Burrowing behaviour of soft-shell clams (Mya arenaria) following human disturbance

The soft-shell clam, Mya arenaria, is an endobenthic marine bivalve species that is abundant in the intertidal and shallow-subtidal zones of eastern Canadian and New England shores. In these coastal environments, clams are subjected to both natural and anthropogenic disturbances that can result in dislodgement from the sediment. This is especially relevant for harvested clam species that are managed by a legal size limit. Herein, live clams that are too small to be retained by harvesters are discarded back onto the sediment surface. These clams must reburrow in order to survive; however, the reburrowing behaviour of these clams after being disturbed is not well known. To fill this knowledge gap, a series of five field trials were conducted in Kouchibouguac National Park, New Brunswick, Canada, during the summer and fall of 2021 to measure reburrowing rates of discarded sub-legal sized (<50 mm) soft-shell clams. Three size groups (20–30, 31–40, and 41–50 mm) of soft-shell clams were excavated and returned to the sediment surface at three different tidal levels inside protective enclosures during low tide for each trial. Time to initiate and complete reburrowing was subsequently observed over a 24 h (h) period. Results showed that reburrowing time was not affected by size class; however, reburrowing time was affected by tidal level, site, and trial. Results suggest that disturbed clams discarded onto submerged sediment surfaces (tidal levels 2 and 3) reburrow quicker than those discarded onto air-exposed sediment (tidal level 1). Reburrowing rates of subtidally-discarded clams were faster during the first three months of the study (June–August) compared to rates observed in the September trials. Comparing reburrowing rates to environmental conditions during the trials suggested that decreasing salinity in September likely drove the slower reburrowing rates compared to rates observed earlier in the season; decreasing temperatures may also have had some influence. These findings suggest that discarding sub-legal sized clams onto submerged sediment will increase the probability of clam survival via faster reburrowing, and that environmental conditions at the time of discard can affect reburrowing rates.