Discriminating between food and space imitation in benthic suspension feeders using self-thinning relationships

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Abstract
Growth and survival have been shown to be density-dependent in many populations of benthic suspension feeders, but it is generally difficult to determine whether food or space is the limiting factor. The study of biomass-density relationships (B-N diagram) in self-thinning situations may provide an additional criterion for such a purpose. It is shown here that the slope of B-N diagrams may vary significantly depending on whether food or space is the limiting factor. When food is limiting, it varies as a function of the exponent relating respiration to individual biomass. When space is limiting, the slope varies as a function of the length-weight relationship, roughness of the substrate at scales significant to the organisms, and the relationship between individual size and maximum population density. Testing the slopes of B-N relationships may, therefore, provide evidence concerning the nature of the limiting factor. Further, when food is the limiting factor, the intercept of B-N diagrams is an index of site quality. An example of the use of the B-N diagram for purposes of managing bivalve aquaculture is provided for mussels cultivated on bouchots (intertidal stakes). It is shown that in this particular case, space and food probably alternated seasonally as limiting factors. In addition, under the hypothesis that the entire population is completely harvested on a single occasion, analysis of the B-N diagram provides an estimate of the optimal sowing density in order to minimize competition and avoid subsequent thinning operations. These results illustrate the utility of managing cultivated populations in terms of biomass and density jointly.