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