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Fishery Bulletin 98(2) 



3 



•g 

 > 



•5 



c 



140 



130 



120 



110 



100 



90 



80 



70 



60 



50 



40 



30 



20 



10 



u 



c 

 u 



3 

 IT 

 V 



Argentine hake 



I commercial size ^ 



10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 



Total length (cm) 



15 20 25 30 35 



Dorsal mantle length (cm) 

 Argentine anchovy 



40 



45 



10 11 12 13 14 15 16 



Total length (cm) 



18 



19 20 



Figure 4 



Repression-estimated size-frequency distribution of Argentine hake (Mer- 

 luccius hubbsn. Argentine shortfin squid illlex argentinus), and Argentine 

 anchovy iEngraulis anchoita) consumed by South American sea lions off 

 Patagonia collected in our study. The arrows indicate commercial sizes for 

 Argentine hake and Argentine shortfin squid. All the sizes for Argentine 

 anchovv are commercial. 



curve in the period 1982-1987. Never- 

 theless, when the Argentine anchovy is 

 excluded from the analysis, no differ- 

 ences were found between the two peri- 

 ods in either at the two overlap indices. 

 Probably, the absence of Argentine 

 anchovy in the period 1982-1987 is 

 related to the small size of the sample 

 collected and to the fact that Argen- 

 tine anchovy was the least important 

 prey species among the six prey spe- 

 cies selected for the overlap analysis 

 (Fig. 3) because the available infor- 

 mation indicates that the abundance 

 of Argentine anchovy was high and 

 roughly constant during the entire 

 study period (Ciechomski and San- 

 chez, 1988; Pajaro et al.5). 



No differences in diet were detected 

 when shore and entangled individu- 

 als were analyzed, even though sev- 

 eral sources of bias could be operating 

 at the same time. The diet informa- 

 tion obtained from individuals found 

 dead onshore could be biased, depend- 

 ing on the degree of digestion of the 

 stomach contents. In our study, the use 

 of several (and complementary) hard 

 remains, such as otoliths and bones, 

 allowed us to reduce this source of bias. 

 Even when a small otolith was totally 

 digested, the fish bones (mostly skull 

 bones) permitted us to identify and 

 quantify these prey. Thus, estimat- 

 ing prey size by regressions avoided 

 underestimating the importance of 

 small or highly digested prey. In some 

 cases, when we used the regression 

 of a related species, the results were 

 likely partially biased. Samples from 

 entangled sea lions probably did not 

 hold any of these biases because all 

 the stomach contents were presum- 

 ably composed of fresh materials. The 

 source of bias in this case would have 

 been due to individuals that fed inside 

 the net. Nevertheless, a comparison 



^ Pajaro, M., R. Sanchez and G. Macchi. 1997. 

 Evaluacion de la biomasa de adultos de.sov- 

 antes de la poblacion nortefia de anchoita 

 {Engraulis anchoita I en el periodo 199.3-1996. 

 Abstracts of the XII Simposio Cientifico- 

 Tecnologico de la Comision Tecnica Mixta 

 del Frente Maritimo, Montevideo, Uruguay, 

 November 12-14. 1997, 4 p. 



