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Fishery Bulletin 102(1) 



diversity of prey estimates, and measures of similarity 

 among rookeries. Crustaceans were not incorporated into 

 the IIMP index because it was not possible to quantify the 

 number of individuals in the samples. 



We used the IIMP Index because it is less sensitive to 

 changes in the number of prey in an individual scat com- 

 pared to PN. For example, if a scat contains a single prey 

 taxon, the IIMP does not change regardless of the number 

 of individuals of that taxon, in that scat. However, as one 

 increases the number of individuals of a given prey taxon 

 in the scat, the PN will also increase for that prey. The 

 IIMP allows each scat to contribute an equal amount of 

 information, whereas PN can be dominated by a few scats 

 with a large number of a single prey-taxon otoliths. In this 

 manner the IIMP is similar to the split-sample frequency 

 of ocurrence (SSFO) index, developed by Olesiuk (1993), 

 where each scat is treated as a sampling unit and does 

 not assume, as does PN, that the distribution of prey hard 

 parts between scats is uniform. However, with the SSFO 

 index, each prey taxon in a given scat is given an equal 

 weight for that scat. If only one species occurs in a sample, 

 its occurrence is scored as 1, if two species occur, each oc- 

 currence is scored as 0.5, and so forth (Olesiuk, 1993). The 

 IIMP index incorporates more information than the SSFO 

 index, regardless of the number of individuals of each taxon 

 in the scat. 4 



Percent number (PN) was used only to show differences 

 between broad prey groups (fishes and cephalopods) and 

 PO was used to review the temporal and spatial changes 

 from each main prey (those with average IIMP of at least 

 10% at any rookery). For all estimations, a single otolith 

 (right or left) or single cephalopod beak (upper or lower) 

 represented one individual prey. We tested the hypothesis 

 that the occurrence of the main prey was independent of 

 the rookery and of the date collection using contingency 

 tables and an estimator of chi-square (x~) (Cortes, 1997). 



Total length of the otoliths (mm) and the linear 

 equation obtained by Alvarado-Castillo 5 were used to 

 estimate the length of the Pacific sardine (total length 

 mm=7. 41+147. 24xotolith length mm); r=0.85, n=2740). 

 Insufficient data did not allow estimating the size of speci- 

 mens from May. All estimated lengths were transformed us- 

 ing loglO, followed by an ANOVA among sampling periods. 

 The size of Pacific sardine consumed by California sea lion 

 was compared to those caught in the commercial fishery. 

 We chose to estimate the size of Pacific sardines because of 

 the broad information available concerning age and growth 

 and other aspects about the fishery and because we found 

 many sardine otoliths in good condition. 



Spatial and temporal correlation in composition of diet 

 was compared by using the Spearman rank correlation co- 



4 Garcfa-Rodriguez, F. J., and J. De la Cruz-Agiiero. In prep. An 

 index to measure the specie prey importance of California sea 

 lion ^Zalophus californianus) based on scat samples. 



'Alvarado-Castillo, R. Unpubl. data. Departamento de 

 Biologia y Pesquerias, Centro Interdisciplinary de Ciencias 

 Marinas. Avenida IPN S/N Col. Palo Playa de Santa Rita, La 

 Paz, Baja California Sur, Mexico 23070. 



efficient (R s ) (Fritz. 1974). Pairs of IIMP values were used 

 for each prey taxon in a pair of sampling events (rookeries 

 or sampling dates) to examine the correlation among them. 

 This analysis was limited to prey that had an IIMP value 

 of 10% or more. Cluster analysis of average IIMP data for 

 the seven rookeries was used to assess the similarity of 

 the diet among rookeries. The dendrogram for the cluster 

 analysis was based on relative Euclidean distances and 

 unweighted pair-grouping methods (UPGMA) strategy 

 (Ludwig and Reynolds, 1988). We included only prey that, 

 for at least one occasion, had IIMP values >10%. 



Trophic diversity was evaluated by using diversity curves 

 (Hurtubia, 1973) developed from IIMP index data. For each 

 date and colony, the cumulative diversity was calculated for 

 increasing numbers of sequentially numbered (but we as- 

 sumed randomly deposited and collected) scat samples. The 

 diversity curves also allowed us to evaluate sample size 

 (Hurtubia, 1973; Hoffman. 1978; Magurran, 1988, Cortes, 

 1997) by identifying the point at which the curve flattens. 

 The diversity was estimated by using the Shannon index: 



H' 



-^P,\nPr 



(2) 



where H' = trophic diversity; 



S = total number of prey taxa; and 



P l = IIMP r and represents the relative abundance 

 of taxon i obtained from each scat and pooled 

 from scat 1 up to the total number of scats 

 collected. 



The values of trophic diversity were then plotted against 

 the number of pooled scats. 



Results 



Identification of prey 



The 1273 scat samples collected during June 1995 through 

 May 1996 (Table 1) yielded fish remains in 97.4% of the 

 samples, cephalopod remains in 11.2%, and crustacean 

 remains in 12.7%. Fish and cephalopods represented 

 95.39; and 4.7%, respectively, of the 5242 individual prey 

 (excluding crustaceans). The occurrence and number 

 of these prey groups changed temporally and spatially 

 (Fig. 3). We identified 92 prey taxa to the species level, 11 

 to the genus level, and 10 to family level from 851 scats 

 (Table 2). Remaining scats had damaged prey structures 

 in a condition that prevented us from identifying species 

 to the genus or family level. 



We found nine main prey with IIMP average values a 10% 

 (Table 3): the Pacific cutlassfish {THchiurus lepturus), the 

 Pacific sardine (Sardinops eaeruleus), the plainfin mid- 

 shipman (Porichthys spp.), myctophid no. 1, the northern 

 anchovy iEngraulis mordax), Pacific mackerel {Scomber 

 japonicus), the anchoveta (Cetengraulis mysticetus), jack 

 mackerel iTrachurus symmetricus), and the lanternfish 

 (unid. myctophid). 



