West Coast of Scotland and Clyde Herring Fisheries Management Plan

Stock Assessment and MSY

Scientific evidence

The following evidence has been collated from the most recent ICES assessment working group report for WoS herring^[4] and from the latest Scottish Government report for Clyde herring^[5].

As set out in Section 3.2 of the JFS, the UK takes an evidence-based approach to fisheries management, making use of the best available scientific evidence. For the purpose of stock management and TAC setting, this tends to focus on the use of advice produced by ICES, although other sources of information, may also be used. This is specifically the case for Clyde herring.

Herring in 6.aN had a zero advice and a monitoring TAC during 2016-2022 under the combined assessment. A series of pelagic acoustic surveys in the Clyde were carried out each October between 2012 and 2018. No acoustic surveys have taken place since 2018, although the MD have now completed planning discussions with regards restarting a Clyde acoustic survey for herring and sprats in 2027.

Despite some evidence of an increasing trend in recent biomass estimates, the survey biomass for this stock in 2025 dropped dramatically and is now the lowest in the timeseries and remains at low levels compared to historical values.

Work on genetic stock identity and spawning distributions of herring conducted under the EASME project (funded through the European Maritime and Fisheries Fund) was used in the ICES Benchmark Workshop on North Sea and Celtic Sea stocks (WKNSCS, ICES 2023a) to split 6a.N autumn spawning herring and 6a.S,7bc winter spawning stocks based on their genetic identity determined from ‘baseline’ samples taken during spawning periods. Results also showed that 6aN autumn spawning herring are genetically indistinguishable from North Sea autumn spawning herring and that uncertainties remain about the genetic composition of stocks in 6a.N during autumn. In particular, results from the EASME project were not able to differentiate 6a.N spring spawning fish from late-spawning 6a.S fish. The consequence of this is that these genetically distinct components currently sit out-side of the assessment and are not accounted for in the estimation of herring abundance WoS. Analysis of acoustic data undertaken as part of the international MSHAS survey, and used in herring stock assessment, quantifies that this population may be of a similar size to the 6aN autumn spawning component.

ICES provides annual advice for WoS herring in area 6aN. Different models are used for assessment and advice, depending on availability and quality of data and uncertainty. The type of assessment is categorised from 1 (quantitative/analytical) to 6 (bycatch only data). The advice type is then given based on MSY principles, including data limited approaches (MSY proxy), and/or following a Precautionary Approach. WoS herring is a Category 3 stock. A genetically split SSB survey index from the Malin Shelf Herring Acoustic Survey is used as an indicator of stock development. The ICES advice for 6aN herring is based on the SSB index for the previous year, multiplied by a constant harvest rate (HR), a biomass safeguard, and a precautionary multiplier. In the 2027 ICES advice for 6aN herring the stability clause has not applied since the index is below the trigger value.

For Clyde herring, a report is produced by the Marine Directorate of the Scottish Government, which collates and analyses the information available on the status of the stock. This informs the TAC level. From 2015-21 and 2023-25, the TAC was set at 583 tonnes. There are currently insufficient data to enable a quantitative stock assessment or determination of stock trends for Clyde herring with the last ICES assessment occurring in 1990. This means that MSY reference points cannot be estimated, and hence MSY-based management is not yet possible.

Assessment of evidence

As set out above, whilst the fisheries policy authorities do not consider there is sufficient evidence to enable them to make an estimate of the MSY for the stock of the 6aN herring fishery, it is considered that there is sufficient evidence to evaluate the likelihood of fishing below FMSY using a MSY proxy-based assessment. The management approach follows the ICES MSY approach under their framework for Category 3 stocks. Autumn-spawning herring in 6.a North and the North Sea (North Sea autumn spawners [NSAS]) belong to the same genetic population but are assessed as two distinct stocks depending on their spatial distribution either side of the 4°W line. While the SSB index was low in 2025, there were considerable herring aggregations just east of the 4°W line, which could potentially indicate a shift in spatial distribution (ICES, 2026a). This brings potential uncertainty to this 2025 index value. The ICES advice for 2027 states that fishing pressure on the stock is above HR_{MSY proxy}, and the stock is below I_trigger. Fisheries policy authorities are committed to improving the data relating to this stock, as well as considering management measures required to ensure the sustainable management of WoS herring. These steps and measures are outlined under the FMP policies section.

For Clyde herring, there is no ICES advice produced, and the fisheries policy authorities do not consider there is sufficient evidence to enable them to make an estimate of the MSY for the stock. Management decisions are therefore based on a separate evidence report produced by the Scottish Government with the current time series extending from 1986 to 2025 for Q1, and from 1997 to 2025 for Q4. Utilising this data, an analysis of mean log number per standard tow is carried out to determine changes in the biomass in the area.

Fisheries policy authorities are committed to improving the data relating to this stock and are currently investigating the feasibility of undertaking an acoustic survey in the Clyde which would be undertaken in co-ordination of other institutes surveys covering the Irish and Celtic seas to make accurate determinations of both stock biomass as well as identity. However, until additional survey data becomes available, management will continue to be based on the Scottish Government science report, and should additional evidence become available this may also be used to inform the management approach.