National Adult Salmon Sampling Project

4. Discussion

The fundamental data pillars for wild Atlantic salmon stock assessment in Scotland are catches, counts, sex ratios, fecundities, sizes and ages. The National Adult Salmon Sampling Programme demonstrated the potential for local managers to contribute valuable information on the ages, size and sex of returning stocks that can be used within population dynamics models to estimate stock status.

Previous work has shown long term changes in the balance between the different sea age classes (George 1982; Summers 1995) and fluctuations in the sizes of salmon returning to Scottish coasts (Summers 1993). Although there is some evidence of a relationship between the size of adult salmon returning from the sea and climatic variables it is not possible to use this to predict size or age of returning fish (Todd et al. 2008; Bacon et al. 2009; Bal et al. 2017).

In contrast to long term temporal changes, there has been less focus on geographic differences in life history parameters of salmon around Scotland. This is a function of the greater size of the stocks and fisheries in the east and north of Scotland affording greater sampling opportunities than on the west. This imbalance is also shown in the number of samples collected during 2021-23. Despite this imbalance, the samples obtained demonstrated some important geographic patterns, particularly in the age and sex of salmon. The link between smolt age and latitude, previously shown at a range-wide scale (Metcalfe & Thorpe 1990), was also shown within Scotland. For sea age, the higher occurrence of 1SW salmon on the west compared to the east (also seen in reported catch statistics; Marine Directorate, 2024) can be seen in the samples. Geographic comparisons of the sex ratio of salmon are likewise limited with Shearer (1992) noting that for the River North Esk “most grilse [1SW] are male and most 2SW fish are female. However, in ‘grilse only’ rivers, the proportions of male and female grilse are about equal.” This general pattern accords with the results presented here where 1SW salmon from the east were more likely to be male and those on the west female (where rivers are equivalent to the ‘grilse only’ rivers noted by Shearer (1992)).

There is limited information on the sex ratio of Scottish salmon, with the majority being collected using visual observations (e.g. Bacon et al. 2009). However, the use of visual methods of sexing were found to underestimate the occurrence of females, both in this study and on the River Tamar (King et al. 2023). Similar to the present study, King et al (2023) found that error varied within and among years highlighting the issues with depending on visual sex identification for stock assessment, and other analyses.

This report highlights how coordinated sampling of adult salmon in recreational or scientific fisheries can be used to gather information on the age, size and sex of adult salmon returning to Scottish coasts. The geographic and temporal differences shown here are not just academically interesting but have real practical implications for management:

• Sea Age. The sea age of salmon in a given stock is related to the size of individuals and the sex ratio and will therefore impact on the estimated egg deposition used to assess stock status. Underestimating the proportion of 1SW salmon (for example Figure 3.13A) would lead to an overestimation of egg deposition.
• Smolt Age. Differences in the smolt age of fish also impact on egg deposition, with females having a lower smolt age tending to contain a greater number of eggs (Hanson et al. 2020).
• Size. Large fish contain more eggs so not using accurate size information could lead to biased stock assessments (Hanson et al. 2020).
• Sex ratio. Currently the sex ratio of 1SW and MSW salmon are treated as fixed when assessing Scottish stocks. This project has provided the first geographic data on these ratios to allow these patterns to be accounted for in the assessments.

Although the current stock assessment methods do account for geographic changes, further information is required - particularly to account for changes over time (e.g. Figure 3.13). If an annual sampling programme can be maintained, it will provide this invaluable information as it matures.

The programme highlighted the utility of national and local bodies working together to collect the information required for management of salmon in Scotland. The sampling relies on the network of Trusts and Boards spread throughout Scotland, their presence at the river bank and their relationships with local fisheries. The national bodies (Fisheries Management Scotland and Marine Directorate) are able to provide funding, project management, coordinated data collection, quality assurance and consistent reading of scales.

Given finite resources available for monitoring salmon populations, the next step will be to investigate the potential of other data sets to provide complementary information to targeted sampling of adult salmon. Rod catches provide some information on the sea age and size of salmon and have very good spatial and temporal coverage. However, there are known inaccuracies in, for example, the reported proportion of 1SW in the catches (MacLean et al. 1996). Further work is therefore required to determine if the two sources can be integrated to provide a more complete picture of adult salmon returning to Scotland.