Can Marine Net Gain Really Turn the Tides for Our Sea’s Biodiversity? – by Amber Connett ACIEEM and Richard White MCIEEM

What is Marine Net Gain?

In England, the Environment Act 2021 introduced a mandatory requirement for new terrestrial developments to deliver biodiversity net gain (BNG; i.e. aiming to leave the natural environment in a better state than before). CIEEM has been at the forefront of BNG policy development and you can find out more about BNG here.

During the development of the biodiversity net gain approach, many groups called for the requirement to extend to the marine environment, although it was widely accepted that marine net gain would look quite different from its terrestrial counterpart, so Defra committed to developing the policy separately to ensure it is fit for purpose.

This summer, Defra launched a consultation on the aims and principles of marine net gain. The proposed principles are that marine net gain:

  1. will measure impacts on habitats and species
  2. will seek to incorporate environmental benefits underpinned by biodiversity
  3. will take a ‘nature first’ approach whilst recognising wider environmental benefits
  4. assessments will not include potentially positive incidental impacts whose benefits are subject to significant uncertainty
  5. requirements will be proportionate and appropriate to the scale and type of development
  6. will be a mandatory requirement. It will apply to all marine development, subject to any minimal thresholds and other exemptions
  7. will incentivise both active interventions and appropriate pressure reduction measures
  8. will incentivise the delivery of strategic interventions in addition to meaningful site-based interventions.
  9. will allow for improvements to designated and non-designated features of Marine Protected Areas to qualify as net gain interventions

We worked with our England Policy Group and Marine and Coastal Special Interest Group to respond to the consultation. This blog sets out some of our key thoughts on the proposals as they stand.


Given the ongoing decline in the state of nature in the marine environment[1], caused by a variety of pressures including pollution, unsustainable fishing levels, and direct loss of and damage to habitats, we are pleased to see the Government recognise the need for nature recovery.

A new strategic and enforceable approach to marine management presents an opportunity for overall reduction of pressures to improve the conservation status of species and habitats, benefitting both people and nature.

Oceans also play a significant role in the planet’s carbon balance, cycling an estimated 83% of all carbon (often referred to as ‘blue carbon’)[2]. Habitats such as coastal salt marsh, seagrass meadows and seabed sediments play a crucial role in this, so it stands to reason that their protection and recovery can have significant benefits in mitigating and adapting to climate change[3].

Key issues

Despite the positive shift in rhetoric, we do have some concerns about how the marine net gain approach will work in practice. The dynamic nature of the marine environment means a fundamentally different approach will be needed to the terrestrial BNG. Individual developments will potentially have much more diffuse impacts than in the terrestrial environment so it is less certain that they can achieve net gain in regard to these impacts.

We must look holistically at the pressures on the marine environment, including cumulative impacts of activities to develop a strategic, ecosystem approach.

As it currently stands, marine net gain is only being proposed for development activities, which would not include fisheries. For the policy to adequately address the pressures leading to biodiversity loss in the marine environment all developments and other activities that have the potential to significantly impact marine habitats and species should be included.

It is also difficult to see how a site-based approach could be taken in some cases. This is simplified if there is a close link between the impact of an activity and net gain measures, e.g. replanting a seagrass bed if it has impacts on seagrass beds, however, measuring losses and gains becomes far more complex when it is not like for like.

While the benefits for other ecosystem services (such as carbon absorption and storage mentioned earlier) should be recognised and maximised where possible, they should not be delivered in lieu of, or prioritised over, biodiversity in an environmental net gain approach.

We support marine net gain being applied to both species and habitats (different from BNG which uses habitats as a proxy measure) as this would allow measures to be implemented for mobile species. However, a major concern with the implementation of marine net gain is whether it is feasible based on our current understanding of marine ecosystems.  Interactions between species, and between species and their habitats, are complex and, beyond the most basic, impacts of human pressures are difficult to predict. For the policy to be measurable and to ensure it is implemented correctly, we would need to develop a clear understanding of:

  • Interactions between species and with their habitats
  • What constitutes an irreplaceable or significant habitat in the marine environment
  • Baselines for delivering net gain
  • Case studies of how marine net gain could work in practice
  • How to incorporate species (which has not yet been achieved in the terrestrial system)
  • How we can ensure longevity of net gain measures in a dynamic ecosystem?
  • Assessing and addressing cumulative impacts

The planned expansion of offshore windfarms is one key element of meeting the UK target of Net Zero by 2050 and marine net gain can be seen as critical to ensuring that climate emergency wins are not made at the expense of marine biodiversity, already heavily pressured by human activities. But this example illustrates the complexity of what we are trying to achieve.  One of the potential impacts of wind farm development is the exclusion of marine species from key feeding areas. This reduction in availability can only be balanced by increasing food supplies elsewhere and it is unlikely that this will be achieved by renewable energy developers alone. Reduction in fishing pressure is one obvious means of ensuring a good food supply for marine species. To achieve this, marine net gain must go beyond a simple protocol for developers, instead forming a core principle of effective marine spatial planning.

Our full response to Defra’s consultation is available on our Resource Hub.

[1] UK Governments failed to achieve Good Environmental Status in our seas by the Marine Strategy Framework Directive 2020 target, failing on 11 out of 15 indicators in 2019.

[2] White R. and Connett A. (2021) Blue Carbon: the Sea, the Coast and the Climate Crisis, In Practice, 114. pp. 52-57

[3] Wildlife and Countryside Link (2022) Tackling the climate crisis through ocean protection.

Richard White MCIEEM

Richard is a marine ecological and conservation consultant with NatureBureau. Leading on carbon crisis work within the business, he is keen to progress the development of small-scale carbon offsetting schemes linking small businesses with local projects that can address both climate and biodiversity crises.  With over twenty-years’ experience in marine conservation he also seeks to highlight the role that coastal and marine habitats have in mitigating climate change and promoting the protection and restoration of these habitats as a key element of climate strategies.

Amber Connett ACIEEM

Amber is CIEEM’s Policy Officer. She leads CIEEM’s Action 2030 project alongside policy activities such as providing the Secretariat for the All-Party Parliamentary Group for Nature. Contact Amber at

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