Density dependence in marine protected populations: a review

Abstract

SummaryThe cessation or reduction of fishing in marine protected areas (MPAs) should promote an increase in abundance and mean size and age of previously exploited populations. Thus density-dependent changes in life-history characteristics should occur when populations are allowed to recover in MPAs. In this review, we synthesize the existing information on resource limitation in marine ecosystems, densitydependent changes in life-history traits of exploited populations and evidence for biomass export from MPAs. Most evidence for compensatory changes in biological variables has been derived from observations on populations depleted by high fishing mortality or on strong year classes, but these changes are more evident in juveniles than in adults and in freshwater rather than in marine systems. It is unclear if adults of exploited marine populations are resource limited. This may suggest that exploited populations are controlled mainly by density-independent processes, which could be a consequence of the depleted state of most exploited populations. MPAs could be a useful tool for testing these hypotheses. If we assume that resources become limiting inside MPAs, it is plausible that, if suitable habitats exist, mobile species will search for resources outside of the MPAs, leading to export of biomass to areas which are fished. However, it is not possible to establish from the available data whether this export will be a response to resource limitation inside the MPAs, the result of random movements across MPA boundaries or both. We discuss the implications of this process for the use of MPAs as fisheries management tools.

Benefits of closed area protection for a population of scallops

“…This is thought to enhance local reproductive output and therefore lead to export of larvae to surrounding areas that are open to fishing (Roberts et al 2001, Gaines et al 2003, Gell & Roberts 2003, Grantham et al 2003. For mobile species, there can also be spillover of juveniles/adults to adjacent areas through emigration or densitydependent dispersal (Lizaso et al 2000, McClanahan & Mangi 2000, Gell & Roberts 2003. Even in fisheries with minimum size limits, protection of juveniles from fishing disturbance may improve both survival and growth during this critical life-history phase (Myers et al 2000).…”

Estimating historical commercial rock lobster (Jasus edwardsii) catch inside Australian State territorial waters for marine protected area assessment: the binomial likelihood method

“…Whether the net effect on stock and ecosystem sustainability is positive or negative depends on the spatial dynamics of reproduction (Tuck and Possingham 2000), density dependence (Sánchez Lizaso et al 2000), movement (McGarvey 2004;Hobday et al 2005), and other spatially variable trophic or habitat interactions.…”

“…This implicitly approximates the binomial by a normal in the neighbourhood of the likelihood maximum, generally a satisfactory approximation for estimating confidence intervals (Cochran 1977;Rice 1995 …”

Spillover from marine reserves related to mechanisms of population regulation

“…The two mechanisms of population regulation that are likely to predominate in marine reserves are densitydependent population growth (DDG) and density-dependent (crowding-accelerated) movement (DDM) (Lizaso et al 2000). In this paper, we use a spatially explicit model to determine how these density-dependent processes differentially influence stock size within the reserve (a measurable surrogate for recruitment effect) and the net export rate of juveniles and adults to the surrounding fished areas (the spillover effect).…”