FISHERY BULLETIN: VOL. 81, NO. 3 



hydration rates (DeMartini and Fountain 198 1) indi- 

 cate that most male S. politus migrate offshore to 

 spawn each night while individual females on average 

 move offshore only once every week. 



Engraulis mordax spawns exclusively at night (Hun- 

 ter and Macewicz 1980) during its peak (January- 

 April) spawning period. Spawning activity may be 

 partially responsible for nocturnal dispersal in this 

 species during this period of year. However, since 

 most E. mordax in our catches were juveniles, its 

 offshore dispersal at night is probably unrelated to 

 spawning. 



We believe that at least part of the general nocturnal 

 dispersal pattern may be explained by the reproduc- 

 tive behavior of Seriphus politus. However, not 

 enough is known about the reproductive habits of the 

 other abundant fishes of this assemblage to assess 

 the overall importance of spawning behavior to noc- 

 turnal dispersal. 



4. Schools disperse at night for individuals to feed 

 on nocturnally active prey. Hobson (1968) stated 

 that some authors have greatly underestimated the 

 extent to which vision can be used at night by pred- 

 atory fishes. Many species of California nearshore 

 fishes possess scotopic visual pigments which have 

 spectral sensitivities best suited for twilight and 

 night vision (Hobson et al. 1981). Five species that 

 were important in our study (Hyperprosopon argen- 

 teum; spotted scorpionfish, Scorpaeno guttata; 

 Seriphus politus; Xenistius californiensis; and Um- 

 brina roncador) were included in Hobson et al.'s 

 (1981) list of fishes that forage at night. Hobson and 

 his colleagues were able to characterize the feeding 

 behavior of these nocturnal species through exten- 

 sive field observations. Midwater planktivores orient- 

 ed in a tail-down attitude in the water column at 

 night. This presumably allowed them to feed on 

 organisms overhead which were silhouetted against 

 back-lighted surface waters. 



Our comparison of day versus night gut fullness has 

 assumed that most planktonic prey are evacuated 

 from foreguts in <12 h at 14°-24°C and that any 

 remaining contents would be in a highly digested 

 state and, therefore, weigh less during nonfeeding 

 periods. These assumptions seem reasonable in light 

 of a recent determination of gastric evacuation rates 

 in Engraulis mordax. At 15°C, foregut excavation 

 rates were <30 min for small E. mordax larvae and 

 about 2 h for the egg yolks and embryos of is. mordax 



(Hunter and Kimbrell 1980). Our interpretations of 

 day-night CI's are based on the further reasonable 

 assumption that gut evacuation rates are not serious- 

 ly confounded by different digestibilities of plank- 

 tonic prey eaten during the day versus at night. 



Engraulis mordax has been described as a diurnal 

 planktivore by Loukashkin (1970). However, a great 

 deal of indirect evidence including 1) the predicted 

 inadequacy of diurnal ration (Leong and O'Connell 

 1969); 2) eye and retinal morphology (O'Connell 

 1972); 3) size selective biting and filtering behavior 

 (Leong and O'Connell 1969; Koslow 1981); and 4) 

 the ability to capture and consume large copepods 

 and euphausids (Loukashkin 1970, cited in O'Con- 

 nell 1972) suggest that E. mordax feeds at night as 

 well as during the day. The results of our gut fullness 

 analysis lend support to the hypothesis of nocturnal 

 feeding in E. mordax. Day-collected fishes did not 

 contain greater amounts of food in the foregut than 

 night specimens, which would be expected if E. mor- 

 dax was strictly a diurnal feeder. Thus, the observed 

 nocturnal dispersal of this species is likely due in 

 large part to feeding behavior. It is also possible that 

 predation pressure interacts with feeding to in- 

 fluence the diel behavior of E. mordax. 



Hobson and Chess (1976) determined that 

 Seriphus politus was primarily a nocturnal feeder. 

 Schools of S. politus migrated offshore at night from 

 shallow water where they had formed resting schools 

 during the day; and specimens collected at night in 

 open water contained large, nocturnally active 

 zooplankters (Hobson and Chess 1976). Our analysis 

 of gut fullness corroborates these findings. Night- 

 captured specimens of S. politus contained a greater 

 amount of (primarily mysid) prey than those of day- 

 captured specimens. Hence we conclude that dis- 

 persal at night facilitates feeding in S. politus. 

 Differential offshore dispersal of juvenile, female, 

 and male Seriphus politus at night is undoubtedly 

 related to the aforementioned breeding as well as 

 feeding behavior. 



Genyonemus lineatus probably feeds day and night 

 as evidenced by gut fullness during both diel periods. 

 A trend toward more food in the foregut was evident 

 in night-collected specimens, but the difference was 

 not statistically significant. Gut contents of G. 

 lineatus collected at dawn and dusk from Long Beach 

 Harbor have also suggested a greater amount of food 

 in dawn-captured specimens, but, as with our study, 

 the difference was not statistically significant at the 

 0.05 level (Richard N. Bray 7 ). Genyonemus lineatus 



depth distributions of immature versus adult queenfish {Seriphus 

 putitus). Manuscr. in prep. Marine Science Institute, University of 

 California, Santa Barbara, CA 93106. 



584 



'Richard N. Bray, California State University, Long Beach, CA 

 90840, pers. commun. April 1982. 





