Recruitment in the swash zone - temporal variations in juvenile recruitment of an exposed sand beach surf clam

This study examined factors affecting juvenile recruitment in the tuatua Paphies donacina on an exposed surf beach in Pegasus Bay, South Island New Zealand between October 1994 and April 1997. Tuatua recruitment followed female gonad maturation with a delay of approximately 2 months. Juveniles (1-10 mm shell length) occurred throughout the study except during June and July 1995 (winter). Average densities (m(-2)) were 28 in winter, 230 in spring, 174 in summer and 108 in autumn. There was high juvenile mortality and/or emigration and the maximum average density of (741 m(-2)) was recorded in November 1996. Recruit densities were similar between years except during winter. Adult tuatua populations were dominated by modal shell lengths greater than 70 mm. There was no significant relationship between adult and juvenile recruit length. Recruit length decreased with recruit density suggesting density dependent competition for space or food resources. Juvenile density increased following increases in food availability (chlorophyll a concentration) but average length of juveniles did not correlate directly with any of the environmental variables. Recruit density decreased as the modal adult length increased and recruit length reduced when adult populations had a high condition index. Because larvae, juveniles and adult are mobile the beach population is characteristic of an open system with adults and juveniles from the resident population affecting post-settlement density-dependent processes.