Structure, sampling gear and environmental associations, and historical changes in the fish assemblage of the Southern Sacramento-San Joaquin Delta

We sampled fishes at 11 fixed sites monthly from January 1993 through December 1994 in the southern Sacramento-San Joaquin Delta. Using three different sampling gears (boat electrofishing, gillnets, and hoopnets), we obtained 988 samples and collected 27,791 fishes representing 33 species. Overall, the catch was heavily dominated by alien species which represented 99% of the total number of fishes we collected. We used partial canonical correspondence analysis (pCCA) to examine the effect of gear type and environmental variables on fish assemblage structure. Gear type strongly influenced the observed assemblage structure accounting for 59% of the variation explained by pCCA. Ictalurids dominated the hoopnet catches. Ictalurids, striped bass, Morone saxatilis, and splittail, Pogronichthys macrolepidotus, dominated the gillnet catches, while centrarchids dominated the electrofishing catches. Electrofishing collected approximately 50% more species than the other two gear types, suggesting that it may be the most favorable of the three to assess presence-absence, although it apparently did not sample large mobile fishes very well. After accounting for the effect of gear type, flow and water temperature had the strongest influence on assemblage structure. The south Delta fish assemblage has changed greatly since it was first described 30 years prior to our study. Two native species have apparently been extirpated and at least eight alien species have established reproducing populations. Our results (1) suggest that, depending upon the goals of the study, the use of a single sampling gear may provide a biased assessment of the south Delta fish assemblage and (2) corroborate the hypothesis that highly altered habitats are vulnerable to the invasion and establishment of alien species.