This data package contains North Coast and Cascades Network (NCCN) Inventory and Monitoring Program fish assemblages tabular data collected during 1994-2011 in Olympic National Park (OLYM). The protocol, publications, and all other associated links can be found in the project reference NCCN Monitoring Fish Assemblages: https://irma.nps.gov/DataStore/Reference/Profile/2182112 Olympic National Park is the only national park in the lower 48 states that contains significant numbers of wild Pacific salmonids, with at least 70 populations in park rivers. These rivers represent some of the largest tracts of contiguous, undisturbed habitat throughout the range of several key fish species of the Pacific Northwest. These watersheds are of high regional importance as freshwater habitat sanctuaries for native fish, where habitat conditions are characterized as having little to no disturbance from development, channelization, impervious surfaces, roads, diversions, or hydroelectric projects. Salmonids are critical to ecosystem function by linking freshwater, marine, and terrestrial ecosystems. Pacific salmonids provide food for over 130 species of aquatic and terrestrial wildlife species and they contribute significant amounts of marine-derived nutrients to freshwater systems through spawned salmon carcasses. Fishery resources are of high ecological and cultural importance in Pacific Northwest National Parks, and significantly contribute to economically important recreational, commercial, and tribal fisheries. Olympic National Park rivers contain federally threatened fish populations (e.g. bull trout, Salvelinus confluentus) and non-commercial fish species such as mountain whitefish (Prosopium williamsoni). Cutthroat trout (Oncorhynchus clarkii) and summer steelhead (Oncorhynchus mykiss) also are important species targeted by recreational anglers. Prior to the inception of this monitoring project there were no existing monitoring programs for non-commercial (e.g., Pacific salmonids) fish species in Olympic Peninsula rivers. In this project, fish communities were monitored via repeated intensive snorkel surveys within approximately five kilometer reference sites from May to December across eight park rivers and five wadeable streams. Specific monitoring objectives of this protocol are to determine seasonal and annual trends in: (1) fish species composition, (2) timing of migration of adult fish, (3) relative abundance, (4) age and size structure, and (5) extent of non-native and hatchery fish. To detect seasonal and annual trends in fish assemblages in reference sites, we relied on repeated and consistent annual sampling at each monitoring site. The general rationale for the repeated sampling of reference sites, about 10 surveys per river per year, is to ensure that we account for the high interannual variability in fish movements and abundances in rivers.