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Spatial and temporal variation in the diet of the California sea lion (Zalophus californianus) in the Gulf of California, Mexico.

Abstract--Between June 1995 and May 1996 seven rookeries in the Gulf of California were visited four times in order to collect scat samples for studying spatial and seasonal variability California sea lion prey. The rookeries studied were San Pedro Martir, San Esteban, El Rasito, Los Machos, Los Cantiles, Isla Granito, and Isla Lobos. The 1273 scat samples collected yielded 4995 otoliths (95.3%) and 247 (4.7%) cephalopod beaks. Fish were found in 97.4% of scat samples collected, cephalopods in 11.2%, and crustaceans in 12.7%. We identified 92 prey taxa to the species level, 11 to genus level, and 10 to family level, of which the most important were Pacific cutlassfish (Trichiurus lepturus), Pacific sardine (Sardinops caeruleus), plainfin midshipman (Porichthys spp.), myctophid no. 1, northern anchovy (Engraulis mordax), Pacific mackerel (Scomber japonicus), anchoveta (Cetengraulis mysticetus), and jack mackerel (Trachurus symmetricus). Significant differences were found among rookeries in the occurrence of all main prey (P [less than or equal to] 0.04), except for myctophid no. 1 (P>0.05). Temporally, significant differences were found in the occurrence of Pacific cutlassfish, Pacific sardine, plainfin midshipman, northern anchovy, and Pacific mackerel (P<0.05), but not in jack mackerel ([chi square]=2.94, df=3, P=0.40), myctophid no. 1 ([chi square]=1.67, df= 3, P=0.64), or lanternfishes ([chi square]=2.08, df=3, P=0.56). Differences were observed in the diet and in trophic diversity among seasons and rookeries. More evident was the variation in diet in relation to availability of Pacific sardine.

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The population of the California sea lion (Zalophus californianus), in the Gulf of California numbers approximately 23,000 individuals, 82% of which inhabit the northern region of the gulf above latitude 28[degrees] (Aurioles-Gamboa and Zavala-Gonzalez, 1994). In this region are found the most important reproductive areas and the highest pup production of the Gulf. Aurioles-Gamboa and Zavala-Gonzalez (1994) suggested that the high concentration of animals in this region is related to high abundance of pelagic fish such as Pacific sardine (Sardinops caeruleus) (also known as South American pilchard, FAO), Pacific mackerel (Scomber japonicus), Pacific thread herring (Opisthonema libertate), and anchoveta (Cetengraulis mysticetus) (Cisneros-Mata et al., 1987 (1); Cisneros-Mata et al., 1991 (2); Cisneros-Mata et al., 1997(3)).

Despite the importance of the northern gulf region, feeding studies of the California sea lion at Gulf of California rookeries have been few and have been conducted at different time periods. Researchers have studied sea lion diet in Los Islotes (Aurioles-Gamboa et al., 1984; Garcia-Rodriguez, 1995), Los Cantiles (Isla Angel de la Guarda), Isla Granito (Sanchez-Arias, 1992; Bautista-Vega, 2000), and Isla Racito (Orta-Davila, 1988). These studies have shown that sea lions consume a variety of prey and that differences exist between the diet of sea lions found at different rookeries within the Gulf of California. At Los Islotes, the most important prey were cusk eel (Aulopus bajacali), bigeye bass Pronotogrammus eos), threadfin bass Pronotogrammus multifasciatus), and splitail bass (Hemanthias sp.) (Aurioles-Gamboa et al., 1984; Garcia-Rodriguez, 1995). At Los Cantiles and Isla Granito important prey were lanternfish (Diaphus sp.), northern anchovy (Engraulis mordax), Pacific cutlassfish (Trichiurus nitens), shoulderspot (Caelorinchus scaphopsis), and Pacific whiting (Merluccius productus) (Sanchez-Arias, 1992; Bautista-Vega, 2000), whereas at Isla Racito, important prey were Pacific sardine (Sardinops caeruleus), Pacific mackerel (Scomber japonicus), grunt (Haemulopsis spp.), rockfish (Sebastes spp.), and Pacific whiting (Merluccius spp.) (Orta-Davila, 1988).

Some California sea lion prey are important fisheries resources in Mexico. The Pacific sardine, for example, is the target of a fishery begun in 1967 and which, along with the northern anchovy, contributed to most of the volume of the catch (200,870 t during the 1995-96 season) obtained in the Gulf (Cisneros-Mata et al. (3)). The central and northern regions of the Gulf of California harbor the greatest abundance of sea lions and schooling fishes, such as the sardine and anchovy. Because of this, knowledge of sea lion feeding habits and their temporal and spatial variability is relevant to understanding the potential interaction between sea lions and fisheries in the area (Orta-Davila, 1988; Sanchez-Arias, 1992; Bautista-Vega, 2000).

In this article, we present the results of concurrent diet studies conducted at various rookeries and haulout areas of sea lions in the northern rookeries of the Gulf of California to examine the prey consumed, and spatial and temporal variability in their diet.

Materials and methods

Scat samples of California sea lions were collected at Isla San Pedro Martir (SPM, 28[degrees]24'00"N, 112[degrees]25'3"W), Isla San Esteban (EST, 28[degrees]42'00"N, 112[degrees]36'00"W), Isla Rasito (RAS, 28[degrees]49'30"N, 112[degrees]59'30"W), Isla Granite (GRA, 29[degrees]34'30"N, 113[degrees]32'15"W), Isla Lobes (LOB, 30[degrees]02'30"N, 114[degrees], 28'30"W), and at two colonies of Isla Angel de la Guarda known as Los Machos (MAC, 29[degrees]20'00"N, 113[degrees]30'00"W), and Los Cantiles (CAN, 29[degrees]32'00"N, 113[degrees]29'00"W, Fig. 1). The total number of California sea lions in these seven rookeries was approximately 15,000 animals (that were hauled out) of which about 12.2% inhabit San Pedro Martir, 34.7% San Esteban, 2.8% El Rasito, 10.0% Los Machos, 8.7% Los Cantiles, 11.0% Isla Granite, and 20.6% Isla Lobes (Aurioles-Gamboa and Zavala-Gonzalez, 1994). All the animals were spread out along the shoreline of each island, except at Isla Angel de la Guarda, where they were clustered within two areas: Los Cantiles, on the eastern shoreline and Los Machos on the western shoreline.

[FIGURE 1 OMITTED]

Scat samples were obtained at reproductive and non-reproductive haulout areas between June 1995 and May 1996. At El Rasito, sampling was done only at one reproductive area; fresh and dried samples were collected (Fig. 2). If for any reason a scat was not collected (because it was found in pieces or in poor condition), it was destroyed and the site was cleared to avoid collection during subsequent trips. All fresh and dried samples collected were pooled to represent each sampling period. We assumed that the diet information corresponded to a time period close to the collection trip, but some dried scats could have been deposited shortly after the last collection.

[FIGURE 2 OMITTED]

Scats were stored in plastic bottles and then dried shortly thereafter to prevent decomposition of fish otoliths and other hard parts (which were used in subsequent prey identification) until the scats could be processed at a later date. The samples were processed by soaking in a weak biodegradable detergent solution for 1 to 7 days before being sifted through nested sieves of 2.00-, 1.18-, and 0.5-mm mesh size. Fish bones and scales, eye lenses of fish and squid, otoliths, cephalopod beaks, and crustacean fragments were extracted from the samples. Cephalopod beaks were stored in 70% ethanol, and the other items were dried and stored in vials. Sagittal otoliths and cephalopod beaks were used to identify teleost fish and cephalopods, respectively. Identifications were made by using photographs and diagrams from Clarke (1962), Fitch (1966), Fitch and Brownell (1968), and Wolff (1984), as well as voucher specimen material from the 1) Center Interdisciplinario de Marinas Ciencias (CICIMAR), 2) Institute Tecnologico y de Estudios Superiores de Monterrey, Guaymas, 3) Los Angeles County Museum of Natural History, California, and 4) Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE), Baja California, Mexico. Prey species identifed to family level were coded by using the family name plus a sequential number. Otoliths from prey species that were not identified to species, genus, or family level were coded with "fish species" plus a number.

