Substrate Additive Studies for the Development of Hardshell Clam Habitat in Waters of Puget Sound in Washington State: An Analysis of Effects on Recruitment, Growth, and Survival of the Manila Clam, Tapes philippinarum, and on the Species Diversity and Abundance of Existing Benthic Organisms

Abstract

To meet demands of expanding tribal and recreational hardshell clam fisheries, the Washington State Department of Fish and Wildlife has been developing new hardshell clam habitat. In Oakland Bay, near Shelton, Washington, two artificial substrates were studied to determine the best substrate for use in improving Manila clam (Tapes philippinarum) production. Treatments consisted of a control of natural substrate; a 10 cm layer of nonuniform size gravel ranging from 6 mm to 19 mm in diameter; and a 10 cm layer of a 50∶50 mixture of gravel and crushed oyster shell ranging from 10 mm to 30 mm in diameter. Effects of these substrates on species diversity and relative abundance of existing, benthic organisms were also investigated. Recruitment of Manila clam seed, less than 10 mm shell length, was significantly higher (p<0.05) in the control plots than in the gravel and gravel + shell plots, which showed no significant differences. Recruitment improved in the gravel and gravel + shell plots after a layer of organic debris and fine sediments collected on them. Manila clam survival was significantly higher in gravel and gravel + shell plots compared to control plots. No significant differences in survival were observed between gravel and gravel + shell plots. Manila clam biomass in the gravel and gravel + shell plots was significantly higher than in control plots. Between the gravel and gravel + shell plots there was no significant difference in biomass. Minor differences in species diversity and abundance of existing benthic organisms were observed among treatments. Gammarid amphipods and nemertean worms were enhanced in gravel and gravel + shell plots. Densities of polychaete worms were slightly lower in gravel and gravel + shell plots compared to control plots.