Effects of density and food supply on postlarval abalone: Behaviour,growth and mortality

摘要

Understanding density dependent processes in abalone is important for sustainable management of fisheries, and optimum aquaculture production. Density and food were manipulated to test their interacting effects on the growth and survival of hatchery produced postlarval Haliotis rubra. Feeding and dispersive behavior was also examined. Initial densities ranged between the densities used in aquaculture and those reported in the natural environment. Higher initial density increased the mortality rates of postlarvae in two experiments. Simultaneous manipulation of diatom food supply and density showed that the effect of density on mortality was mostly independent of food. Growth was strongly related to food supply, but also significantly reduced by high initial density, and these effects were additive. When diatom density was low, postlarvae did not increase rasping rates so the effect was that food was consumed more slowly. This explains the strong effect of food supply on growth. No aggressive interactions between feeding postlarvae were observed, but they graze intensively, leaving small distinct patches that have been cleared of algal food. Postlarvae encountering patches without food move rapidly and apparently randomly in search of food. The dispersal rate of postlarvae increased with density, presumably because post larvae spent more time searching for food. This searching is likely to be energetically expensive, and would increase at high densities because more cleared patches are created. This may explain why high initial densities of postlarvae had persistent reduced growth even when food was abundant, but does not explain the increased mortality at high density irrespective of food supply. We hypothesize that this effect may be attributable to other factors, such as the increased transmission of a pathogen at high density.