Three indices were used to describe the diet of sea lions. Percent number (PN) represents the percentage of the number of individuals for each prey taxon over the total number of individuals found in all scat samples. Percent of occurrence (PO) represents the percentage of scats having a given prey taxon and indicates the percentage of the population that is consuming a particular prey species. The third index, index of importance (IIMP) incorporates PN and PO and is defined as

(1) IIM[P.sub.i] = 1/U[U.summation over j=1][X.sub.ij]/[X.sub.j],

where [x.sub.i] = number of individuals of taxon i in scat j;

[X.sub.j] = total number of individuals from all taxa found in scat j; and

U = total number of samples with prey.

The IIMP, developed for scat analysis (Garcia-Rodriguez, 1999), was used to determine the importance of prey species, their spatial and temporal variation in the diet, 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 compared 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 occurrence 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 ([chi square]) (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+(47.24 x otolith length mm); r=0.85, n=2740). Insufficient data did not allow estimating the size of specimens from May. All estimated lengths were transformed using log10, 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 coefficient ([R.sub.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 [greater than or equal to] 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 assumed 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:

(2) H' = -[S.summation over i=1][p.sub.i]ln[p.sub.i],

where H' = trophic diversity;

S = total number of prey taxa; and

[P.sub.i] = IIM[P.sub.i], and represents the relative abundance of taxon i obtained from each seat 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 seats.

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 95.3% 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.

[FIGURE 3 OMITTED]

We found nine main prey with IIMP average values [greater than or equal to] 10% (Table 3): the Pacific cutlassfish (Trichiurus lepturus), the Pacific sardine (Sardinops caeruleus), the plainfin midshipman (Porichthys spp.), myctophid no. 1, the northern anchovy (Engraulis mordax), Pacific mackerel (Scomber japonicus), the anchoveta (Cetengraulis mysticetus), jack mackerel (Trachurus symmetricus), and the lanternfish (unid. myctophid).

Spatial and temporal variability of the main prey

The importance (IIMP) of the Pacific cutlassfish was greater in Los Cantiles (28.4%), Isla Lobos (20.8%), and Isla Granito (48.5%) than at other sites (Fig. 4). The Pacific sardine was the dominant prey at Los Machos and to the south. There was a significant correlation across the seasons between Los Machos and El Rasito (r=0.998, P=0.04), but not between Los Machos and Isla Granito r=0.602, P=0.59). The IIMP of sardine was also correlated between El Rasito and San Esteban (r=0.95, P=0.04). The plainfin midshipman did not show a clear pattern, but its presence in the diet increased northeastward from Isla Angel de la Guarda. Lanternfishes, especially myctophid no. 1, were the dominant prey at San Pedro Martir, San Esteban, and El Rasito. The presence of Pacific mackerel was positively correlatcd with the presence of the Pacific sardine. The anchoveta was only found at Isla Lobes, and jack mackerel at El Rasito, San Pedro Martir, and Isla Granito.

[FIGURE 4 OMITTED]

The changes in the PO of the main prey coincided with the variations of the IIMP. The occurrence of Pacific cutlassfish, Pacific sardine, plainfin midshipman, northern anchovy, Pacific mackerel, and jack mackerel was significantly different (P<0.04) among rookeries. Myctophid no. 1 showed no significant difference in ocurrence ([chi square]=11.04, df=6, P=0.09); but when all lanternfishes were pooled, their occurrence among rookeries was significantly different ([chi square]=11.13, df=6, P=0.04). We found significant temporal differences in the occurrence of Pacific cutlassfish, Pacific sardine, plainfin midshipman, northern anchovy, and Pacific mackerel (P<0.05), but no significant differences were found among seasons in the occurrence of jack mackerel ([chi square]=2.94, df=3, P=0.40), myctophid no. 1 ([chi square]=1.67, df=3, P=0.6428), or lanternfish ([chi square]=2.08, df=3, P=0.5562).

Size of Pacific sardine consumed by sea lions

The estimated size of the Pacific sardine found in scat was between 101.8 mm and 179.7 mm (mean length of 150.4 mm [+ or -] 13.7 mm). Significant differences were found among sampling periods (F=4.79, df=2, P=0.01), specifically between June and January (Newman-Keuls test; P=0.04) and between September and January (Newman-Keuls test; P=0.01). The average size was 147.4 mm ([+ or -]16.1 mm) in June, 151.7 mm ([+ or -]13.0 mm) in September, and 136.5 mm ([+ or -]13.7 mm) in January (Fig. 5). A similar pattern was found in Los Cantiles, Los Machos, and Isla Granito.

[FIGURE 5 OMITTED]

Spatial and temporal correlation in diet

We identified 25 prey taxa that had an IIMP index value of [greater than or equal to] 10% (Table 3) for a given collection. The Spearman rank correlation coefficient of IIMP between any pair of rookeries during June, September, January, and May was not significant (P>0.08). There was no positive correlation among any pair of sampling periods for any rookery (P>0.14), except between January and May at San Pedro Martir ([R.sub.S]=0.64, P<0.05) and El Rasito ([R.sub.S]=0.66, P<0.05) and between January and June as well as between January and May at Isla Lobos ([R.sub.S]=0.56, P=0.05; and [R.sub.S]=0.59, P=0.05, respectively).

The similarity in diet was related to the distance between rookeries. A clustering for the seven rookeries was obtained from these 25 prey taxa (Fig 6). We arbitrarily used a "cutoff" distance of 0.3 and 0.4 on the dendrogram as reference points for identifying clusters. The group obtained by using the first distance (0.3) showed four feeding areas: one located in the south (area I: San Pedro Martir, San Esteban, and El Rasito), another in Canal de Ballenas (area II: Los Machos) and two in the north (area III: Los Cantiles and Isla Lobes; and area IV: Isla Granite). When the second distance (0.4) was used, the seven rookeries grouped into two clusters: 1) the southern region and Canal de Ballenas, and 2) the region north of Angel de la Guarda.

[FIGURE 6 OMITTED]

Spatial and temporal variability in trophic diversity

Temporal variability in trophic diversity was evident between the rookeries (Fig. 7). In general, two patterns could be differentiated: one in which the diversity varied little throughout the year and the other in which diversity was high in January and low in September. The rookeries San Pedro Martir and Isla Lobos showed the first pattern and Los Machos and Isla Granito (and to a lesser extent, San Esteban and El Rasito) showed the second pattern. In September, when diversity was low, the dominant prey at San Esteban, El Rasito, and Los Machos was Pacific sardine, whereas at Isla Granito, it was Pacific cutlassfish (Fig. 4). The curves obtained for Los Cantiles showed a clear pattern of diversity only in September, although the trend in the January curve would suggest a higher diversity in January than in September.

[FIGURE 7 OMITTED]

Discussion

Stomach acids attack otoliths, affecting their size and number and consequently the estimate of prey occurrence and importance. Erosion of otoliths during digestion has been analyzed in studies of pinnipeds in captivity. Bowen (2000) reviewed nine studies that estimated the proportion of otoliths recovered in scat samples to obtain a prey-number correction factor (NCF). He found that NCF is greater than 1.0 because many prey species are not recovered in the scat samples. Additionally, the erosion level can be significantly different among prey species (Bowen, 2000) because of differences in the shape and microstructure of otoliths. Therefore, estimates of biomass based on scat analysis should be carefully interpreted because the consumption of some prey species can be under- or overestimated. Correction factors are needed to compensate for differential erosion for the prey species of each pinniped.

In this study the most important prey of California sea lions were pelagic fish with small, thin, and fragile otoliths (Nolf, 1993). The lanternfish also have small otoliths--perhaps smaller than those of any other prey taxa found in the scats. Their true importance in California sea lion feeding may be underestimated because of erosion caused by stomach acids (Da Silva and Neilson, 1985; Murie and Lavigne, 1985; Jobling and Breiby, 1986; Jobling, 1987; Tollit et al., 1997). Similarly, the presence of cephalopods may have been underestimated because their jaws are composed of chitin, which is harder to digest, and frequently are regurgitated (Pitcher, 1980; Hawes, 1983). However, the high resistance to digestion of cephalopod beaks allows recovery of them in good shape. Thus they are a good choice to use in such diet analyses (Lowry and Carretta, 1999).

