5. Assessment of Potential Adverse Effects on Integrity: Annex II Diadromous Fish

5.1. Introduction

  1. This section provides background information and an explanation for the approach taken to assess the potential impacts of the Array on European sites designated for Annex II diadromous fish.
  2. As stated in section 3.1, the potential for LSE2 was identified for the Annex II diadromous fish features of nine SACs, which are listed in Table 5.1   Open ▸ and illustrated in Figure 5.1   Open ▸ .

 

Table 5.1:
European Sites Designated for Annex II Diadromous Fish Features for which an Appropriate Assessment is Presented

Table 5.1: European Sites Designated for Annex II Diadromous Fish Features for which an Appropriate Assessment is Presented

Figure 5.1:
Location of European Sites Designated for Annex II Diadromous Fish for which an Appropriate Assessment is Required

Figure 5.1: Location of European Sites Designated for Annex II Diadromous Fish for which an Appropriate Assessment is Required


  1. LSE2s on the SACs presented in Table 5.1   Open ▸ were identified for the construction and operation and maintenance phases of the Array, and these are outlined in Table 5.3   Open ▸ A range of designed in measures have been committed to as part of the Array, these are presented, where relevant, in section 5.3.

 

Table 5.2:
Potential For Impact to Annex II Diadromous Fish

Table 5.2: Potential For Impact to Annex II Diadromous Fish

 

  1. The Stage Two Appropriate Assessment (considering effects of the Array both alone and in-combination) for European sites designated for Annex II diadromous fish are presented in section 5.3 and 5.4 respectively. A summary of Assessments undertaken within this Part of the RIAA is provided in section 7.
  2. Freshwater pearl mussel has been considered within this section (specifically as a qualifying feature of the River Dee SAC, River South Esk SAC, River Spey SAC, and the River Oykel SAC) because the larval stage of its life cycle is reliant on salmonid species, such as Atlantic salmon (see paragraph 16). The potential for adverse effects to freshwater pearl mussel, if they occur at all, would be indirect and would occur as a result of direct effects on Atlantic salmon, which are a relevant host species for freshwater pearl mussel within the SACs assessed.

5.2. Baseline

  1. Baseline information on the relevant screened in Annex II diadromous fish and freshwater pearl mussel features has been gathered through a comprehensive desktop study of existing datasets and materials. Full detail is provided in volume 2, chapter 9 and volume 3, appendix 9.1 of the Array EIA Report (Ossian OWFL, 2024).
  2. Within the Array EIA Report, a broad fish and shellfish ecology study area was defined and agreed with SNCBs for the purposes of the baseline characterisation. It encompassed a buffer of 100 km around the site boundary and included the Firth of Forth. A buffer of 100 km was used as it represented a precautionary Zone of Influence (ZoI) of potential impacts due to underwater noise. The fish and shellfish ecology study area provides wider context for the spatially and temporally variable Annex II diadromous fish species and encompasses a potential area in which they may migrate to and from the various SACs assessed in this Part of the RIAA ( Figure 5.1   Open ▸ ). Based on feedback from SNCBs, an additional three SACs in the Moray Firth (and therefore out with the fish and shellfish ecology study area) have been brought forward for Appropriate Assessment ( Figure 5.1   Open ▸ , see Table 2.1   Open ▸ ).

5.2.1. River Dee SAC

                        Site description

  1. At its closest point, the River Dee SAC is located 80.57 km north-west from the site boundary. The entire length of the River Dee is designated as a SAC due to its importance for Atlantic salmon and freshwater pearl mussel. The River Dee originates in the Cairngorms and flows through southern Aberdeenshire to reach the North Sea at Aberdeen. The site covers an area of 2,334.48 ha and is designated for the following Annex II diadromous fish features: Atlantic salmon and freshwater pearl mussel (NatureScot, 2020c). 

