Impacts of habitat, predators, recruitment, and disease on soft-shell clams Mya arenaria and stout razor clams Tagelus plebeius in Chesapeake Bay

Soft-shell clams Mya arenaria and razor clams Tagelus plebeius in Chesapeake Bay, USA, have declined in numbers since the 1970s, with severe declines since the 1990s. These declines are likely caused by multiple factors, including habitat loss, predation, recruitment limitation, disease, warming, and harvesting. We surveyed Chesapeake Bay to examine influential factors on bivalve populations, focusing on habitat (mud, sand, gravel, shell, or seagrass), predators (crabs, fish, and cownose rays), recruitment, disease, and environment (temperature, salinity, and dissolved oxygen). M. arenaria and T. plebeius were found more often in habitats with complex physical structures (seagrass, shell) than any other habitat. Pulses in bivalve density associated with recruitment were attenuated through the summer and fall when predators are most active, indicating that predators likely influence temporal dynamics in these species. Presence of M. arenaria, which is near the southern extent of its range in Chesapeake Bay, was negatively correlated with water temperature. Recruitment of M. arenaria in the Rhode River, Maryland, declined between 1980 and 2016. Infection by the parasitic protist Perkinsus sp. was associated with stressful environmental conditions, bivalve size, and environmental preferences of Perkinsus sp., but was not associated with bivalve densities. It is likely that habitat loss, predators, and low recruitment are major factors keeping T. plebeius and M. arenaria at low densities in Chesapeake Bay. Persistence at low densities may be facilitated by habitat complexity (presence of physical structures), whereas further reductions in habitats such as seagrass and shell hash could result in local extinction of these important bivalve species.