6. Scientific Information on the Status of Herring (Clupea harengus) in the Clyde Sea (ICES Statistical Rectangles 39E4 – 40E5) in 2023
Eleanor MacLeod, Campbell Pert and Steven O'Connell
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 1960s, with catches typically less than 500 tonnes over the last 20 years. Excluding 180 tonnes caught in 2021, catches have remained at zero tonnes since 2014.
Scientific surveys suggest that the herring population currently found in the Clyde is heavily dominated by young age classes (1- and 2- year old herring). These fish are below the regulation minimum landing size (20 cm) for this area.
Clyde herring are known to be linked to other herring stocks to the west of Scotland, which are currently at low biomass levels.
There is no evidence that herring stocks had significantly decreased since 2021. However, there is no scientific evidence to suggest an increase above the 2021 TAC of 583 tonnes would be appropriate. Similarly, the data do not suggest that the TAC should be reduced below the 2022 level of 466 tonnes.
Due to the lack of available data for Clyde herring, the current control measures should remain in place. Fishers should ensure compliance with the measures specific to this fishery.
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, have altered the fishery and the fish stocks of the Clyde.
The herring fishery 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 1950s and mid-1960s. From the mid-1960s to 1980s 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). In 2022, due to the cancellation of the west coast survey in quarter 1 following the research vessel Scotia suffering a mechanical breakdown, no fishery-independent data on herring in the Firth of Clyde were available. The TAC was therefore set at 466 tonnes, as a precautionary measure.
Over time, the fishery has been dominated by Scottish and Northern Irish vessels. The majority of the catch was taken by Scottish vessels from 1995 to 2000, but by Northern Irish vessels from 2000 to 2012. From 2013 to 2022 there were no 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 – 2020 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 shows there have been no strong year classes found in the Clyde catches since the 1990s.
|Scotland||2 506||2 530||2 991||3 001||3 395||2 895||1 568||2 135||2 184||713||929||852||608||392|
|Discards||1 253||1 265||2 3082||1 3442||6792||439||245||-3||-3||-3||-3||-3||-3||-3|
|Agreed TAC||3 000||3 000||3 100||3 500||3 200||3 200||2 600||2 900||2 300||1 000||1 000||1 000|
|Total||4 021||4 361||5 770||4 800||4 650||3 612||1 923||2 343||2 259||731||929||853||608||586|
|Agreed TAC||1 000||1 000||1 000||1 000||1 000||1 000||1 000||1 000||1 000||1 000||1 000||800||800||800|
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 Scotland 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. Length information was collected from 693 herring and otoliths were taken and aged from 229 fish. Fish ranged in size from 18 to 31 cm, with a modal length of 27 cm (Figure 3). Landings were composed of fish spanning the full age range (1 – 10 years). Approximately 40% of the fish landed were four years or older.
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 landing caught at the outer edge of the Firth of Clyde in 2012 (pers. comm. SFO Campbeltown). Overall, 6 samples were collected from the 12 trips. Similar to 2011, 5 of these samples were collected in the inner Firth of Clyde. A total of 679 herring were measured and 189 fish were aged (Table 2).
|Year||Landings||Landings sampled||Proportion Sampled||Measured herring||Aged herring|
In 2012 fish ranged from 18 to 36 cm, with a modal length of 24 cm. Landings were dominated by fish between 1 – 3 years (Figure 4). The large landings from the outer Clyde were composed solely of smaller younger herring (modal length 23.5 cm and age range of 1 – 5 years, 95% of fish aged 3 or below). In contrast the landings from the inner Clyde were more similar in composition to 2011 (modal length 27.5 cm, spanning the full age range, 67% of fish landed were 4 years or older).
The dominance of the large sample from the outer Clyde results in the numbers at age in the landings being very different in 2012 compared to 2011. The proportion of fish at older ages in 2012 is much lower than in 2011, but this is entirely driven by the composition of the one large landing from the outer Clyde.
In 2013 the fishery was very small, with only 21 tonnes of registered landings (Table 1). Half of the 4 trips carried out by Scottish vessels were sampled (Table 2). A total of 420 herring were measured and 280 fish were aged.
Fish ranged from 11.5 to 29 cm, with a modal length of 27 cm that was similar to previous years. Landings were dominated by age 5 fish (35%, Figure 5).
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 no samples were provided by the vessel. 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 tends to be low. Landings are dominated by different ages over the three years (Figure 6), although the sample data in 2012 are dominated by the one large landing in the outer Clyde. Length against age plots (Figure 7) 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 years available are in good agreement with one another.
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 MS 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 have been no observer trips since 2019 due to the Covid-19 pandemic, 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 8).
