MORTALITY-RATES OF 0-GROUP PLAICE (PLEURONECTES-PLATESSA L), DAB (LIMANDA-LIMANDA L) AND TURBOT (SCOPHTHALMUS-MAXIMUS L) IN EUROPEAN WATERS .1. STATISTICAL-ANALYSIS OF THE DATA AND ESTIMATION OF PARAMETERS

The early demersal phase of those species of flatfish with inshore nursery areas are usually accessible with relatively modest demands on gear and logistics. However, the design of the sampling programme, adjustment for gear efficiency, and statistical analysis of the resulting catch data, raise a number of questions which have not hitherto been addressed comprehensively. This paper presents a systematic analysis of all the published data known to us of demersal 0-group flatfish in nursery grounds adjacent to the North Sea and British coastal waters, together with some hitherto unpublished data. The analysis showed that in most cases catches from repeated tows of a beam trawl can be averaged, after appropriate correction for gear efficiency, to give representative estimates of fish density for a succession of sampling dates. From the time series of density estimates thus obtained a selection of the period from maximum density until the early part of the following year proved to give a consistent window for the estimation of mortality rates by use of regression methods. A total of 57 data sets was analysed (45 plaice, 11 dab and 1 turbot). Of these 48 gave essentially linear log/time trends, of which 7 showed some degree of curvature, (4 concave upwards and 3 concave downwards). In one further case the trend remained essentially horizontal (flat-topped) until near the end of the sampling window and then declined sharply. The remaining 8 trends were so erratic as to preclude any further conclusions to be drawn. There was no obvious regularity in the occurrence of either the concave or flat-topped patterns by location or in time. Estimates of density and mortality rate are presented, together with estimates of standard errors of these estimates. The interpretation of these estimates in relation to the design and frequency of the sampling procedure is discussed, with special reference to the different topographies of the areas surveyed.