Effect of an Artificial Pond on Migrating Brown Trout Smolts

Abstract

In southern Sweden, nutrient runoff from agricultural areas causes extensive eutrophication of inland and coastal waters. To reduce nutrient transport, especially nitrogen, resource managers are creating wetlands and ponds within stream systems. During 1996 and 1997, we studied the migration of smolts of brown trout Salmo trutta through one of these artificial ponds, situated in Önnerupsbäcken Creek in southern Sweden. Because the former migration route through the stream was severed, migration through the pond was compared with an upstream reference section in the stream. Smolts were captured by a smolt trap about 2 km upstream of the pond and marked with electronic passive integrated transponders before being released into the stream. Downstream of the smolt trap, loss rate, swimming speed, and timing of movements of the brown trout were measured in the reference stream section and in the pond as the smolts passed each of three submerged antennae placed in the stream and at the pond's inlet and outlet. We found that the median time it took the tagged smolts to swim a stream length of 1,280 m (first and second antennae) was 4 h in 1996 and 23 h in 1997. Most movement (75%) occurred between 2100 and 0400 hours. The loss rate in the stream was estimated to be 23% and 42% for 1996 and 1997, respectively. The shortest distance across the pond was 285 m, and the median time required for swimming through the pond (second and third antennae) was 52 h in 1996 and 26 h in 1997. Compared with the results in the stream, the timing of movement out of the pond was evenly spread over the day, and the loss rate was estimated to 81.5% and 77% for 1996 and 1997, respectively. Thus, migration time was slower and mortality rate was higher than in the stream section, presumably due to predation in the pond. Although we cannot make any conclusions about how migration has been altered by the pond relative to the former stream route, the differences in migration between the pond and the reference section suggest that there may be negative impacts of new lentic habitats that outweigh the gains achieved by the remedial measure.