A numerical index of prey species importance may over-or underestimate the dominance of prey species in the diet because it does not consider the weight of the prey. For IIMP, a numerical index that assumes a similar weight for all prey species, the true importance of the individual large prey in the diet can be underestimated and the importance of individual small prey can be overestimated. This problem is also present when the PO, PN, and the SSFO index are used because these are all based only upon the number and occurrence of otoliths and cephalopods beaks. As when using PN, and the SSFO, the IIMP does not work for species that cannot be enumerated, such as crustaceans.

Given the tendencies of the trophic diversity curves, the sample size was suitable in almost all cases. However, at San Pedro Martir a few more samples would have been useful to fully depict the diet. At Los Cantiles, except during September 1995, the samplings should have been more intense because the flattened portion of the diversity curves are not clear. The information, therefore, that comes from those samples could be biased. However, the number of scats that we analyzed contained a high percentage of the consumed species, especially the main prey.

The results of this study indicate that the California sea lion consumed mainly fish and some crustaceans and cephalopods. According to the PN index, fish were more important than cephalopods in the diet of sea lions. In addition, fish were more frequent (PO) than crustacean and cephalopods.

Crustaceans were represented in a similar manner in scats from all rookeries. Cephalopods, however, were more important at San Pedro Martir and San Esteban, probably because they are more common towards the southern gulf. Species of the suborder Oegopsida, which includes oceanic species (Roper and Young, 1975), were most commonly found in scats from these rookeries. Orta-Davila (1988) and Sanchez-Arias (1992) have also noted the low consumption of cephalopods at the northern rookeries. Fish were the most diverse and commonly eaten prey. In contrast to cephalopods, fish were slightly less important in the southern region.

The availability and abundance of the various prey resources influenced the diet of the sea lions in the Gulf of California. The distribution pattern of Pacific sardine closely agrees with its importance in the sea lions diet. The Pacific sardine occurred in high concentrations around Angel de la Guarda and Isla Tiburon during the summer and along

the coast of southern Sonora during the winter, where spawning occurs (Cisneros-Mata et al. (3)). Sardines were consumed in the Canal de Ballenas region during the summer (September), when they are very abundant. Larger size Pacific sardines were consumed by sea lions most frequently during the summer when adult sardines occur more frequently in the Canal de Ballenas. As adult sardine left. Canal de Ballenas (Cisneros-Mata et al., 1997), the proportion of young individuals in the diet of sea lions increased. The fish eaten by sea lions were apparently smaller than those captured by the commercial fisheries. The average estimated size of the sardines consumed was 150.4 mm, whereas the average size of commercially caught fish during the 1995-96 season was 162.4 mm (Cisneros-Mata et al. (3)). This 7% difference in size may have been caused by an underestimation of otolith size because of digestive erosion (Jobling and Breiby, 1986). If this is so, then the size of Pacific sardines consumed by sea lions is similar to the size of those captured by the fishery.

Isla Lobos was the only rookery where Pacific sardine was not consumed. This finding differs from those of Cisneros-Mata et al. (3) which show the Pacific sardines present as far north as Isla Lobos. However, their study period was during the 1991-92 El Nino episode, whereas our study occurred during normal oceanographic conditios in 1995-96.

Less is known about the spatial and temporal availability of other important prey. As with commercial captures (Arvizu-Martinez, 1987), Pacific mackerel occurred together with Pacific sardine. Similar variations in occurrence for both species have been noticed from stomach content analyses of the giant squid (Dosidicus gigas) (Ehrhardt, 1991). Lanternfishes were abundant north of Isla Angel de la Guarda (Robison, 1972); however they were not important in the diet of the California sea lion in this region. Their greater importance in the diet at southern rookeries was probably due to the absence of more preferred prey such as Pacific sardine, Pacific cutlassfish, or anchoveta. The consumption of northern anchovy tended to be less important towards Canal de Ballenas, where Pacific sardine reached its maximum importance. The low spatial overlap of these two species has also been noted in other studies. The anchoveta was present only at Isla Lobos. This is an estuarine-lagoon species, typical of coastal lagoons of northern Sinaloa and Sonora (Castro-Aguirre et al., 1995). The presence of this prey in Isla Lobos is possibly due to the sandy coast (Walker, 1960), which is similar to that of the Sinaloa-Sonora coast.

The diet of California sea lions differed among rookeries, probably due to differences in feeding sites and prey availability. Antonelis et al. (1990) studied the foraging characteristics of the northern fur seal (Callorhinus ursinus) and the California sea lion at San Miguel Island and found differences between foraging areas among species. The northern fur seal was found most frequently foraging in oceanic water within 72.4 km from the island, whereas Califorinia sea lions forgaged more often in the shallower neritic zone, within 54.2 km from the island. Different foraging distances in California sea lions from San Miguel Island were found by Melin and DeLong (1999). During the nonbreeding season a higher percentage of foraging locations occurred at distances less than 100 km, whereas during the breeding season most of the foraging locations occurred at distances greater than 100 km. These differences are probably due to the increased California sea lion population in San Miguel; this increase in population forces sea lions to exploit new areas as a density-dependent response to population growth. Although, these differences could also be due to variability in the distribution of' prey (Melin and DeLong, 1999), as suggested by Antonelis and Fiscus (1980), foraging areas might change with season and annual variations in prey availability and abundance.

Foraging areas in the Gulf of California could lie closer to rookeries than those recorded for San Miguel Island sea lions because the diet was different among rookeries in spite of the shorter distance between them (54.2 km). At Los Islotes, Baja California Sur, adult females fed within 20 km of the colony (Duran-Lizarraga, 1998). Kooyman and Trillmich (1986a, 1986b) reported similar data in sea lion colonies of the Galapagos Islands. In the northern region of the Gulf of California, feeding range could be shorter than that at Los Islotes because of the higher concentration of food at high nutrient concentrations (phosphate, nitrate, nitrite, and silicate) in Canal de Ballenas that is associated with strong tidal mixing (Alvarez-Borrego, 1983).

Four foraging zones were discerned from dietary differences in sea lions from the seven rookeries studied. Zone I, which included San Pedro Martir, San Esteban, and El Rasito, was characterized by the consumption of lanternfish; zone II, which included Los Machos was characterized by the consumption of Pacific sardine and Pacific mackerel; zone III, which included Isla Granito, by the consumption of Pacific cutlassfish and the northern anchovy; and zone IV, Los Cantiles and Isla Lobos, was characterized by the consumption of the plainfin midshipman and the Pacific cutlassfish. These four zones may indicate differences in habits used by sea lions or may indicate different oceanographic conditions exploited by sea lions. The eastern coast of the Gulf of California displays high photosynthetic pigment concentrations, associated with upwelling induced by winds from the northwest in the winter. These conditions may make Canal de Ballenas one of the most important for the distribution of Pacific sardine during the summer.

Trophic diversity varied spatially and temporally. San Pedro Martir and Isla lobos sea lions seem to depend on a more stable feeding areas compared to sea lions at rookeries on Isla Granito and Los Machos, where changes in diversity of consumed species indicated that sea lions feed on fewer species during certain times of the year. Similar results in relation to the changes in diversity were also noticed in the rookeries of the Channel Islands and Farallon Islands, California (Bailey and Ainley, 1982; Antonelis et al., 1984; Lowry et al., 1990; Lowry et al., 1991). Perhaps the tendency to have the highest values of diversity and little seasonal variation at San Pedro Martir is the result of this rookery being located in a zone of transition between two biogeographical areas. This geographical position confers greater environmental heterogeneity and greater ecological diversity (Walker, 1960).

California sea lions in the upper region of the Gulf of California obtain the main portion of their diet from a relatively small number of species. The decrease in abundance of any of these food resources can seriously affect the population, particularly at Isla Granito and Los Machos because sea lions from these rookeries depend on a few species.
Table 1

Number of scats collected at each rookery for each sampling period.

