Assessing relative and cumulative impact on the marine environment

Pressures on the marine environment are steadily increasing with new uses including renewable energy and new pressures including warming and more acidic seas.
Diagrams showing trawled regions over a period of 20 years.

IMAGE: The 20-year cumulative effect of trawl fisheries on four groups of demersal fish species. Darker areas indicate higher cumulative impact. Key ecological features identified in the south east are overlaid in green.

Some of these pressures will threaten conservation values and the impacts can be cumulative. Predicting the location and magnitude of these impacts can be challeging. A scientifically robust and pragmatic approach is needed that can estimate the overall cumulative impact of pressures, as well as detect how different pressures contribute to the overall impact (their relative impact). This will provide managers and regulators with information to underpin policy, regulatory or management responses. In this project, fisheries and biodiversity conservation scientists worked with the Department of the Environment to examine and advise on the development of cost-effective approaches to understanding and estimating cumulative impact on marine biodiversity assets in South-east Australia.


The project team identified three broad approaches that have been applied to understand and estimate cumulative impact on biodiversity assets on the continental shelf in South-eastern Australia: expert elicitation, qualitative mathematical models and quantitative mathematical models. Expert elicitation is a relatively simple, low cost approach used by the Department in marine bioregional planning to assess the potential for cumulative impact on conservation values, including biodiverse submarine canyons and shelf rocky reefs (identified as Key Ecological Features).

Qualitative mathematical models were used by CSIRO to identify ecological indicators sensitive to cumulative impact on submarine canyons and shelf reefs. They identified important ecosystem components, and how these are linked and affected by pressures. High-resolution statistical models were used to retrospectively predict the cumulative impacts of 20 years of trawling on demersal fish communities on the continental shelf of South-eastern Australia.

An examination of these three approaches highlighted their relative strengths and weaknesses in terms of simplicity, cost, data requirements and uncertainty. It also demonstrated how each approach can complement each other when arranged in hierarchical manner, from simple, rapid and low cost approaches to more complex and costly approaches. This hierarchical approach to estimating cumulative impact supports timely decision making based on available information, enabling additional scientific understanding to be incorporated when available, and identifying potential research investments.

New knowledge and opportunities

The application of many tools and approaches for assessing cumulative impact is limited by the availability of knowledge, data and resources, and uncertainty. It might be desirable to have a single tool that could always be used to meaningfully assess cumulative impact and attribute relative impact, but the circumstances in which this is possible are limited.

The hierarchical approach developed here provides a framework for identifying the minimum set of tools required to understand the potential for cumulative impact and progress to more sophisticated approaches to assess cumulative and relative impact where this is required and possible. For example, the project identified three different processes to produce a three-level hierarchical approach to assess the cumulative impact of trawling.

Outputs and outcomes

Collaboration with the Department and fisheries scientists has enabled the Hub’s researchers to propose a hierarchical approach to understanding and estimating cumulative impacts and relative impacts that is scientifically robust, practical and incorporates tools already used to inform decision making processes.


Piers Dunstan
(03) 6232 5382