The indicator for tracking the status of Scotland’s commercial fish stocks is the estimated % of commercial stocks fished at sustainable levels. This is based on the average of two sub-indicators:
- The proportion of the 95% confidence interval for fishing mortality that is below the sustainable fishing threshold.
- The proportion of the 95% confidence interval for the spawning stock biomass that is above the sustainable biomass threshold.
Fishing mortality is a measure of the rate at which fish are captured by commercial fishing, including both landings and discards, for different key commercial stocks. Spawning stock biomass is the combined weight of adult fish that are of sufficient size to spawn (breed) and produce replacements. Sustainability thresholds are defined by relevant MSY (maximum sustainable yield) reference points.
This combination indicator measures sustainable fishing by looking at two related measures. The first sub-indicator measures whether the rate of fishing is at a level that is low enough to enable the fish stocks to replenish, or recover to similar or greater levels within one year. The second sub-indicator measures whether the biomass of the remaining fish stocks are large enough to ensure sufficient spawning and hence young fish in the population in future years.
You can download the data underlying this indicator from the statistics.gov.scot open data platform.
The required data for this indicator for each relevant stock are taken from the International Council for the Exploration of the Sea (ICES) assessments of annual fishing mortality and the corresponding ICES estimates for the MSY reference points for fishing mortality and spawning stock biomass, where available. See the definitions section for which stocks are included.
Definitions and methods:
In each year, the ICES assessments of fishing mortality will be compared to corresponding ICES estimates for the MSY reference point for fishing mortality (Fmsy). As these are estimates with uncertainty, the sub-indicator value will then be equal to the proportion of the 95% confidence interval for the fishing mortality of each of the key commercial stocks that at or below the MSY reference point for fishing mortality (Fmsy).
ICES also provide similar assessments of spawning stock biomass and the biomass level (MSY Btrigger) that should produce MSY if fished at Fmsy. As these are estimates, the sub-indicator value will then be equal to the proportion of the 95% confidence interval for the spawning stock biomass of each of the key commercial stocks at or above the MSY reference point for this biomass (MSY Btrigger).
An average of the proportions calculated for each of the sub-indicators and each of the key commercial fish stocks is calculated to produce an overall percentage for each year to track progress. The aim is to increase the percentage of commercial stocks fished at sustainable levels by:
- Increasing the proportion the 95% confidence interval of the fishing mortality for each of the commercial stocks that is at or below Fmsy and;
- Increasing the proportion the 95% confidence interval of the spawning stock biomass for each of the commercial stocks is at or above MSY Btrigger.
- For Nephrops stocks, the ICES assessments report harvest rate (rather than fishing mortality) by functional unit (rather than ICES Area). For the indicator, the proportion of functional units for which the estimated harvest rate is below the corresponding MSY harvest rate is determined, and this proportion is then included in the overall indicator estimation.
- For most stocks, estimates of both fishing mortality and the MSY fishing mortality reference points are uncertain. This is why the confidence intervals have been used as this prevents small changes to the reference points causing large changes in the indicator that may not be significant.
- The indicator is presented as a simple mean – there is no weighting by abundance or landed value.
- The MSY reference point estimates are not necessarily fixed, but can be adjusted by ICES following significant changes in fishery data or assessment methodology. For the purposes of the indicator, the fishing mortality estimate and the spawning stock biomass estimate in each advice year is compared with the most up-to-date estimate of the relevant MSY reference point.
Definitions of terminology used:
Commercial fish stocks: fish stocks for which harvesting takes place in large quantities and that are marketed for human consumption or industrial uses.
The determination of which stocks to include in the indicator was made based on the value of stocks landed by Scottish vessels by value, as listed in Scottish Sea Fisheries Statistics (Table 5) for 2020 and the availability of suitable data. The following species are included: Mackerel, Nephrops, Haddock, Cod, Whiting, Herring, Hake, Blue whiting, Saithe, Plaice, and Megrim. Note that Monkfish, while a valuable species, is not included as MSY reference points are not available.
Maximum sustainable yield (MSY): is the largest catch that can be captured from a fish stock under existing environmental conditions that can be sustained over an indefinite time period. It represents the level of fishing at which a fish stock is able to continue replenishing itself at the maximum rate. In some cases, MSY is provided as a range to reflect significant uncertainty in its estimation, although only point estimates are used in the indicator calculations.
Fishing mortality: the rate of loss of fish from a stock due to death resulting from fishing activity. Denoted by F.
MSY fishing mortality rate: Fmsy is the fishing mortality rate that should lead to the maximum sustainable yield, given sufficient biomass.
Size-based proxy: size-based proxies for Fmsy are used in cases where age cannot be determined (e.g. Nephrops).
Spawning stock: Mature fish that have reached the age and size for sexual maturity and are able to spawn/reproduce.
Sustainable spawning stock biomass threshold (MSY Btrigger): This is the estimated minimum overall weight of mature fish that are needed to ensure that a fish stock is able to produce sufficient replacements to ensure a stable or increasing fish population.
Criteria for Change:
This evaluation is based on evidence of positive change in the defined indicator.
A guide for the general methodology of indicators can be found here: https://www2.gov.scot/About/Performance/scotPerforms/NPFChanges/Methodology