MOYLE ET AL.: NONCOEVOLVED ASSEMBLAGE OF ESTUARINE FISHES 



abundance with numbers and biomass lowest in 

 winter and spring and highest in late summer. Fishes 

 were least abundant when river outflows were 

 highest and most abundant when salinities and tem- 

 peratures were highest. 



2. There was an overall decline in fish abundance 

 and species diversity through the study period. 



3. Of the 21 species that occurred in the marsh on 

 a regular basis, 14 were residents, 4 were winter 

 seasonals, and 3 were spring/summer seasonals. 

 Another 21 species occurred sporadically, in small 

 numbers. These were mainly marine and freshwater 

 species that presumably could become established 

 in the marsh if environmental conditions changed 

 significantly. 



4. The abundant resident species fell into two 

 groups, one made up of native species that concen- 

 trated in the small dead-end sloughs and the other 

 a mixture of introduced and native species that were 

 widely distributed in the marsh, but most abundant 

 in the larger sloughs. 



5. The structure of the fish assemblage (i.e., the 

 pattern of distribution and abundance) was fairly 

 consistent over the 54-mo period. 



The seasonal pattern of fish abundance was due 

 to a number of factors, most importantly 1) varia- 

 tion in sampling efficiency, 2) influxes of young-of- 

 year fish, 3) favorable environmental conditions for 

 most fish species in late summer, and 4) abundance 

 of Neomysis mercedis. When outflows were high, 

 water levels in the marsh were high and showed lit- 

 tle tidal fluctuation. Therefore trawling was less ef- 

 ficient because there was more water and more 

 flooded vegetation available as cover for fish. How- 

 ever, even under these conditions most of the sam- 

 pling areas were rarely more than 2 m deep, so our 

 trawl covered at least half the water column, and 

 large catches were common, especially early in the 

 study. Therefore, variation in sampling efficiency 

 may have exaggerated the peaks and valleys of the 

 catch curves (Figs. 4, 5) but was unlikely to obscure 

 the general trends in abundance Probably the most 

 important contributor to the seasonal patterns was 

 the increase in young-of-year striped bass, splittail, 

 prickly sculpin, and tule perch, in June through 

 August. These species (and others, to a lesser extent) 

 became vulnerable to our trawl at 30-40 mm SL, and 

 catches of several hundred individuals in a 5-min tow 

 were made on occasion. The rapid growth of these 

 species during summer (Daniels and Moyle 1983; 

 Herbold and Moyle, unpubl. data) indicated that en- 

 vironmental conditions, including warm tempera- 

 tures and moderate salinities, were favorable for 



them and for other euryhaline species (ag., staghorn 

 sculpin, starry flounder). These same conditions also 

 favored N. mercedis, a small shrimp that is an im- 

 portant food item in summer diets of most of the 

 fishes (Herbold fn. 4). It is possible that the summer 

 peak in fish abundance may be due also in part to 

 fishes moving in to take advantage of an abundant 

 food resource The decline in N. mercedis abundance 

 in late summer may be related in part to fish preda- 

 tion, although it is presumably related mainly to 

 their seasonal movements within the entire estuary 

 (Orsi and Knutson 1979). 



The overall decline in fish abundance over the 

 study period seemed to be due to two factors: varia- 

 tion in reproductive success of major species and the 

 fact that 1982 and 1983 were years of unusually high 

 precipitation and runoff, so freshwater conditions 

 prevailed throughout the summer months of both 

 years. Splittail showed an unusually strong year class 

 in 1978, which dominated the 1979, and, to a lesser 

 extent, 1980 samples (Daniels and Moyle 1983). 

 Catches of splittail in 1979 were typically 2-5 times 

 greater than in subsequent years. Striped bass, tule 

 perch, and carp also showed peaks of abundance in 

 1979 and had low abundances in 1982-83, with one 

 or two peaks of abundance in between. Except for 

 carp, the peaks were largely due to influxes of young- 

 of-year fish. The reason for the abundance of the 

 1978 year class of fish was presumably related to 

 1978 being a year of high, but not excessive, outflows. 

 Increased reproductive success during high outflow 

 years has been documented for striped bass (Stevens 

 1977), splittail (Daniels and Moyle 1983), American 

 shad, chinook salmon, and longfin smelt (Stevens and 

 Miller 1983). However, under extreme outflow con- 

 ditions (such as existed in 1982 and 1983), young- 

 of-year fish are apparently carried downstream to 

 areas below the marsh (San Francisco and San Pablo 

 Bay) where chances of survival may be less (Stevens 

 1977). 



Drought also contributed to the variation in the 

 fish fauna. During 1976 and 1977, severe drought 

 reduced freshwater inflows to the marsh, resulting 

 in sustained high salinities. Freshwater fishes de- 

 clined dramatically during the drought period (Herr- 

 gesell et al. 1981) and the fishery for catfish (main- 

 ly white catfish and black bullhead) was greatly 

 reduced (Baracco 1980). The catfish populations did 

 not recover during the study period, but the regular 

 appearance of young-of-year white catfish in our 

 trawls in late 1983 indicated a recovery may be in 

 progress. Other freshwater fishes found in the marsh 

 (Table 1) showed no signs of increasing. Most were 

 represented in our samples by <10 individuals that 



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