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Abstract/ Summary

A comparative field experiment was initiated at two intertidal flats in southern Maine (Wells – Webhannet River; Portland – Fore River) in May 2014 to examine the interactive effects of tidal height and predator exclusion on the growth and survival of cultured individuals of the soft-shell clam, Mya arenaria L. (xShell length = 12.95 ± 0.20 mm). Experimental units (0.018 m2) were placed near the upper and lower intertidal and filled with ambient sediments at both sites. Clams were added to units at a density of 660 ind. m-2. Predator exclusion included five treatments: 1) none (controls); 2) flexible netting (4.2 mm aperture); 3) flexible and rigid netting (6.4 mm); 4) Pet screen over the top of the units; and 5) Pet screen over the top and bottom of the units. Netting was designed to exclude green crabs, Carcinus maenas, whereas Pet screen was designed to exclude crabs and nemertean worms, Cerebratulus lacteus. Experimental units at each site were collected in October 2014, after 151 days in the field.

Survival did not vary significantly across tidal heights at either site. Less than 5% of clams in control units were recovered vs. > 50% survival in protected units at both sites. Pet screen did not enhance survival at either site compared with flexible netting. Growth was faster at the lower vs. upper intertidal at one site (by ca. 20%), but not at the other, and was depressed between 50-60% in units protected with Pet Screening compared to open and netted units. Mean final shell length in open and netted units pooled across sites ranged from 25-40 mm. Wild, 0-year class recruits of Mya were observed at both sites, and were generally more abundant in lower vs. upper intertidal units. Few recruits occurred in control units, but mean abundance of recruits was an order of magnitude greater in units protected with Pet screen (3843.9 ± 1737.9, n = 44) vs. flexible netting (726.4 ± 400.5, n = 44) pooled across both sites.





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