Towing of Atlantic salmon smolts from River Storelva as a mitigation measure –Smolt cohort 2018

Summary

Aluminum (Al) in estuaries is known to affect fish health and survival within aquaculture pens. NIVA have provided information demonstrating how Al originating in freshwater is mobilized to toxic forms of Al that can accumulate on fish gills at increasing salinities. Gill-Al concentrations increase rapidly when salinity is in the range from 1 to 4 psu. Moreover, concentrations decrease when salinity increases from 5 to 7 psu, and drop to low levels at salinities higher than 10 psu. Survival is reduced when the gill-Al concentrations are high. Telemetry studies performed in 2007 and 2008 demonstrated that fewer smolts migrated through the fjord and reached saline coastal waters when the inner basin contained brackish water. Later tag-recapture studies from River Storelva have documented that Al in the estuaries could reduce smolt-to-adult survival of wild Atlantic salmon. To mitigate this problem and increase smolt survival, we caught and towed smolts by boat past the inner fjord basin that contained brackish water and released them in the Sandnesfjorden with salinities in the range 14-16. A total of 1748 Atlantic salmon smolts were towed during the 2018 smolt run. The total number of smolts caught at the two catch stations seems to stabilize around 2000 individuals annually in River Storelva.