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

Soft-shelled clams, Mya arenaria, and razor clams, Tagelus plebeius, in Chesapeake Bay have been in decline since the 1970s, with severe declines since the early 1990s. These declines are likely caused by multiple factors including warming, predation, habitat loss, recruitment limitation, disease, and harvesting. A bivalve survey was conducted in Chesapeake Bay to examine influential factors on bivalve populations, focusing on predation (crab, fish, and cownose rays), habitat type (mud, sand, gravel, shell, or seagrass), environment (temperature, salinity, and dissolved oxygen), recruitment, and disease. M. arenaria and T. plebeius were found more often in more-complex habitats such as seagrass or 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. Mya arenaria, which is near the southern extent of its range in Chesapeake Bay, was negatively associated with high water temperatures. Recruitment of M. arenaria in Rhode River, MD, declined steadily 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, low recruitment, and predators are major factors keeping T. plebeius and M. arenaria at low densities in Chesapeake Bay. Persistence at low densities may be facilitated by complex habitats, whereas further reductions in habitats such as seagrass could result in local extinction of these important bivalve species.