                        Feature accounts

                        Atlantic salmon
  1. As a diadromous species, Atlantic salmon live in both freshwater and marine environments throughout their life cycle. Adult Atlantic salmon spawn in the rivers that they were born in (referred to as ‘natal rivers’) from November to December, and females lay their eggs in gravel depressions known as ‘redds’. These are immediately fertilised by a male, and the female will cover the fertilised eggs with gravel (NatureScot, 2023a). The eggs typically hatch in early spring and are known as ‘fry’ when they reach around 3 cm in length. They develop into ‘parr’ once they develop markings on their sides, and live in the river for two to three years before migrating to the sea as ‘smolts’ and ‘post-smolts’ (NatureScot, 2023a). After one to three years at sea, adult Atlantic salmon migrate back to their natal river to spawn again. Individuals that only spend one year at sea, before migrating back to spawn are referred to as ‘grilse’, which reach 2 kg to 3 kg in weight (Malcolm et al., 2010, NatureScot, 2023a, Scottish Government, 2019). Individuals that spend two to three years at sea before returning to spawn are referred to as ‘multi-sea-winter salmon’ and are typically larger than grilse as they’ve spent more time feeding in marine environments (usually off the coast of Greenland) (Malcolm et al., 2010, NatureScot, 2023a, Scottish Government, 2019). Around 90% to 95% of Atlantic salmon will die after spawning, but those which survive may spawn again (NatureScot, 2023a).
  2. Malcolm et al. (2015) used metadata to assess the timing of smolt emigration across Scotland. This suggested that most fish leave rivers between mid-April and the end of May. These results do not include the period spent by smolts in the coastal environment after leaving their natal rivers. There was also evidence that smolt emigration is becoming earlier (by around 1.5 days per decade over a period of around 50 years) (Malcolm et al., 2015).
  3. Various cues are involved during their return migration to their natal rivers; in earlier phases, sun position and Earth’s magnetic field seem to play a role in oceanic orientation (Hansen et al., 1998). Tidal phase and time of day have also been suggested as important factors for their upstream migration (Smith et al., 1997). Migration upstream of estuaries have also been observed to be nocturnal, occurring during ebb tides (Smith et al., 1997). In the final phase of the upstream migration, olfactory cues direct Atlantic salmon up the river (Hasler et al., 1983). For smolts migrating downstream, migratory activity has been identified to be associated with night time while daytime was utilised more for prey detection and predator avoidance  (Hedger et al., 2008). Upon reaching the North Sea, the post-smolts are transported by water currents towards northern Norway and then into the Norwegian Sea (Jonsson et al., 1993). Further evidence from Atlantic salmon from the east coast of Scotland (i.e. from the River Dee, River Tay and River North Esk) recaptured in Greenland and the Faroe Islands waters showed that smolts emigrated west to feed and grow (Malcolm et al., 2010).
  4. This is further supported by evidence from the Moray Firth (Gardiner et al., 2018b, Newton et al., 2019, Newton et al., 2017), which suggests that smolts migrating from their rivers in the Moray Firth head directly across the North Sea relatively rapidly. Newton et al. (2017) also showed the majority remained predominantly within the upper 1 m of the water column during migration. Mortality of smolts was considered mainly attributable to predation and there was a strong relationship between group survival, early migration and group size.  It is thought that this route, rather than moving in a coastal direction upon leaving their natal rivers, allows them to take advantage of east flowing currents which cross the North Sea. This fast progress away from the coast limits exposure to predators occurring close to the coast. Similar evidence of a rapid easterly migration out into the North Sea has also been shown for the River Dee in Aberdeenshire (Gardiner et al., 2018a). Therefore, it could be assumed that smolts from other east coast rivers (e.g. the River Tay, River Dee, River South Esk, River Tweed, and River Teith) would move in a similar fashion.
  5. Between 2018 to 2021, a tagging study was conducted on juvenile Atlantic salmon and sea trout migrating from the River Dee (River Dee Trust et al., 2023). Atlantic salmon were found to travel at an average speed of 0.45 m/s from the river mouth to around 4 km offshore, before dropping to 0.24 m/s between 4 km to 20 km offshore (River Dee Trust et al., 2023). Individuals mostly swam within the top 3 m of the water column and swam in an easterly direction in the first 4 km, before following a south-easterly trajectory. This pattern was consistent over the three years of the study, suggesting that this is a regular migratory route and that Atlantic salmon must make a northerly course adjustment at some point in their migration to reach higher latitude feeding grounds in the Norwegian Sea (River Dee Trust et al., 2023). The authors concluded that this easterly and south-easterly migratory trajectory suggests that Atlantic salmon could be present in offshore areas of the North Sea.
  6. Atlantic salmon numbers have declined throughout their geographic range, including in Scottish rivers (JNCC, 2024e, NatureScot, 2020c). The most recent wild Atlantic salmon total rod catch data reported 42,204 individuals in 2022, the fourth lowest record since the start of the statistics on salmon fisheries in Scotland by Marine Scotland in 1952 (Scottish Government, 2023). This is an increase from the 35,693 recorded in 2021 (Scottish Government, 2022), but still provides evidence of the population declines of Atlantic salmon in Scotland.
  7. Atlantic salmon are of considerable cultural and conservation importance (Hindar et al., 2011). The species is subject to many pressures in both marine and freshwater environments. These include pollution, the introduction of non-native salmon stocks, physical barriers to migration, exploitation from netting and angling, physical degradation of spawning and nursery habitat, and increased marine mortality (Oslo Paris Convention (OSPAR) Commission (2024)). Since 2016, as a result of the Salmon Conservation Regulations, Atlantic salmon caught in coastal waters must be released. This was implemented to prevent the killing of Atlantic salmon in coastal waters and estuaries to protect stocks that were in poor conservation status.
  8. Alongside other salmonids (such as sea trout Salmo trutta), Atlantic salmon are host species for the parasitic larval phase of freshwater pearl mussel. Freshwater pearl mussel are strictly freshwater species and have seen population declines throughout their UK range. A decline in Atlantic salmon stocks is one of the factors corresponding to this population decline. As stated in paragraph 16, the potential impact s to Atlantic salmon can directly affect populations of Annex II freshwater pearl mussel, which has been assessed alongside Atlantic salmon in this Part of the RIAA.
                        Freshwater pearl mussel
  1. Freshwater pearl mussels are long-lived freshwater molluscs that live in beds of clear, well-oxygenated, and fast-flowing rivers that are free of turbidity and pollution (JNCC, 2024d). They burrow into sandy substrates, often between boulders and pebbles (JNCC, 2024d). During their parasitic larval stage, they are dependent upon a healthy population of salmonids (young Atlantic salmon or sea trout) which act as host species (Taeubert et al., 2017). Freshwater pearl mussel larvae attach to the gills of salmonid fish in mid to late summer and drop off the following spring. When they detach from their hosts they must land in sandy or gravelly substrates to settle and grow to adulthood (JNCC, 2024d). In suitable conditions they can live for over 100 years and grow up to 20 cm.
  2. They are filter feeders, and are therefore particularly vulnerable to pollution and other changes in water quality (NatureScot, 2023b). Other threats to freshwater pearl mussels include hydrological alterations (including river engineering and abstractions), habitat degradation of river beds and banks, illegal fishing, and availability of host salmonids (JNCC, 2024d, NatureScot, 2023b).
  3. The freshwater pearl mussel is widely distributed in Europe and north-eastern North America, but has suffered serious decline and is threatened with extinction or is highly vulnerable in every part of its former range (JNCC, 2024d, NatureScot, 2023b). It is listed as ‘critically endangered’ in Europe by the International Union for the Conservation of Nature and Natural Resources (IUCN) (Moorkens, 2011). The species was formerly widespread throughout western and northern parts of the UK. However, England and Wales are each now believed to support only a single recruiting population. In Northern Ireland the species formerly occurred widely in several catchments but is now restricted to a few sites (NatureScot, 2023b). Many UK rivers now contain only scattered individuals, with no juvenile mussels recorded; such populations may become extinct due to lack of recruitment. Despite serious declines in both range and total population, Scotland is the remaining European stronghold for the species, supporting functional populations in over 50 rivers, mainly in the Highlands. Of the 26 SACs designated for freshwater pearl mussel, 19 are in Scotland, and four of these are included in this Part of the RIAA (e.g. the River Dee SAC, River South Esk SAC, River Spey SAC, and River Oykel SAC).

