Avoidance and mitigation measures are the primary strategies for managing potential significant impacts of activities on the marine environment, and form the basis of management frameworks regulating activities in the marine environment. These strategies, however, may not address all impacts associated with the activities. There may also be some impacts on the marine environment (e.g. severe storm events) that are unable to be predicted, and cannot be avoided or mitigated. These impacts are regarded as residual risks.

The assessment summary identifies residual risks for the marine environment using a classification system outlined in the SoE Approach report (Jackson et al. 2016). The method for applying the classifications does not follow a formal risk assessment framework, but instead uses best judgement based on available information. Because of the subjective nature of the assessment, the authors recommend that the assessment summary be used as a guide only, as it may not necessarily reflect the output of a robust, quantitative approach to risk assessment.

Residual risks have been divided into 2 groups: those associated with pressures that are currently managed; and those associated with pressures that are currently not managed, or whose impacts are unable to be predicted, and cannot be avoided or mitigated. The identified risks associated with pressures that are currently managed are based on a ‘business as usual’ situation, where current management processes and frameworks continue as they are. Risks are classified according to their likely impacts on populations at a national scale, not on an individual or a local scale.

Each classification includes a pressure (e.g. commercial fishing) and the component of the environment affected by that pressure (e.g. bycatch species). Where a component being affected is not identified, the impact relates to the whole marine environment. The residual risks are discussed below.

Our understanding of the likelihood of occurrence and the associated impacts of residual risks varies, depending on our understanding of impacts and the management frameworks in place to mitigate these impacts.

Residual risks associated with ongoing pressures—climate change, fishing (commercial, recreational and traditional use), oil and gas extraction and production, marine debris, and anthropogenic noise—are identified as ‘almost certain’ in their occurrence. Impacts range from catastrophic to unknown. In these cases, pressures will continue to affect the marine environment; however, the impact of these pressures varies, depending on the management frameworks currently in place.

Residual risks associated with climate change are currently the most extensive and are likely to have the largest impacts. Those associated with marine debris are also considered extensive and likely to have major impacts on the marine environment. These residual risks are classified in such a manner because jurisdictions currently do not have management frameworks in place that can mitigate or reduce impacts. As a result, risks are widespread rather than being truly residual in nature. Management frameworks currently in place for residual risks associated with most other pressures result in reductions in either likelihood (e.g. commercial fishing impacts on habitats) or impact (e.g. planned oil and gas exploration and production activities).

The classification of residual risks identifies some areas where current management frameworks may be less effective and require improvement. For example, although management of the impacts of commercial fishing is considered effective, and therefore residual risk to target species is considered to be minor, there is less information on bycatch species, thus increasing the potential impact on these species to moderate. The impacts of recreational fishing and traditional use of resources are also considered moderate, because less information is available and their management is only partially effective.

Several residual risks have unknown impacts, yet are considered to be almost certain (anthropogenic noise, ocean acidification) or likely (cumulative impacts), indicating a need for further information in these areas. Identification of unknown impacts highlights key challenges in developing strategies for managing impacts—that, is the paucity of data and understanding about the broader ecological impacts of industries on the marine environment, the effectiveness of current management, and likely future trends.

Ecological risk assessment methods are available to identify known residual risks in a quantitative and often hierarchical framework (Hobday et al. 2011). These risk assessments can play a central role in an adaptive management process, integrating stakeholder values and identifying how to improve future management effectiveness (Van den Brink et al. 2016). Such frameworks should not be confused with ecological impact assessments, which frequently fail to explicitly identify risks, potential impacts and likelihoods (Gibbs & Browman 2015). Ecological risk assessment frameworks have been developed in Australia to identify risks associated with fishing (e.g. Smith et al. 2007, Hobday et al. 2011, Fletcher 2015) and introduced species (e.g. Hayes 2003, Hayes & Sliwa 2003). Application of quantitative frameworks to other pressures is lacking, identification of residual risks associated with these pressures is less clear, and opportunities for earlier and more effective avoidance and mitigation may be missed.

Biodiversity loss because of development or use once avoidance and mitigation options have been fully exercised can, in some cases, be compensated for through environmental offsets. Such approaches have been implemented as part of the approval process of the EPBC Act and as part of approval processes operating in a number of states. According to an Australian Government policy released in 2012 under the EPBC Act, companies have to compensate for their residual adverse impacts on national environmental assets by implementing biodiversity offsets, with the aim of ensuring no net environmental loss (DSEWPaC 2012f). The suitability of any offsets proposed is considered as part of the decision-making process associated with the assessment of actions under the EPBC Act. Offsets must meet 10 requirements associated with the delivery of conservation outcomes, proportional to impacts on the environment; monitoring, auditing and enforcing of offsets once in place; and management of risks associated with the offsets failing to meet their targets (DSEWPaC 2012f). Similar policies have been set out for various sectors by some state governments (e.g. the marine fish offset policy developed by the Queensland Department of Agriculture, Fisheries and Forestry). Examples of marine offsets implemented under Australian Government legislation include support for sea ranger programs, conservation programs for particular species, and education and management programs (Richert et al. 2015).

Community responses to the implementation of offsets vary, depending on how closely related the offset activity is to the residual risk, the location of the offset, and social acceptance of governance systems in place at the time (Rogers 2013). Offsets that may be construed as providing future predicted benefits to counter current real loss are especially contentious (e.g. Pascoe et al. 2011).

Evaluation of the success of offsets in achieving environmentally sustainable outcomes is often not integrated into approval processes (Maron et al. 2012). A review of offsets proposed for the Great Barrier Reef under the EPBC Act found that offset strategies were often vague, were submitted before residual impacts were properly described and quantified, and were often finalised after the development was approved—thereby increasing the risk that developments with inadequate offsets and unmeasurable benefits were approved. Offsets often comprised indirect rather than direct offsets, thereby failing to meet the target of 90 per cent direct offsets set out in the policy. Additionally, sites where offsets were implemented were not coordinated with regional planning and zoning, and were limited only to the period of the development, increasing the risk of degradation over time and future development impacts. Budgets set out for offsets were found to be arbitrary, and lacking in transparency and justification, and frameworks for monitoring offsets were lacking. It was concluded that many of the offsets approved did not provide protection for matters of national environmental significance under the EPBC Act (Bos et al. 2014).

Frameworks to assess offset management strategies are now becoming available. They allow testing of strategies under alternative scenarios relating to both ecological and societal responses to development or use, and the implementation of associated offsets (e.g. Thébaud et al. 2015). The recently released Policy statement: advanced environmental offsets under the Environment Protection and Biodiversity Conservation Act (DoE 2016) may also provide improved environmental outcomes through implementation of offsets before any impact occurs, a more strategic delivery of offsets, and a streamlined environmental assessment process.