EBELING and BRAY: ACTIVITY OF REEF FISHES 



Table 4.— Vertical-zone variation in numbers of fishes observed along a transect line compared between day and night. Vertical zones 

 are defined in Table 1; the M measure of a species' change in vertical position between day and night is defined in the text. 



'Individuals are assumed to bury themselves at night. 



tion for the day-night contrast with samples 

 pooled (tau = 0.13), were nonsignificant (P>0.05). 

 A Wilcoxon signed-ranks test for paired (day- 

 night) observations indicated that numbers of 

 eight species did not differ significantly between 

 day and night, while numbers of four species 

 actually increased (Table 3). 



Two species commonly observed during the day 

 were either seldom or not seen at night: Phaner- 

 odon furcatus and Oxyjulis californica. Although 

 we often saw individuals of P. furcatus browsing 

 on bryozoan-encrusted algae (mainly Gelidium 

 sp.) along a crest of the reef during the day, we 

 rarely observed them at night along the crest and 

 never observed them during regular transects. We 

 commonly saw small groups of 0. californica in the 

 mid-water zone above the transect lines during the 

 day. At dusk, however, Oxyjulis individuals bury 

 themselves in rubble and sand on the reef and 



remain covered until dawn (Herald 1961; Feder et 

 al. 1974; Bray and Ebeling 1975). 



Only one species was seen at night but never 

 during the day. Hyperprosopon argenteum was the 

 second-most abundant species recorded at night, 

 although it was never seen around the transect 

 line during daylight hours. In over 6 yr of obser- 

 vations, we have seen this species in kelp beds on 

 only a few occasions during the day. Schools of H. 

 argenteum commonly occur in shallow waters 

 along sandy beaches and shallow reefs during the 

 day, so it appears that at least some of the larger 

 individuals migrate offshore to kelp beds at dusk. 

 To reach Naples Reef, fish near the surf would 

 have to swim approximately 1.6 km offshore. 



Resemblance between the day and night sam- 

 ples of S = 25 species within each of the four 

 vertical zones was measured by coefficients of 

 similarity or "overlap" (cf. Colwell and Futuyma 



707 



