Management of Manila clam beds: I. Influence of seed size, type of substratum and protection on initial mortality

Abstract

Commercial Manila clam beds in the Eo estuary (NW Spain) were studied from 1991 to 1996 to determine the influence of seed size, level of protection and substratum type on mortality early in the culture period. We detected a significant effect of size on mortality in both protected and unprotected beds, as, even with predator control, smaller seed showed poorer survival when planted in the field. A significant influence of substratum on mortality was also evident, with lowest mortality in sand–gravel beds. Clams reached a size-refuge at 1 g (16 mm in shell length) when protected with nets from predators and at 2 g (21 mm in length) when unprotected, showing how the net reduced the availability of clams to predators. Therefore, planting larger clams in sand–gravel beds and protecting them with nets will enhance clam survival.