 June 1995 September 1995 January 1996

San Pedro Martir (SPM) 22 33 88
San Esteban (EST) 50 66 91
El Rasito (RAS) 11 56 58
Los Cantiles (CAN) 20 58 47
Isla Granito (GRA) 24 20 41
Los Machos (MAC) 39 32 36
Isla Lobos (LOB) 72 139 72
Total 238 404 433

 May 1996 Total

San Pedro Martir (SPM) 29 172
San Esteban (EST) 67 274
El Rasito (RAS) 25 150
Los Cantiles (CAN) 35 160
Isla Granito (GRA) 19 104
Los Machos (MAC) 0 107
Isla Lobos (LOB) 23 306
Total 198 1273

Table 2

Prey of California sea lion identified from scat samples collected at
Isla San Pedro Martir, Isla San Esteban, Isla El Rasito, Los Cantiles,
Isla Granito, Los Machos and Isla Lobos from June 1995 through May
1996. n ind. = number of individuals in the sample; PN = percent
number; n occurr = number of occurrences; PO = percentage of
occurrence; IIMP = index of importance.

Scientific name Common name n Ind.

Trichiurus lepturus Pacific cutlassfish 306
Sardinops caeruleus Pacific sardine 358
Porichthys spp. midshipman 456
Myctophid no. 1 lanternfish 714
Engraulis mordax northern anchovy 430
Scomber japonicus Pacific mackerel 103
Cetengraulis mysticetus anchoveta 410
Loliolopsis diomedeae squid 77
Trachurus symmetricus jack mackerel 111
Merlaccius spp. Pacific whiting 55
Pontinus spp. scorpionfish 61
Enoploteuthid no. 1 squid 95
Caelorinchus scaphopsis shoulderspot 65
Octopus sp. no. 1 octopus 24
Sebastes macdonaldi Mexican rockfish 42
Citharichthys sp no. 1 sanddab 120
Fish species no. 1 -- 49
Haemulopsis leuciscus white grunt 176
Peprilus snyderi salema butterfish 163
Prionotus spp. searobin 12
Prionotus stephanophrys lumptail searobin 53
Argentina sialis Pacific argentine 19
Fish species no. 2 -- 55
Hemanthias peruanus splittail bass 60
Fish species no. 3 -- 9
Micropogonias ectenes slender croaker 13
Lepophidium spp. cusk-eel 9
Fish species no. 4 -- 10
Sebastes exsul buccanner rockfish 15
Cranchiid no. 2 Squid 20
Haemulon flaviguttatum yellowspotted grunt 11
Selar crumenophthalmus bigeye scad 24
Fish species no. 5 -- 33
Paralabrax sp. no. 1 sea bass 9
Synodus sp. no. 3 lizardfish 10
Lepophidium prorates prowspine cusk-eel 5
Fish species no. 6 -- 9
Synodus sp. no. 1 lizardfish 25
Octopus sp, no. 2 octopus 8
Gonatus berryi squid 5
Mugil cephalus striped mullet 1
Paranthias colonus Pacific creole-fish 1
Balistes polylepis finescale triggerfish 13
Physiculus nematopus charcoal mora 30
Hemanthias spp. sea bass 9
Fish species no. 7 -- 10
Uroconger varidens conger eel 8
Larimus spp. drum 8
Apogon retrosella barspot cardinalfish 5
Dosidicus gigas squid 8
Merluccius productus Pacific whiting 1
Fish species no. 8 -- 2
Synodus sp. no. 2 lizardfish 12
Scorpaena sonorae Sonora scorpionfish 2
Eucinostomus spp. mojarra 13
Fish species no. 9 -- 3
Cynoscion reticulatus striped weakfish 23
Fish species no. 10 -- 10
Caulolatilus affinis bighead tilefish 4
Paralabrax auroguttatus goldspotted sand bass 18
Fish species no. 11 -- 3
Cranchiid no. 5 squid 1
Bodianus diplotoenia mexican hogfish 1
Prionotus sp. no. 1 searonbin 2
Strongylura exilis california needlefish 1
Synodus spp. lizardfish 6
Fish species no. 12 -- 3
Fish species no. 13 -- 2
Fish species no. 14 -- 3
Fish species no. 15 -- 2
Fish species no. 16 2
Porichthys sp. 1 midshipman 1
Fish species no. 17 -- 5
Calamus brachysomus Pacific porgy 5
Fish species no. 18 -- 1
Fish species no. 19 -- 5
Ophididae no. 1 -- 1
Fish species no. 20 -- 5
Sebastes sinesis blackmouth rockfish 2
Symphurus spp. tonguefish 3
Fish species no. 21 -- 2
Pronotogrammus multifasciatus threadfin bass 8
Fsh species no. 22 -- 2
Fish species no. 23 -- 2
Orthopristis reddingi Bronze-striped grunt 16
Fish species no. 24 -- 2
Fish species no. 25 -- 1
Cranchiidae no. 4 squid 2
Fish species no. 26 -- 2
Histioteuthis heteropsis squid 1
Scorpaenidae no. 1 -- 1
Fish species no. 27 -- 3
Fish species no. 28 -- 1
Fish species no. 29 -- 1
Cranchiidae no. 3 squid 1
Bollmannia spp. goby 1
Fish species no. 30 -- 1
Cranchiidae no. 1 squid 1
Paralabrax maculatofasciatus spotted sand bass 1
Ophidion scrippsae basketweave cusk-eel 1
Physiculus spp. cod, codling, mora 2
Ophididae no. 2 -- 4
Unid. Carangidae jacks 8
Unid. Engraulidae anchovies 1
Unid. Haemulidae grunts 13
Unid. Labridae wrasses 1
Unid. Mycthophidae lanternifishes 216
Unid. Ophididae cusk-eel 2
Unid. Scianidae drums 13
Unid. Scorpaenidae scorpionfishes 30
Unid. Serranidae sea bass 13
Unid. Triglidae searobins 1
 Unid. Fishes 39
 Unid. Cephalopods 4
 Unid. fishes (very eroded) 381
 Remains of cephalopods
 Remains of crustaceans

