Density-dependence inside a marine protected area increases natural mortality and stunts the growth of a spiny lobster

Sustainable fisheries management often requires the modelling of stocks under unfished conditions, when the influence of population densities on animal growth and mortality can be substantial. This can be especially true for species such as spiny rock lobster, which are very habitat specific. Using western rock lobster (Panulirus cygnus) tag-recapture data from adjacent and similar fished and unfished areas, the key life history parameters of natural mortality and growth were examined and compared under different population density scenarios. In an area representative of virgin biomass levels, lobsters exhibited reduced growth rates and a substantially higher rate of natural mortality than in the adjacent, less densely populated fishing grounds. This research highlights the non-stationary nature of growth and natural mortality in this species, a concept which is poorly understood and rarely acknowledged in stock assessment models. Additionally, these results indicate that the perceived benefits of fishery closures, such as spill over and increased reproductive output, may not be as simple as is often assumed, due to the reduced growth and increased mortality of the protected stock relative to the fished population.