Density dependence of growth and production in a Wadden Sea population of the cockle Cerastoderma edule

Abstract

In suspension-feeding bivalves, growth rates generally remain at a high level over abroad range of numerical densities. However, data series on growth and production that cover awide density range in a natural marine benthic system appear to be scarce. A long-term monitoringstudy (1973-2013) of zoobenthos on tidal flats in the western Wadden Sea yielded twiceannualdata on numbers and weights of (among others) cockles Cerastoderma edule, allowingestimates of their annual and cohort production by the weight-increment summation method. Innearly all years, cockle densities did not exceed a few 100s m-2, and their individual weight incrementsshowed no significant relationship with density. Only the appearance of one exceptionallystrong year class in 2011 caused higher densities over an extensive area for >1 yr. Growth rates inall cockles were significantly reduced in the second growing season of this strong cohort, whentheir densities amounted to >500 m-2. Seasonal and annual survival did not show a significantrelationship with density. As a consequence of the reduction in growth of its members, productionof the single strong cohort was substantially less than expected on the basis of its initial abundance.In all smaller cohorts, the positive relationship between recruit numbers and cohort productionwas close to linear. However, when the higher density of the one strong year class wasincluded, an inverse-U shaped line would better characterize this relationship. Appearances ofvery strong bivalve cohorts are rare events in the Wadden Sea: bivalve stocks on the tidal flatsusually appear to be far below the carrying capacity of the area for suspension-feeding animals.