AQUATIC RESOURCE PROGRAM REPORT Executive Summary

The fishes and invertebrates of the Cosumnes River basin and its floodplain were intensively studied from 1999 through 2002 to determine their basic distribution, abundance, and ecology. The purpose of the studies, done in conjunction with studies on fluvial geomorphology, water chemistry, and other aspects of the physical and chemical environment, was to develop the information needed for management to favor native aquatic organisms. The Cosumnes watershed was chosen for this study because (1) it is the largest river left in Central California without a dam across its mainstem, (2) it has a newly restored floodplain that was amenable to study on the basis of access and scale, (3) it was known to support a small run of chinook salmon, and (4) it could be compared with a neighboring watershed, the Mokelumne River, which is highly regulated. Faunal inventory, lower basin. Surveys of the fauna of the tidal portion of the river (which includes the floodplain) and associated sloughs, revealed a diversity of fishes and aquatic invertebrates. The invertebrates, with the exception of crayfish, the Asian clam, and a few other rare forms, were largely native, diverse, and abundant. The fishes, on the other hand were a diverse collection of largely alien species. Resident natives, with the exception of Sacramento sucker, were uncommon and the fish fauna was largely dominated by alien fishes such as largemouth bass, sunfishes, common carp, and golden shiner. Fish studies, upper basin. The fishes of the upper watershed were studied over a three year period to answer the following questions:1. Was the native fish fauna still present? 2. Why were alien fishes so abundant in a river system with a 'natural' flow