SEAwise report on the effects of fishing on food webs and community diversity aimed at populating the MSFD Descriptor 4 and based on food web and end-to-end modelling.

The SEAwise project works to deliver a fully operational tool that will allow fishers, managers, and policy makers to easily apply Ecosystem Based Fisheries Management (EBFM) in their fisheries. This SEAwise report describes work that aims to provide a strong evidence base supporting the use of ecological indicators in fisheries management through statistical studies and end to end modelling. Our aim is to support the use of ecological indicators to guide managers towards strategies that lead to an ecologically safe space for fisheries, which we define as a range of states that provide yields for sustainable fisheries, maintains ecosystem functions and leads to low risk of overexploitation of species and loss of diversity. We show from an analysis of the past, using stock assessment model products for the northeast Atlantic and observations from scientific survey data of the North Sea, that fishing has already impacted ecosystems. Fishing on commercial fish stocks altered the balance of functional groups within the ecosystem of the northeast Atlantic, with depletions in the biomass of fish during the 1980s followed by stock rebuilding since the early 2000s, such that the biomasses of benthivorous fish and planktivorous fish are now much greater than that of piscivorous fish, which may be a sign of an improving ecosystem. Within the North Sea, fishing also appeared to have depleted the biomass of fish during the 1980s (particularly in the southern North Sea) and led to a change in the composition of species and the proportion of large individuals in the ecosystem (particularly in the northern North Sea).

We use comparative ecosystem modelling, employing multiple model types, to investigate the response of foodweb and community diversity indicators under a range of fishing strategies. We consider ‘no fishing’ scenarios, under prevailing environmental conditions, as a measure of the unimpacted state of the ecosystem i.e., its carrying capacity. Our modelled unimpacted state is our ‘yardstick’ - a standard from which we can measure the impact on the ecosystem due to current levels of fishing in the current conditions. We propose that the depletion of species, the difference in biomass of each species from their unimpacted levels under prevailing environmental conditions, can provide a basis from which to evaluate the risk of loss across impacted groups within the ecosystem.

We investigate if any of our selected food web and community indicators, including those used within the regional sea convention OSPAR, can support the use of assessment thresholds that may help managers reduce the risk of depletion of species or degradation of ecosystems by fishing. We explore management scenarios, linked to changes in levels of fishing impact due to current fisheries (so not favouring one type of fishing over another), to demonstrate how reducing or increasing activity levels will alter the risk of depletion of species and demonstrate how ecological indicators are expected to change given alterations in the structure of the ecosystem. We contrast these management scenarios to additional scenarios in which seabed-impacting gears only are restricted to avoid damage to habitats. This latter scenario promotes fishing on pelagic fish, such as herring and sprat, in the future and inhibits trawling for demersal fish such as sole, plaice and cod.

In each model, we find clear predictable relationships between fishing mortality overall (averaged across all model groups) and the depletion risk within the ecosystem. The biomass of apex predators typically decreases as fishing mortality increases. Similarly, as the size-structure of demersal fish communities decreases, risk within the communities increase. In contrast, the balance of trophic guilds in the ecosystem and the Shannon diversity overall or by group respond in differing ways (increases and decreases with increased fishing) dependent on the type fishing strategies modelled. The indicator targets for recovery and risk-based limits proposed here will be explored further in WP6 of the project.

Read more about the project at www.seawiseproject.org