COS 276-5 - Optimal dynamic spatial closures can improve fishery yield and reduce fishing-induced habitat damage

Background/Question/Methods

Fishing gear that contacts the seabed, such as the widely used bottom trawl, produces a significant amount of seafood globally but can result in seafloor habitat damage. Spatial closures may provide a valuable tool for mitigating habitat damage, but their overall effect on fish harvest depends on numerous factors, including how fish respond to changes in habitat quality. Empirical evidence suggests that habitat quality may affect both fish population productivity and movement; however, these important interactions have been excluded from most previous spatial management analyses. Additionally, past fishery models with fishing-induced habitat damage have largely focused on management with permanent spatial closures (MPAs, marine protected areas), ignoring the possibility of dynamic spatial closures that change through time. We develop an n-patch dynamic model of a fishery with fishing-induced habitat damage, open-access fishing effort dynamics, and effects of habitat quality on both fish population growth and movement behavior. Using an approximate dynamic programming algorithm, we are able to find near-optimal policies of dynamic spatial closures. We compare these dynamic policies with conventional policies, e.g. permanent closures (MPAs) and non-spatial management at maximum sustained yield (MSY) harvest rates.

Results/Conclusions

We find that the relative performance of different policy types is strongly impacted by the effects of habitat quality on the stock. Dynamic spatial closures often considerably outperform MPAs under strong habitat effects, but offer fewer or no benefits under weak habitat effects. Additionally, dynamic spatial closures can closely match or increase yield relative to our non-spatial MSY benchmark, while also generally providing increased habitat protection. These results suggest that dynamic spatial closures can be a useful management policy for open-access fisheries. We further explore these interactions with multi-objective optimization where habitat protection is included as an explicit component of the management goal, and show that our main conclusions are robust to stochastic fluctuations in recruitment.