Scientific name Common name PN

Trichiurus lepturus Pacific cutlassfish 5.837
Sardinops caeruleus Pacific sardine 6.829
Porichthys spp. midshipman 8.699
Myctophid no. 1 lanternfish 13.621
Engraulis mordax northern anchovy 8.203
Scomber japonicus Pacific mackerel 1.965
Cetengraulis mysticetus anchoveta 7.821
Loliolopsis diomedeae squid 1.469
Trachurus symmetricus jack mackerel 2.118
Merlaccius spp. Pacific whiting 1.049
Pontinus spp. scorpionfish 1.164
Enoploteuthid no. 1 squid 1.812
Caelorinchus scaphopsis shoulderspot 1.240
Octopus sp. no. 1 octopus 0.458
Sebastes macdonaldi Mexican rockfish 0.801
Citharichthys sp no. 1 sanddab 2.289
Fish species no. 1 -- 0.935
Haemulopsis leuciscus white grunt 3.357
Peprilus snyderi salema butterfish 3.110
Prionotus spp. searobin 0.229
Prionotus stephanophrys lumptail searobin 1.011
Argentina sialis Pacific argentine 0.362
Fish species no. 2 -- 1.049
Hemanthias peruanus splittail bass 1.145
Fish species no. 3 -- 0.172
Micropogonias ectenes slender croaker 0.248
Lepophidium spp. cusk-eel 0.172
Fish species no. 4 -- 0.191
Sebastes exsul buccanner rockfish 0.286
Cranchiid no. 2 Squid 0.382
Haemulon flaviguttatum yellowspotted grunt 0.210
Selar crumenophthalmus bigeye scad 0.458
Fish species no. 5 -- 0.630
Paralabrax sp. no. 1 sea bass 0.172
Synodus sp. no. 3 lizardfish 0.191
Lepophidium prorates prowspine cusk-eel 0.095
Fish species no. 6 -- 0.172
Synodus sp. no. 1 lizardfish 0.477
Octopus sp, no. 2 octopus 0.153
Gonatus berryi squid 0.095
Mugil cephalus striped mullet 0.019
Paranthias colonus Pacific creole-fish 0.019
Balistes polylepis finescale triggerfish 0.248
Physiculus nematopus charcoal mora 0.572
Hemanthias spp. sea bass 0.172
Fish species no. 7 -- 0.191
Uroconger varidens conger eel 0.153
Larimus spp. drum 0.153
Apogon retrosella barspot cardinalfish 0.095
Dosidicus gigas squid 0.153
Merluccius productus Pacific whiting 0.019
Fish species no. 8 -- 0.038
Synodus sp. no. 2 lizardfish 0.229
Scorpaena sonorae Sonora scorpionfish 0.038
Eucinostomus spp. mojarra 0.248
Fish species no. 9 -- 0.057
Cynoscion reticulatus striped weakfish 0.439
Fish species no. 10 -- 0.191
Caulolatilus affinis bighead tilefish 0.076
Paralabrax auroguttatus goldspotted sand bass 0.343
Fish species no. 11 -- 0.057
Cranchiid no. 5 squid 0.019
Bodianus diplotoenia mexican hogfish 0.019
Prionotus sp. no. 1 searonbin 0.038
Strongylura exilis california needlefish 0.019
Synodus spp. lizardfish 0.114
Fish species no. 12 -- 0.057
Fish species no. 13 -- 0.038
Fish species no. 14 -- 0.057
Fish species no. 15 -- 0.038
Fish species no. 16 0.038
Porichthys sp. 1 midshipman 0.019
Fish species no. 17 -- 0.095
Calamus brachysomus Pacific porgy 0.095
Fish species no. 18 -- 0.019
Fish species no. 19 -- 0.095
Ophididae no. 1 -- 0.019
Fish species no. 20 -- 0.095
Sebastes sinesis blackmouth rockfish 0.038
Symphurus spp. tonguefish 0.057
Fish species no. 21 -- 0.038
Pronotogrammus multifasciatus threadfin bass 0.153
Fsh species no. 22 -- 0.038
Fish species no. 23 -- 0.038
Orthopristis reddingi Bronze-striped grunt 0.305
Fish species no. 24 -- 0.038
Fish species no. 25 -- 0.019
Cranchiidae no. 4 squid 0.038
Fish species no. 26 -- 0.038
Histioteuthis heteropsis squid 0.019
Scorpaenidae no. 1 -- 0.019
Fish species no. 27 -- 0.057
Fish species no. 28 -- 0.019
Fish species no. 29 -- 0.019
Cranchiidae no. 3 squid 0.019
Bollmannia spp. goby 0.019
Fish species no. 30 -- 0.019
Cranchiidae no. 1 squid 0.019
Paralabrax maculatofasciatus spotted sand bass 0.019
Ophidion scrippsae basketweave cusk-eel 0.019
Physiculus spp. cod, codling, mora 0.038
Ophididae no. 2 -- 0.076
Unid. Carangidae jacks 0.153
Unid. Engraulidae anchovies 0.019
Unid. Haemulidae grunts 0.248
Unid. Labridae wrasses 0.019
Unid. Mycthophidae lanternifishes 4.121
Unid. Ophididae cusk-eel 0.038
Unid. Scianidae drums 0.248
Unid. Scorpaenidae scorpionfishes 0.572
Unid. Serranidae sea bass 0.248
Unid. Triglidae searobins 0.019
 Unid. Fishes 0.744
 Unid. Cephalopods 0.076
 Unid. fishes (very eroded) 7.268
 Remains of cephalopods
 Remains of crustaceans

 n
Scientific name Common name Occurr.

Trichiurus lepturus Pacific cutlassfish 128
Sardinops caeruleus Pacific sardine 88
Porichthys spp. midshipman 95
Myctophid no. 1 lanternfish 119
Engraulis mordax northern anchovy 43
Scomber japonicus Pacific mackerel 42
Cetengraulis mysticetus anchoveta 15
Loliolopsis diomedeae squid 35
Trachurus symmetricus jack mackerel 41
Merlaccius spp. Pacific whiting 25
Pontinus spp. scorpionfish 26
Enoploteuthid no. 1 squid 23
Caelorinchus scaphopsis shoulderspot 25
Octopus sp. no. 1 octopus 17
Sebastes macdonaldi Mexican rockfish 18
Citharichthys sp no. 1 sanddab 23
Fish species no. 1 -- 25
Haemulopsis leuciscus white grunt 21
Peprilus snyderi salema butterfish 33
Prionotus spp. searobin 9
Prionotus stephanophrys lumptail searobin 14
Argentina sialis Pacific argentine 13
Fish species no. 2 -- 27
Hemanthias peruanus splittail bass 22
Fish species no. 3 -- 6
Micropogonias ectenes slender croaker 9
Lepophidium spp. cusk-eel 3
Fish species no. 4 -- 3
Sebastes exsul buccanner rockfish 10
Cranchiid no. 2 Squid 12
Haemulon flaviguttatum yellowspotted grunt 3
Selar crumenophthalmus bigeye scad 12
Fish species no. 5 -- 19
Paralabrax sp. no. 1 sea bass 5
Synodus sp. no. 3 lizardfish 3
Lepophidium prorates prowspine cusk-eel 4
Fish species no. 6 -- 5
Synodus sp. no. 1 lizardfish 10
Octopus sp, no. 2 octopus 7
Gonatus berryi squid 5
Mugil cephalus striped mullet 1
Paranthias colonus Pacific creole-fish 1
Balistes polylepis finescale triggerfish 4
Physiculus nematopus charcoal mora 12
Hemanthias spp. sea bass 6
Fish species no. 7 -- 8
Uroconger varidens conger eel 5
Larimus spp. drum 6
Apogon retrosella barspot cardinalfish 4
Dosidicus gigas squid 5
Merluccius productus Pacific whiting 1
Fish species no. 8 -- 2
Synodus sp. no. 2 lizardfish 5
Scorpaena sonorae Sonora scorpionfish 1
Eucinostomus spp. mojarra 5
Fish species no. 9 -- 3
Cynoscion reticulatus striped weakfish 7
Fish species no. 10 -- 1
Caulolatilus affinis bighead tilefish 3
Paralabrax auroguttatus goldspotted sand bass 4
Fish species no. 11 -- 2
Cranchiid no. 5 squid 1
Bodianus diplotoenia mexican hogfish 1
Prionotus sp. no. 1 searonbin 2
Strongylura exilis california needlefish 1
Synodus spp. lizardfish 5
Fish species no. 12 -- 3
Fish species no. 13 -- 1
Fish species no. 14 -- 1
Fish species no. 15 -- 1
Fish species no. 16 2
Porichthys sp. 1 midshipman 1
Fish species no. 17 -- 3
Calamus brachysomus Pacific porgy 2
Fish species no. 18 -- 1
Fish species no. 19 -- 2
Ophididae no. 1 -- 1
Fish species no. 20 -- 3
Sebastes sinesis blackmouth rockfish 1
Symphurus spp. tonguefish 1
Fish species no. 21 -- 1
Pronotogrammus multifasciatus threadfin bass 2
Fsh species no. 22 -- 2
Fish species no. 23 -- 1
Orthopristis reddingi Bronze-striped grunt 1
Fish species no. 24 -- 1
Fish species no. 25 -- 1
Cranchiidae no. 4 squid 2
Fish species no. 26 -- 2
Histioteuthis heteropsis squid 1
Scorpaenidae no. 1 -- 1
Fish species no. 27 -- 2
Fish species no. 28 -- 1
Fish species no. 29 -- 1
Cranchiidae no. 3 squid 1
Bollmannia spp. goby 1
Fish species no. 30 -- 1
Cranchiidae no. 1 squid 1
Paralabrax maculatofasciatus spotted sand bass 1
Ophidion scrippsae basketweave cusk-eel 1
Physiculus spp. cod, codling, mora 1
Ophididae no. 2 -- 1
Unid. Carangidae jacks 3
Unid. Engraulidae anchovies 1
Unid. Haemulidae grunts 11
Unid. Labridae wrasses 1
Unid. Mycthophidae lanternifishes 71
Unid. Ophididae cusk-eel 1
Unid. Scianidae drums 9
Unid. Scorpaenidae scorpionfishes 18
Unid. Serranidae sea bass 6
Unid. Triglidae searobins 1
 Unid. Fishes 16
 Unid. Cephalopods 4
 Unid. fishes (very eroded) 231
 Remains of cephalopods 14
 Remains of crustaceans 162

