Aspects of erosion of restored trout spawning beds in two streams in Northern Sweden

Sammanfattning: Many rivers and streams in northern Sweden have been channelized due to timber floating. This has severely degraded the spawning habitats for salmon and trout, and therefore a common action when restoring channelized rivers is to establish new spawning beds by adding gravel to the streambed. However, little is known about the longevity of these spawning beds, and erosion caused by both water discharge and spawning fish may move gravel/pebbles out the spawning bed, gradually decreasing their functionality. In this thesis, I studied erosion of eight spawning beds (approx. 5 x 2 m), constructed in two different streams, Storkvarnbäcken and Mattjokkbäcken, in northern Sweden. In total, 643 pebbles were tagged with half-duplex passive integrated transponder tags and placed on newly constructed spawning beds, which made it possible to study their movement over multiple years by scanning the spawning beds with PIT-tag reader/antennas. The overall objectives for this study were to quantify and describe erosion process of constructed spawning beds. More specifically, I investigated the effect of streambed slope and pebble size on the distance substrate were moved by erosion processes, and on the likelihood of substrate remaining on the spawning bed. Further, I evaluated different methods used to position PIT-tagged pebbles in boreal streams (conventional GPS, High Precision GPS, and Laser distance meter), and specifically investigated precision and accuracy of commonly used GPS-systems integrated in PIT-tag reader units. Overall, recovery of PIT-tagged pebbles by scanning spawning beds with PIT-tag antennas were high despite multiple years between the deployment of pebbles and the scanning. 85 % of pebbles were found in Storkvarnbäcken 8 years after deployment, and 60 % were found in Mattjokkbäcken 6 years after deployment. There was a big difference between the two study streams in the proportion of recovered pebbles that had remained on the spawning beds (82% remaining in Storkvarnbäcken, and 45% in Mattjokkbäcken), and the probability of pebbles being found outside the spawning bed were significantly higher in Mattjokkbäcken. The average distance pebbles had been moved by erosion was 1m in Storkvarnbäcken, translating into 0.13 m / year, and 6.21 m in Mattjokkbäcken (1,04 m /year). However, there was a considerable variation in erosion rate over time, and in Storkvarnbäcken the average distance moved was 0.48m the first month after deployment, after which erosion-rate decreased to less than 0.08/year, suggesting that average annual erosion rates should be used carefully. There was a positive relationship between the slope of the spawning bed and the distance pebbles were moved by erosion, and pebbles located on spawning beds with a steeper slope had moved longer distances on average. Within the range of 0-3% slope, average distance increased from <1 m to >7 m. Erosion caused small pebbles (45-55 mm) to move a significantly longer distance than medium sized pebbles (65-75 mm), however no difference in distance moved could be found between small and large size (85-95 mm) pebbles. In average, GPS integrated in PIT-tag reader units had very poor accuracy compare to high-precision GPS (Mean Absolute Error (MAE) 4.7m), and to laser-distance meter (MAE 2.5m). Also, the precision of pebble distances calculated using integrated GPS were low, (±2.87m for mean distances in Storkvarnbäcken, ±1.34m for Mattjokk). The MAE for the high precision GPS was somewhat higher than when using the laser, which indicates that the high precision GPS is closer to the true value. Results are discussed based on the many interacting factors underlying erosion rates and spawning bed longevity, and the way the results could be used as guidelines for construction of trout spawning beds.