Indirect effects of clam digging on the viability of soft-shell clams, Mya arenaria L.

Abstract

A series of laboratory experiments was conducted to determine whether non-lethal burial or exposure on the sediment surface could alter the normal living depth of Mya arenaria in sand and mud. After 2 weeks, clams (∼ 3 to 7 cm shell length) which were buried under 1 to 15 cm of medium-fine sand were found deeper than the controls, whereas clams (∼ 1 to 7 cm length) which were exposed on sand (and had subsequently reburrowed) were able to re-establish their normal living depth. In contrast, clams (∼ 1 to 7 cm length) buried under 1 to 15 cm of mud attained their normal living depth within two weeks, but exposed clams (∼ 1 to 7 cm length) reburrowed to abnornally shallow depths. An increased likelihood of predation at shallow sediment depths was compounded by the 60% slower reburrowing speed of exposed clams in mud than in sand. An additional experiment determined the approximate depth of burial under which at least 50% of a laboratory clam population could survive (LD 50). For small (< 3 cm in shell length), mid-size (3 to 5 cm) and large (> 5 cm) clams in sand, LD 50 values were < 75 cm, > 75 cm and between 50 and 75 cm, respectively. In mud, the LD 50 of clams < 5 cm in length was < 25 cm, and between 25 and 50 cm for clams ≥ 5 cm in length. It was concluded that the indirect mortality level associated with commercial and recreational harvesting will be much greater on intertidal mudflats than on sandflats.