Modelling the effect of introducing MPAS in a commercial fishery: a rock lobster example

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. If an MPA displaces fishing effort into the area that remained open to fishing, then fishing mortality CF) would be expected to rise. The effect of this increase in F would depend on the level of stock depletion, with three possible main outcomes: 1. If the population was only lightly depleted and above the level of BMSY then fishing the open areas harder increases the depletion level but Fenders the stock more productive. Depending on the exact level of depletion and the increase in Fr a new equilibrium was reached. 2. Highly depleted population, below BMsY; fishing the stock harder depletes it further, making it even less productive, if fishing maintained, leads to a fishery collapse. 3. If stock already close to collapse, then displaced effort would be so ineffective that the MPA could act to increase recruitment levels and make the whole stock relatively more productive. The model suggested that introducing large MPAs may be harmful without a reduction in catch at least equivalent to that displaced from the MFA. An MFA without concomitant catch reduction could lead to further stock depletion in open regions. This can lead to a new equilibrium or fishery collapse, depending on the level of stock depletion when the MPA was introduced. If the fishery was close to or already collapsed, an MFA was likely to be beneficial to stock recovery because of its contribution to recruitment.