Herring in the Firth of Clyde: recommending the total allowable catch for 2025: Consultation

6. Scientific Information on the Status of Herring (Clupea harengus) in the Clyde Sea (ICES Statistical Rectangles 39E4 – 40E5) in 2025

Prepared by Campbell Pert and Helen Dobby

Marine Directorate of the Scottish Government, 375 Victoria Road, Aberdeen, AB11 9DB

6.1 Executive Summary

The herring fishery in the Clyde has declined from its peak in the 1960’s, with annual catches typically less than 500 tonnes over the last 20 years. Excluding 180 tonnes caught in 2021, there have been no commercial catches recorded in the Clyde since 2014.

The 2024 surveys suggest an apparent increase in biomass, particularly during Q4, as well as the presence of an adult cohort which gives rise to some optimism that Clyde herring stocks are increasing. However, given the relatively poor state of several herring stocks to the west of the British Isles and the high likelihood that juvenile herring in the Clyde are from one or more of these stocks, consideration should be given to continuing those protections currently in place for Clyde herring.

There is no evidence to indicate that stocks have significantly decreased since 2022, with Q4 2024 data suggesting another year-on-year increase in stock biomass at this time of year although this is tempered by a potential small decrease during Q1. However, there is insufficient scientific evidence to suggest that an increase above the 2024 TAC of 583 tonnes would be appropriate. Similarly, the data does not suggest that the TAC should be reduced below the current level.

Due to the lack of available data for Clyde herring, the current control measures should remain in place as a precautionary measure, and awareness raised among fishers to ensure compliance with these measures.

6.2 History of the Fishery

The Firth of Clyde is a fjordic-like system, reaching over 100 km into the southwest coast of Scotland, which has a centuries-long history of fishing. Advances in fishing gears and vessels, and a succession of fishery management regulations^[2], have altered the fishery and the fish stocks of the Clyde.

Herring in the Firth of Clyde was one of the most economically important species to fishers during the first half of the 20th century. Annual landings of herring between 1900 and 1940 were typically 10,000 to 20,000 tonnes/year, and landings reached a peak between the late 1950’s and mid-1960’s. From the mid 1960’s to 1980’s landings fluctuated between 2,000 and 5,000 tonnes (Figure 1). A Total Allowable Catch (TAC) was first introduced in 1984; the TAC was 1,000 tonnes from 1993 until 2007 and has been gradually reduced since (Table 1). The TAC had been set at 583 tonnes per year from 2015 to 2021 before being reduced to 466 tonnes in 2022 and then returning to 583 tonnes in 2023. The reduction in 2022 was a precautionary measure following the research vessel “Scotia” suffering a mechanical breakdown, meaning the cancellation of the west coast survey in quarter 1 (Q1) and therefore no fishery-independent data on herring in the Firth of Clyde were available for that year.

Over time, the fishery has been dominated by Scottish and Northern Irish vessels. Most of the catch was taken by Scottish vessels from 1995 to 2000, but by Northern Irish vessels from 2000 to 2012. From 2013 to 2024 there were no commercial landings from the Clyde, apart from 2021 where a Northern Irish vessel reported a single landing of 180 tonnes (Table 1)

Since 1995 there has been virtually no unallocated catch or discarding reported for Clyde herring. Landings data are available from 1955, with data for 1982 – 2024 presented in Figure 1 and Table 1. Catch and sampling data are incomplete, resulting in a large proportion of the data being estimated; sampling effort has been low and the numbers-at-age data are unavailable from 2002-2010 and in 2021. In the historical data (with numbers at age) it is possible to track cohorts of herring as they move through the population. Figure 2 suggests there have been no strong year classes found in the Clyde commercial catches since the 1990s but in more recent years this is due to a lack of landings rather than a true reflection of the stock present.

¹ Calculated from estimates of weight per box and/or by-catch in the sprat fishery

² Based on sampling.

³ Estimated to be at a low level, currently unknown

6.3 Landings Data Collected Since 2011

In 2011 the targeted sampling of Clyde herring landings was successfully resumed in collaboration with the Marine Directorate (MD) Compliance fishery office in Campbeltown. During the 2011 herring season a total of six samples were collected from seven hauls (Table 2) in the inner Firth of Clyde (Figure A1)

The 2012 fishery was carried out by two Scottish vessels operating as a pair team, alongside one Northern Irish vessel. The two Scottish vessels made a total of 11 trips, whereas the Northern Irish vessel made one trip that accounted for 61% of the total landings (Table 1). Landings were dominated by this one large haul caught at the outer edge of the Firth of Clyde (pers. comm. SFO Campbeltown). Overall, six samples were collected from the 12 trips. Similar to 2011, five of these samples were collected in the inner Firth of Clyde (Figure A2).

In 2013 the fishery was very small, with only 21 tonnes of registered landings (Table 1). Half of the four trips carried out by Scottish vessels were sampled (Table 2). A total of 420 herring were measured and 280 fish were aged (Figure A3).