                        Conservation objectives

  1. Conservation objectives for the River Dee SAC have been developed by NatureScot as part of a CAP (NatureScot, 2020c). Conservation objectives for all qualifying features of this SAC are:
  • to ensure that the qualifying features of the River Dee SAC are in favourable condition and make an appropriate contribution to achieving FCS; and
  • to ensure that the integrity of the River Dee SAC is restored by meeting objectives 2a, 2b, 2c for each qualifying feature (and 2d for freshwater pearl mussel) (NatureScot, 2020c).
  1. Conservation objectives for freshwater pearl mussel are as follows:
  • 2a. Restore the population of freshwater pearl mussel as a viable component of the site;
  • 2b. Restore the distribution of freshwater pearl mussel throughout the site;
  • 2c. Restore the habitats supporting freshwater pearl mussel within the site and availability of food; and
  • 2d. Restore the distribution and viability of freshwater pearl mussel host species and their supporting habitats (NatureScot, 2020c).
  1. Conservation objectives for Atlantic salmon are as follows:
  • 2a. Restore the population of Atlantic salmon, including range of genetic types, as a viable component of the site;
  • 2b. Restore the distribution of Atlantic salmon throughout the site; and
  • 2c. Restore the habitats supporting Atlantic salmon within the site and availability of food (NatureScot, 2020c).
  1. The overarching conservation objectives detailed in paragraph 69 are related to the species-specific conservation objectives 2a to 2c for Atlantic salmon and 2a to 2d for freshwater pearl mussel. Therefore, the assessment of the Array alone and in-combination with other plans and projects focuses on the individual species-specific conservation objectives. This allows a proportionate approach, as by demonstrating that potential impact s associated with the Array alone and in-combination with other plans and projects will not have an adverse effect on the species-specific conservation objectives (2a to 2c/2d), the overarching conservation objectives will therefore not be impaired.