Scientific name Common name PO

Trichiurus lepturus Pacific cutlassfish 15.041
Sardinops caeruleus Pacific sardine 10.341
Porichthys spp. midshipman 11.163
Myctophid no. 1 lanternfish 13.984
Engraulis mordax northern anchovy 5.053
Scomber japonicus Pacific mackerel 4.935
Cetengraulis mysticetus anchoveta 1.763
Loliolopsis diomedeae squid 4.113
Trachurus symmetricus jack mackerel 4.818
Merlaccius spp. Pacific whiting 2.938
Pontinus spp. scorpionfish 3.055
Enoploteuthid no. 1 squid 2.703
Caelorinchus scaphopsis shoulderspot 2.938
Octopus sp. no. 1 octopus 1.998
Sebastes macdonaldi Mexican rockfish 2.115
Citharichthys sp no. 1 sanddab 2.703
Fish species no. 1 -- 2.938
Haemulopsis leuciscus white grunt 2.468
Peprilus snyderi salema butterfish 3.878
Prionotus spp. searobin 1.058
Prionotus stephanophrys lumptail searobin 1.645
Argentina sialis Pacific argentine 1.528
Fish species no. 2 -- 3.173
Hemanthias peruanus splittail bass 2.585
Fish species no. 3 -- 0.705
Micropogonias ectenes slender croaker 1.058
Lepophidium spp. cusk-eel 0.353
Fish species no. 4 -- 0.353
Sebastes exsul buccanner rockfish 1.175
Cranchiid no. 2 Squid 1.410
Haemulon flaviguttatum yellowspotted grunt 0.353
Selar crumenophthalmus bigeye scad 1.410
Fish species no. 5 -- 2.233
Paralabrax sp. no. 1 sea bass 0.588
Synodus sp. no. 3 lizardfish 0.353
Lepophidium prorates prowspine cusk-eel 0.470
Fish species no. 6 -- 0.588
Synodus sp. no. 1 lizardfish 1.175
Octopus sp, no. 2 octopus 0.828
Gonatus berryi squid 0.588
Mugil cephalus striped mullet 0.118
Paranthias colonus Pacific creole-fish 0.118
Balistes polylepis finescale triggerfish 0.470
Physiculus nematopus charcoal mora 1.410
Hemanthias spp. sea bass 0.705
Fish species no. 7 -- 0.940
Uroconger varidens conger eel 0.588
Larimus spp. drum 0.705
Apogon retrosella barspot cardinalfish 0.470
Dosidicus gigas squid 0.588
Merluccius productus Pacific whiting 0.118
Fish species no. 8 -- 0.235
Synodus sp. no. 2 lizardfish 0.588
Scorpaena sonorae Sonora scorpionfish 0.118
Eucinostomus spp. mojarra 0.588
Fish species no. 9 -- 0.353
Cynoscion reticulatus striped weakfish 0.823
Fish species no. 10 -- 0.118
Caulolatilus affinis bighead tilefish 0.353
Paralabrax auroguttatus goldspotted sand bass 0.470
Fish species no. 11 -- 0.235
Cranchiid no. 5 squid 0.118
Bodianus diplotoenia mexican hogfish 0.118
Prionotus sp. no. 1 searonbin 0.235
Strongylura exilis california needlefish 0.118
Synodus spp. lizardfish 0.588
Fish species no. 12 -- 0.353
Fish species no. 13 -- 0.118
Fish species no. 14 -- 0.118
Fish species no. 15 -- 0.118
Fish species no. 16 0.235
Porichthys sp. 1 midshipman 0.118
Fish species no. 17 -- 0.353
Calamus brachysomus Pacific porgy 0.235
Fish species no. 18 -- 0.118
Fish species no. 19 -- 0.235
Ophididae no. 1 -- 0.118
Fish species no. 20 -- 0.353
Sebastes sinesis blackmouth rockfish 0.118
Symphurus spp. tonguefish 0.118
Fish species no. 21 -- 0.118
Pronotogrammus multifasciatus threadfin bass 0.235
Fsh species no. 22 -- 0.235
Fish species no. 23 -- 0.118
Orthopristis reddingi Bronze-striped grunt 0.118
Fish species no. 24 -- 0.118
Fish species no. 25 -- 0.118
Cranchiidae no. 4 squid 0.235
Fish species no. 26 -- 0.235
Histioteuthis heteropsis squid 0.118
Scorpaenidae no. 1 -- 0.118
Fish species no. 27 -- 0.235
Fish species no. 28 -- 0.118
Fish species no. 29 -- 0.118
Cranchiidae no. 3 squid 0.118
Bollmannia spp. goby 0.118
Fish species no. 30 -- 0.118
Cranchiidae no. 1 squid 0.118
Paralabrax maculatofasciatus spotted sand bass 0.118
Ophidion scrippsae basketweave cusk-eel 0.118
Physiculus spp. cod, codling, mora 0.118
Ophididae no. 2 -- 0.118
Unid. Carangidae jacks 0.353
Unid. Engraulidae anchovies 0.118
Unid. Haemulidae grunts 1.293
Unid. Labridae wrasses 0.118
Unid. Mycthophidae lanternifishes 8.343
Unid. Ophididae cusk-eel 0.118
Unid. Scianidae drums 1.058
Unid. Scorpaenidae scorpionfishes 2.115
Unid. Serranidae sea bass 0.705
Unid. Triglidae searobins 0.118
 Unid. Fishes 1.880
 Unid. Cephalopods 0.470
 Unid. fishes (very eroded) 27.145
 Remains of cephalopods 1.645
 Remains of crustaceans 19.036

