On the Density Effect in Populations of the Mussel, Mytilus galloprovincialis

Abstract

In natural populations of the mussel, Mytilus galloprovincialis, the higher the population density is, the lighter the average weight of individuals is. That is, the population density and the average weight of individuals are inversely proportional. In addition, the population density is directly proportional to the biomass. Therefore, individuals are small and their growth is delayed where the biomass is high in the populations. When samples are compared between surface and bottom layers, the average size of individuals in the surface layers are significantly larger than that in the bottom layers. This seems due to the difference in growth rate between the two layers. In places where population density is high, the number of dead individuals is large. Dead mussels are almost all small in size. Where the population has a high density and large standing crop, the growth is retarded and thinning out occurs, and consequently the biomass is reduced. Where the density is low and the standing crop is small, the growth of the population continues as a kind of self-regulation within mussel populations.