Setting thresholds for good status in marine ecosystem management

Environmental management aims to achieve desirable ecosystem states and avoid adverse or degraded states. Assessing ecosystem state requires quantitative indicators of condition and thresholds above which state is defined as 'good' (Rice et al., 2012; Levin et al., 2016). For example, for MPAs designated under the Habitats Directive, conservation status takes account of parameters such as the population abundance, the natural range of a species and the structure and function of a habitat. Meeting the ecological expectations placed on MPAs requires avoiding adverse impacts from fisheries and other human activities and identifying thresholds in ecosystem state below which the system becomes degraded and also no longer confers any socio-economic benefits. Identifying where this threshold between good or favourable status and bad or poor status lies has used a wide variety of approaches and has been haphazard and subjective (Dorrough et al., 2020). For example, existing approaches range from identifying statistically derived breakpoints in the ecosystem state-pressure relationships to expert-elicitation (Dorrough et al., 2020; Hillebrand et al., 2020). Furthermore, some thresholds aim to maintain the state itself, while others aim to maintain the ecosystem functions or biodiversity that are supported by the state. Different approaches may result in very different thresholds for prompting management actions, and these may also have very different social impacts and consequences on dependent communities. There is therefore an urgent need to identify the various pros and cons of the different thresholds for desirable ecosystem states identified. Such thresholds are, for example, needed by the Joint Nature Conservation Committee (JNCC) in the management of bottom trawl fisheries in MPAs in the UK; by the International Council for the Exploration of the Sea (ICES) for advice to the EU on how to implement the Marine Strategy Framework Directive (MFSD, ICES, 2021); by OSPAR and the UK government for assessing the state of biodiversity for implementation of the MSFD (OSPAR) and UK Marine Strategy (UKMS); and for plans and projects assessed through the Habitat Regulations to determine if adverse effect applies and, if it does, what compensation measures could best support the status. Current and increasing interest in NetGain is also relevant, as that requires an understanding of current conservation status and what measures could be taken to improve that. Indeed, it is not possible to determine if biodiversity indicators are in Good Environmental Status under the MSFD/UKMS without thresholds; this is currently the case for pelagic habitats, benthic habitats, and food web indicators, which are unable to be assessed for environmental status because of the lack of thresholds. From a policy perspective, therefore, it is imperative that thresholds are agreed so biodiversity can be robustly assessed, and appropriate management actions implemented.