                        Condition assessment

  1. The condition of Atlantic salmon was assessed in 2011, and in 2014 for freshwater pearl mussel (NatureScot, 2020c). The outcomes of these feature condition assessments were as follows:
  • freshwater pearl mussel: unfavourable – declining; and
  • Atlantic salmon: favourable – maintained.
  1. Freshwater pearl mussel was assessed as being in unfavourable condition due to the low number and density of freshwater pearl mussels present. This is due to low levels of juvenile recruitment, water flow, river morphology, the presence of filamentous algae, and water quality at the SAC (NatureScot, 2020c).

5.2.2. River South Esk SAC

                        Site description

  1. At its closes point, the River South Esk SAC is located 107.13 km west from the site boundary. The site is located in Angus in Eastern Scotland and covers 471.85 ha. The site is designated solely for Atlantic salmon and freshwater pearl mussel (NatureScot, 2020e).

                        Feature accounts

                        Atlantic salmon
  1. The ecology of Atlantic salmon is as described above in section 5.2.1, and not repeated here. The SAC supports a large, high-quality Atlantic salmon population. The River South Esk has a strong nutrient gradient along its length, rising in the nutrient-poor Grampians and flowing for half of its length through the rich agricultural lands of Strathmore. The high proportion of the South Esk which is accessible to salmon and the range of ecological conditions in the river allows it to support the full range of life-history types found in Scotland, with sub-populations of spring, summer salmon and grilse all being present (JNCC, 2024g).
                        Freshwater pearl mussel
  1. The ecology of freshwater pearl mussel is as described above in section 5.2.1, and not repeated here,. This species is abundant in the River South Esk and is highest in the middle reaches of the river where they attain densities > 20 m2. The conservation importance of the site is further increased by the abundance of juveniles which comprise approximately 20% of the population. The presence of juvenile freshwater pearl mussels >20 mm long indicates that there has been successful recruitment since monitoring began in 1996 (JNCC, 2024g).

                        Conservation objectives

  1. Conservation objectives for the River South Esk SAC have been developed by NatureScot as part of a CAP (NatureScot, 2020e). Conservation objectives for all qualifying features of the SAC are:
  • to ensure that the qualifying features of the River South Esk SAC are in favourable condition and make an appropriate contribution to achieving FCS; and
  • to ensure that the integrity of the River South Esk SAC is restored by meeting objectives 2a, 2b, 2c for each qualifying feature (and 2d for freshwater pearl mussel) (NatureScot, 2020e).
  1. Conservation objectives for freshwater pearl mussel are as follows:
  • 2a. Restore the population of freshwater pearl mussel as a viable component of the site;
  • 2b. Restore the distribution of freshwater pearl mussel throughout the site;
  • 2c. Restore the habitats supporting freshwater pearl mussel within the site and availability of food; and
  • 2d. Restore the distribution and viability of freshwater pearl mussel host species and their supporting habitats (NatureScot, 2020e).
  1. Conservation objectives for Atlantic salmon are as follows:
  • 2a. Restore the population of Atlantic salmon, including range of genetic types, as a viable component of the site;
  • 2b. Restore the distribution of Atlantic salmon throughout the site; and
  • 2c. Restore the habitats supporting Atlantic salmon within the site and availability of food (NatureScot, 2020e).
  1. The overarching conservation objectives detailed in paragraph 78 are related to the species-specific conservation objectives 2a to 2c for Atlantic salmon and 2a to 2d for freshwater pearl mussel. The assessment of the Array alone and in-combination with other plans and projects focuses on the individual species-specific conservation objectives. This allows a proportionate approach, as by demonstrating that potential impacts  associated with the Array alone and in-combination with other plans and projects will not have an adverse effect on the species-specific conservation objectives (2a to 2c/2d), the overarching conservation objectives will therefore not be impaired.