Discards from 2009 – 2019 are dominated by small herring (Figure 9), and are often smaller in size than herring sampled from the commercial sampling programme (Figures 3 - 5). 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 9).
6.5 Survey Data
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 Prior to 2012
Acoustic surveys were carried out in July from 1987 to 1990 and again in 2008 – 2009 (Table 3, Figure 10, Figure 11). 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 10). The biomass was predominately within the boundaries of the Clyde area, and composed solely of aged one and two (juvenile) herring (Figure 11) rather than herring of a size targeted by the fishery (over 20 cm).
Acoustic Surveys 2012 – 2018
A new series of pelagic acoustic surveys in the Clyde were carried out each October between 2012 and 2018, apart from 2013 where the survey was cancelled due to vessel breakdown (Figure 10). In 2015 and 2018 intermittent faults with the 38kHz 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 11).
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 a product of changes in survey approach, the results are not directly comparable.
Distributions of herring in the surveys were similar between years, with the majority of 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. Apart from 2016, biomass estimates in recent years are significantly lower than estimates from the previous survey series (Figure 10). Contrasting earlier surveys, herring biomass in more recent years is almost entirely composed of immature herring (Figure 11).
No acoustic surveys have taken place since 2018.
Bottom Trawl Survey Data
Marine Scotland carries out two annual bottom trawl surveys in West of Scotland waters, with stations in the Firth of Clyde. The time series extends back from 1986 and 1997 to 2023 and 2022 for quarter 1 and quarter 4 respectively. In 2022 the quarter 1 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 remains low, and uncertainty around these figures remains high due to the low number of tows carried out each year (Figure 12). Catches are consistently higher in quarter 1 compared to quarter 4, which may be explained by the herring overwintering in areas further up the Clyde sea lochs where this survey does not operate.
An analysis of the spatial distribution of catches in the IBTS surveys suggests that herring are not evenly distributed throughout the Clyde, but have a patchy distribution. Therefore the stock perception can be strongly influenced from year to year depending on whether a large haul, such as those seen in Ayr Bay some years, is taken (Figure 13, Figure 14).
An analysis of the length-frequencies of herring from IBTS catches shows the Clyde herring stock being dominated by small fish in the 0 – 2 age range with very few older, typically larger fish being observed, particularly in recent years (Figure 15 – Figure 18). With a minimum landing size of 20 cm in place for this area, it is likely that a fishery targeting herring in the Clyde would encounter significant quantities of undersize fish.
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 these 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 MS survey vessel to practically survey 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 quite opportunistic). It was mentioned that locating commercially viable sized herring 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 3) 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 2015 have encountered some larger mature herring, but these made up a very small proportion of the overall biomass and almost none were encountered in the 2017 or 2018 acoustic or the Q1 trawl surveys.
It is possible to some extent that this is due to differences in the fishing practices employed between commercial and research vessels. The commercial catches are mainly taken in very shallow water inaccessible to the survey vessel, and only located after prolonged searching. This would indicate a typical pattern of decreased stock size in pelagic fish, where school size is maintained but the number of schools is decreased and the encounter rate in the survey reduced. The lack of catch data since 2014 hinders further comparisons with recent survey results. However, the distinct shift in age composition was also observed in the results from the MSS bottom trawl survey series indicating that this shift in perception of the age composition is not caused by a failure of the survey to capture older fish. The re-appearance of mature fish in the 2016 acoustic survey (confirmed in the bottom trawl survey that year) indicates that the survey in its present form is able to detect the presence of these older fish.
The herring biomass in the Firth of Clyde is predominately composed of young immature individuals. It is not known whether these juvenile herring originate from herring spawning within the Clyde, but based on studies from other nursery areas there is a strong likelihood that herring in the Clyde comprise a mixture of different biological populations; the mix of stocks is uncertain but it is likely that the Clyde also contains juvenile Irish Sea herring (for example). With the improvements in genetic testing for stock management a longer term aspiration could be to try and address this knowledge gap via genetic analysis.
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 Scotland herring stocks. The present data collection programme will continue to contribute to the assessment of the stocks.
The continued absence of adult fish in surveys in 2021 and the loss of the quarter 1 bottom trawl survey in 2022 warranted continued conservative management of this resource. Given the poor state of several of the 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 continue to offer protection for those herring in the Clyde so as to not adversely impact neighbouring stocks.
In light of the available data, there is no evidence to indicate that stocks have significantly decreased since 2021, the last year for which Q1 survey data was available. There is also no evidence to support an increase in the TAC above the 2021 level. Similarly, there is no strong case for further reducing the TAC as long as the strict control measures (section 5) already in place remain enforced. Efforts should be taken to ensure catches by vessels in the Clyde are properly sampled, and fishers should ensure compliance with the measures specific to this fishery.
There is a problem
Thanks for your feedback