MORTALITY-RATES OF 0-GROUP PLAICE (PLATESSA-PLATESSA L), DAB (LIMANDA-LIMANDA L) AND TURBOT (SCOPHTHALMUS-MAXIMUS L) IN EUROPEAN WATERS .3. DENSITY-DEPENDENCE OF MORTALITY-RATES OF 0-GROUP PLAICE AND SOME DEMOGRAPHIC-IMPLICATIONS

This last of our three linked contributions on the dynamics of North Sea plaice is concerned with the detection and measurement from demographic data of the density-dependence of mortality rate during the early demersal phase of the life history. A mathematical expression is developed for the survival trajectory of a cohort subject to an instantaneous relative mortality rate which is a linear function of the logarithm of its density. This is used to analyse three independent data sets; a. pairs of densities at or soon after settlement in the Wadden Sea, b. estimates of seasonal mortality rates (M d-1) and initial density of 0-group plaice cohorts derived by linear regression and c. autumn estimates 0- and I-group fish from the ICES Young Fish Surveys. After correcting for various sources of bias, these each gave statistically significant estimates of the density-dependent mortality coefficient-mu-2, of 0.015, 0.0044 and 0.0010 per day, respectively. The same theoretical treatment of density-dependent mortality is used to develop an equation predicting the progressive 'damping' of the extremes of inter-year-class variation with age. The above three estimates of the density-dependent mortality coefficient-mu-2 applied in sequence provide more than sufficient 'damping' to explain the very low variability of recruitment and long-term stability which is characteristic of the North Sea plaice stock.