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The Effect of Introducing MPAs on a Commercial Fishery - A Rock Lobster Example

Malcom Haddon and Colin Buxton

Tasmanian Aquaculture and Fisheries Institute, HOBART, AUSTRALIA

Theme: TH4

The effect of introducing a large marine protected area (MPA) into a managed commercial fishery was investigated using a spatially-explicit, size-structured model. The stock dynamics approximated the biology of Tasmanian rock lobsters in that adult movement was very limited while larval dispersal was widespread. The strategy used to explore the effects of MPAs involved: Initialing a stock of numerous populations in an equilibrium, unfished state; harvesting to deplete the model populations to a known level using selective fishing mortality; and introducing the maximum sustainable harvest rate for the given level of depletion either with or without a large MPA. If introducing an MPA displaces a significant amount of fishing effort into the area that remains open to fishing, then fishing mortality (F) will rise in the open area. There is an asymmetry to the effect of this increase in F depending on the level of stock depletion, with three main outcomes: 1) if the population is only lightly depleted and is above the Brood Management Stock Yield (BMSY) level then fishing the open areas harder will increase the level of depletion that, at the same time, should render the stock more productive-depending on the exact level of depletion and the degree of increase in F, a new equilibrium should be produced; 2) if the depletion level is high and the population is below BMSY then fishing the stock harder will merely deplete it further making it even less productive-if the excess harvest rate is maintained this could lead to a fishery collapse; (3) if the stock is already depleted to such an extent that the fishery is close to or is already collapsed, then the displaced effort can be so ineffective that the MPA could act to increase recruitment levels and make the whole stock relatively more productive. The model suggests that the introduction of large MPAs may not be beneficial if there is no reduction in catch at least equivalent to that displaced from the MPA. An MPA with no concomitant catch reduction will lead to further stock depletion in the open regions. This further depletion may lead to a new equilibrium or fishery collapse, and will depend on the level of stock depletion when the MPA is introduced. If a fishery is already collapsed an MPA is likely to be beneficial to stock recovery because of its contribution to recruitment.