DIEL AND SEASONAL VARIATION IN ABUNDANCE AND DIVERSITY OF 

 SHALLOW- WATER FISH POPULATIONS IN MORRO BAY, CALIFORNIA 



Michael H. Horn' 



ABSTRACT 



More than 11,000 fishes weighing over 197 kg and representing 21 species were caught in bag seine 

 hauls taken at quarterly periods (November 1974. May and August 1975, February 1976) in the 

 southeastern section of Morro Bay. During each sampling period, nine seine hauls were completed, one 

 at each 3-hour interval over a 24-hour cycle. Atherinops affinis, Cymatogaster aggregata , andLeptocot- 

 tus armatus accounted for 82''"f of the individuals collected, and A. affinis, C. aggregata, and Mustelus 

 californicus constituted 84*7^ of the biomass obtained. Larger numbers of individuals and greater 

 biomass were collected in night hauls, but nearly equal numbers of species were captured during the 

 day and night. The largest number of species and individuals and greatest biomass were obtained in 

 May, a period of high reproductive activity, whereas the smallest values of these three parameters were 

 recorded in August. Diversity (H') for numbers peaked in May (1.56) but reached a maximum for 

 biomass in November ( 1.91). Lowest diversity for both numbers (0.86) and biomass (0.79) was recorded 

 in February. Total diversity was 1.63 for numbers and 1.59 for biomass. Wide ranging similarity values 

 (PS) between consecutive sampling periods for numbers (24-64%) and biomass (2 1-76%) demonstrated 

 the marked seasonality of the shallow-water fish populations of the bay and primarily reflected the 

 fluctuations in numbers or biomass of the four most abundant species (above). 



The pattern of total diversity and seasonal similarity for Morro Bay fishes was consistent with a 

 recent model that utilizes diversity and similarity indices together as measures of environmental 

 quality. Analysis of data from three other localities indicated that the model has the potential for 

 application in a variety of temperate bay-estuarine habitats. 



Morro Bay (Figure 1), an estuary located on the 

 central California coast (lat. 35°20' N), is one of 

 the largest and least altered coastal wetlands in 

 California and a critically important aquatic 

 habitat. It supports abundant invertebrate 

 populations and is an integral part of the Pacific 

 flyway for migratory, water-associated birds 

 (Gerdes et al. 1974). The bay is the site of rookeries 

 for two species of herons, and the two endangered 

 bird species, California least tern and peregrine 

 falcon, utilize the resources of the bay. Steelhead 

 occur in the tributary streams and a sizeable sport 

 fishery exists in the bay. Although more than 60 

 species of fishes are known to occur in Morro Bay 

 (Fierstine et al. 1973), little is known of the 

 dynamics and organization of the fish com- 

 munities. This lack of information provided the 

 impetus for the present study. 



The main purpose of the study was to assess in 

 terms of abundance, diversity, and species compo- 

 sition, the diel (24 h) and seasonal variation of the 

 fish community occurring in the shallow waters of 



'Department of Biology, California State University, Ful- 

 lerton, CA 92634. 



the bay. In addition, the investigation was de- 

 signed to provide a preliminary test in Morro Bay 

 of the relationship proposed by Haedrich (1975) 

 that indices of diversity (measuring species 

 richness and equitability ) and similarity (measur- 

 ing seasonal composition and succession) as com- 

 munity parameters can be used together as indi- 

 cators of environmental quality of temperate bays 

 and estuaries. Based on trawl collections of fishes 

 in nine Massachusetts estuaries and embayments, 

 Haedrich (1975) showed that in habitats of low 

 annual (or total) diversity little seasonal change is 

 reflected in high similarity from season to season 

 whereas in locations of high annual diversity 

 lower similarity indicates a greater degree of sea- 

 sonal change. Low diversities characterized areas 

 of high pollution and higher diversities those of 

 lesser pollution. Because of the reportedly low 

 levels of environmental stress (including human- 

 induced pollution) in Morro Bay (Gerdes et al. 

 1974), the expected outcome of the present study 

 was that total diversity would be relatively high 

 and similarity between seasons relatively low or 

 show a wide range of values. Comparisons of total 

 diversity and seasonal similarity were made be- 

 tween Morro Bay samples and bag seine collec- 



Manuscript accepted December 1979. 

 FISHERY BULLETIN: VOL. 78, NO. 3, 1980. 



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