Since 2013 there has only been one official landing of Clyde herring. In 2021 a single landing of 180 tonnes was landed into Belfast by a Northern Irish vessel. However, this landing was not sampled as the fish were caught and landed four weeks before Marine Directorate staff were made aware. Attempts to obtain biological data via colleagues at the Agri-Food and Biosciences Institute (AFBI) were unsuccessful as their staff were not present either during the landing. Therefore, no landings sample data for this year are presented.

From the three years of landings samples available, the proportion of herring older than age 5 tended to be low. Landings were dominated by different ages over the three years (Figure A4), although the sample data in 2012 were dominated by the one large landing in the outer Clyde. Length against age plots (Figure A5) suggests a fairly rapid growth in the first few years followed by slower growth over a protracted period. This plot does not fully track the growth of individual year classes and should be treated as a preliminary analysis. The data from the three available years are in good agreement with one another.

Additional analysis on the 2011, 2012 and 2013 landing data can be found in Annex A.

Additional licence conditions for vessels operating in the Clyde fishery require the supply of GPS data from the vessels operating in the area and a haul-by-haul log of catches. Together with the specific sample information described above, this will play an important role in furthering our knowledge of the state of the stock.

6.4 Discard Data

Examination of the dataset held by MD suggests there has not been consistent sampling of herring discards in the Clyde throughout the timeseries. Until 1988 discards were estimated but from 1989 to 2008, discard levels are unknown (Table 1). From 2009-2019, discard estimates from the Nephrops fishery are available from observer trips. It should be noted that there are relatively few trips sampled each year, so estimates are highly variable. There were no observer trips 2020 - 2023 due to the Covid-19 pandemic, and only very limited trips in 2024 due to difficulty in accessing vessels so estimates in the most recent years are not available. Estimates of discards from 2012 - 2019 have been low compared to earlier years and historical levels (Figure A6).

Discards from 2009 – 2019 are dominated by small herring (Figure A7), and are often smaller in size than herring sampled from the commercial sampling programme (Figures A1, A2, A3). It is possible that the Nephrops fishery could have an impact on juvenile herring that have not yet matured and had the opportunity to spawn. However, in some years a larger proportion of the herring discarded by the Nephrops fishery are of commercial landing size (Figure A7).

6.5 Survey Data

6.5.1 Historical and Acoustic Surveys

Spring trawl surveys were carried out from 1985 to 1993. Egg surveys were conducted by the FRV “Clupea” during March – April from 1988 to 1993 around Ballantrae Bank and South Arran. These surveys aimed to collect samples of spring-spawning herring in spawning condition and to discover areas of herring spawn on the gravel beds in these areas.

Acoustic surveys were carried out in July from 1987 to 1990, again in 2008 and 2009 and 2012-2018 (Table A1, Figure A8 and A9). These surveys aimed to provide an estimate of age-disaggregated abundance/biomass indices using acoustic equipment alongside pelagic trawls.

The 2008 and 2009 acoustic surveys showed a higher biomass than in earlier years (Figure A8). The biomass was predominately within the boundaries of the Clyde area, and consisted solely of aged one and two (juvenile) herring (Figure A9) rather than herring of a size targeted by the fishery (over 20 cm).

A series of pelagic acoustic surveys in the Clyde were carried out each October between 2012 and 2018, apart from 2013 when the survey was cancelled due to vessel breakdown (Figure A8). In 2015 and 2018 intermittent faults with the 38 kHz transducer meant that it was not possible to estimate total biomass for these years. Information on size and age composition are still available for those years (Figure A9).

The size and capabilities of survey vessels used in the older and more recent surveys are comparable, as are the methods used. However, there are differences in season (summer vs. winter survey) and total area covered. Given that the fishery has mainly taken place during the early winter it was deemed more appropriate to conduct acoustic surveys that coincide with the fishery when acoustic surveys resumed in 2012. While the lack of adult herring in the more recent surveys is not thought to be a consequence of changes in survey approach, the results are not directly comparable.

Distributions of herring in the surveys were similar between years, with most herring encountered in the inner Clyde to the west and south of the Isle of Bute and in the deeper parts to the east and southeast of Arran. Except for 2016, biomass estimates were significantly lower than estimates from the previous survey series (Figure A8). Contrasting earlier surveys, herring biomass in 2014 – 2018 consisted almost entirely of immature herring (Figure A9).

No acoustic surveys have taken place since 2018.

6.5.2 Bottom Trawl Survey Data

The MD carries out two annual bottom trawl surveys in West of Scotland waters, with stations in the Firth of Clyde. The time series extends from 1986 to 2024 for Q1, and from 1997 to 2024 for Q4. In 2022 the Q1 survey was cancelled due to a vessel mechanical breakdown. While bottom trawl surveys are not ideal for capturing the dynamics of a pelagic species such as herring, they provide a long-continued time series and are sometimes included in stock assessments for herring stocks.

