Brown Trout Spawning Migration in Fragmented Central European Headwaters: Effect of Isolation by Artificial Obstacles and the Moon Phase

Abstract

AbstractThe spawning migrations of 123 brown trout Salmo trutta were studied in six highland streams in the Elbe River catchment area, Czech Republic, in central Europe. Trout were observed by using radiotelemetry from August to November in headwater stretches isolated by artificial obstacles without fish ladders. The length of isolated headwater stretches ranged between 5.7 and 16.1 km. Migration distance per day and total migration length over the study period (total migration) were analyzed. In total, 1,957 individual fish positions were recorded. In general, the brown trout spawning migration reflected the seasonality with respect to temperature. Migration distance per day was low in August, reached a maximum in October, and then decreased in November. In isolated headwaters, trout adapted their migrations to the length of available free migration stretches, as both migration descriptors (migration distance per day and total migration) increased according to this variable. Moon phase appeared to be the key factor that influenced the timing of brown trout migration activity on a daily basis. High migration distance per day occurred during the eclipse of the moon, whereas the lowest migration distance per day occurred during the full moon. Furthermore, migration distance per day decreased with increasing river slope. The other variables tested (sex and physicochemical parameters, such as flow, pH, conductivity, and dissolved oxygen) did not affect brown trout migration activity. The results indicated that despite the restrictions of upstream migrations by lateral obstacles without fish ladders, brown trout migration activity in artificially isolated headwaters with pristine morphology is preserved. Furthermore, the analyses revealed a significant impact of the moon phase on brown trout migration that has not been previously described for this species. Although the observed migration occurred over a short period of time, brown trout did not adopt sedentary behavior, as has been described in the river stretches upstream from natural obstacles, such as waterfalls.