          Condition assessment

  1. The condition of Atlantic salmon was assessed in 2011, and in 2009 for freshwater pearl mussel (NatureScot, 2020e). The outcomes of these feature condition assessments were as follows:
  • freshwater pearl mussel: unfavourable – no change; and
  • Atlantic salmon: unfavourable – recovering. 
  1. Freshwater pearl mussel was assessed as being in unfavourable condition due to the low number and density of freshwater pearl mussels present. This is due to low levels of juvenile recruitment, biological oxygen demand, and disturbance of mussel beds through largely historical fishing (NatureScot, 2020e).

5.2.3. Tweed Estuary SAC

                        Site description

  1. At its closest point, the Tweed Estuary SAC is located 128.65 km south-west from the site boundary. The site, located in Northumberland, encompasses the Tweed Estuary, a long and narrow estuary discharging into the North Sea. The site covers an area of 155.93 ha.  The site is designated for Annex I habitats and Annex II river lamprey and sea lamprey (with the latter being applicable to this Part of the RIAA) (Natural England, 2018).

                        Feature accounts

                        Sea lamprey
  1. The sea lamprey is a primitive, jawless fish which resembles an eel. It is the largest of the three lamprey species found in the UK (with the other two being river lamprey and brook lamprey Lampetra planeri) (NatureScot, 2023c). As a diadromous species, the sea lamprey requires both freshwater and marine habitats during different phases of its life cycle. Adults require estuaries and easily accessible rivers with clean gravel substrates in which to spawn. Hatched larvae are referred to as ammocoetes, which drift downstream with the current and settle in nursery habitats with fine, soft substrate in well-oxygenated and slow-flowing freshwater (NatureScot, 2023c). Sea lamprey ammocoetes may remain in their freshwater nurseries for up to eight years before the metamorphose into adults and migrate to the marine environment, where they are parasitic predators on a range of different fish species (Hume, 2017; Maitland, 2003). These prey species include large fish, such as salmonids, but sea lamprey have been recorded as a parasitic predator on at least 54 different fish and marine mammal species (Silva et al., 2014). Sea lamprey remain at sea for 18 to 24 months, before migrating upstream into freshwater spawning habitats between April and May. They spawn in May and June, and die after spawning (JNCC, 2023c). During spawning, they have preference for warm waters (JNCC, 2023c). In contrast to Atlantic salmon (see paragraph 58), there is no evidence of homing behaviour to natal rivers in sea lamprey (Scottish Government, 2019).
  2. Sea lamprey are rarely captured in coastal and estuarine waters, suggesting that they are solitary hunters and widely dispersed at sea, and can be found at considerable depths (up to 4,099 m) (Scottish Government, 2019). As they are parasites during the adult marine phase of their life cycle, their distribution at sea is largely dictated by their host species (Scottish Government, 2019). As such it is not expected that they will be particularly attracted to structures associated with offshore wind developments. However, this is not certain, as there is limited information available on the utilisation of the marine environment by sea lamprey. It is a possibility that sea lamprey will be present in the vicinity of the Array.
  3. The sea lamprey occurs over much of the Atlantic coastal area of western and northern Europe (from northern Norway to the western Mediterranean) and eastern North America, and is reasonably widespread in UK rivers (JNCC, 2023c). However, sea lamprey has declined in parts of its UK range and has become extinct in a number of rivers. It appears to reach its northern limit of distribution in Scotland and does not occur north of the Great Glen (see section 5.2.6) (JNCC, 2023c).
  4. The Tweed Estuary SAC, together with the River Tweed SAC (see section 5.2.4), was designated for its significant presence of sea lamprey (Natural England, 2018). Sea lamprey migrates through the Tweed Estuary to reach freshwater spawning grounds in the main River Tweed and its tributaries in late May and June. Downstream juvenile migration occurs in July to September (Maitland, 2003). Given the difficulty in surveying sea lamprey larvae, they were only recorded in the lower part of the main river in the Tweed catchment in a 2004 dataset (NatureScot, 2020a). It is unclear whether or not this reflects the actual distribution of sea lamprey or whether it is due to the difficulties of sampling sea lamprey as reports of spawning and adults were historically more widespread over the catchment (NatureScot, 2020a). The 2004 dataset also presented evidence for three years of spawning sea lamprey in the lower Tweed (NatureScot, 2020a). However, a more recent survey in 2013 did not find any records of sea lamprey ammocoetes (Campbell, 2013).
  5. Sea lamprey use the Tweed Estuary for migration and feeding between their freshwater spawning grounds and coastal waters. Man-made features such as weirs and dams, as well as polluted sections of the River Tweed may impede migration (Natural England, 2018). Within the adjacent freshwater River Tweed SAC (see section 5.2.4), sea lamprey need clean gravel for spawning, and marginal silt or sand for juveniles to burrow in (Natural England, 2018).

