Movement and Mortality of Adult Brown Trout in the Motupiko River, New Zealand: Effects of Water Temperature, Flow, and Flooding

—Management of the effects of water quality and flow on fisheries requires an understanding of the factors that control fish movements. We used radiotelemetry to monitor the movements of adult brown trout Salmo trutta in a New Zealand river over 11 months (September 2004 to August 2006) and linked those movements to the changes in flow and water temperature. Individual fish moved up to 41 km during the study. However, most fish moved less than 1 km. All of the trout that showed little movement throughout the summer were living in relatively deep pools that presumably provided cover. The rates of movement declined steadily over the spring–summer period, as flow decreased and water temperature increased. The percentage of fish moving was positively related to the average daily flow during the interval between tracking occasions and negatively related to the average daily water temperature, less than 20% of the tagged fish moving once temperatures were above 198C. A severe, 50-year flood occurred in March 2005 and was associated with the mortality of 60–70% of the remaining tagged fish, confirming that flood-induced mortality can affect a substantial proportion of an adult brown trout population. Research on river-resident salmonids over the last 15 years has indicated that widespread movement is much more common than previously thought (Gowan et al. 1994; Young 1996; Young et al. 1997; Diana et al. 2004; Mellina et al. 2005; Heggenes et al. 2007). Movement can be within an extended home range of several hundred meters (Rodriguez 2002; Diana et al. 2004; Mellina et al. 2005; Roghair 2005) up to largerscale movements of many kilometers (Clapp et al. 1990; Bettinger and Bettoli 2004; Gresswell and Hendricks 2007; Heggenes et al. 2007). Large-scale movements can be important for providing valuable permanent or seasonal fisheries in particular reaches of rivers (Meyers et al. 1992; Meka et al. 2003) and also help explain patterns in size distribution of fish throughout catchments (Hughes 1999). One of the main reasons for large-scale trout movement is to seek spawning habitat (Northcote 1992). However, the importance of spawning migrations in river trout populations probably depends on the location of good spawning gravels relative to the position of good adult trout habitat (Northcote 1992). Trout may also move to seek better food resources and feeding habitat or thermal conditions so they can maximize their energy intake and growth potential (Gowan and Fausch 2002; Hughes 1998, 1999; Hughes et al. 2003). Water temperature is a key variable controlling trout growth and survival. If it is too cold, trout will stop growing; however, high temperatures will also restrict growth and increase mortality (Elliott 1994). Trout also seek refuge from adverse environmental conditions in winter (Cunjak 1996) and summer (Nielsen et al. 1994; Ebersole et al. 2003). Floods may also initiate longitudinal or lateral movement in fish, either to take advantage of easier passage during high flow conditions (Dedual and Jowett 1999; Natsumeda 2003) or in an effort to avoid the impact of floods (Jowett and Richardson 1994; Schwartz and Herricks