Marine Scotland Science Report 01/15: Status of Scottish Salmon and Sea Trout Stocks 2014

Introduction

Assessment of the number of salmon and sea trout returning to Scottish rivers is important for determining conservation status and management of fisheries. For salmon, stock assessment is informed by a number of sources of scientifically useful information including reported rod catch, counters and fixed traps which provide indices of adult salmon abundance. Fixed traps may also provide data on juvenile emigrants. These data sources have different strengths and weaknesses.

Trap counts are generally accurate but localised to small tributaries, whereas rod catches are imprecise but geographically widespread. Counter data provide intermediate accuracy and at present, relatively low geographic coverage. Juvenile electrofishing surveys are relatively insensitive to variations in the numbers of spawning adults, until they decline to a particularly low level, due to density dependent mortality in early life stages. This process can result in similar numbers of juveniles being produced across a broad range of returning adult numbers. Furthermore, a strategically structured sample of juvenile salmon in Scotland is not currently available. Thus, consideration of rod catch, counter and trap data together provides the best option for interpreting the status of Scottish salmon stocks at the present time and it is these indices that we consider in this document.

Salmon generally home to the areas where they reared in fresh water and as a consequence they form geographically localised breeding groups that may evolve biological characteristics that are adapted to their specific local environment. Run-timing is one such character. Based on available data, earlier up-river migration tends to be associated with higher altitude spawning. Although it is not currently possible to manage salmon at the fine scale of specific breeding groups, run-timing groups are of significance to fisheries and offer a practical solution for separation of stocks. Through examination of different run-timing groups (stocks) it is possible to obtain insights into the likely relative status of salmon from different regions of river catchments. However, it should also be noted that run timing may vary from year to year. Here, abundance indices are considered as three broad run-timing groups referred to as spring (February-May), summer (June-August) and autumn (September-October) stocks.

Marine Scotland Science ( MSS) do not have access to any reliable counter data where sea trout are clearly differentiated from salmon. In addition, although an experimental trapping facility on the River Shieldaig provides some local information on survival of trout at sea, there are no trap data from catchments supporting substantial or self-sustaining sea trout populations. Rod catches are, therefore, the only available source of data for understanding the general status of Scottish sea trout stocks. Sea trout generally home to the areas where they reared in fresh water and, as with salmon, form distinct local stocks. However, there is no evidence linking run-timing to specific spawning areas and hence the rod catches are examined only in terms of the aggregated annual total.

The aim of this document is to present a simple summary of the various data collected by Scottish Government that indicate the status of adult salmon and sea trout stocks.