FISHERY BULLETIN: VOL. 78, NO. 3 



creased. This pattern continued through the Au- 

 gust and November sampling periods and was in 

 sharp contrast to that recorded for either A. affinis 

 or C. aggregata. In August, a small number of 

 relatively large L. armatus were collected. The 

 corresponding biomass accounted for more than 

 l&7c of the total sample. In November, a small 

 number of larger individuals were captured. The 

 corresponding biomass made up >16% of the total 

 sample. 



Although only 29 individuals of a fourth species, 

 M. californicus , were captured during the study, 

 the fish ranked second in biomass and accounted 

 for almost 28% of the weight of the total collection 

 (Table 1). The largest number of M. californicus 

 were caught in February when they were concen- 

 trated in the channels as a result of the spring low 

 tide. The corresponding biomass accounted for 

 >79% of the total February sample (Figure 2). 

 Mean size was 834 mm total length and mean 

 weight was 2,077 g. Too few specimens were col- 

 lected to compare the abundance and mean size of 

 individuals in day and night samples. 



DISCUSSION 



The results of this study indicate that the 

 shallow-water fish populations of Morro Bay un- 

 dergo both diel and seasonal variations in abun- 

 dance (numbers and biomass), diversity, and 

 species composition. A relatively small number of 

 species (three) accounted for a large proportion 

 (82% ) of the total number of individuals collected. 

 These findings are consistent with the results of 

 several other studies of temperate bay-estuarine 

 fish populations that have been reviewed by Allen 

 and Horn (1975). A pattern that emerged from 

 these studies was that at least 75% of the sampled 

 fishes belonged to five or fewer species even though 

 many more species were collected. 



In terms of overall diel variation, more indi- 

 viduals and greater biomass were obtained in 

 night samples but nearly equal numbers of species 

 were collected during the day and night. Very few 

 species, usually the rarer forms, were captured 

 either only during the day or only at night. 



Although surprisingly little is known concern- 

 ing day-night differences in utilization of various 

 habitats by fishes (McCleave and Fried 1975), most 

 of the information that is available on trawl or 

 seine samples in inshore waters indicate that 

 greater catches, either of species, individuals, or 

 biomass, are obtained at night (e.g., Hoese et al. 



1968; Allen 1976; Livingston 1976). McCleave and 

 Fried (1975) collected fewer total individuals at 

 night and equal numbers of species day and night 

 with a beach seine in a Maine tidal cove; however, 

 they found that four numerically important 

 species were either present only at night or more 

 abundant at night. 



Diurnal-nocturnal activity patterns and day- 

 time gear avoidance, particularly by larger fish, 

 are two factors among a complexity of cir- 

 cumstances that produce day-night differences in 

 abundance and composition of net-caught fishes. 

 Little is known about the first factor for bay- 

 estuarine fishes although McCleave and Fried 

 (1975) reviewed the diel patterns of a few inshore 

 species. They and Hoese et al. (1968) both consid- 

 ered the second factor to be of importance in their 

 respective studies. In my study, gear avoidance 

 probably was one of the factors causing the 

 generally smaller (numerically) daytime catches. 

 However, size differences of day vs. night captured 

 individuals of the three most abundant species 

 were insignificant in almost all cases thus casting 

 doubt on the assumption that the larger fish avoid 

 the seine in the daytime. This reasoning is 

 perhaps most relevant for L. armatus, the only 

 nonschooling member of the three-species group. 



Quarterly fluctuations in biomass (totals, diver- 

 sity//', and PS values) were of greater magnitude 

 than those for numbers, but both parameters ex- 

 pressed the seasonal dynamics offish populations 

 in the shallow waters of the bay. In February low 

 numbers and biomass diversity but relatively high 

 total biomass represented an early influx of pre- 

 reproductive adults. The peak numbers and 

 biomass reached in May corresponded to an abun- 

 dance of A. affinis and C. aggregata of mature size 

 (see species accounts below) as well as the pres- 

 ence of several other species in wide size ranges. 

 Reduced numbers and biomass in August but high 

 PS values for both numbers and biomass between 

 May and August indicated that young-of-the-year 

 fishes remained in the shallow waters while larger 

 individuals migrated out of the sampling area. 

 The large number of individuals and high biomass 

 diversity recorded for November was the result of a 

 relatively even distribution of biomass among 

 juvenile fishes which continued to utilize the in- 

 shore areas late in the year. 



Seasonal abundance and diversity were only 

 partly attributable to variations in physical fac- 

 tors. Salinity was not an important factor because 

 values were relatively high and varied little in the 



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