Scientific name Common name IIMP

Trichiurus lepturus Pacific cutlassfish 16.392
Sardinops caeruleus Pacific sardine 10.020
Porichthys spp. midshipman 9.297
Myctophid no. 1 lanternfish 7.901
Engraulis mordax northern anchovy 5.199
Scomber japonicus Pacific mackerel 3.403
Cetengraulis mysticetus anchoveta 2.404
Loliolopsis diomedeae squid 2.399
Trachurus symmetricus jack mackerel 2.273
Merlaccius spp. Pacific whiting 2.206
Pontinus spp. scorpionfish 1.842
Enoploteuthid no. 1 squid 1.754
Caelorinchus scaphopsis shoulderspot 1.728
Octopus sp. no. 1 octopus 1.614
Sebastes macdonaldi Mexican rockfish 1.496
Citharichthys sp no. 1 sanddab 1.220
Fish species no. 1 -- 1.153
Haemulopsis leuciscus white grunt 1.093
Peprilus snyderi salema butterfish 1.025
Prionotus spp. searobin 0.855
Prionotus stephanophrys lumptail searobin 0.814
Argentina sialis Pacific argentine 0.754
Fish species no. 2 -- 0.737
Hemanthias peruanus splittail bass 0.602
Fish species no. 3 -- 0.592
Micropogonias ectenes slender croaker 0.547
Lepophidium spp. cusk-eel 0.532
Fish species no. 4 -- 0.511
Sebastes exsul buccanner rockfish 0.505
Cranchiid no. 2 Squid 0.501
Haemulon flaviguttatum yellowspotted grunt 0.468
Selar crumenophthalmus bigeye scad 0.431
Fish species no. 5 -- 0.384
Paralabrax sp. no. 1 sea bass 0.373
Synodus sp. no. 3 lizardfish 0.341
Lepophidium prorates prowspine cusk-eel 0.335
Fish species no. 6 -- 0.324
Synodus sp. no. 1 lizardfish 0.324
Octopus sp, no. 2 octopus 0.308
Gonatus berryi squid 0.274
Mugil cephalus striped mullet 0.265
Paranthias colonus Pacific creole-fish 0.265
Balistes polylepis finescale triggerfish 0.245
Physiculus nematopus charcoal mora 0.244
Hemanthias spp. sea bass 0.234
Fish species no. 7 -- 0.233
Uroconger varidens conger eel 0.189
Larimus spp. drum 0.174
Apogon retrosella barspot cardinalfish 0.173
Dosidicus gigas squid 0.171
Merluccius productus Pacific whiting 0.167
Fish species no. 8 -- 0.159
Synodus sp. no. 2 lizardfish 0.132
Scorpaena sonorae Sonora scorpionfish 0.130
Eucinostomus spp. mojarra 0.129
Fish species no. 9 -- 0.127
Cynoscion reticulatus striped weakfish 0.124
Fish species no. 10 -- 0.122
Caulolatilus affinis bighead tilefish 0.114
Paralabrax auroguttatus goldspotted sand bass 0.110
Fish species no. 11 -- 0.102
Cranchiid no. 5 squid 0.097
Bodianus diplotoenia mexican hogfish 0.087
Prionotus sp. no. 1 searonbin 0.087
Strongylura exilis california needlefish 0.083
Synodus spp. lizardfish 0.146
Fish species no. 12 -- 0.074
Fish species no. 13 -- 0.065
Fish species no. 14 -- 0.060
Fish species no. 15 -- 0.058
Fish species no. 16 0.056
Porichthys sp. 1 midshipman 0.052
Fish species no. 17 -- 0.049
Calamus brachysomus Pacific porgy 0.043
Fish species no. 18 -- 0.042
Fish species no. 19 -- 0.041
Ophididae no. 1 -- 0.040
Fish species no. 20 -- 0.039
Sebastes sinesis blackmouth rockfish 0.039
Symphurus spp. tonguefish 0.038
Fish species no. 21 -- 0.036
Pronotogrammus multifasciatus threadfin bass 0.029
Fsh species no. 22 -- 0.027
Fish species no. 23 -- 0.021
Orthopristis reddingi Bronze-striped grunt 0.020
Fish species no. 24 -- 0.020
Fish species no. 25 -- 0.016
Cranchiidae no. 4 squid 0.014
Fish species no. 26 -- 0.014
Histioteuthis heteropsis squid 0.014
Scorpaenidae no. 1 -- 0.011
Fish species no. 27 -- 0.011
Fish species no. 28 -- 0.010
Fish species no. 29 -- 0.008
Cranchiidae no. 3 squid 0.006
Bollmannia spp. goby 0.006
Fish species no. 30 -- 0.005
Cranchiidae no. 1 squid 0.004
Paralabrax maculatofasciatus spotted sand bass 0.003
Ophidion scrippsae basketweave cusk-eel 0.003
Physiculus spp. cod, codling, mora 0.003
Ophididae no. 2 -- 0.002
Unid. Carangidae jacks 0.141
Unid. Engraulidae anchovies 0.248
Unid. Haemulidae grunts 0.509
Unid. Labridae wrasses 0.005
Unid. Mycthophidae lanternifishes 4.895
Unid. Ophididae cusk-eel 0.098
Unid. Scianidae drums 0.643
Unid. Scorpaenidae scorpionfishes 1.078
Unid. Serranidae sea bass 0.176
Unid. Triglidae searobins 0.002
 Unid. Fishes 1.819
 Unid. Cephalopods 0.373
 Unid. fishes (very eroded)
 Remains of cephalopods
 Remains of crustaceans

Table 3

Prey of California sea lions having IIMP index values [greater than or
equal to] 10% in at least one sampling period for seven rookeries in
the Gulf of California, Mexico. Blank indicate that species were not
recorded in diet; "--" means unavailable data.

 Prey species June 1995

San Pedro Engraulis mordax 29.7
Martir myctophid no. 1 29.0
 Porichthys spp. 11.2
 Prionotus stephanophrys 0.6
 enopleoteuthid no. 1
 Sebastes macdonaldi
 Haeumulopsis leuciscus
San Esteban Trichiurus lepturus 24.9
 Sardinops caeruleus 10.0
 unid. Myctophidae 13.8
 myctophid no. 1 2.8
 enopleoteuthid no. 1
 Sebastes macdonaldi
 fish species no. 1
El Rasito Porichthys spp. 26.2
 unid. Myctophidae 16.4
 Scomber japonicus 13.8
 Pontinus spp. 11.5
 Octopus sp. no. 1 11.5
 myctophid no. 1 6.6
 Sardinops caeruleus 1.6
 Trachurus symmetricus
 Caelorinchus scaphopsis
Los Machos Sardinops caeruleus 21.0
 Scomber japonicus 19.0
 Merluccius spp. 15.4
 Trichiurus lepturus 11.7
 Sebastes macdonaldi 1.8
Los Cantiles Porichthys spp. 66.7
 Trichiurus lepturus 22.2
 Engraulis mordax 3.7
 myctophid no. 1
 Sardinops caeruleus
 fish species no. 3
 unid. Fishes
 unid. Scianidae
 Lepophidium spp.
 Loliolopsis diomedeae
Isla Granito Engraulis mordax 49.3
 Trichiurus lepturus 22.0
 unid. Myctophidae 1.7
 Sardinops caeruleus 0.9
 Porichthys spp. 0.5
 Cithartchthys sp. no. 1
Isla Lobos Cetengraulis mysticetus 32.7
 Trichiurus lepturus 25.2
 Porichthys spp. 9.0
 Loliolopsis diomedeae 4.9
 Peprilus snyderi

 Prey species September 1995

San Pedro Engraulis mordax
Martir myctophid no. 1 10.5
 Porichthys spp. 2.0
 Prionotus stephanophrys 3.3
 enopleoteuthid no. 1 27.3
 Sebastes macdonaldi 10.4
 Haeumulopsis leuciscus
San Esteban Trichiurus lepturus 3.4
 Sardinops caeruleus 34.1
 unid. Myctophidae 3.4
 myctophid no. 1 11.8
 enopleoteuthid no. 1 16.9
 Sebastes macdonaldi 2.1
 fish species no. 1
El Rasito Porichthys spp. 4.0
 unid. Myctophidae 1.5
 Scomber japonicus 3.2
 Pontinus spp. 5.1
 Octopus sp. no. 1
 myctophid no. 1 5.1
 Sardinops caeruleus 40.1
 Trachurus symmetricus 22.0
 Caelorinchus scaphopsis 3.6
Los Machos Sardinops caeruleus 64.1
 Scomber japonicus
 Merluccius spp.
 Trichiurus lepturus 5.4
 Sebastes macdonaldi
Los Cantiles Porichthys spp. 15.5
 Trichiurus lepturus 38.2
 Engraulis mordax 0.4
 myctophid no. 1 17.6
 Sardinops caeruleus 6.8
 fish species no. 3 0.9
 unid. Fishes 0.9
 unid. Scianidae
 Lepophidium spp.
 Loliolopsis diomedeae
Isla Granito Engraulis mordax 7.8
 Trichiurus lepturus 70.1
 unid. Myctophidae 1.1
 Sardinops caeruleus
 Porichthys spp. 18.2
 Cithartchthys sp. no. 1
Isla Lobos Cetengraulis mysticetus 0.1
 Trichiurus lepturus 27.7
 Porichthys spp. 10.3
 Loliolopsis diomedeae 2.2
 Peprilus snyderi 23.5

