HORN: ABUNDANCE AND DIVERSITY OF MORRO BAY FISHES 



sampling area, an indication of the marine charac- 

 ter of the bay. Tidal ranges were smallest in May, 

 the period of highest fish abundance, and largest 

 in February, the month of lowest diversity. Tem- 

 perature, the environmental factor most fre- 

 quently recognized as having a major influence on 

 temperate, shallow-water fish populations (e.g., 

 Allen and Horn 1975; Subrahmanyam and Drake 

 1975; Wallace 1975; Hoff and Ibara 1977), did not 

 consistently correspond to changes in abundance 

 and diversity of Morro Bay fish populations: The 

 largest increase in mean temperature (4° C) oc- 

 curred between February and May, the transition 

 period marked by the greatest increase in abun- 

 dance and diversity. In contrast, the greatest de- 

 cline in temperature between sampling periods ( 8° 

 C from August to November) was accompanied by 

 a substantial increase in abundance and diversity. 



The life history patterns of the three most abun- 

 dant species, A. affinis, C. aggregata, and L. ar- 

 matus, serve not only to help clarify the seemingly 

 conflicting responses to temperature of the fish 

 populations but to illustrate the strategies of 

 utilization of a bay-estuarine environment by in- 

 shore fishes. These patterns, recognized in previ- 

 ous studies, are discussed in turn below for each of 

 the three species and related to the data I recorded 

 in Morro Bay. 



In his study of A. affinis in Newport Bay in 

 southern California, Fronk (1969) recognized 3 

 age classes based on length frequencies ( 80-90 mm 

 fork length in the first year; 120-130 mm by the 

 second year; 150 mm after the third year) and 

 found that spawning occurred from February to 

 August (peak in May) when the fish were in their 

 second and third years of life. These findings 

 correspond to the seasonal length frequencies and 

 abundance I recorded for A. affinis in Morrow Bay. 

 In February, the bimodal size distribution in- 

 cluded small numbers of both immature and 

 larger individuals, some of which had mature 

 gonads. May was marked by a high abundance of 

 large fish, many of which released eggs and milt 

 upon capture. The substrate of the sampling area 

 apparently is an optimal spawning site since it is 

 known (Frey 1971) that A. affinis attaches its eggs 

 to eelgrass and low-growing algae such as Grnci- 

 laria sp. The egg masses were frequently found on 

 the vegetation that was obtained in the seine 

 hauls. By August, the number of small juveniles 

 had increased but adult numbers had decreased. 

 The overwhelming domination of the November 

 catch by first (mainly) and second year fish is con- 



sistent with the apparent movement of postrepro- 

 ductive adults out of the shallow spawning areas 

 that become nursery grounds for the juvenile fish. 



In his study of C. aggregata in Anaheim Bay in 

 southern California, Odenweller ( 1975) identified 

 three age classes based on otolith rings and length 

 frequencies. Fish in their first year ranged be- 

 tween 3 1 and 87 mm SL ( x 57 mm), in their second 

 year between 68 and 115 mm ix 88 mm) and in 

 their third year between 81 and 117 mm (x 101 

 mm). Cymatogaster aggregata gives birth in the 

 spring, primarily in May, according to Bane and 

 Robinson (1970) and Odenweller (1975). Both 

 Bane and Robinson ( 1970) and Allen ( 1976) found 

 that in Newport Bay (southern California) the 

 majority of adults migrate out of the bay after 

 breeding in the spring leaving juveniles to utilize 

 the area as a nursery ground. These adults appar- 

 ently return to the bay to bear young the following 

 spring. The seasonal abundance and size frequen- 

 cies of C. aggregata in Morro Bay are in accord 

 with the patterns found in the southern California 

 bays and estuaries. The absence of the fish in Feb- 

 ruary, its abundance in a wide size range in May 

 and the presence of almost only small juveniles in 

 August are indicative of the existence of the mi- 

 gratory breeding pattern in Morro Bay. The 

 November catch, consisting of a small number of 

 juveniles slightly larger than the August indi- 

 viduals, is further support for the existence of the 

 pattern. Most of the young-of-the-year, which 

 mature at or soon after birth (Bane and Robinson 

 1970), apparently moved out of the shallows after 

 mating in the late summer or early fall. 



According to studies carried out by Jones ( 1962) 

 in Tomales Bay (near San Francisco) and San 

 Francisco Bay and Tasto (1975) in Anaheim Bay 

 (near Los Angeles), L. armatus is a winter 

 spawner with the peak in January and February. 

 Sexual maturity is reached near the end of the first 

 year of life at approximately 120-150 mm SL for 

 females and 110-120 mm SL for males. Tasto 

 (1975) found that the Anaheim Bay population 

 consisted almost entirely of juvenile fish and that 

 postspawning mortality was apparently high, 

 based on the absence of the older fish in the 

 population, a sharp reduction in the catch per unit 

 effort of adults during the breeding season and the 

 capture of only two spent females. The data I ob- 

 tained on L. armatus in Morro Bay are generally 

 consistent with those of the two studies cited. Fol- 

 lowing the February sample, which was composed 

 almost entirely of small juveniles, the frequency of 



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