Individuals do not live in isolation but are part of populations and communities. The resources in a given habitat have to be divided not only between individuals of the same species but also with species that have similar food preferences. Obviously, the amount of food available for an individual affects its growth rate. One can quantify density-dependent effects in a modification of the von Bertalanffy growth equation. The density-dependent asymptotic length LlB (note the index B to distinguish it from asymptotic length without the density effect) may be expressed as
where g is a competition coefficient defining how strongly the asymptotic length decreases with increasing biomass B (Figure 3). In an analysis of fisheries data across different species, 9 out of 16 fish stocks examined showed density-dependent growth. Also, the effect of density dependence was stronger in unproductive areas that originally harbored low fish biomass compared to richer areas with denser fish populations.
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