Sampling for plaice eggs was carried out in the Southern Bight of the North Sea in January 1981 and 1982. Egg distributions were centred in the patch of warmer water entering the southern North Sea from the English Channel. Compared with seasonal values from previous years the estimates of daily egg mortality within the limited survey period were high in both 1981 (0.141) and 1982(0.106). The positive buoyancy of plaice eggs was determined both from measurements of egg density (mean of 1.02552 g cm −3 at 8°C, standardized using a derived coefficient of thermal expansion of 14.0 × 10 −5 per °C) and their passive rate of vertical ascent (mean of 5.2 m h −1 ). Vertical-distribution sampling using both a Lowestoft Multiple Changing Net and a Longhurst-Hardy Plankton Recorder showed eggs at all stages of development to be distributed throughout the water column, despite their positive buoyancy. The implied high mixing rates were attributed to both wind and tidally-induced mixing. An eddy diffusivity coefficient (K) in the order of 250 cm s −1 was calculated using the buoyancy and vertical-distribution data. It is suggested that in some years the effects of rough weather in increasing egg mortality may have an influence on recruitment strength.
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Coombs et al. (1990) studied this question.