EFFECT OF THE ORGANOCHLORINE PESTICIDE TECHNICAL CHLORDANE ON THE POPULATION-STRUCTURE OF SUSPENSION AND DEPOSIT FEEDING BIVALVES

As part of a study designed to assess the impact of the pesticide technical chlordane on marine infauna, a 3 mo field trial was conducted to identify sensitive species and life stages and to determine the time-scale of their response. Technical chlordane was applied to a hitherto uncontaminated site to achieve a near-surface sediment concentration of about 7.5 ng (g dry fines)-1. After 5 to 13 tidal cycles, the density of common infaunal suspension and deposit feeding bivalves began to decline and continued to do so for at least a further 31 tides. Populations of Chione stutchburyi and Tellina liliana decreased to about 30 and 40% respectively of their initial densities. Analysis of the size structure of the 2 populations showed that the decrease resulted primarily from the loss of juveniles 0.5 to 2.5 mm in size. Juveniles within this size range gradually repopulated the site between Tides 44 and 112, when chlordane residues in the near-surface sediment dropped to 3 ng (g dry fines)-1 following a storm. Numerous post-metamorphic specimens of Chione stutchburyi (76) and Tellind liliana (163) were caught in drift nets placed around the study area, suggesting that the mobility of post-settlement juveniles may play an important role in reducing the density of small shellfish in contaminated sediment. The potential movement of post-settlement juveniles away from contaminated sediments could provide a useful technique for impact assessment.