The use of market sampling to generate maturity ogives and to investigate growth, sexual dimorphism and reproductive strategy in central and south-western North Sea sole ( Solea solea L.)

Abstract

The influence of body size, age, geographical distribution, year and season on the sexual maturation of Dover sole ( Solea solea L.) in the North Sea was investigated using market sampling data collected from fish landed by commercial fishing vessels at English ports on a monthly basis since 1983. A comparison was made with maturity estimates for sole derived from research vessel survey data. The market sampling data were summarized using general linear modelling and used to generate maturity ogives for sole. There are a number of uncertainties with this approach, stemming from the use of the commercial fishery to provide the fish samples and reliance on visual (macroscopic) staging to ascertain the stage of sexual maturation of the gonads. Despite the inability to resolve all uncertainties, it was felt that it should be feasible to derive general-purpose maturity ogives from market sampling data, and that these should be sufficiently reliable for use in assessing the spawning stock biomass of North Sea Dover sole. Male and female sole were shown to have different growth and reproductive strategies, with the emphasis being directed towards maximizing egg production.

Current stock assessments for sole assume that sexual maturation is knife-edged, with all fish maturing at 3 years of age. Evidence is presented to show that this approach is likely to inflate estimates of spawning stock size, since not all 3-year-olds spawn, and the proportion that do spawn varies annually. Size selection pressures resulting from commercial fishing practices are also likely to further inflate estimates of the spawning stock size. In addition, reproductive investment in first time spawning females was found to be substantially lower than in older established spawners, and it is the latter which are the first to become depleted through the pressure of sustained size-selective fishing. As a consequence, the current view of the state of well being of the North Sea sole spawning stock is likely to be overly optimistic.