VARIABILITY IN STOCK ASSESSMENT OF COCKLES (CERASTODERMA-EDULE L) IN THE OOSTERSCHELDE (IN 1980-1990), IN RELATION TO ENVIRONMENTAL-FACTORS

The edible cockle (Cerastoderma edule L.) is a dominant suspension fee der in the Oosterschelde, a 351 kM2 tidal bay in the SW Netherlands. T o establish its role in the benthic foodweb, and to assess the impact of human activities, data on density, age composition, biomass and gro wth were collected from several tidal flats in the Oosterschelde betwe en 1980 and 1990. To estimate the overall biomass development of the c ockle, a simple model was used, in which three growing seasons are def ined for the cockle population. A standard individual growth curve was constructed. A negative exponential mortality function was assumed to estimate the number of recruits. By combining the estimated number of recruits, the estimated specific mortality rate and the standard indi vidual growth curve, numbers and biomass of each age group in the Oost erschelde population were estimated. Average biomass (including shell organics) per m2 of tidal flat in August varied from 140 g AFDW in 198 0 to 21 g AFDW in 1989, implying a total cockle stock on all tidal fla ts of 19170 to 2350 tonnes AFDW (72 x 10(3) to 9 X 10(3) tonnes flesh) , respectively. A comparison of results from field surveys and the rec onstructed stock estimations showed large deviations. However, an unce rtainty analysis performed on the model showed that most field data fi tted within the minimum and maximum biomass calculated. Total biomass is largely dependent on the strength of certain year classes. In this respect, the year classes 1979, 1982, and 1985 were good. Effects of t he construction of the storm-surge barrier and the compartmentalisatio n dams could not be demonstrated. The year-to-year variation in cockle stocks, assessed in the way described in this paper should be regarde d as relative, because a systematic survey of the intertidal flats was not performed every year, but population dynamics from selected stati ons were used instead.