Migration of a fast‐growing population of brown trout (Salmo trutta l.) through a fish ladder in relation to water flow and water temperature

The ascent of brown trout spawners ( Salmo trutta L.) in the fish ladder at the Hunder dam on the River Gudbrandsdalslågen, eastern Norway was correlated with water flow and water temperature from 1983 to 1990. Ascent took place from the end of June to the middle of October. The majority of the trout entered the fishway during August and the first half of September. Before arriving at the fish ladder, ascending trout must pass through two sections of river with differing flow regimes and optimum flows for migration. Conditions in the section below the dam are critical for ascent. When discharge over the dam exceeded about 180m 3 s −1 , it was difficult for the fish to locate the entrance to the fishway. High discharges often occurred in June and July, delaying or preventing ascent. At flows below 20m 3 s −1 it was difficult for fish larger than 80 cm to approach the ladder entrance. When flow decreased to below 10 m 3 s −1 , ascent was difficult for all sizes. Water temperature was of less importance. Lengths of brown trout ascending the ladder were most often between 55 and 75 cm. Large fish arrived earlier than small fish, females before males, and naturally recruited fish before stocked fish.