Movement is amongst the most important of animal behaviors, because it allows animals to respond to conditions within their environment to increase growth, survival and reproductive success (Kahler et al. 2001). The scale and pattern of fish movement varies widely among species. Some of the longest and most directed movements are completed by anadromous[1] salmonids in North America rivers or the piracema in South America as they migrate from headwater streams as young fish to the ocean and then migrate back to their natal streams as adults to spawn. Most species of stream fish do not migrate to the ocean, but instead appear to establish home ranges of various sizes and occupy those ranges for various amounts of time. Species such as smallmouth bass (Micropterus dolomieu) or green sunfish (Lepomis cyanellus) spend a majority of their time within a home pool, but do make trips to up and downstream areas (Todd and Rabeni 1989, Smithson and Johnston 1999).
The interpretation and importance of movement for these less mobile species has changed overtime. Early researchers concluded that most adult stream fishes restricted their movements to a small home range (Gerking 1959), failing to focus much attention on evidence of movement away from the home pool, with the exception of spawning runs. Researches are re-examining the premise of what is now termed the restricted movement paradigm (Gowan et al.1994), and are discovering that this paradigm is insufficient to fully define movement by stream fishes (Smithson and Johnston 1999).
Some species of resident stream fish, such as cutthroat trout (Oncorhynchus, clarki) were thought to spend their lives in 20 meter to 50 meter stream reaches (Miller 1957). However, recent studies have shown that most trout have relatively large home ranges (Young 1994 in Fausch and Young 1995) and at times move long distances (Hilderbrand and Kershner 2000). Bjornn and Mallett (1994) studied rainbow trout (Oncorhynchus mykiss), bull trout (Salvelinus confluentus) and cutthroat trout in Salmon River, Idaho and found that over several months many fish moved greater than 8 kilometers and some had moved greater than 40 kilometers. Hilderbrand and Kershner (2000) found that similar results for other species of trout indicate that large scale movement is a widespread pattern among resident stream salmonids.