- 2 Jun 2012
In 2010, the Esk District Salmon Fishery Board applied to the Scottish Government for measures that were designed to reduce overall exploitation on early running salmon and also sea trout stocks in the River South Esk. The location of the South Esk is shown on map (below).
Scottish Ministers responded by acknowledging that there may be issues with these stocks and instructed Marine Scotland (MS) to carry out an investigation with a focus on the early running component of the River South Esk salmon stock. As a result, MS are undertaking a 3 year project with a view to identifying both the underlying causal factors for the apparent decline in these stocks and the appropriate management responses and remedial actions.
The first step in this project is to determine from where in the South Esk catchment early running salmon stocks originate. This will be achieved by radio tagging salmon captured in the coastal net fishery to the south of the river and tracking them to their spawning locations. In addition it will be established whether genetic tools can be developed to identify spawning locations of a larger sample of early-running salmon.
Once the spawning areas have been identified, then it may be possible to assess the status of the stocks in these areas. The suitability of the habitat for supporting the freshwater stages will be assessed to identify where bottlenecks to salmon productivity may be occurring. This will involve investigating the relative impacts of the physical, chemical and biological factors that may be responsible for limiting smolt production. It may also be necessary to evaluate the potential for removal of obstacles to migration.
Year 1: radio tracking study to determine from where in the South Esk catchment early running salmon stocks originate
These web pages are intended to provide an introduction to the radio tracking study and a mechanism for providing regular updates of progress.
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.
Tagging the fish
Salmon are being tagged using well established procedures. Each fish is fitted with a miniature radio transmitter (Lotek coded MCFT2-16-CE tags measure 46mm in length with a diameter of 16mm and weigh 16g in air) applied under a scientific licence from the Home Office. After recovery, the fish is released back into the sea and its location can be determined using tracking devices once it enters fresh water.
Fish for tagging were intercepted by coastal nets operated by Usan Salmon Fisheries. The extent of the netting stations and the release area can be seen on map (below). Triangles show the furthest extent of netting stations used in the project and the release point, to the north of Usan harbour, is indicated by a red circle.
It was planned to radio tag 150 salmon over the period February 16th (the start of the netting season) to 31st May. The aim was to tag approximately 10 fish in February, 30 in March, 50 in April and 60 in May, the monthly target number being spread evenly across available weeks in each month. This target has been met, with 153 fish having been tagged over the period February to May.
- The numbers of fish tagged (available to download below) between February and May together with target numbers
In addition, multi sea winter fish (those greater than 70cm fork length) were tagged and released in the same area during the first two weeks in September. In total, 9 fish were tagged in week 1 (Sept 4th to 7th) and 3 were tagged in week 2 (Sept 11th to 14th).
Tracking the fish
Radio receivers (Lotek SRX-DL) are used as fixed stations and will record the identity of individual fish as they pass given points on a river system together with date and time. Further, mobile receivers (Lotek SRX 600) are used as active tracking devices and will record the identity of individual fish as the receiver passes tagged fish in the river together with date, time and GPS coordinates.
Tagged salmon in the South Esk
Up to 15 SRX-DL receivers will be deployed on the river South Esk and these, together with the active tracking receivers and aerial surveys, provide information on the movements of individual tagged fish during their riverine migration. Locations of the receivers can be seen on map (below).
Tracking on the South Esk for the first year of the project has been successfully completed. Marine Scotland Science would like to thank all who have helped with the work. The data are currently being analysed and a summary will be provided here in due course.
Tagged salmon in east coast salmon rivers
In addition to the South Esk, fixed receivers have also been deployed in the lower reaches of some other Scottish east coast salmon rivers: the Tay, the North Esk and the Dee (through collaboration with the Dee Fishery Trust). Survey flights have been undertaken to check for the presence of tags in the rivers North and South Esks, Lunan, Bervie, Dee, Don and Tay.
- Numbers of fish recorded to date (available to download below) - correct as of 17 December.
Coloured bars indicate the numbers of tagged fish either detected in each river or taken by coastal fisheries. For any given river, the darker colours indicate the numbers of tagged fish currently in the river while the lighter coloured bars indicate the numbers of fish which have entered the river and have subsequently either returned to the sea, been removed by fisheries or are thought to have regurgitated their tag. Included in the figure is: -
- one fish, tagged on 29th May, which was recorded in the North Esk between 2nd and 8th June and was subsequently first detected in the South Esk on 10th June. This fish is now thought to have regurgitated its tag.
A total of four fish tagged in September have been detected in monitored rivers: two in the North Esk and one each in the Dee and Tay.
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