Interactive effects of shell hash and predator exclusion on 0-year class recruits of two infaunal intertidal bivalve species in Maine, USA

Abstract

Biotic and abiotic factors affecting survival and growth of recently settled infaunal bivalves not only determine the strength of 0-year class populations, but the structure and function of benthic soft-bottom communities. At mudflats across north-central Casco Bay, a 517 km2 embayment in the western Gulf of Maine, USA, intertidal sediments are generally acidic (pore water pH range = 7.09–7.85), which can negatively affect settlement and subsequent recruitment success of infaunal bivalves due directly or indirectly to shell dissolution. In addition, predation on bivalves by invasive green crabs, Carcinus maenas, and native consumers is intense in this region. Two commercially important infaunal bivalve species occupy these sediments (softshell clams, Mya arenaria; hard clams, Mercenaria mercenaria). Fisheries managers, legislators, and others have suggested that adding crushed shells of M. arenaria to the surface of mudflats can ameliorate negative effects of acidic sediment pore water through chemical buffering. We investigated the interactive effects of modifying surface sediments using crushed and weathered shells of Mya and predator exclusion on abundance and size of 0-year class individuals of these two bivalve species in large-scale plots (9.3 m2) and small-scale experimental units (EU; 182.4 cm2). Field experiments were conducted over three years (170–204 days yr−1; each initiated prior to spawning and continuing well after settlement had ceased). Shell hash in large-scale plots (mean particle size = 19.3 mm) varied across three levels (0, 0.63, and 1.27 kg m−2), and between 0 and 1.27 kg m−2 in EU where shell size varied from 1.9–19.3 mm. Small-scale experiments also used granite chips that, like crushed shells, increased habitat heterogeneity but did not buffer sediments. Density and size of both bivalve species at the end of most field trials were significantly greater in predator-exclusion treatments vs. controls independent of shell treatment. In all trials, neither Mya nor Mercenaria responded positively to the presence of shell additions. Fisheries managers should focus attention on mitigating effects due to predators instead of spreading shell hash to buffer intertidal sediments.