4.4. Herring

4.4.1. Desktop Study

  1. Herring are a commercially important pelagic fish in the North Sea which are targeted in the vicinity of the site boundary. However, the herring stock collapsed entirely in the 1970s as a consequence of overfishing (Scottish Herring, 2023). Since then, stocks have shown signs of recovery supported by a herring recovery plan implemented for the North Sea in 1996 and a ban on discards for pelagic fisheries, including for herring, from 2015. Active management is however still required to avoid a recurrence of the collapse (Dickey-Collas et al., 2010).
  2. Herring are listed on the Scottish Biodiversity List (SBL) and as a Scottish Priority Marine Feature (PMF) and are therefore considered a high priority species for conservation actions in Scotland (Fauchald et al., 2011; Casini et al., 2004). Herring also plays an important ecological role as they are a key prey species for numerous fish, marine mammals and birds.
  3. Herring populations in the North Sea are divided into three stocks during the breeding season with their own spawning and nursery grounds and migration routes (Daan et al., 1990; Coull et al., 1998). The Buchan/Shetlands stock spawns off the Scottish and Shetlands coasts in August and September, the Banks/Dogger stock spawns in the central North Sea from August to October and Southern Bight/Downs stock spawns in the English Channel between November and January. The Buchan/Shetlands stock off the coast of Scotland is the closest to the site boundary and the fish and shellfish ecology study area. The stock’s spawning and nursery grounds are presented in Figure 4.3   Open ▸ .
  4. Herring nursery grounds are widespread across the entire North Sea and the west coast of Scotland with higher intensity grounds occurring in coastal waters (Ellis et al., 2012) where post larval juveniles that are yet to reach sexual maturity remain to feed. Later, they migrate further offshore where they feed until reaching sexual maturity (ICES, 2006).
  5. Spawning for herring usually takes place in shallow areas between approximately 15 m and 40 m depth. Female herring lay eggs once a year in a single batch, with inter-stock variations on number, size and weights of the eggs (Cefas, 2001). For example, a 28 cm female from the Buchan/Shetlands stock could produce approximately 67,000 eggs per year, whereas a similarly sized individual from the Southern Bight/Downs stock (English Channel) could produce approximately 42,000 (Barreto and Bailey, 2014). The sticky eggs are deposited on a variety of sediments from coarse sand and gravel to shells and small stones, although gravel has been suggested as their preferred spawning habitat. Muddy sediments are considered unsuitable for herring spawning as the fine particles can stick to the eggs and block the pores thus increasing egg mortality via asphyxiation, resulting from the prevention of oxygen transfer through the pores. After incubating for between one and three weeks (water temperature dependent), autumn-spawned herring larvae become pelagic and drift in the plankton from the western North Sea to the eastern North Sea using water currents (Dragesund et al., 1980). Larvae using the Moray Firth as a nursery ground originate from west Scotland and larvae using the Firth of Forth have unclear origins (Department of Energy and Climate Change (DECC), 2004). The pelagic larvae will feed on fish eggs, copepods, euphausiids and juvenile sandeels (Last, 1989). The specific substrates on which herring spawn make herring particularly sensitive to impacts from habitat loss and disturbance. In addition, herring are considered hearing specialists with an increased sensitivity to underwater noise and are therefore vulnerable to injury or disturbance from activities which generate underwater noise, such as pile driving (refer to volume 3, appendix 10.1).

4.4.2. Site-Specific Surveys

  1. Herring spawning grounds are most accurately mapped using a combination of herring larval data and particle size data, as recommended by Boyle and New (2018). In order to characterise herring spawning habitats in the vicinity of the site boundary, these two factors have been considered to accurately determine where the key herring spawning ground for the Buchan stock are located, following the Boyle and New (2018) guidelines. That is, the area where herring are known to spawn most frequently, noting that there is some natural variability in spawning.

                        Particle size data

  1. As outlined in section 3.3, site-specific survey data were collected in 2022 and, alongside desktop studies, were used to assess the extent of suitable spawning habitat for herring within the site boundary. PSA was undertaken on the sediment samples collected which allowed classification of the sediment types according to Reach et al. (2013), as described in Table 4.4   Open ▸ . These classifications were originally developed for the marine aggregates industry, drawing on work from Greenstreet et al. (2010a) investigating spatial interactions between the aggregate application areas and herring spawning habitat.

 

Table 4.4:
Herring Potential Spawning Habitat Sediment Classifications Derived from Reach et al. (2013)

Table 4.4: Herring Potential Spawning Habitat Sediment Classifications Derived from Reach et al. (2013)

 

  1. Habitat suitability classifications for herring spawning, based on site-specific data, illustrated that the overwhelming majority (95%) of the site boundary has unsuitable sediment composition for herring spawning. Just four stations out of 78 within the site boundary were considered suitable for herring spawning (two preferred and two marginal) ( Table 4.4   Open ▸ ). These stations were sparsely distributed in the north-west and centre of the site boundary (see volume 3, appendix 8.1).
  2. Figure 4.7   Open ▸ illustrates site-specific survey data alongside EMODnet seabed substrate data. The EMODnet seabed substrate data can also be used to assign habitat suitability for herring spawning, showing sandy gravel (between 30% and 80% gravel and a sand:mud ratio above 9:1) and gravel (above 80% gravel) as preferred spawning habitat and gravelly sand (between 5% and 30% gravel and a sand:mud ratio above 9:1) as marginal spawning habitat. Where no shading is present in Figure 4.7   Open ▸ , the habitat in that area is unsuitable for herring spawning. Overall, the results from the site-specific surveys data align with EMODnet seabed substrate data with the site boundary being encompassed by unsuitable habitat for herring to spawn. Preferred habitats are located directly north of the site boundary, in line with spawning grounds from Coull et al., (1998).
  3. It is worth noting that the EMODnet seabed substrate data is of lower resolution and accuracy than the results of the site-specific survey data, due to interpolation between known data points, but provides an overall broadscale picture of the surrounding substrate within the region.
  4. Figure 4.7   Open ▸ also shows the wider area comprising the Buchan Stock spawning habitat. This shows more extensive areas of marginal spawning habitat to the north of the site boundary (0.62 km away), coinciding with the area mapped by Coull et al. (1998) and a smaller area towards the south (87.45 km away). These patterns in sediment composition are considered in the context of herring larval abundances, as discussed in paragraph 80.
  5. Figure 4.8   Open ▸ expands on the site-specific data presented in Figure 4.7   Open ▸ , as it includes results of PSA conducted on sediment samples obtained from the OneBenthic Portal in the fish and shellfish ecology study area outside of the site boundary (Cefas, 2019a). Of the 669 samples obtained from the OneBenthic Portal, 559 were assessed as unsuitable, 53 as marginal, and 57 as preferred spawning habitat.

