A fish counter network for monitoring Scottish salmon stocks

3. Data requirements

The assessment of stocks to inform the development of annual Conservation Regulations (and international management by NASCO) requires information on the abundance of adult salmon within rivers throughout the year. Counter data is used directly in the assesssments where such data are available. The abundance of adult salmon in areas without a fish counter is estimated using a Generalised Linear Mixed Model (GLMM) (SG, 2018) using information on season and river flow to estimate a correction factor based on the relationship between catches and counts of fish for rivers with fish counters. The relationship between catch and count is derived from monthly data from seven fish counters and two net fisheries and the rod catches upstream of the counters/fisheries.

The abundance estimates are converted into estimates of egg deposition and compared to numbers required for the stock to meet its egg requirement (conservation limit). This involves the use of additional data drawn from sources including Official Statistics on salmon catches and programmes of adult fish sampling.

Finally, the conservation status of each stock is defined by the probability of the stock meeting its egg requirement over a five-year period and an annual report on the status of stocks is published (The status of salmon in Scotland: 2025 - gov.scot; SG, 2025). Estimates of these parameters in 2024 were derived from:

• in-river fish counters (three rivers)
• catch returns (all rivers, most recent five years)
• records of the sizes and ages of salmon throughout Scotland (39 sites between 1963 and 2018)
• information on the sex ratios of different age classes of salmon across Scotland (10 sites between 1966 and 2016)
• the relationship between catches and fish counts (nine rivers, pre-2016 data)

The ICES WGNAS recommend that stock abundance is monitored for one in 30 salmon rivers (ICES, 2012). Scotland has 215 salmon rivers in 173 assessed areas, meaning at least seven fish counters are required to meet that recommendation. Guidance for the use of GLMM, indicates that the absolute minimum number of whole-river counts required each year to model the stock assessment for Scotland is six (Bolker et al., 2009, 2024). To provide resilience, ensure that at least six/seven counts are available each year and provide acceptable robustness to the data, GLMM guidance suggests that ten counters are required (Bolker et al., 2024). Those counters should provide balanced geographic coverage.