Dynamics of shallow-water juvenile flatfish nursery grounds: application of the self-thinning rule

Annual development of density and mean weight of juvenile plaice Pleuronectes platessa was examined at 2 nursery grounds (Port Erin Bay, Isle of Man, Irish Sea and Admucknish Bay, Firth of Lorne, west coast of Scotland) over a number of years. In both cases there was evidence of 'self-thinning' in the populations when the density was high, although in Port Erin Bay the slope suggested a relationship different from the standard - 4 ⁄3 rule proposed for animal populations. The site specific dynamic thinning lines (dynamic carrying capacities) were consistent with the dynamic thinning line (-4⁄3 rule) for both populations. The 2 nursery grounds were different, with growth rates and densities generally higher on the Ardmucknish Bay nursery ground compared with Port Erin Bay. Similarly, the estimated total biomass of plaice was higher in Ardmucknish Bay than Port Erin Bay, but there was considerable inter-annual variation. Maximum plaice biomass generally occurred from August to September for both sites. In Port Erin Bay this coincided with the maximum consump- tion rates. The size structure was described using the Gini coefficient (G) and the timing of size struc- ture development by the centre of gravity (CG). The size structure of the populations differed and in both cases varied during the season in response to settlement and mortality. Over the study periods, population density was only occasionally high enough to reach the site specific self-thinning line. This suggests that these populations rarely approached the carrying capacity of the nursery grounds.