Biotic and abiotic factors influencing growth and survival in wild and cultured individuals of the soft-shell clam, Mya arenaria L., in eastern Maine

Soft-shell clams, Mya arenaria L., have been harvested commercially in Maine since the mid-1800's. Numerous attempts to manage the wild resource since that time have focused on licensing restrictions, minimum size limitations, and closed seasons. These traditional approaches depend on extensive natural sets each year. When predation rates on small clams are high, or year-class phenomena result in the lack of successful recruitment, these management plans fail. One method of counteracting the poorly understood, highly variable early life-history processes is to enhance stocks with individuals raised in a shellfish hatchery. The collection of works in this thesis examine the ecology of both wild and hatchery-reared individuals at several field sites in eastern Maine.; I tested the hypothesis that Mya attains a size refuge from the naticid gastropod, Euspira heros (Say), at an intertidal site near Dennysville. Snails preyed heavily on all sizes of clams offered (15-51 mm), but concentrated their feeding on animals 30-40 mm. Laboratory tests and field observations suggest that prey size refuge is a function of tidal height: small snails ({dollar}<{dollar}20 mm) occupy higher positions on the shore and can attack only small clams ({dollar}{bsol}leq{dollar}30 mm) whereas large snails ({dollar}{bsol}geq{dollar}20 mm) are found at lower tide levels and can prey on clams {dollar}>{dollar}30 mm. Another study investigated the role of seagrass, Zostera marina L., in protecting juvenile clams from predation. Contrary to results of previous investigations using infaunal suspension-feeding bivalves, aquatic vegetation does not serve as a spatial refuge for small clams. It harbors equally their crustacean predators that use seagrass beds to escape higher-order predators.; To enhance existing stocks with hatchery-reared individuals or farm cultured clams on private leases, animals {dollar}{bsol}geq{dollar}8 mm in shell length and at densities between 333-666/m{dollar}{bsol}sp2{dollar}, should be planted in early April near the mid-tide level and protected from large epifaunal predators using flexible mesh plastic netting (6.4 mm aperture). To decrease interference with clam feeding, netting should be raised above the sediment surface 5-10 cm.