Figure 4.7:
Herring Spawning Habitat Preference Classifications from EMODnet and Site-specific Survey Data

Figure 4.7: Herring Spawning Habitat Preference Classifications from EMODnet and Site-specific Survey Data


Figure 4.8:
Herring Spawning Habitat Preference Classifications from EMODnet and Spawning Sediment Classifications from the Site-Specific Survey Data and from the OneBenthic Portal

Figure 4.8: Herring Spawning Habitat Preference Classifications from EMODnet and Spawning Sediment Classifications from the Site-Specific Survey Data and from the OneBenthic Portal


                        International herring larvae survey data

  1. Herring spawning grounds can also be identified through monitoring of herring larval abundances, alongside sediment composition data. The IHLS conducts monitoring programmes where larvae numbers are recorded around the UK coastline and throughout the North Sea. Herring larvae are identified as being recently hatched by their size, with small herring larvae assumed to have been hatched recently and in close proximity to the area where eggs were laid (ICES, 2006; 2022b). The IHLS present herring larvae counts by size per m2, with larvae <10 mm long used as a cut off point for recently hatched larvae (ICES, 2022b).
  2. High abundances of herring larvae are a good indication of recent spawning activity local to where these were sampled. These data were plotted for each year from 2007 to 2016 in Figure 4.9   Open ▸ to Figure 4.11   Open ▸ showing the changing spatial distribution of herring spawning relative to areas of historic spawning grounds as identified by Coull et al. (1998), in line with guidance from Boyle and New (2018).
  3. Due to lack of IHLS survey data between 2017 and 2018, and a change in reporting strategy from IHLS, since 2019, more recent herring larvae data were not available for analysis. However, an ICES scientific report (ICES, 2021) noted that IHLS data for 2019 to 2020 in the Buchan area was in the same order of magnitude as previous years, therefore, it can be assumed that there are no significant changes from the results presented for 2007 to 2016 outside of normal annual variations.
  4. These data show that the spawning ground adjacent to the north-west of the site boundary identified by Coull et al. (1998) has recorded persistently high levels of spawning activity with relatively little variation from 2007 to 2016. The spawning area identified to the south-west of the site boundary has had variable spawning levels from 2007 to 2016. It is worth noting that spatial variability of larval densities may be a result of the timing of data collection and/or variation in ocean and tidal current speeds and direction, which may account for some of the variability shown to the south-west of the site boundary. Both spawning areas identified through Coull et al. (1998) and the IHLS heat maps are supported by habitat suitability data from EMODnet, as shown in Figure 4.7   Open ▸ and Figure 4.8   Open ▸ by the large patches of favourable and marginal spawning habitat directly to the north and further south-west of the site boundary, which corresponds with spawning areas identified through particle size data and IHLS larval data.
  5. Each year of data were also presented cumulatively over the ten year period between 2007 and 2016 ( Figure 4.12   Open ▸ ) to gain an understanding of where the most common spawning grounds were over the time period. As per Figure 4.12   Open ▸ , a persistent hotspot of high larval density is present towards the north-west of the site boundary. It should be noted that this is consistent with Coull et al. (1998), which demonstrates its continued relevance when assessing spawning grounds. These data align with what was reported in the post-consent fish monitoring strategy report for Seagreen Alpha and Bravo (now referred to as Seagreen 1 Offshore Wind Farm and Seagreen 1A Project) (Seagreen Wind Energy Ltd., 2019).
  6. No high intensity spawning grounds identified by Coull et al. (1998) directly overlap with the site boundary. This is supported by the habitat suitability data from both site-specific sampling effort and broadscale EMODnet seabed substrates (following classifications in Reach et al., 2013), as shown in Figure 4.7   Open ▸ . The large patches of gravelly sand and >5% mud content reported provide unsuitable spawning habitat throughout much of the site boundary, with only four discrete areas of marginal/preferred spawning habitat identified out of 78 stations.

Figure 4.9:
Herring Larval Density from IHLS Data Sets from 2007 to 2010

Figure 4.9: Herring Larval Density from IHLS Data Sets from 2007 to 2010


Figure 4.10:
Herring Larval Density from IHLS Data Sets from 2011 to 2014

Figure 4.10: Herring Larval Density from IHLS Data Sets from 2011 to 2014


Figure 4.11:
Herring Larval Density from IHLS Data Sets from 2015 and 2016

Figure 4.11: Herring Larval Density from IHLS Data Sets from 2015 and 2016


Figure 4.12:
Herring Cumulative Larval Density from IHLS Data Sets from 2007 to 2016

Figure 4.12: Herring Cumulative Larval Density from IHLS Data Sets from 2007 to 2016