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Report: understanding fish mortality in relation to marine developments

Natural Resources Wales appointed ABPmer to improve understanding of how fish mortality estimates are incorporated in marine fish stock assessments in the North and Irish Sea.


Fish mortality can be caused by natural processes such as predators or disease, as well as by human activities including fishing and some marine developments. A robust understanding of marine fish mortality, from both natural and anthropogenic factors, and how these are considered when calculating marine fish stock abundances, is needed to underpin advice to regulators and developers.

Natural Resources Wales (NRW) appointed ABPmer to provide an understanding of how natural and anthropogenic fish mortality estimates are incorporated in marine fish stock assessments in the North and Irish Sea. This will help to inform advice to regulators based on an understanding of whether potential mortality from marine developments is accounted for in fish stock assessments.

ABPmer authored a report on behalf of NRW to present our findings:

Marine Fish Mortality Considerations as part of Maximum Sustainable Yield Calculations

Under the Common Fisheries Policy, the EU manages fish stocks to achieve their Maximum Sustainable Yield (MSY). MSY is a broad conceptual objective, aimed at achieving the highest yield over the long term by keeping fish stocks at a level producing maximum growth. Fish stock assessment is used to assess the current status of a fish stock and determine appropriate management measures such as Total Allowable Catches (TACs) to achieve and maintain stocks at MSY level.

Natural mortality (M) is a key variable and source of uncertainty in stock assessments. It is defined as ‘deaths of fish from all causes except fishing (e.g. ageing, predation, cannibalism, disease and perhaps increasingly pollution)’. It includes physical (abiotic conditions such as temperature), biological (such as predation, disease and starvation) and anthropogenic (such as from disturbance and pollution) sources. An understanding of what is captured by the estimate of natural mortality therefore informs whether potential mortality from anthropogenic developments needs to be considered over and above the stock assessment outputs or not.

The findings of the report were:

  • The estimation of the level of natural mortality has progressed from a general assumption of M=0.2, to age-specific estimates, and estimates that take account of levels of predation mortality specifically.
  • Natural mortality is higher for younger ages (smaller sizes) and larval stages, a consideration that is increasingly reflected in stock assessments with age-dependent estimates.
  • One of the key sources of mortality is predation. Multi-species models have been developed that consider predator-prey interactions, allowing predation mortality to be estimated based on the biomass of upper-trophic level species. These estimates are included in assessments for North Sea species, and models are also being developed for the Irish Sea.
  • Background natural mortality accounts for other sources of mortality, e.g. disease, or anthropogenic activities such as power station water intakes and installation of offshore energy production infrastructure. Significant changes to the level of natural mortality from these other sources would result in the natural mortality estimate being updated.
  • Extreme events affecting mortality, such as temperature changes, are not usually incorporated into natural mortality estimates, but would be accounted for in the estimate of year class strength assessment models, and hence incorporated into the stock assessment as the fish reach a catchable size.

The report concluded that, at the scale of a stock in a sea basin or a widely-distributed stock, the impact of mortality from a specific local anthropogenic development is likely to be insignificant and already accounted for in the natural mortality estimates. Where an individual development has the potential to impact on local stocks that are spatially limited in their distribution, a specific assessment for that development can be carried out under Environmental Impact Assessment requirements.

The greater understanding of the components of natural mortality included in this report will help statutory nature conservation bodies provide advice to regulators in relation to EIA requirements for marine developments that have the potential to affect fish stocks.

Image © Andrew Pearson