The wild Atlantic salmon is an iconic Scottish species, however, their numbers are in decline. The total salmon rod catch in Scottish rivers reached its lowest number in 2018, having been in a steep decline since 2010. Coastal net catches have also been in decline since the 1960s, with most net fisheries having been bought out for conservation reasons.
In response to this, the Scottish Government introduced the Conservation of Salmon (Scotland) Regulations 2016. These state that an assessment of the conservation status of salmon must be carried out for all rivers, based on both the number and condition of salmon. Marine Scotland carry out this assessment each year, giving each river a grade which determines how it should be managed i.e. on which rivers salmon can be caught and kept and which ones require caught salmon to be returned to the river (known as catch and release). To do this they use data from scientists and anglers to estimate how many salmon eggs will be deposited in each river and to determine the likelihood of these egg deposits reaching the level required to maintain sustainable salmon stocks.
Marine Scotland has produced this video to explain the methods used:
This blog post describes the methods used to carry out this assessment which have recently been updated. These are also covered in detail in SPICe briefing SB 19-48 about wild salmon which also discusses the different reasons behind the salmon’s decline. Assessment methods involve a number of discrete steps:
Step 1: Converting rod catch to numbers of returning adult salmon
A basic model is created to calculate the percentage of salmon caught by anglers. On rivers which have fish counters (automatic devices measuring the number of fish passing along a river), the number of fish counted can be compared to the number of fish caught by anglers further upstream. This is then used to produce a model which can estimate salmon abundance for rivers which do not have fish counters.
Step 2: Estimate the number of salmon in each river
The model also accounts for variation in the relationship between fish counters and rod catch due to the time of year and flow rate.
For a given count, catches tend to be:
- lower during the summer than the spring or autumn
- lower in low flow conditions where salmon appear to be harder to catch compared to high flow conditions.
Data on the return profile of rivers (i.e. the seasonal variation of salmon counts) and flow rate allows estimates to be made of the numbers of salmon returning outside of the fishing season.
Step 3: Calculate the number of eggs which will be deposited
The estimate of adult salmon abundance is used to work out how many eggs are likely to be deposited. This requires knowing how many fish are female and their size.
The percentage of female salmon is calculated using historical data about how the percentage of female fish in each age class (i.e. one or multi-sea-winter fish) differs with month, year and location. The number of eggs produced by each female is related to its age and size and is also estimated using historical data on the egg content of salmon in relation to month, year and location.
Step 4: Compare the number of eggs to the egg targets
Each river has an egg target – the number of eggs required to maintain salmon stocks at sustainable levels. This is estimated from rivers where information is available on the relationship between the number of eggs deposited and the number of adults those eggs will produce. Uncertainties involved in estimating egg numbers and targets are combined to calculate the percentage chance that an egg requirement has been reached for the previous five years.
Step 5: Assign each river a conservation status grade
The grade of the river is determined by the percentage chance that the egg requirement has been reached for the previous five years. These are defined as:
- Grade 1: >80% chance the egg requirement has been met over the past 5 years
- Grade 2: 60-80% chance the egg requirement has been met over the past 5 years
- Grade 3: <60% chance the egg requirement has been met over the past 5 years
The flow chart below summarises the different types of data that go into each step of the assessment.
The river grade determines the management which is required, as set out by Marine Scotland. This advice is as follows:
- Grade 1: Exploitation is sustainable. No additional management action is currently required.
- Grade 2: Catch and release should be promoted strongly to reduce exploitation.
- Grade 3: Catch and release is mandatory as exploitation is unsustainable.
Based on these methods and using data from the 2013 – 2017 period, the assessment for the 2019 fishing season categorized Scotland’s rivers as follows:
- 49 rivers as grade 1
- 30 rivers as grade 2
- 94 rivers as grade 3
Although these results reduce the number of grade 3 rivers compared to previous assessments (122 in 2018), it must be noted that this is solely the result of changes in the assessment methods used for 2019. This includes changes to the egg requirements which are now based specifically on data from Scottish rivers, rather than the more generic data previously used. These improvements are expected to make river assessments more accurate.
Applying the updated methods to the previous three 5-year periods, results in a progressive increase in the number of grade 3 rivers as shown by the figure below. This indicates that the conservation status for wild salmon is declining.
Kate Ashley, Research Intern, SPICe