BRAY: INFLUENCE OF WATER CURRENTS AND ZOOPLANKTON DENSITIES 



tures of time and energy, and the threat of 

 increased predation (e.g., Pyke et al. 1977). The 

 differences in foraging movements between juve- 

 nile and adult blacksmith may indicate that 

 although juveniles apparently benefit from for- 

 aging at the incurrent end, the cost of migrating to 

 and maintaining station at the incurrent end 

 might outweigh the benefit of greater food intake 

 there. Smaller fish have lower cruising speeds and 

 expend relatively more energy in swimming a 

 given distance (Bainbridge 1958, 1960), so juve- 

 niles sheltering at the excurrent end may find it 

 too costly to swim across the reef. Juveniles 

 already at the incurrent end may remain near the 

 bottom, because they find it too costly to maintain 

 station in strong roidwater currents. Hobson and 

 Chess (1976) observed that in strong currents, 

 blacksmith abandon open places for the lee of kelp 

 plants. Similarly, when currents are strong over 

 tropical reefs, diurnal planktivores approach the 

 bottom (Hobson and Chess 1978). 



Predation pressures may limit the movements 

 of juveniles, which are vulnerable to many more 

 predators than are the adults. Covich (1976:242) 

 presented a simple graphic model that showed 

 how predation can influence distances traveled by 

 foragers if the threat of predation increased far- 

 ther away from shelter; he stated, "Often the risk 

 of predation to the forager and the distribution of 

 resources are the major interacting variables that 

 regulate consumer movement." In the tropics, 

 juvenile fishes remain closer to reefs than adults, 

 and at dusk when predation is most intense, 

 smaller individuals seek shelter first (Hobson 

 1972, 1979). Many coral reef fish seek nearby 

 shelter when predators approach (e.g., Hartline 

 et al. 1972), and relocation experiments indicate 

 that damselfishes released away from shelter 

 are quickly eaten (Mariscal 1970; Nolan 1975). 

 Similarly, the threat of predation might dis- 

 courage juvenile blacksmith from aggregating in 

 midwater at the incurrent end of Naples Reef. 

 Paralabrax clathratus ranked second in abun- 

 dance in my midwater surveys (Table 1), with 

 larger individuals tending toward the incurrent 

 end. Although these predators may exceed 700 

 mm TL (Miller and Lea 1972), they probably 

 would have difficulty consuming large black- 

 smith. However, I have observed kelp bass >400 

 mm TL attacking juveniles, and gut analyses 

 indicate they feed on a variety of small fishes, 

 including juvenile blacksmith (Quast 1968d; Love 

 and Ebeling 1978). Additional predators include 



other residential as well as open-water fishes, and 

 marine birds and mammals. 



Zooplankton Distribution Patterns 

 Residents Versus Nonresidents 



Comparing differences in plankton densities 

 across a reef has often been used to estimate the 

 importance of plankton to the energetics of reef 

 communities (e.g., Johannes and Gerber 1974). 

 However, others have shown that inshore reefs 

 also contain resident zooplankters with different 

 habitat preferences. Many of these "demersal 

 plankters" form a nocturnal component that either 

 hides in the reef during the day and emerges at 

 night (e.g., Alldredge and King 1977) or resides in 

 deeper water during the day and moves into 

 shallow areas at night (Hobson and Chess 1978, 

 1979). Thus, as several authors have pointed out 

 (e.g., Alldredge and King 1977), differences in 

 plankton densities across a reef might reflect the 

 habitat preferences or patchiness of resident zoo- 

 plankton, rather than the consumption of extrinsic 

 zooplankton by fish or other reef residents. 



It is doubtful that the incurrent-excurrent dif- 

 ferences in plankton densities at Naples Reef 

 resulted from sampling resident, demersal zoo- 

 plankton. All samples were taken around midday 

 in the water column away from reef or kelp 

 substrata. At this time, most demersal forms 

 hide in or near shelter or in deeper water (All- 

 dredge and King 1977; Hobson and Chess 1976, 

 1978, 1979). Furthermore, typical reef residents — 

 mysids, cumaceans, polychaetes, and decapods — 

 were insignificant components in the plankton 

 collections, while the groups that were consis- 

 tently less abundant at the excurrent end — 

 cladocerans and larvaceans — have not been re- 

 ported as residential forms. However, it would be 

 risky to conclude that the observed decline in 

 zooplankton density across Naples Reef was en- 

 tirely a consequence of predation by fishes and 

 invertebrates of the kelp-bed community. I did not 

 follow a specific parcel of water as it drifted across 

 the reef; in fact, I sampled the excurrent end of 

 the reef first in five of eight collections. Thus, 

 at least some of the differences in plankton den- 

 sities between the two ends may have been due to 

 my sampling different patches of plankton. This 

 would explain, e.g., the greater numbers of small 

 copepods at the excurrent end in one collection 

 (Table 4). 



839 



