Small marine reserves do not provide a safeguard against overfishing

Marine reserves aim to protect harvested species within their boundaries and can therefore provide insurance against the effects of overfishing. To evaluate whether marine reserves are meeting this expectation, we compiled over 40 years of rock lobster (Jasus edwardsii) monitoring data and examined long-term trends in three relatively small marine reserves (c.4-8 km(2)) in northern New Zealand, a region that has experienced large-scale declines in the fishery over the last decade. In all three reserves, lobster density initially increased by at least a factor of three following protection. However, over the last 10 years, lobster populations have experienced large declines in all three reserves with densities now 59-80%, and spawning stock biomass (SSB) now 51-86%, below historic levels. Using peak lobster density in each reserve as an unfished reference point, current densities on fished reefs are <15% of unfished levels and SSB is currently only 3-12% of peak population biomass in reserves. These results highlight the poor state of the wider stock and declines in the reserves follow regional declines in catch-per-unit-effort in the fishery, suggesting that lobster populations within these reserves are not fully protected from fishing. Adult J. edwardsii are known to undertake seasonal foraging excursions that carry them beyond the offshore boundaries of these reserves where they are susceptible to fishing-related mortality. We propose that the observed declines are likely a result of direct harvest of individuals beyond reserve boundaries and that larger reserves which encompass both a species' longshore and offshore movements are needed.

Automated summary

Analysis of over 40 years of monitoring data from three relatively small marine reserves in northern New Zealand shows that lobster populations initially increased by at least a factor of three following protection, but have experienced large declines over the last 10 years. These declines suggest that lobster populations within these reserves are not fully protected from fishing, and larger reserves that encompass both a species' longshore and offshore movements may be needed to address this issue.