FISHERY BULLETIN: VOL. 78, NO. 3 



MORRO ROCK 



120° SO' 



Figure l.— Morro Bay, Calif. Shaded rectangle is the sam- 

 pling area. 



tions taken in three other California bay- 

 estuarine habitats with similar ichthyo faunas. 



THE STUDY AREA 



Morro Bay is characterized by expansive tidal 

 flats, central channels, and extensive eelgrass 

 beds. During spring low tides, the bay is essen- 

 tially reduced to a series of channels. Although 

 two creeks empty into the bay, salinities are rela- 

 tively uniform and approach those of the sea, mak- 

 ing the bay more of a marine lagoon than a true 

 estuary. 



The study was conducted during quarterly 

 periods (November 1974, May and August 1975, 

 February 1976) in the shallow mudflat and chan- 

 nel area of the southeastern section of Morro Bay 

 adjacent to Baywood Park (Figure 1). The sub- 

 strate of the area was characterized by a relatively 

 uniform mud-sand material, a large percentage of 

 which was covered mainly by eelgrass, Zostera 

 marina, and also a red alga, Gracilaria sp., and a 

 green alga, Ulva sp. Water depth over the study 

 area was as great as 2 m during high tide periods. 



Water temperature and salinity were recorded at 

 30 cm depth at the time each fish sample was 

 taken. Temperatures (x±l SD, n = 9) were 

 13.7°±1.4° C in February, 17.9°±0.5° C in May 

 19.6°±2.9° C in August, and 11.8°±1.9° C in 

 November. Salinity values were 30.9±0.7%o in 

 February 30.0±0.8%o in May 31.1±1.0%o in Au- 

 gust, and 31.8±1.7%o in November. Tidal ranges 

 during the 24-h sampling periods varied from 1.0 

 m (3.3 ft) in May 1975 to 2.2 m (7.3 ft) in February 

 1976. 



METHODS 



Fish sampling was performed with the use of a 

 seine 3 m deep by 29.2 m long with a 2.2 x 2.2 x 2.2 

 m bag of 6 mm mesh size. The seine was set paral- 

 lel to the beach from a 3 m skiff and hauled to 

 shore with polypropylene lines. The distance the 

 seine was set from the water's edge was 60 m 

 except at extreme low tides when there was water 

 only in the channels. At these times (the tows at 

 1500 h and 1800 h in February), successive hauls 

 covering small areas were made until the total 

 area sampled was approximately equal to that of 

 single hauls at higher tide periods. Samples were 

 taken at randomly selected intervals along a 400 

 m stretch of shore. The total sampling area was 

 approximately 2.4 ha (0.4-0.5% of the total area of 

 the bay) and each seine haul covered about 0.18 ha. 

 Based on visual surveys, this stretch of inshore 

 habitat was typical (in terms of substrate, depth, 

 and position relative to the mouth and main chan- 

 nel) of the rather uniform shallow-water condi- 

 tions in the bay. 



During each of the four sampling periods, seine 

 hauls were made at 3-h intervals over a 24-h cycle 

 for a total of nine samples per visit. For day-night 

 comparisons, the second of the two 0900-h samples 

 was not included each period so that equal num- 

 bers of day and night samples (four) were com- 

 pared. All fishes captured, or aliquots of the 

 largest catches of abundant species, were iden- 

 tified and sorted, and their standard lengths (SL) 

 and weights recorded. 



The Shannon- Wiener information function H' 

 was calculated as a measure of diversity in which 



H' = - IP, log P, 



i = l 



where P, is the proportion of individuals (or 



760 



