Spatial and temporal variation in the diet of the California sea lion (Zalophus californianus) in the Gulf of California, Mexico.
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.
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]
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
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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Francisco J. Garcia-Rodriguez
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): email@example.com
Manuscript approved for publication 9 October 2003 by Scientific Editor.
Manuscript received 20 October 2003 at NMFS Scientific Publications Office.
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