This theme and its research objective questions reflect the necessity to develop a greater understanding of diadromous fish populations and the requirement to strengthen risk based assessments. The aim of this theme is to achieve a better knowledge base on Atlantic salmon populations to improve the resource management. This will essentially contribute not only to sustainable OMRE development but also to other coastal activities such as aquaculture, harbour development and fisheries.
The priority research objectives aimed at improving understanding of diadromous fish populations in Scotland, as an element in the assessment of the potential impacts of various activities on these populations are summarised below.
1. Understanding and detecting changes to salmon at a population level
At present, salmon populations are assessed predominantly using rod catch returns, which are useful in that they cover a wide geographic range and time series but are subject to potential systematic bias and inaccuracy. Rod catch data can be bolstered significantly using information from traps and counters.
Current actions: MS is promoting the development of a network of fish counters at key locations to improve the power to detect changes in numbers of returning adult salmon. The first stage in this process is contracting work to review costs, technological constraints and options of installation of a range of designs of fish counters across a range of river types. MSS is also investigating options for compiling and analysing existing counter data from across Scotland, this will be contingent on the cooperation of those running existing counters.
Future actions: stock assessment is an evolving science that underpins many components of salmon fisheries management. Improvement of stock assessment is likely to be considered in the current Scottish Government review of Freshwater Fisheries; future actions are subject to the outcomes of that process.
2. Can potential changes in population levels be determined as significant with respect to conservation status and fisheries?
Any change in stock size may generally be expected to have proportional effects on fisheries. However, it is unlikely that a simple proportional relationship exists between losses and conservation status of salmon populations. The stock-recruitment relationship, production of smolts (recruitment) generally increases with the number of spawners (stock) until the stock is sufficient to saturate freshwater rearing habitats with eggs. A critical target (the conservation limit) is generally defined as a point on the stock-recruitment curve that avoids the populations falling below the saturation level. However, the options for deriving robust conservation limits in Scottish water have yet to be adequately explored.
Current actions: MSS is currently exploring and refining options for deriving robust conservation limits for salmon in Scotland.
Future actions. Compare likely impacts of OMRE, aquaculture and fishing with fisheries takes and risk of populations and river stocks falling below conservation limits.