                        Conservation objectives

  1. The conservation objectives for the Tweed Estuary SAC were developed by Natural England (2018). These high-level objectives ensure that, subject to natural change, the integrity of the site is maintained or restored as appropriate, and that the site contributes to achieving the FCS of its qualifying features, by maintaining or restoring:
  • the extent and distribution of qualifying natural habitats and habitats of the qualifying species;
  • the structure and function (including typical species) of qualifying natural habitats;
  • the structure and function of the habitats of the qualifying species;
  • the supporting processes on which qualifying natural habitats and the habitats of qualifying species rely;
  • the populations of each of the qualifying species; and
  • the distribution of qualifying species within the site (Natural England, 2018).
  1. The second conservation objective: ‘the structure and function (including typical species) of qualifying natural habitats’ is only relevant to the Annex I habitat features of the Tweed Estuary SAC and is therefore not included further in this assessment on Annex II diadromous fish features.
  2. Supplementary advice on conservation objectives (last updated on 09 May 2023), provide the site-specific attributes and targets specific to the sea lamprey feature of the site. All targets for the sea lamprey feature have been set as ‘Maintain’ by Natural England, using expert judgement based on knowledge of the sensitivity of the feature to activities that are occurring/have occurred on the site (Natural England, 2023b). A summary of the conservation targets for sea lamprey are as follows:
  • maintain the unrestricted usage of the estuary by adult and juvenile sea lamprey including for migratory passage and juvenile development;
  • maintain the reproductive and recruitment capability of the species;
  • maintain the presence and spatial distribution of the species and their ability to undertake key life cycle stages and behaviours;
  • maintain connectivity of estuarine features to surrounding rivers, freshwater, marine and coastal habitats, to ensure larval dispersal and recruitment, maintain nursery grounds for mobile species, and to allow movement of migratory species;
  • maintain the extent and spatial distribution of the following supporting habitats: water column;
  • maintain the abundance of preferred food items required by the species;
  • maintain the natural physico-chemical properties of the water;
  • maintain all hydrodynamic and physical conditions such that natural water flow is not significantly altered or constrained;
  • reduce aqueous contaminants to levels equating to High Status according to Annex VIII and Good Status according to Annex X of the Water Framework Directive (WFD), avoiding deterioration from existing levels. This target was set using the Environmental Agency 2019 water body classifications data;
  • maintain the dissolved oxygen concentration at levels equating to High Ecological Status (specifically ≥ 5.7 mg/L (at 35 salinity) for 95% of year) avoiding deterioration from existing levels. This target was set using the Environmental Agency 2019 water body classifications data;
  • maintain water quality at mean winter dissolved inorganic nitrogen levels where biological indicators of eutrophication (opportunistic macroalgal and phytoplankton blooms) do not affect the integrity of the site and features, avoiding deterioration from existing levels. This target was set using the Environmental Agency 2019 water body classifications data; and
  • maintain natural levels of turbidity (e.g. concentrations of suspended sediment, plankton and other material) in areas where this species is, or could be present (Natural England, 2023b).

                        Condition assessment

  1. The condition of the Tweed Estuary SAC’s features had not been assessed at the time of writing (Natural England, 2018).

5.2.4. River Tweed SAC

                        Site description

  1. At its closest point, the River Tweed SAC is located 133.40 km south-west from the site boundary. The site, located in Eastern Scotland and Northumberland and Tyne and Wear encompasses 3,742.62 ha of the River Tweed’s catchment and 1,285 km of watercourse (NatureScot, 2020h). The site is designated for Annex I habitats and Annex II species, including diadromous fish species relevant to this assessment: Atlantic salmon and sea lamprey.

