River Frome salmon population
Frome parr: salmon (bottom), trout (middle) and potential salmon/trout hybrid (top). © Rasmus Lauridsen/GWCT
BACKGROUND
At the Salmon & Trout Research Centre at East Stoke we carry out research on all aspects of salmon and trout life history and have monitored the run of adult salmon on the River Frome since 1973. The installation of full river coverage PIT-tag systems in 2002 facilitated the study of life history traits of salmon and trout at not only population level, but also at the level of individuals. The PIT-tag installation also enabled us to quantify the smolt output. The River Frome is one of only 14 index rivers around the North Atlantic to report on the marine survival of wild salmon populations to the International Council for the Exploration of the Sea (ICES).
Figure 1 Estimated spring smolt population 1995-2016 15,000
An otter captured crossing the salmon counter in July last year. © GWCT
Smolts Traditionally, smolt trapping at East Stoke starts on 1 April but in 2016 we started a week earlier because sea trout smolts start their migration earlier than salmon smolts and the first young-of-the-year trout tagged in 2015 were expected to migrate to sea in 2016. Nearly 2,900 salmon smolts were caught in the trap and the total salmon smolt
run in 2016 was estimated at 9,534 (see Figure 1), which is slightly up on the five-year average (9,033) but a little down on the 10-year average (10,597). In an average year, 10% of the fish caught are tagged but in 2016 only 8% were tagged, which reflects the difficult river conditions during the 2015 tagging campaign where we fell 1,500 salmon parr short of deploying the targeted 10,000 tags. In addition to the salmon smolts detected, just under 80 trout smolts were
detected, the first fish from our new trout research programme. However, the bulk of trout tagged in 2015 (2,700+) are expected to emigrate in 2017 as most Frome trout spend two years in the river before smolting, unlike the salmon where 98% smoltify after just one year in the river.
Adults The early run of multi-sea-winter salmon returning to the Frome was strong in 2016, whereas most grilse were late migrating past East Stoke in autumn. These late grilse will most likely have been in the lower river since the summer – this is a well-known pattern on the Frome but is particularly pronounced in years with dry summers, like 2016. Overall, the run of adult salmon was average (see Figure 2) but exceeded the conserva- tion limit unlike 2013 and 2014. 2016 marked the return of the first cohort of grilse with our new tags. The new tag
system proved itself very successful with an adult detection efficiency of more than 95% for the systems at East Stoke. This is a big improvement on our old PIT system which
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14 | GAME & WILDLIFE REVIEW 2016
www.gwct.org.uk
Smolt numbers (95% CL)
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