Reilly et al : Diets of pelagic juvenile Sebsstes off central California 



513 



the 42 shortbelly rockfish that were classified as small 

 came from deep stations. Results presented earlier 

 (Table 7) showed that the diet of shortbelly rockfish 

 varied significantly with depth in 1985 (i.e., fewer 

 euphausiid eggs and copepod juveniles at deep sta- 

 tions). Thus, the conclusion that small fish consumed 

 large prey in 1985 is, to some degree, confounded with 

 this spatial effect. 



Discussion 



The five species of pelagic juvenile rockfish examined 

 in this study consumed pelagic zooplankton almost 

 exclusively. Relatively few prey types made up the 

 major portion of the diet each year. Various life history 

 stages of calanoid copepods and euphausiids dominated. 

 Carlson and Haight (1976) reported that copepods and 

 euphausiids were important in the diet of pelagic juve- 

 nile Pacific ocean perch S. alutics. Singer (1985) recent- 

 ly reported that settled juveniles of several rockfish 

 species consumed copepods and zoea larvae in a cen- 

 tral California kelp forest. Other studies (Robb and 

 Hislop 1980, Bowman 1981, Conway 1980) have also 

 demonstrated that calanoid copepods and euphausiids 

 are extremely important foods to pelagic juvenile fishes 

 in the northeastern Pacific Ocean. These studies 

 demonstrate that the diets of pelagic juvenile rockfishes 

 are similar to those of other species possessing pelagic 

 juvenile life stages. 



A significant finding of this study is that Sebastes 

 spp. juveniles periodically forage heavily on euphau- 

 siid eggs. Euphausiid eggs have not been previously 

 reported as a prey item of pelagic juvenile rockfish and 

 yet they were a very important dietary component both 

 in 1985 and 1987. During those years, euphausiid eggs 



averaged over 37% of the prey items 

 consumed by the five species studied. 

 However, euphausiid eggs were ab- 

 sent from samples collected in 1984 

 and were a minor component in 1986. 

 Some species of euphausiids brood 

 their eggs prior to hatching (e.g., Nyc- 

 tiphanes spp.), whereas other species 

 release eggs upon fertilization (e.g., 

 Euphausia pacifica and Thysanoessa 

 spinifera). Since adult euphausiids 

 were not found in any stomachs in 

 1985 (Table 3), and since only the lat- 

 ter genera were encountered in large 

 swarms in the study area in 1987 

 (Smith and Adams 1988), rockfish 

 must have consumed eggs after re- 

 lease. It was not expected that a non- 

 motile prey would constitute such an 

 important food resource to pelagic juvenile rockfish. 

 The appearance of eggs in clumped masses in guts sug- 

 gests that eggs were not individually picked from the 

 plankton. 



Another interesting finding was the consumption of 

 fish larvae by bocaccio juveniles. A total 15-20% of all 

 bocaccio sampled in 1984 and 1987 contained larval 

 fish. In our surveys, bocaccio grow faster and reach 

 larger sizes as pelagic juveniles ( > 1 00 mm SL) than do 

 other species (Woodbury and Ralston 1991). They are 

 also distributed at shallower depths (Lenarz et al. 

 1991). 



We used the I^ statistic to rank the importance of in- 

 dividual prey items in the diet. This statistic differs 

 from a similar statistic used by Hobson (1974) in that 

 it is the product of proportional frequency of occur- 

 rence and percent by number, rather than percent by 

 volume. Use of this statistic allowed us to characterize 

 the prey types consumed by Sebastes in each of the 4 

 years studied. No obvious species-specific patterns 

 emerged in the absence of a temporal component. 



Our results indicate that pelagic juvenile Sebastes 

 tend to respond similarly to environmental fluctuations 

 in their food base, suggesting an opportunistic feeding 

 strategy. Intraspecific dietary overlap between interan- 

 nual pairings was much lower than were interspecific 

 interannual pairings. On a relative basis, interannual 

 differences in diet were tracked similarly among the 

 five species we examined. Annual changes in diet are 

 likely to reflect annual differences in the composition, 

 availability, and abundance of prey. 



It was not possible to infer from our results whether 

 or not food is limiting to pelagic juvenile rockfishes, 

 given the relatively large interannual variation in the 

 diet among these species and the likelihood that varia- 

 tion in the availability of prey is likely responsible. 



