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If You Can’t See It How Can You Count It?

Bill
Beaumont
September 21 2011

Game & Wildlife Conservation Trust
Salmon & Trout Research Centre
East Stoke
Wareham
Dorset
BH20 6BB

Fish have one very large disadvantage for those who study them – it is very difficult and often impossible to see them! Fishery scientists therefore often resort to a wide range of methods and technological wizardry that help overcome this problem. On the river Frome at East Stoke in Dorset scientists have developed one of the most technologically advanced Atlantic salmon monitoring facilities in Europe. The river Frome is the most westerly chalk stream in England and hosts a large variety of wildlife. Historically it was also renowned as a good river for catching large 20-40 lb salmon. In the early 1970’s it was chosen as a site to test new and developing technology for counting adult salmon when they go up rivers to spawn. The most successful of these methods was a resistivity fish counter. In this, electronics measure the electrical resistance of an area of water. When a fish crosses this area the resistance of the water is changed and thus the fish is detected and counted. In various forms this system has been used at East Stoke to count the adult salmon since 1973.

This monitoring has shown that the adults suffered a dramatic 75% decline in the early 1990’s. This dramatic reduction was also found in most of the fisheries in the North Atlantic, indicating that the marine survival of the fish was becoming a major problem. Attempting to manage and conserve the salmon at sea is an impossible task; however, it is possible to manage the juvenile fish while they are growing in the rivers. To understand the pressures on these juvenile fish, in the mid 1990’s the scientists began trying to estimate the number of smolts that go out to sea. The task of counting these small fish seemed at first impossible. They are only about 12 cm long and often migrate when the river is in a spring spate. Once again cutting edge technology came to the rescue in the form of small Passive Integrated Transponder (PIT) tags. These tags (about the size of a grain of rice), each having a unique id code, are harmlessly implanted into the fish in the autumn. Then in spring they are detected and recorded by sophisticated electronic detectors at East Stoke.

A great Salmon on the Frome

Two years ago this unique research facility looked like it would close. Fortunately the Game & Wildlife Conservation Trust took on the management of the research and the staff and therefore the groundbreaking research is continuing. It allows the scientists to assess the numbers of juvenile salmon present in the autumn, both count the smolts that survive from these autumn fish and determine what parts of the river are good (or bad) for juvenile survival. The work then completes the cycle by counting the returning adult fish. This research will help identify where survival of salmon in the river can be improved and help offset the problems that the fish encounter in the marine environment. The results will be applicable to a wide range of rivers in the UK and will enable targeted intelligent management of salmon populations and will give vital information on factors influencing the life history characteristics of this species with a view to halting the decline and restoring salmon populations to former levels.

An annual report detailing the salmon numbers and the research carried out at the Salmon & Trout Research Centre at East Stoke can be found on the GWCT web site: www.gwct.org.uk