An analysis of mean log number per standard tow reveals that the mean catch rate of herring in the Clyde has fluctuated over the years, although uncertainty around these figures remains high due to the relatively low number of tows (typically 4 – 6) carried out each year (Figure 3). Historically, catches have consistently been higher in Q1 compared to Q4, although there does appear to be a change in this trend with catches increasing markedly in Q4 over the last three years with mean catch rates in 2024 now higher in Q4 than Q1. These findings differ from further north on Scotland’s west coast where there have been large spawning aggregations of spring spawning herring observed in March at varying locations since 2018. These observations could suggest an increasing number of herring overwintering in the Clyde or inward migration from other west UK herring populations.

An analysis of the spatial distribution of catches in the IBTS surveys suggests that herring are not evenly spread throughout the Clyde but have a patchy distribution. Therefore, the stock perception can sometimes be influenced from year to year depending on whether a large haul is taken, such as those seen in Ayr Bay in some years (Figure 4 and 5).

An analysis of the length frequencies of herring from IBTS catches in 2024 indicates that the Clyde herring stock contains a range of size classes although similar to previous years much of these are small fish in the 0 – 2 age range (Figure 6, A10, A11, A12, A13). However, both the Q1 and Q4 data would indicate a cohort within the minimum landing size of 20 cm in place for this area, and a second group potentially adding to the commercial size biomass in 2025/26.

6.6 Assessments

The last analytical assessment for Clyde herring was performed in 1990.

6.7 Industry Perception

The commercial samples taken from 2011 – 2013 indicate that a significant portion of the landed fish were age 4 or older. The failure of the 2012 – 2018 surveys to capture the adult fish that form the basis of the fishery was seen as problematic. The issue was discussed with industry members pursuing the herring fishery in meetings in 2014, 2015 and 2019.

Many of the commercial landings sampled were caught close inshore in areas that are too shallow for the survey vessel to safely traverse or deploy fishing gear due to its size. There were additional concerns that the acoustic surveys were carried out during the day while adult herring are perceived by fishers to rise off the bottom at night time, becoming easier to detect and catch. The difference in the timing of the fishery and acoustic surveys (particularly the summer surveys of 2008 and 2009) was discussed as a potential cause of differences in perception due to a possible migration of adult herring into the Clyde after completion of the survey. The timing of the fishery does not appear to be guided by the availability of large herring, but rather the availability of time to pursue the fishery (which is largely opportunistic). It was mentioned that locating herring of commercially-viable sizes is time consuming and requires detailed local knowledge.

6.8 Discussion and Conclusion

The lack of adult herring in the acoustic surveys conducted in the Clyde in recent times (2008, 2009, 2012 – 2018) and the discrepancy with the information collected from the fishery in 2011 – 2013 has been a cause for concern. Historical acoustic surveys (1987 – 1990; Table A1) in the Firth of Clyde were carried out in similar fashion to the more recent surveys but did not have problems locating and sampling schools of larger adult herring.

The acoustic survey in 2016 and the trawl surveys in Q4 since 2021 have encountered some larger mature herring present in the Clyde. However, much of the herring biomass in the Firth of Clyde consists predominately of young immature individuals. It is not known whether these juvenile herring originate from herring spawning within the Clyde, as demonstrated by studies in the 1950’s, but based on studies from other nursery areas there is a strong likelihood that herring in the Clyde consist of a mixture of different biological populations. However, while the mix of stocks is uncertain, it is likely that the Clyde also contains juvenile Irish Sea as well as 6aN herring. It would be beneficial to try and address this knowledge gap via genetic analysis, although other constraints, primarily sourcing samples, has, to date, proved difficult.

It is not possible to estimate Maximum Sustainable Yield (MSY) from the currently available data sources, particularly given the likely mixing with other West of UK herring stocks. The present data collection programme will continue to contribute to any future assessment of the stocks.

The 2024 surveys do suggest an apparent increase in biomass, particularly during Q4, as well as the presence of an adult cohort which gives rise to some optimism that Clyde herring stocks are increasing. However, given the relatively poor state of several herring stocks to the west of the British Isles and the high likelihood that juvenile herring in the Clyde are from one or more of these stocks, consideration should be given to continuing those protections currently in place for Clyde herring as a precautionary measure. Ensuring the protection of juvenile herring in the Clyde ensures there are likely to be no adverse impacts on neighbouring stocks.

Considering the available data, there is no evidence to indicate that stocks have significantly decreased since 2022, with Q4 2024 data suggesting another year-on-year increase in stock biomass at this time of year although this is tempered by a potential small decrease during Q1. Accordingly, there is no strong case for reducing the TAC in 2025 but similarly there is currently insufficient data to support an increase above the 2024 level. However, the continued increase in stock biomass for the last three years, and the increased presence of mature adult fish, are very positive signs and one which MD scientists will continue to monitor (and review advice if necessary). Similar to previous years, the strict control measures^[3] already in place should remain. Additionally, efforts should be taken to ensure any catches by vessels in the Clyde are properly sampled and awareness raised among fishers intending to fish in the area for herring to ensure compliance with fishery stipulations.