Why are fisheries important?
The waters around our coastline contain highly valued fish stocks, supporting local communities through the provision of commercial fishing and recreational angling opportunities for both local residents and visitors. These activities create employment for those directly involved in fishing and a range of other indirectly benefited businesses such as shops and restaurants.
Our coastal fisheries are also ecologically important, forming a key part of ecosystems and food chains. Many types of fish provide a food source for other species; sandeels, for example, are predated upon by other fish, such as sea trout and bass, as well as sea birds.
Some fish species are given additional legal protection due to their scarcity at a national or international level. These examples include the migratory Twaite and Allis shad and river and sea lamprey.
How do we protect them?
The siting of lagoons in tidal waters anywhere around the UK, or world, requires detailed consideration of their impact on fish stocks. This initial assessment involves gathering information and evidence and asking a series of questions which include:
What type of fish are living in the area and what is their population structure? This will include the diversity of species and the presence of different life-stages (fish of same species but with different ages).
Where they are living in relation to the proposed lagoon site? For example, are the fish bottom-dwelling fish such as plaice, or fish that swim higher in the water such as herring.
When are they present in the area? Some fish are only present for certain periods of the year or even part of the day if they are diurnal.
Why the fish are present? Some fish are migratory, such as salmon, lamprey or shad, and may be migrating past the Swansea Bay Tidal Lagoon site en-route to nearby rivers where they spawn. Other species, such as herring, are believed to spawn in the area of the proposed Swansea Bay Tidal Lagoon.
How are fish affected by lagoons?
Whilst some marine fish species may be resident all year round, others will be more seasonal, only appearing during the winter months (cod) or summer (bass). Some fish will live in coastal waters or pass through them during part of their life-cycle with the remainder of their lives being spent in the freshwater catchments that feed into the sea – such fish include migratory salmon, sea trout and European eel. We therefore need to consider the potential impact on these species both in the sea and freshwater and ensure lagoons are designed to minimise impacts and provide suitable offsetting where we believe a more significant impact is occurring.
Whilst the construction of tidal lagoons may impact on fish spawning, nursery and feeding areas, the greatest risk is often perceived to be that of entrainment in the turbines. We assess the impacts on fish stocks by modelling the encounter rate of fish with the generating structures and also the injury rates incurred as fish pass through the operating turbines. We consider the impacts of fish both entering the lagoons on a flood tide and exiting again during the ebb.
Individual Based Modelling (IBM) is used to predict encounter rates of fish and the lagoon. This method considers the movements of the fish, their response to certain marine features and for migratory fish, their ability to follow the scent (or olfactory) trail produced by their native river.
Fish can only physically enter a lagoon via the turbines during flood generation (for approximately one third of time). They will be unable to swim against the ebb generation flows passing out through the turbines (approximately one third of time). Fish will not be able to enter or exit the lagoon during the ‘holding period’ when the lagoon is effectively closed off from the sea outside (approximately one third of time). Fish may be able to enter and exit the lagoon via any shipping locks and sluices (where these are included in the design).
The injury of fish in the turbines is predicted using a model called ‘STRIKER’. This model considers the water conditions within the turbine that could cause harm to fish. A very small proportion of fish have been predicted to be affected in Swansea. They can be injured internally and externally due to adverse pressure change, water movement (known as shear stress), explosion of air bubbles (known as cavitation) and physical collisions with structures such as blades.
What if we have an impact?
Modelling results for Swansea show that overall, very low fish mean annual mortality rates are predicted using the IBM and STRIKER approach, with rates for the majority of species being less than 1% and rates for all species being less than 3%. These annual mortality rates are well within the natural annual variation that occurs within fish stocks of the species concerned. We will seek to offset for any impact on species above 2% annual mortality.
Such information is vital so that we can model the likely impacts of the lagoon construction and operation on the existing fish stocks and take action to reduce our impact. We will do this through:
Mitigation – redesigning elements of the lagoon so that fewer fish are harmed. Examples: moving from 4-blade fixed-speed turbines to 3-blade variable speed turbines which reduced predicted impacts on shad and herring at our Swansea lagoon, or provision of new spawning areas to replace those lost.
Offsetting – where we predict a significant impact on fish species we will try and make improvements elsewhere which will produce a commensurate number of fish to those lost.
Enhancement – having completed our mitigation and offsetting, we will consider going beyond the requirements so that an overall improvement is delivered.
How do we know if our modelling predictions are accurate?
We are already actively monitoring the fish populations around the Bristol Channel and Severn Estuary using various netting techniques in seasonal surveys.
Radio-tagging and tracking of a number of fish has already taken place and more is planned in order to learn more about fish behaviours, particularly of migratory fish, as they move between the sea and adjacent river catchments.
Hi-tech sonar cameras are being trialled with a view to using them on the turbines themselves to assess fish behaviours in the vicinity of the turbine housing structure. Within the operating turbines we are proposing to use electronic detectors (‘Sensorfish’) to record the flow conditions as fish pass through the structure.
What if we don’t construct lagoons?
We have to generate our electricity from somewhere. If lagoons don’t provide the necessary power, then we will remain reliant on the burning of fossil fuels, such as coal, to generate power. This comes at a cost to our fisheries.
Coal and other traditional power stations require vast quantities of cooling water which suck huge numbers of fish through intakes and kills millions of fish each year.
Burning fossil fuels contributes to harmful emissions which contribute to climate change and acidification of upland water bodies in particular. Climate change effects can include more frequent and larger flood events, potentially washing out fish eggs, to warmer, drier conditions leading to dried-up river beds or temperatures at which certain species cannot survive.
The provision of coal, for example, has historically left an environmental and economic legacy with at least 17 minewater treatment systems still being paid for in Wales alone and poor water quality from abandoned mines resulting in failing Water Framework Directive Waterbodies.