Modeling MPAs in regulated fisheries: assessing the role of larval dispersal distance
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Transcript of Modeling MPAs in regulated fisheries: assessing the role of larval dispersal distance
Modeling MPAs in regulated fisheries: assessing the role of larval dispersal
distance
Carey McGilliard and Ray Hilborn
July 12, 2006
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
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.
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?
One Stage Model
BirthFishing
MovementNatural
Mortality
Larval stage Adult stage
Recruitment
Births
FishingBehavior
FishingRegulations
Movement occurs in one dimension: e.g. along the coastline and not between offshore and coastal environments
How is the Marine Protected Area implemented?
Previously unregulated fishery(Post-dispersal density dependent mortality)
Long Distance Larval Dispersal Short Distance Larval Dispersal
Fishery at MSY
Long Distance Larval Dispersal Short Distance Larval Dispersal
Fishery recovering from an overfished state
Long Distance Larval Dispersal Short Distance Larval Dispersal
Fishery recovering from an overfished state (II): Lower TAC only
Only MPA (Long distance dispersal) Only lower TAC
Fishery recovering from an overfished state (III): Implement MPA & lower TAC
Long Distance Larval Dispersal Short Distance Larval Dispersal
Why doesn’t population abundance increase more significantly in scenarios with an MPA?
Long Distance Larval Dispersal Short Distance Larval Dispersal
What happens to CPUE?
Long distance dispersal
At MSY
Recovering, Lower TAC +MPA
Recovering, MPA
Unregulated
Short distance dispersal
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.
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
Acknowledgements
• SAFS fellowship funding
• ARCS Foundation
• Other committee members: Andre Punt & Tim Essington
• Hilborn lab members, Punt lab members