Garcia-Rodriquez and Aurioles-Gamboa: Spatial and temporal variation in the diet of Zalophus californianus 



51 



Table 1 



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



June 1995 



San Pedro Martir (SPM) 



SanEsteban(EST) 



ElRasito(RAS) 



Los Cantiles (CAN) 



IslaGranito(GRAl 



Los Machos (MAC) 



IslaLobos(LOB) 



Total 



22 

 50 

 11 

 20 

 24 

 39 

 72 

 238 



September 1995 



January 1996 



33 

 66 

 56 

 58 

 20 

 32 

 139 

 404 



91 

 58 

 47 

 41 

 36 

 72 

 433 



Mav 1996 



29 

 67 

 25 

 35 

 19 

 

 23 

 198 



Total 



172 

 274 

 150 

 160 

 104 

 107 

 306 

 1273 



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 sea- 

 sons between Los Machos and El Rasito (r=0.998. P=0.04), 

 but not between Los Machos and Isla Granito U-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 

 correlated with the presence of the Pacific sardine. The 

 anchoveta was only found at Isla Lobos, and jack mackerel 

 at El Rasito, San Pedro Martir, and Isla Granito. 



The changes in the PO of the main prey coincided with 

 the variations of the IIMP. The occurrence of Pacific cut- 

 lassfish. Pacific sardine, plainfin midshipman, northern 

 anchovy, Pacific mackerel, and jack mackerel was signifi- 

 cantly different (P<0.04) among rookeries. Myctophid no. 

 1 showed no significant difference in ocurrence <x 2 =11.04, 

 df=6, P=0.09); but when all lanternfishes were pooled, 

 their occurrence among rookeries was significantly differ- 

 ent (x 2 =H.13,df=6,P=0.04). We found significant temporal 

 differences in the occurrence of Pacific cutlassfish. Pacific 

 sardine, plainfin midshipman, northern anchovy, and Pa- 

 cific mackerel (P<0.05), but no significant differences were 

 found among seasons in the occurrence of jack mackerel 

 (* 2 =2.94, df=3, P=0.40), myctophid no. 1 <x 2 =1.67, df=3, 

 P=0.6428), or lanternfish <x 2 =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 ±13.7 mm). Significant differences were found 

 among sampling periods (P=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 (±16.1 mm) in 

 June, 151.7 mm (±13.0 mm) in September, and 136.5 mm 

 ( ±13.7 mm ) in January ( Fig. 5 ). A similar pattern was found 

 in Los Cantiles, Los Machos, and Isla Granito. 



Spatial and temporal correlation in diet 



We identified 25 prey taxa that had an IIMP index value 

 of >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 correla- 

 tion among any pair of sampling periods for any rookery 

 (P>0.14), except between January and May at San Pedro 

 Martir (P s =0.64, P<0.05) and El Rasito (P s =0.66, P<0.05) 

 and between January and June as well as between Janu- 

 ary and May at Isla Lobos (R s =0.56, P=0.05; and P 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 "cut- 

 off" distance of 0.3 and 0.4 on the dendrogram as reference 

 points for identifying clusters. The group obtained by us- 

 ing 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 Lobos; and area IV: Isla Granito). 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. 



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 



