Offshore wind vs. fishing and Marine Net Gain.

Offshore wind farms have environmental and social impacts. Birds can be killed by the turbine blades (though far more are killed in other ways each year [8]). Fishermen have to stay away from wind farms for fear of snagging their nets on the turbine support structures [9]. So overall, are they a good thing or a bad thing? Under current legislation, developments requiring planning permission need to show an expectation of 10% increase in biodiversity. This rule is called Biodiversity Net Gain (BNG); marine Net Gain (MNG) is the same thing for marine developments such as wind farms. It is not yet mandatory in legislation. The  consultation on it concluded in 2022 with this response: 'Further work is required on the MNG assessment framework to define how this can be established, as well as close working with approaches to strategic compensation for offshore wind development. The government will work with stakeholders to clarify further the distinction between the policy areas as delivery mechanisms are established.'

Measuring Biodiversity Net Gain (BNG)

MNG will apply to wind farms, oil and gas drilling and pipeline infrastructure, undersea cables etc.

Biodiversity or BNG [1] is measured in habitat units and it takes account of:

• Habitat size
• Condition (relative to the optimum for the type)
• Rarity
• Strategic significance

Multipliers are used to account for some other factors including

• Distance from the development site to the location where biodiversity is affected
• Difficulty in implementing the net gain successfully, which introduces risk
• Delay between action to improve biodiversity and achieving the required improvement.
  • This is implemented as a discount rate which quantifies the value of future diversity, in the same way we discount future income. The rate used is 3.5%/year, consistent with the UK Treasury Green Book.

Some habitats, such as veteran trees, are effectively irreplacable and require bespoke agreements with the local planning authority. You will need an ecologist to make this assessment. If your proposal does not meet the required net gain you can ‘buy’ BNG units.  A multiplier is applied that penalises units far away from the impact of the development [1]. As a further complication, there are three types of habitat: area (measured in ha), hedgerows and watercourses (both measured in km). Losses in units of one type must be offset by increases of the same type. The land must be secured for 30 years and you are encouraged to devise and implement a habitat management and monitoring plan (HMMP). Local planning authorities may charge a fee for monitoring for section 106 applications (https://www.local.gov.uk/pas/events/past-events/biodiversity-net-gain-local-authorities/biodiversity-net-gain-faq)The multipliers for location are:

Where	Multiplier
Within the same Local Planning Authority (LPA) or National Character Area (NCA) as the impacted site	x 1.0
Within a neighbouring NCA or LPA	x0.75
For intertidal zones, outside the Marine Plan Area (MPA) for the impacted site and neighbouring MPAs	x0.5

Marine Net Gain

BNG covers land down to the mean low water mark; below that, MNG, when it is implemented, will apply. MNG also applies to some activities such as fishing as well as infrastructure development [3]

Activities which cause damage to marine biodiversity include:

• Overfishing
• Noise from shipping, seismic surveys and oil drilling [4-5]
• Military Sonar [4]
• Where fisheries are displaced due to human activity, the fishing fleet has to voyage further to achieve the same catch thus increasing fuel use and GHG emissions. For example, since Brexit, Norwegian fishermen may not use the UK’s exclusive economic zone. This restriction halved the catch for each trip and doubled the number of trips. Overall, the GHG emissions per kg mackerel more than doubled [6].
BNG applies primarily to habitats but marine fauna are highly mobile and so MNG takes this into account.

MNG does not apply to fishing activities or fishing infrastructure as this is already covered by other legislation (Fisheries Act 2020).

How marine projects can increase biodiversity

MNG may be increased in many ways [4] including:

• Transplanting or reseeding seagrass and seaweed
• Seagrass meadows can sequester carbon 35 times faster than a tropical rain forest. They also act as a nursery for some marine species, stabilise the sea bed and improve water quality.
• Advanced mooring systems
  • Traditional mooring systems often drag cables across the sea bed, damaging plants (including seagrass) growing there
• New oyster beds [4]
• Oysters filter and clean the water
• Oyster beds and seaweed support crustaceans and sponges and provide a nursery habitat for fish.
• Restore saltmarshes (85% lost since 1860)

As with BNG, MNG is calculated based on area, with multipliers for rarity and condition. There will also be multipliers for risk.

References

[1] The Statutory Biodiversity Metric – User Guide DeFRA (2024)

[2] Principles of Marine Net Gain consultation response Wildlife and Countryside Link 2022
https://pml.ac.uk/news/protecting-the-ocean-through-marine-net-gain/

[3] Consultation on the principles of marine net gain (DeFRA) 2022

[4] Creating a Project Pipeline for MNG in the Southern North Sea 2024

[5] How Noise Pollution is Devastating Marine Life Marine Biodiversity and Sustainability, 2025


[6] Recycled Concrete Aggregate for Oyster Aquaculture Waste 2024, 2(3), 201-217; 

[7] Kim J.N. Scherrer, Tom J. Langbehn, Gabriella Ljungström, Katja Enberg, Sara Hornborg, Gjert Dingsør, Christian Jørgensen, Spatial restrictions inadvertently doubled the carbon footprint of Norway’s mackerel fishing fleet,
Marine Policy, Volume 161,2024, 

[8] MIT Climate Portal December 2023

[9] Spatial Conflict in Offshore Wind Farms: Challenges and Solutions for the Commercial Fishing Industry UKERC (2025)