Could MPAs mitigate the effects of fishing activities and aggregates extraction on a versatile ecosystem?
Marine Protected Areas (MPAs) are a developing tool for ecosystem and fisheries management. Their efficiency depends on characteristics such as their locations, dimensions and the rules that apply within the MPAs. MPAs also interact with conventional management measures and can impact other human activities.
Predicting possible consequences of MPAs is essential to find management scenarios that maximise efficiency and minimize adverse effects of MPAs. Such predictions can be made by means of spatially explicit marine ecosystem models representing fish populations, fishing activities and their management, MPAs, and other human activities such as aggregates extraction.
These models gathering a lot of information are complex. Besides information fed to the model is uncertain, reducing the reliability of model outputs. To deal with these issues we develop a method based on the information gap theory in order to identify management measures and MPA scenarios that allow reaching management goals robustly.
Our method combining sensitivity analysis and an info-gap approach allows us to demonstrate that management goals (in terms of spawning biomass or fishing mortality) can be reached by 2020 for sole and plaice in the eastern English Channel provided some criteria concerning management measures and population dynamics are met.
We implement a deterministic approach based on decision theory in a complex model of the Eastern English Channel. Using the ISIS-Fish modelling platform, we model populations of sole and plaice in this area. Real levels of fish abundance and TAC are used for the first three years, then projections are made with a fixed fish abundance and TACs computed by means of a Harvest Control Rule. We test a wide range of values for ecosystem, fleet and management parameters. The 2008-2018 period is simulated. From these simulations, we identify management rules controlling fish harvesting that allow reaching management goals recommended by ICES (International Council for the Exploration of the Sea) working groups while providing the highest robustness to uncertainties on ecosystem parameters.
For sole as well as plaice, no combinations of management measures could be identified that always allow reaching management goals accounting for ‘‘Nature uncertainty’’. However, management goals on sole spawning biomass can be reached with the Harvest Control Rule currently applied in the Eastern English Channel, provided mean weight-at-age is not lower than 24% of the reference level and recruitment not lower than 22% of the reference level. The model also provides a way to deal with stronger variations on these natural parameters: reducing the target fishing mortality below 0.22 allows dealing with 37% variations in recruitment. Management goals are more difficult to reach for plaice, but a target fishing mortality below 0.23 allows reaching target SSB level by 2018 for this species.
Three scenarios were tested for MPAs in the Eastern English Channel (full access to no access). At the scale of the entire study area these scenarios have little impact because total fishing efforts remain constant. Consequences appear at small spatial scales due to changes in the allocation of fishing effort.
We built a complex, spatially explicit, model of two flatfish populations and three benthic groups in the eastern English Channel using the ISIS-Fish modelling platform. Fish species modelled are sole (Solea solea) and plaice (Pleuronectes platessa). Benthos is modelled as three groups gathering several taxa, based on their mobility. Human activities modelled include fishing and aggregates extraction. Populations and human activities can interact in time and space. Simulations are run for twelve years to encompass a 2008-2020 period. 2020 corresponds to the year when good ecosystem state should be reached in EU marine ecosystem according to the Marine Strategy Framework Directive. We model conventional management measures as well as possible spatial management scenarios and search for robust combinations using the method developed by Gasche et al.1.
The uncertainty analysis indicates that the current ecosystem state should allow reaching management goals defined for sole and plaice in this ecosystem. However, natural variability that can be tolerated on environmental parameters is low. We evidence no positive effects of MPAs on fish populations, either at the scale of the eastern English Channel or at smaller scales gathering several bays on the French and English coasts. On the contrary, effects of MPAs are very important on benthic taxa because they are not mobile at the level of a model cell. In particular, we show that MPAs can efficiently maintain high benthos abundances in protected areas, but at the cost of severely depleted abundances in adjacent areas that are not protected.
We develop a method using linear mixed-effect models to quantify the importance of several variables in a BACI analysis. We use it to quantify the impacts of aggregates extraction on benthic communities: decreases in benthos density are strong and no significant recovery can be evidenced, even several years after extraction.
The density of benthic species was measured for six years on an experimental dredging area located in the Bay of Seine. We search for effects of aggregates extraction by means of a BACI-like analysis. Contrary to usual BACI analyses, we go beyond just evidencing effects of aggregates extraction and explicitly quantify the impacts of several variables defining the aggregate extraction activity on the density of benthos in the study sites. These relationships between aggregates extraction and benthos density are quantified by means of mixed-effects models.
Recovery of benthos density after extraction was also studied. Our results concerning mobility guilds show that burrowers are negatively impacted by dredging, whereas mobile species are less impacted. When grouping species according to their diet, impacts of dredging are significant on carnivorous species. Detritus feeders are positively impacted by dredging, at least for a few months after dredging, which may underline their opportunistic behaviour. In several cases, increases in abundance after dredging cannot be linked to a recovery time and are due to strong inter-annual variability in benthos abundance. Overall the recovery of benthos density at the guild level is hard to evidence, suggesting that it may occur at longer time scales than that studied here.
Relevance for Policy:
- Common Fisheries Policy
- Directive on Maritime Spatial Planning and Integrated Coastal Management (forthcoming)
- Integrated European Maritime Policy (IMP)
- Marine Strategy Framework Directive