TROPHIC INTERACTIONS AMONG FISHES AND ZOOPLANKTERS 

 NEAR SHORE AT SANTA CATALINA ISLAND, CALIFORNIA^ 



Edmund S. Hobson and James R. Chess^ 



ABSTRACT 



Predation pressures from fishes have influenced major evolutionary trends among shallow-water 

 zooplankters, as concluded from study at Santa Catalina Island, Calif. The predominant zooplank- 

 tivorous fishes near shore are actinopterygians, an evolutionary line that has centered around 

 generalized visually feeding, large-mouthed predators. Historically, zooplankters threatened by these 

 fishes have faced selective pressures favoring reduced size, transparency, and/or nocturnal planktonic 

 habits. At present, most zooplankters in the nearshore water column by day are very small (<2 mm, 

 approximately); included are cladocerans, copepods, and various larval forms. Their small size precludes 

 capture by most large-mouthed fishes, thus providing protection in daylight, when the visual sense of 

 generalized predatory fishes is most effective. Larger zooplankters in the water column by day, for 

 example chaetognaths, tend to be transparent. The advantage of transparency to organisms 

 threatened by visually feeding predators is obvious, and is only briefly mentioned here. Zooplankters 

 having sizes (most >2 mm) and other features making them vulnerable to large-mouthed fishes tend to 

 enter the water column only at night, when darkness off'ers some security from visually feeding 

 predators. Included are polychaetes, mysids, cumaceans, gammaridean and caprellid amphipods, 

 tanaids, isopods, and carideans. 



Because successful defensive features of prey create pressures that modify the offensive features of 

 predators, the tendencies toward reduced size and nocturnal habits among zooplankters have generated 

 appropriate adaptations among planktivorous fishes. Fishes that prey as adults on zooplankters during 

 the day (e.g., blacksmith, Chromis pxinctipinnis) have specialized features, including a small highly 

 modified mouth, that permit even relatively large individuals to take the tiny organisms which 

 constitute the daytime zooplankton. Some other fishes are diurnal planktivores only as small juveniles 

 and assume different feeding habits as they grow larger (e.g., kelp perch, Brachyistius frenatus; 

 senorita, Oxyjulis californica; smaller juvenile olive rockfish, Sebastes serranoides). Fishes that prey on 

 zooplankters at night (e.g., larger juvenile olive rockfish; kelp rockfish, Sebastes atrovirens; queenfish, 

 Seriphus poHtus; walleye surfperch, Hyperprosopon argeyiteum; and salema, Xenistius californiensis) 

 take the larger organisms that join the zooplankton after dark. In their feeding morphologies and body 

 form, these large-mouthed fishes have diverged less than their diurnal counterparts from the 

 generalized predators that give rise to them all. They have, however, acquired specialized features, 

 including large eyes, suited to detect and capture prey in the dark. 



Interactions among predators and their prey 

 are best recognized by viewing assemblages of 

 animals that occur together in nature. Further- 

 more, many trophic interactions become apparent 

 only upon considering the changes that occur from 

 day to night, and from one season to another. 

 These convictions shaped studies of feeding rela- 

 tions among tropical reef fishes undertaken 

 between 1962 and 1970 (Hobson 1965, 1968, 1972, 

 1974), and similarly influenced work done in warm 

 temperate waters from 1972 to 1975. This more 

 recent work centered on the inshore habitats at 

 Santa Catalina Island, Calif, (lat. 33°28'N, long. 



'Contribution no. 21 from the Catalina Marine Science Center, 

 University of Southern California. 



^Southwest Fisheries Center Tiburon Laboratory, National 

 Marine Fisheries Service, NOAA, Tiburon, CA 94920. 



118°29'W), where most of the attention was di- 

 rected at fishes that forage on the benthos (Hobson 

 and Chess in prep.). The present report, however, 

 deals with that segment of the work involving 

 certain fishes and trophically related zooplankters 

 that interact in the water column near shore. 



Only a few studies have considered feeding 

 habits in natural assemblages of marine fishes. 

 Limbaugh (1955) and Quast (1968) made the major 

 contributions in southern California, but these 

 important studies represent only a beginning. 



The present study goes beyond earlier inves- 

 tigations by considering the organisms taken by 

 the fishes as prey against a broader consideration 

 of the array of similar forms present that would 

 seem to have been equally accessible. The selection 

 of specific prey, however, is only partially 

 developed in discussing these data. Selectivity will 



Manuscript accepted March 1976. 

 FISHERY BULLETIN; VOL. 74, No. 3, 1976. 



567 



