Review of Cabling Techniques and Environmental Effects Applicable

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Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind Farm Industry – Technical Report operations. This is seen as a localised and temporary displacement of fish, which in isolation is generally not a significant impact on natural fish resources. Most species of marine fish spawn in the water column and so changes to the seabed through the placement of a cable do not have severe long-term implications. However, disruption to the spawning of species that do utilise the seabed, such as Atlantic herring (Clupea harengus), sandeels (Ammodytes tobianus) and dogfish, should be minimised through alternative routing or timing to avoid spawning areas or periods respectively. In general, CEFAS recommend that construction on or in the seabed should be carried out outside of the spawning season for substrate spawners e.g. February to April for spring spawning herring. As this is an important operating window, it may be prudent to determine the extent of spawning areas within the cablelaying corridor. In the absence of data regarding the importance of sites for spawning, additional studies may be required to determine whether mature fish in spawning condition are present in the area during the spawning season and/ or whether eggs and larval stages are present (see CEFAS, 2004). Nursery grounds (areas favoured by juvenile fish) are also important habitats, although in many locations such habitats may be widespread. If the cable pathway crosses through an important nursery ground, then the relative importance of the site to that region should be assessed. As most fish species are relatively opportunistic predators, particular feeding areas are not well defined. However, some species of fish may congregate in certain areas at particular times of the year to feed on particular prey species. The routing and laying of the cable should therefore minimise disruption to such sites wherever possible. Habitat disturbance can be a more significant issue to benthic (associated with the seabed) mobile fish resources such as many shellfish species. This is particularly relevant for areas which are important for certain shellfish life stages where there is reduced mobility, namely crustacean over-wintering areas and settlement areas for juvenile shellfish. Settlement areas are discussed further in ‘Smothering’ section below. The true extent of seasonal migration by many fish and shellfish species is becoming better understood. Again the severity of impact from cable installation activities is likely to increase for species that migrate across the seabed, namely crustacean species such as lobster (Hommarus gammarus), spider crab (Hyas araneus) and edible crab (Cancer pagarus). Routing and timing of cable-laying operations to avoid disruption to this seasonal activity may be required. For example, for the Norfolk (Cromer) Round 1 Offshore Wind Farm, mitigation cited in the Environmental Statement recommended that the export cable, which covered an inshore area believed to be important for “pairing” (or mating) of edible crab, should be installed outside this period (July-September) (Norfolk 114
Potential impacts and mitigration measures Offshore Wind, 2002). The presence of this local crab fishery, which is considered to be of local and national importance, led to a FEPA consent condition requiring a crab surveillance programme to be formulated (Cefas, pers. comm.). Molluscan shellfish species, such as king scallop (Pecten maximus), mussel (Mytilus edulis), native oyster (Ostrea edulis) and cockle (Cerastoderma edule), do exhibit avoidance behaviour over very short distances, but are less mobile than crustacean species and likely to be directly impacted by trenching operations. For most shellfish beds any resulting mortality along the width of the cable route is generally not a significant loss in relation to the remaining population. For small, isolated beds, however, the physical damage and disturbance from cablelaying operations can be significant. Mussels settling on the seabed attach to surrounding hard surfaces, including other adjacent mussels. This behaviour can alter the seabed substrate with the formation of biogenic reefs. These reefs can extend over significant areas of seabed creating a rich habitat for other species as well as suitable habitat for further mussel settlement. Such reefs are widespread, but generally found in large shallow inlets and bays in estuarine areas (Holt et al., 1998; BMT Cordah, 2003). Ploughing through these reefs can in some instances destabilise the reef by leaving exposed edges prone to damage by wave and current action. While biogenic reefs formed by Mytilus are found to be more tolerant of disturbance than other types of biogenic reef such as Sabellaria reefs (Holt et al., 1998) (see Section 5.3), these species-rich habitats should be avoided where possible. Noise & vibration The potential impact of noise and vibration on the natural fish resource within the affected area will be largely dependant upon the ‘hearing’ sensitivity of the fish species concerned. In this context three main types of fish are recognised: ● ● ● Hearing specialists: These species, including herring and sprat, ‘hear’ sound through the acoustico-lateralis system; a collective term for the inner ear and lateral line. Sound vibrations are also detected by a gas-filled swim bladder which is connected to the inner ear via a gas duct; Hearing specialists with mid range sensitivity: These species, including cod, mackerel and salmon, are hearing specialists but are deemed less sensitive to noise, due largely to the lack of a gas duct between the inner ear and the swim bladder; and Non-hearing specialists: Typical species include flatfish such as dabs, plaice and sole and elasmobranchs such as dogfish. These species are non hearing specialists and do not possess a swim bladder. 115
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REVIEW OF CABLING TECHNIQUES AND EN
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Review of Cabling Techniques and En
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Abbreviations AIS Automatic Identif
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TTS Temporary Threshold Shift TWA T
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3 Cable types and 18 installation t
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