                        Feature accounts

                        Atlantic salmon
  1. The ecology of Atlantic salmon is as described above in section 5.2.1, and not repeated here,. The River Tweed supports a very large, high-quality population of Atlantic salmon, with sub-catchments in both Scotland and England. The river is the best example in the UK of a large river showing a strong nutrient gradient along its length, with oligotrophic conditions in its headwaters, and nutrient-rich lowland conditions just before it enters the sea at Berwick (JNCC, 2024k). The river supports the full range of salmon life history types, with sub-populations of spring, summer salmon and grilse all being present  (JNCC, 2024k). Research by Gauld (2014) and Gauld et al. (2016) suggested that Atlantic salmon mainly spawn in the main lower stretches of the channel of the River Tweed. The extensive system supports a significant proportion of the Scottish salmon resource. In recent years, the salmon catch in the River Tweed is the highest in Scotland, with up to 15% of all salmon caught (JNCC, 2024k).
                        Sea lamprey
  1. The ecology of sea lamprey is as described above in section 5.2.3, and not repeated here,. In the English waters of the River Tweed SAC, sea lamprey are believed to spawn in the lower reaches of the main river although features such as weirs and dams may impede migration to spawning grounds (Natural England, 2022). Sea lamprey seems to be relatively poor at ascending obstacles to migration (in comparison to river lamprey), and are frequently restricted to the lower reaches of rivers (Natural England, 2022). Within the River Tweed SAC, there are excellent examples of the features that sea lamprey needs for survival, including areas of silt, sand, gravel and cobbles in the middle to lower reaches of the river which provide suitable spawning and nursery habitat (Natural England, 2022).

                        Conservation objectives

  1. The River Tweed SAC crosses the border between England and Scotland. Management of the River Tweed SAC is shared by Natural England and NatureScot and conservation objectives for the site have been published by both SNCBs (Natural England, 2022, NatureScot, 2020h). In this assessment, both sets of conservation objectives have been consulted as the features being assessed are diadromous fish, and therefore may migrate to and from the English or Scottish parts of the SAC.
  2. A CAP for the River Tweed SAC has been developed by NatureScot (NatureScot, 2020h). Conservation objectives for Atlantic salmon and sea lamprey are:
  • to ensure that the qualifying features of the River Tweed SAC are in favourable condition and make an appropriate contribution to achieving FCS; and
  • to ensure that the integrity of the River Tweed SAC is restored by meeting objectives 2a, 2b, 2c for each qualifying feature (NatureScot, 2020h).
  1. Conservation objectives for Atlantic salmon are as follows:
  • 2a. Maintain the population of Atlantic salmon, including range of genetic types, as a viable component of the site;
  • 2b. Maintain the distribution of Atlantic salmon throughout the site; and
  • 2c. Maintain the habitats supporting Atlantic salmon within the site and availability of food (NatureScot, 2020h).
  1. Conservation objectives for sea lamprey are as follows:
  • 2a. Maintain the population of the lamprey species’ as viable components of the site;
  • 2b. Maintain the distribution of the lamprey species throughout the site; and
  • 2c. Maintain the habitats supporting the lamprey species within the site, and availability of food (NatureScot, 2020h).
  1. The overarching conservation objectives detailed in paragraph 98 are related to the species-specific conservation objectives 2a to 2c for Atlantic salmon and sea lamprey. The assessment of the Array alone and in-combination with other plans and projects focuses on the individual species-specific conservation objectives. This allows a proportionate approach, as by demonstrating that potential impacts associated with the Array alone and in-combination with other plans and projects will not have an adverse effect on the species-specific conservation objectives (2a to 2c), the overarching conservation objectives will therefore not be impaired.
  2. Conservation objectives and related supplementary advice developed by Natural England apply to those parts of the SAC lying in England (Natural England, 2022). The high-level objectives for the site are:
  • to ensure that the integrity of the site is maintained or restored as appropriate, and ensure that the site contributes to achieving the FCS of its Qualifying Features, by maintaining or restoring:

–           the extent and distribution of qualifying natural habitats and habitats of qualifying species;

–           the structure and function (including typical species) of qualifying natural habitats;

–           the structure and function of the habitats of qualifying species;

–           the supporting processes on which qualifying natural habitats and the habitats of qualifying species rely;

–           the populations of qualifying species; and

–           the distribution of qualifying species within the site (Natural England, 2022).

