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 19 degrees C. 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.