hypothesis that females feed in 

 more coastal and shallower waters than do males. 



The red octopus lives mostly in caves on rocky bot- 

 toms (Re, 1998) and is the most important prey spe- 

 cies of female South American sea lions. Red octopus 

 reach maturity around 120 mm DML and 850 g of 

 total weight (Re, 1998); male sea lions consumed 



