FISHERY BULLETIN: VOL. 84, NO. 1 



had presumably been washed into the marsh from 

 freshwater habitats upstream. However, black crap- 

 pie and perhaps other centrarchids contributed to 

 the local fishery prior to the drought, mainly in the 

 upper ends of the larger sloughs, so a recovery can 

 be expected. 



Despite the decline in freshwater fishes during the 

 drought, there was no corresponding major increase 

 in the abundance of euryhaline marine species 

 characteristic of nearby San Francisco Bay (Herr- 

 gesell et al. 1981). Marine species (such as northern 

 anchovy, Pacific herring, and shiner perch) general- 

 ly appeared in our samples in late summer when 

 salinities were highest, in parts of the marsh closest 

 to Suisun Bay. 



Considering the annual and long-term variations 

 in fish abundances and the fact that the fish assem- 

 blage is made up of a mixture of native and intro- 

 duced species, the consistency of the assemblage 

 structure during the study is surprising. Coevolution 

 has obviously little role in an assemblage in which 

 the most abundant species (striped bass) entered in 

 1879 and other abundant species entered in the 

 1960's (yellowfin goby) and 1970's (inland silversides) 

 (Moyle 1976). The apparent consistency in structure 

 seemed to be the result of 1) two introduced species, 

 striped bass and carp, that were consistently abun- 

 dant in the marsh, 2) the group of native resident 

 fishes that was persistent in deadend sloughs, and 

 3) the native fishes that moved in and out of the 

 marsh on a seasonal basis. 



This does not mean that the structure observed 

 during this study will persist indefinitely. A number 

 of changes in the fish fauna may already be occur- 

 ring. For example, the presence of young-of-year 

 white catfish in 1983 and 1984 may signify a shift 

 of the assemblage towards catfishes and centrar- 

 chids, such as existed before the 1976-77 drought. 

 Striped bass are presently in a long-term decline in 

 abundance, a trend which seems to be continuing 

 (Kelley et al. 1982). Past history indicates that new 

 introductions of fishes into the system are likely: 

 specifically, the white bass, Morone chrysops, has 

 recently become established in part of the San Joa- 

 quin drainage and may become a major new predator 

 in the Sacramento-San Joaquin Estuary if planned 

 eradication attempts fail (California Department of 

 Fish and Game unpubl. data). Furthermore, addi- 

 tional diversions of freshwater from the estuary are 

 planned (Herrgesell et al. 1981), and major modifica- 

 tions to the marsh channels are planned or under- 

 way (Baracco 1980), so the environment, especially 

 in the dead-end sloughs, may change significantly. 

 It is difficult to predict what the combined effects 



of all these changes will be on the present fish 

 assemblage, but extinctions of both native and intro- 

 duced species in the estuary have occurred in the 

 past (Moyle 1976) and could occur again in the future 

 The structure of the fish assemblage of Suisun 

 Marsh is similar in may respects to the structure of 

 the fish assemblages of other large estuaries (e.g., 

 Markle 1976; Meeter et al. 1979), despite the impor- 

 tance of recently introduced species and the stabi- 

 lizing influence humanity has had on the pattern and 

 amount of freshwater inflow (Kahrl 1978). In most 

 such estuaries, as in the Sacramento-San Joaquin, 

 the assemblages are dominated by juvenile fishes, 

 and most species have substantial populations out- 

 side the estuary. As in Suisun Marsh, the fish assem- 

 blages of such estuaries are made up of a relatively 

 small number of the species available in nearby 

 marine and freshwater environments. Presumably, 

 the species composition of an estuarine assemblage 

 is determined in large part by the ability of the 

 species to tolerate the particular set of environmen- 

 tal conditions that exist there Since these conditions 

 may change with short-term climatological changes, 

 the fish assemblages may change as well (Meeter et 

 al. 1979; Marais 1982). Thus coevolution is given lit- 

 tle chance to operate in estuarine systems in general. 

 In this context, it is not surprising that the fish 

 assemblage of the Suisun Marsh behaves ecologically 

 in a way similar to fish assemblages in most other 

 estuarine systems. Because resource partitioning is 

 commonly observed among estuarine fishes (Sheri- 

 dan and Livingston 1979; Whitfield 1980), competi- 

 tion may be an important process in determining the 

 structure of estuarine fish assemblages (Thorman 

 1982), a hypothesis we are currently investigating 

 in the Suisun Marsh. 



ACKNOWLEDGMENTS 



This project was supported by the California 

 Department of Water Resources (DWR) and by the 

 Agricultural Experiment Station, University of 

 California (Project No. 3930-H). It would not have 

 been possible without the support and encourage- 

 ment of Randall L. Brown, Central District, DWR. 

 Numerous volunteers assisted the sampling effort, 

 but especially Larry Brown, Sonia Cook, Bart 

 Daniel, Lynn Decker, Tim Ford, Bret Harvey, Ned 

 Knight, Tim Takagi, Bruce Vondracek, Eric Wikra- 

 manayake, and Wayne Wurtsbaugh. The manuscript 

 was reviewed in various drafts by Larry Brown, Beth 

 Goldowitz, Ned Knight, and Eric Wikramanayake. 

 The manuscript was "processed" by Donna 

 Raymond. 



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