Modeling MPAs in regulated fisheries: assessing the role of larval dispersal distance

1. Modeling MPAs in regulated fisheries: assessing the role of larval dispersal distance Carey McGilliard and Ray Hilborn July 12, 2006

2. Outline • Motivation for the study • A review of the model • Long vs. short dispersal distance • The unregulated fishery • The fishery at MSY • The fishery recovering from an overfished state • Conclusions

3. Motivation • Most previous research focuses on MPAs in otherwise unregulated fisheries • Some MPAs are introduced into managed fisheries, sometimes fisheries that are not heavily overfished. • Hilborn et al. (2006) use a one-stage spatial model to show that an MPA does not always improve abundance and catch.

4. Hilborn et al. (2006) is a one-stage model with short movement distances What is the role of larval dispersal when an MPA is implemented in a regulated fishery?

5. Fishing Regulations One Stage Model Fishing Birth Natural Mortality Movement Fishing Behavior Births Larval stage Adult stage Recruitment

6. How is the Marine Protected Area implemented? Movement occurs in one dimension: e.g. along the coastline and not between offshore and coastal environments

7. Previously unregulated fishery(Post-dispersal density dependent mortality) Long Distance Larval Dispersal Short Distance Larval Dispersal

8. Fishery at MSY Long Distance Larval Dispersal Short Distance Larval Dispersal

9. Fishery recovering from an overfished state Long Distance Larval Dispersal Short Distance Larval Dispersal

10. Fishery recovering from an overfished state (II): Lower TAC only Only MPA (Long distance dispersal) Only lower TAC

11. Fishery recovering from an overfished state (III): Implement MPA & lower TAC Long Distance Larval Dispersal Short Distance Larval Dispersal

12. Why doesn’t population abundance increase more significantly in scenarios with an MPA? Long Distance Larval Dispersal Short Distance Larval Dispersal

13. What happens to CPUE? Long distance dispersal At MSY Recovering, Lower TAC +MPA Recovering, MPA Unregulated

14. Short distance dispersal

15. Conclusions: Abundance • An MPA improves population abundance in an unregulated fishery with declining abundance for both long and short larval dispersal distances • Applying an MPA to a regulated stock does not always improve stock status and sometimes causes population decreases (as in the scenario starting at MSY). • Abundance in stocks with long larval dispersal distances will fare as well as or better than abundance for stocks with short larval dispersal distances when an MPA is implemented.

16. Conclusions: Catch and CPUE • Catch and CPUE collapse when dispersal distances are short, regardless of initial stock state • CPUE is a little more than ½ of CPUE at MSY conditions in all scenarios with an MPA

17. Acknowledgements • SAFS fellowship funding • ARCS Foundation • Other committee members: Andre Punt & Tim Essington • Hilborn lab members, Punt lab members