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 feeder in the Oosterschelde, a 351 km² tidal bay in the SW Netherlands. To establish its role in the benthic foodweb, and to assess the impact of human activities, data on density, age composition, biomass and growth were collected from several tidal flats in the Oosterschelde between 1980 and 1990. To estimate the overall biomass development of the cockle, a simple model was used, in which three growing seasons are defined 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 individual growth curve, numbers and biomass of each age group in the Oosterschelde population were estimated. Average biomass (including shell organics) per m² of tidal flat in August varied from 140 g AFDW in 1980 to 21 g AFDW in 1989, implying a total cockle stock on all tidal flats of 19170 to 2350 tonnes AFDW (72 x 10³ to 9 X 10³ tonnes flesh), respectively. A comparison of results from field surveys and the reconstructed stock estimations showed large deviations. However, an uncertainty analysis performed on the model showed that most field data fitted 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 the construction of the storm-surge barrier and the compartmentalisation dams could not be demonstrated. The year-to-year variation in cockle stocks, assessed in the way described in this paper should be regarded as relative, because a systematic survey of the intertidal flats was not performed every year, but population dynamics from selected stations were used instead.