INFLUENCE OF WATER CURRENTS AND ZOOPLANKTON DENSITIES ON 



DAILY FORAGING MOVEMENTS OF BLACKSMITH, 



CHROMIS PUNCTIPINNIS, A PLANKTIVOROUS REEF FISH 



Richard N. Bray* 



ABSTRACT 



The blacksmith, Chromis punctipinnis , one of the most abundant fishes in kelp forests off southern 

 California, daily emerges from rock shelters and moves to specific locations where it forages in the 

 midwater on zooplankton. Midwater transects taken over a reef that experiences occasional reversals 

 in water currents indicated that large blacksmith (greater than 150 mm total length) consistently 

 gathered at the incurrent end of the reef. These movements are probably related to the availability of 

 food. Replicate plankton tows taken near the ends of the reef demonstrated that zooplankton densities 

 were greater at the incurrent end. Experiments in which large fish were placed in cages suspended in 

 midwater at both reef ends demon.strated that individuals foraging at the incurrent end consumed 

 more prey. Small blacksmith (less than 125 mm total length) did not undergo foraging movements. 

 Instead, most remained in the shallower portions of the reef, close to cover, even though caging 

 experiments and collections of free-living individuals indicated that these fish would consume more 

 prey if they moved upcurrent. Since small as well as large blacksmith benefit from foraging at the 

 incurrent end, the size-specific differences in foraging movements probably reflect differences in the 

 cost of migrating in terms of time, energy, and predation. 



Many fishes on temperate and tropical reefs feed 

 heavily on zooplankton. They eat either by day or 

 by night, and school or shelter when inactive 

 (Hobson 1972, 1973, 1974; Hobson and Chess 1976). 

 Some planktivores limit their movements to the 

 water column above their shelters (e.g., Sale 1971; 

 Hobson 1972, 1973); the distribution and small- 

 scale movements of these parochial species have 

 been the subject of recent quantitative investiga- 

 tions (e.g., Stevenson 1972; Hobson and Chess 

 1978; de Boer 1978). Other planktivores undergo 

 extensive horizontal movements (Hobson 1972, 

 1973), and have received less attention. Nonethe- 

 less, migrating planktivores are often extreme- 

 ly abundant and probably import substantial 

 amounts of extrinsic energy — drift zooplankton 

 — into reef communities (Stevenson 1972). 



The blacksmith, Chromis punctipinnis , aplank- 

 tivorous pomacentrid that may grow to a total 

 length (TL) of 300 mm (Miller and Lea 1972), 

 is one of the most abundant fishes inhabiting 

 the inshore rocky reefs of southern California. 

 In aggregations of up to several hundred indi- 



' Department of Biological Sciences, University of California, 

 Santa Barbara, Calif.; present address: Department of Biology, 

 California State University Long Beach, CA 90840. 



Manuscript accepted March 1980. 

 FISHERY BULLETIN: VOL 78, NO. 4, 1981. 



viduals, they feed throughout the day on a variety 

 of zooplankton, including larvaceans, copepods, 

 cladocerans, and various larvae (Hobson and 

 Chess 1976). At dusk, they descend to the reef 

 surface where they shelter in holes and crevices 

 until dawn (Ebeling and Bray 1976; Hobson and 

 Chess 1976). Many tropical congeners of the black- 

 smith have similar activity patterns (e.g., Hobson 

 1965, 1972; Collette and Talbot 1972; Emery 1973). 

 During preliminary observations on rocky reefs 

 near Santa Barbara, Calif., I found that black- 

 smith often forage and shelter in different areas. 

 Large numbers of blacksmith emerge from shel- 

 ters at dawn, assemble into a school, and move to a 

 location above a reef, where they disperse into a 

 loose aggregation in the midwater and forage on 

 zooplankton. At least some blacksmith near Santa 

 Catalina Island, Calif., show a similar pattern 

 (Hobson and Chess 1976). In commenting on the 

 location of daytime foraging aggregations, Lim- 

 baugh (1955) observed that, "Thick schools of 

 young to half-grown blacksmith often form where 

 a plankton-rich current enters the kelp bed." Such 

 a response to water currents would enable black- 

 smith to be among the first of many vertebrate and 

 invertebrate planktivores to forage on plankton as 

 it is swept across the reef community. However, 



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