Short-term variations in settlement and early spat mortality of Iceland scallop, Chlamys islandica (O.F. Müller)

Abstract

A field experiment was carried out from 20 September to 9 November 1992 in the Baie des Chaleurs (Gulf of St. Lawrence) to examine the temporal pattern of Iceland scallop ( Chlamys islandica) spat settlement on artificial filamentous collectors. One hundred sixty five cubic Vexar cages (10 × 10 × 10 cm) filled with 150 g (wet weight) of dead hydroids ( Tubularia larynx) perisarcs were deployed and retrieved using a sequential time exposure schedule [collectors were deployed in decreasing numbers (30 to 5) every ≈ 5 days and retrieved in the reverse order (5 to 30) of their immersion]. This approach gave independent samples with respect to time and made it possible to estimate (1) near instantaneous settlement density, (2) short-term post-settlement mortality, (3) cumulative collection efficiency, and to compare (4) settlement onto substrata immersed for different periods of time. Simultaneous measurement of environmental factors (tidal range, current speed, seawater temperature and salinity) allowed an assessment of the relative influence of environmental factors in determining settlement density. The spat collectors used in this study were successful in collecting many thousands of Iceland scallop post-larvae. Settlement took place during 1 month, from the middle of September to the middle of October 1992 and collectors immersed for 10, 15, and 20 days collected significantly more spat than those immersed for 5 days. After the settlement season, spat mortality remained low for at least an additional 25 days (the end of the observation period). The daily mean density (DMD) of newly settled spat during each ≈ 5-day interval varied significantly in time and there was no significant correlation between DMD and the environmental factors. Different hypotheses concerning larval flux (no. of larvae × mean current speed) and their probability of coming into contact and being retained by a collector (retention efficiency) are proposed to explain the temporal pattern of spat collection observed in this study. Some suggestions are made to enhance collection efficiency of artificial scallop spat collectors.