  1. The second conservation objective: ‘the structure and function (including typical species) of qualifying natural habitats’ is only relevant to the Annex I habitat features of the River Tweed SAC and is therefore not included further in this assessment on Annex II diadromous fish features.
  2. Supplementary advice on conservation objectives (published on 31 August 2022) (Natural England, 2022) provides the site-specific attributes and targets specific to the Atlantic salmon and sea lamprey features of the SAC. Conservation targets for Atlantic salmon are summarised here:
  • restore the population to that expected under un-impacted conditions, allowing for natural fluctuations;
  • maintain juvenile densities at those expected under un-impacted conditions throughout the site, taking into account natural habitat conditions and allowing for natural fluctuations;
  • restore the distribution of spawning to reflect un-impacted conditions through the site, and avoid reductions in existing levels;
  • maintain or where necessary restore the distribution and continuity of the feature and its supporting habitat, including where applicable its component vegetation types and associated transitional vegetation types, across the site;
  • maintain or where necessary restore the total extent of the habitats which support the feature at 156.20 km (the entire length of the English portion of the river), including habitat mosaics, supply of coarse and fine sediment, water flows, underlying soil types, water quality, vegetation, and thermal regime;
  • the movement of other characteristic biota should not be artificially constrained;
  • maintain or where necessary restore the feature's ability, and that of its supporting habitat, to adapt or evolve to wider environmental change, either within or external to the site;
  • ensure non-native species categorised as 'high-impact' in the UK under the WFD are either rare or absent but if present are causing minimal damage to the feature;
  • maintain or, where necessary, restore concentrations and deposition of air pollutants to at or below the site-relevant Critical Load or Level values given for this feature of the site;
  • ensure exploitation (e.g. netting or angling) of Atlantic salmon is undertaken sustainably without compromising any components of the population, including multi-sea winter fish and seasonal components of the adult run;
  • ensure fish stocking introductions do not interfere with the ability of the river to support self-sustaining populations of the feature; and
  • maintain a sufficient proportion of all aquatic macrophytes to allow them to reproduce in suitable habitat and unaffected by river management practices (Natural England, 2022).

 

  1. Natural England (2022) conservation targets for sea lamprey are summarised here:
  • maintain or where necessary restore juvenile densities at those expected under unimpacted conditions throughout the site, taking into account natural habitat conditions and allowing for natural fluctuations;
  • maintain or where necessary restore the abundance of the population to a level which is close to that expected under unimpacted conditions throughout the site (subject to natural habitat conditions and allowing for natural fluctuations), whilst avoiding deterioration from its current level as indicated by the latest mean peak count or equivalent;
  • maintain or where necessary restore the distribution and continuity of the feature and its supporting habitat, including where applicable its component vegetation types and associated transitional vegetation types, across the site;
  • maintain or where necessary restore the total extent of the habitats which support the feature at 156.20 km (the entire length of the English portion of the river), including habitat mosaics, nutrient regimes, supply of coarse and fine sediment, water flows, underlying soil types, water quality, vegetation, and thermal regime;
  • the movement of other characteristic biota should not be artificially constrained;
  • all exploitation (e.g. netting or angling) of sea lamprey should be undertaken sustainably without compromising any components of the population;
  • ensure fish stocking/introductions do not interfere with the ability of the river to support self-sustaining populations of the features;
  • all intakes and discharges likely to trap a significant number of individuals of characteristic species are being adequately screened;
  • ensure non-native species categorised as 'high-impact' in the UK under the WFD are either rare or absent but if present are causing minimal damage to the feature;
  • maintain or, where necessary, restore concentrations and deposition of air pollutants to at or below the site-relevant Critical Load or Level values given for this feature of the site; and
  • maintain the feature's ability, and that of its supporting processes, to adapt or evolve to wider environmental change, either within or external to the site (Natural England, 2022).

                        Condition assessment

  1. The condition of Atlantic salmon was assessed in 2011, and in 2018 for sea lamprey (NatureScot, 2020h). The outcome of the feature condition assessment was as follows:
  • Atlantic salmon: favourable – maintained; and
  • sea lamprey: unfavourable – declining (NatureScot, 2020h).
  1. Sea lamprey was assessed as being in unfavourable condition at this SAC due to a restricted distribution within the Tweed catchment. The Mertoun weir at St Boswells is potentially a significant obstacle for sea lamprey, although other fish species are largely successful at migrating upstream of the structure (NatureScot, 2020h). Despite the presence of a fish pass, river flow rates over the weir, particularly during upstream migrations, may not be suitable for the species. As a result, sea lamprey may be largely restricted to the lower Tweed. Better evidence is required to provide a true picture of sea lamprey distribution at this SAC and the impacts of the Mertoun weir on this (NatureScot, 2020h).
  2. Although the conservation objectives for sea lamprey are phrased in order to ‘maintain’ the species’ population, distribution and habitats (paragraph 100), the condition assessment concluded the sea lamprey feature of this SAC to be in unfavourable declining condition (paragraph 106). Therefore, the conservation objectives for the sea lamprey feature of this SAC have been assessed throughout as ‘maintained or restored’ to account for this condition assessment.