| FISHERIES - AUTUMN SALMON MIGRATION
Wading downstream with the inflatable PIT tag antenna (green device) which links to a GPS enabling us to determine the distance travelled by individual fish during the autumn shift. Understanding migration
Rasmus Lauridsen looks at the phenomenon of a second migration period of salmon juveniles taking place in late autumn
S
almon from the River Frome display a classic spring smolt migration peaking at the end of April. However, with the installation of full river coverage of our PIT tag detectors at East Stoke in 2002, it became clear that a second migration period of juveniles takes place in late autumn. This autumn migration of parr usually happens during the first few high flow events after mid-October and has been observed on a number of rivers in the UK, France and
“Autumn migration of parr has been observed on a
number of rivers in the UK, France and America”
America. It has been proposed that this autumn movement is an early sea migration, but it has also been suggested that these are weak individuals that quickly die. We have conducted tests and found that these autumn migrants are unable to tolerate saltwater, but we have also detected some of these autumn migrants as returning adults. As some fish obviously survive but are not adapted for saltwater, this migration must be
36 | GAMEWISE • SUMMER 2017
a downstream shift within freshwater rather than an early migration to sea. Observations from stationary PIT tag detectors provide us with information on tagged individuals passing fixed points on the river so we know when and where the individual was tagged and when it passed our detectors.
With the increased detection range of our new PIT tag equipment it has been possible to develop an inflatable floating antenna that can be used for mobile scanning of the river for PIT tags. In the autumn of 2016 we used one of these mobile antennas to scan the main river channel from Dorchester to Wool (20 kilometres) before and after the main autumn shift (see picture above).
During these mobile tracking surveys, we detected more than 1,500 individual tags and because the mobile antenna is linked to a GPS (see right), we can determine the distance travelled by these individuals between tagging and before the autumn shift. For individuals detected both before and after the autumn shift period, we will be able to determine distance travelled during the autumn shift.
www.gwct.org.uk/fishing
The mobile tracking data will enable us to better describe and understand the autumn movement for the population as a whole and not just the individuals that migrate past the stationary antennae. This will ultimately bring us closer to understanding who moves and why.
Rasmus Lauridsen is a freshwater ecologist in our fi sheries research team at East Stoke. He primarily does research on the migration strategy of young salmon and the drivers and consequences of different life history choices.
Heat map of PIT tag detections on the Moreton carrier of the Frome. The more intense the red colour the higher the density of tags.
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