Many rivers in Scotland enjoy a long rod-fishing season when catchable salmon enter rivers following migration home from their feeding grounds on the high seas. This duration is maintained due to differences in the characteristics of salmon distributed throughout the river catchments. For example, work previously undertaken by Marine Scotland and others show that fish returning to North East Scottish rivers such as the Dee and Spey in spring tend to originate from and return to the high altitude burns, whereas autumn salmon tend to belong to populations in the lower tributaries and river main-stem.
This characteristic of river populations can result in complexity in the management process as a good understanding of stock structuring is fundamental to maintaining the full economic value of salmon fisheries. Currently, we are noticing changes in the characteristics of populations of salmon, probably in some way a consequence of climate change. These changes make interpretation of trends in rod catches challenging because there is no certainty that the season in which each population returns is constant. Overall, there has been a reduction in "spring salmon", which return early in the year and are of relatively high economic value. This downward trend, however, has generally abated or shown signs of recovery over the last few years.
Unfortunately, no such recovery in spring salmon has been evident in reported catches for the River South Esk where, unlike in neighbouring rivers, there has been a sustained decline. Since 2005, a raft of measures has been applied to the management of salmon in the South Esk, principally to reduce overall exploitation of these fish. The Esk District Salmon Fishery Board, in 2010, applied to Scottish Minsters for additional conservation measures that would further reduce exploitation.
However, it has become clear that reducing exploitation has not stemmed the trend of decline in returning fish, although the reduction in coastal netting has probably allowed some salmon to escape into fresh water. Furthermore, the fact that the decline of spring salmon on the South Esk is out of line with other east coast rivers suggests that factors local to the river may be causing a demise of the fishery and population strength. Recognising that if issues affecting the salmon stocks in the River South Esk can be identified they might be addressed by management action, Scottish Ministers instructed MS to carry out a scientific investigation.
The investigation will address several key questions including:
- What proportion of spring salmon captured in coastal nets near the South Esk are destined to return to each of the local rivers? This information is crucial for determining the likely effect that regulation of the net fishery would have on the South Esk rod fishery and conservation status of the population.
- What region within the South Esk the spring fish originate? Having established the freshwater origin(s) of the spring salmon, it may be possible to assess the status of the stocks and if necessary investigate the reasons for and perhaps develop plans to deal with the causes of population decline.
The first step in this project will be to attach miniature radio transmitters to salmon captured in the coastal net fishery to the south of the South Esk so that they can be tracked to their spawning locations. Salmon stop feeding as they finish their migration back to fresh water, so will retain tags within the stomach with no adverse effects. Movements of the fish in the river will be recorded using radio receivers and recording devices mounted along the migration route. The project will also test whether the latest genetics tools might be used to find the origins of fish captured in the coastal nets. The theoretical possibilities of using such a genetics approach are exciting, but the methods available to date have not provided sufficient resolution to give the information needed in this case.