Lead scientists: Celine Artero & William A Beaumont
Game & Wildlife Conservation Trust
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An assessment was made of salmon and sea trout smolt mortality in the estuarine phase of their seaward migration.
Four estuaries were chosen to represent four different morphologies:
The river Frome’s Poole Harbour: an enclosed estuary
The river Tamar estuary: a long tapering estuary.
The river Scorff (France): a widely tapering estuary.
The river Bresle (France): a man-made harbour controlled by lock gates.
Fish were caught, tagged with acoustic tags, and released in the lower reaches of their respective rivers. They were tracked using passive receivers in the lower river and estuaries.
A total of 480 Salmon and 360 trout smolts were tagged.
Fish passage characteristics through the estuaries was very variable both within the different estuaries and between different morphology estuaries.
Mortality of fish also varied between estuaries with the Frome having the highest overall mortality of both trout and salmon smolts (49% and 27% respectively).
Estuary morphology, speed of passage and distance to travel were found to be the principal causes of the mortality. For all sites mortality rates for trout was consistently lower than for salmon. The restricted exit of Poole Harbour was also thought to be a contributary factor in the high mortality rates of this site.
A proposal to mitigate the high mortality of salmon smolts migrating across Poole Harbour, by translocating the smolts from the lower Frome to the open sea, is being considered. However, there are serious concerns that need to be addressed before this can take place.