 Prey species January 1996

San Pedro Engraulis mordax 2.1
Martir myctophid no. 1 9.0
 Porichthys spp. 6.8
 Prionotus stephanophrys 3.3
 enopleoteuthid no. 1 0.8
 Sebastes macdonaldi
 Haeumulopsis leuciscus 16.7
San Esteban Trichiurus lepturus
 Sardinops caeruleus
 unid. Myctophidae 4.3
 myctophid no. 1 8.9
 enopleoteuthid no. 1
 Sebastes macdonaldi 9.7
 fish species no. 1 1.7
El Rasito Porichthys spp. 2.3
 unid. Myctophidae 8.1
 Scomber japonicus 3.7
 Pontinus spp. 4.1
 Octopus sp. no. 1 2.9
 myctophid no. 1 21.4
 Sardinops caeruleus 0.9
 Trachurus symmetricus 5.0
 Caelorinchus scaphopsis 13.5
Los Machos Sardinops caeruleus 16.8
 Scomber japonicus 10.9
 Merluccius spp. 8.2
 Trichiurus lepturus
 Sebastes macdonaldi 11.3
Los Cantiles Porichthys spp.
 Trichiurus lepturus
 Engraulis mordax 14.3
 myctophid no. 1 4.8
 Sardinops caeruleus 19.0
 fish species no. 3 14.3
 unid. Fishes 19.0
 unid. Scianidae 14.3
 Lepophidium spp. 14
 Loliolopsis diomedeae
Isla Granito Engraulis mordax
 Trichiurus lepturus 2.0
 unid. Myctophidae 12.6
 Sardinops caeruleus 18.7
 Porichthys spp. 4.6
 Cithartchthys sp. no. 1 21.7
Isla Lobos Cetengraulis mysticetus 6.8
 Trichiurus lepturus 15.8
 Porichthys spp. 23.2
 Loliolopsis diomedeae 11.6
 Peprilus snyderi 5.2

 Prey species May 1996

San Pedro Engraulis mordax 0.5
Martir myctophid no. 1 20.5
 Porichthys spp. 15.5
 Prionotus stephanophrys 10.9
 enopleoteuthid no. 1
 Sebastes macdonaldi
 Haeumulopsis leuciscus 6.0
San Esteban Trichiurus lepturus 3.0
 Sardinops caeruleus 4.2
 unid. Myctophidae 10.9
 myctophid no. 1 18.8
 enopleoteuthid no. 1
 Sebastes macdonaldi 1.4
 fish species no. 1 11.0
El Rasito Porichthys spp.
 unid. Myctophidae 16.4
 Scomber japonicus 2.5
 Pontinus spp. 10.9
 Octopus sp. no. 1 7.7
 myctophid no. 1 6.8
 Sardinops caeruleus 7.3
 Trachurus symmetricus 23.4
 Caelorinchus scaphopsis 10.5
Los Machos Sardinops caeruleus --
 Scomber japonicus --
 Merluccius spp. --
 Trichiurus lepturus --
 Sebastes macdonaldi --
Los Cantiles Porichthys spp.
 Trichiurus lepturus 53.1
 Engraulis mordax
 myctophid no. 1
 Sardinops caeruleus
 fish species no. 3
 unid. Fishes
 unid. Scianidae
 Lepophidium spp.
 Loliolopsis diomedeae 21.1
Isla Granito Engraulis mordax
 Trichiurus lepturus 100.0
 unid. Myctophidae
 Sardinops caeruleus
 Porichthys spp.
 Cithartchthys sp. no. 1
Isla Lobos Cetengraulis mysticetus 27.8
 Trichiurus lepturus 14.3
 Porichthys spp. 35.5
 Loliolopsis diomedeae 3.5
 Peprilus snyderi

 Prey species Average

San Pedro Engraulis mordax 8.1
Martir myctophid no. 1 17.3
 Porichthys spp. 8.9
 Prionotus stephanophrys 4.5
 enopleoteuthid no. 1 7.0
 Sebastes macdonaldi 2.6
 Haeumulopsis leuciscus 5.7
San Esteban Trichiurus lepturus 7.8
 Sardinops caeruleus 12.1
 unid. Myctophidae 8.1
 myctophid no. 1 10.6
 enopleoteuthid no. 1 4.2
 Sebastes macdonaldi 3.3
 fish species no. 1 3.2
El Rasito Porichthys spp. 8.1
 unid. Myctophidae 10.6
 Scomber japonicus 5.8
 Pontinus spp. 7.9
 Octopus sp. no. 1 5.5
 myctophid no. 1 10.0
 Sardinops caeruleus 12.5
 Trachurus symmetricus 12.6
 Caelorinchus scaphopsis 6.9
Los Machos Sardinops caeruleus 34.0
 Scomber japonicus 10.0
 Merluccius spp. 7.9
 Trichiurus lepturus 5.7
 Sebastes macdonaldi 4.4
Los Cantiles Porichthys spp. 20.6
 Trichiurus lepturus 28.4
 Engraulis mordax 4.6
 myctophid no. 1 5.6
 Sardinops caeruleus 6.5
 fish species no. 3 3.8
 unid. Fishes 5.0
 unid. Scianidae 3.6
 Lepophidium spp. 3.5
 Loliolopsis diomedeae 5.3
Isla Granito Engraulis mordax 14.3
 Trichiurus lepturus 48.5
 unid. Myctophidae 3.9
 Sardinops caeruleus 4.9
 Porichthys spp. 5.8
 Cithartchthys sp. no. 1 5.4
Isla Lobos Cetengraulis mysticetus 16.9
 Trichiurus lepturus 20.8
 Porichthys spp. 19.5
 Loliolopsis diomedeae 5.6
 Peprilus snyderi 7.2


Acknowledgments

We wish to thank Secretaria de Marina, Armada de Mexico, for its great support during the field activities, and the Consejo Nacional de Ciencia y Tecnologia (CONACYT) for funding this study under grant number 26430-N. The Secretaria de Medio Ambiente, Recursos Naturales y Pesca (SEMARNAP) provided permits for field work (DOO.-700-(2)01104 and DOO.-700(2).-1917). We would like to thank Robert Lavenberg and Jeff Siegel for allowing us the use of otoliths from the collection at the Natural Museum History of Los Angeles County and also Lawrence Barnes for his logistical support during the stay of first author at Los Angeles; we also thank Manuel Nava for allowing us the use of otoliths from the collection in Tecnologico de Monterrey, Campus Guaymas. We are also grateful to Unai Markaida for his assistance in prey identification based on the examination of cephalopods beaks. We thank Mark Lowry fur commenting on an earlier draft of the paper, Norman Silverberg for reviewing the manuscript in English, and two anonymous reviewers fur their valuable suggestions and criticism. The first author would like to thank Centro Interdisciplinario de Ciencias Marinas-IPN for a scholarship (PIFI, Programa Institucional para la Formacion de Investigadores) assigned for postgraduate studies.

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(2) Cisneros-Mata, M. A., M. O. Nevarez-Martinez, G. Montemayor-Lopez, J. P. Santos-Molina, and R. Morales-Azpeitia. 1991. Pesqueria de sardina en el Golfo de California de 1988/89-1989/90. Informe Tecnico, 80 p. Centro Regional de Investigaciones Pesqueras de Guaymas. INP. SEPESCA. Calle 20 No. 605 Sur Col. La Cantera. Guaymas, Son. CP. 85400.

(3) Cisneros-Mata, M. A., M. O. Nevarez-Martinez, M. A. Martinez Zavala, M. L. Anguiano-Carranza, J. P. Santos Molina, A. R. Godinez-Cota, and G. Montemayor-Lopez. 1997. Diagnosis de la pesqueria de pelagicos menores del Golfo de California de 1991/92 a 1995/96. Informe Tecnico, 59 p. Centro Regional de Investigaciones Pesqueras de Guaymas. INP. SEMARNAP. Calle 20 No. 605 Sur Col. La Cantera. Guaymas, Son. CP. 85400.

(4) Garcia-Rodriguez, F. J., and J. De la Cruz-Aguero. In prep. An index to measure the specie prey importance of California sea lion (Zalophus californianus) based on scat samples.

(5) Alvarado-Castillo, R. Unpubl. data. Departamento de Biologia y Pesquerias, Centro Interdisciplinario de Ciencias Marinas. Avenida IPN S/N Col. Palo Playa de Santa Rita, La Paz, Baja California Sur, Mexico 23070.

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Francisco J. Garcia-Rodriguez

David Aurioles-Gamboa

Centro Interdisciplinario de Ciencias Marinas-Instituto Politecnico Nacional

Departamento de Biologia Marina y Pesquerias

Apdo. Postal 592

La Paz, Baja California Sur, Mexico

E-mail address (for F. J. Garcia-Rodriguez): figrodri@cibnor.mx

Manuscript approved for publication 9 October 2003 by Scientific Editor.

Manuscript received 20 October 2003 at NMFS Scientific Publications Office.
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