Environmental Factors Influencing Bacterial Levels Associated with Rangia Cuneata (Gray) in North Carolina

Résumé du livre

Rangia cuneata (Gray), 1831, is a mactrid clam that frequents oligo- to mesohaline systems of the mid-Atlantic and southern states. Recent interest in commercial harvesting and marketing the clam for human consumption as a fresh seafood product prompted an investigation into its sanitary quality. The purpose of this research was to: (1) extend the data base on Rangia fecal coliforms (FC) and standard plate counts (SPC) reported by Comar et al. (1979) from Albemarle Sound, N.C.; (2) investigate the effect of environmental factors on clam SPC; and, (3) determine SPC variance among clams. A median MPN (most probable number) of 80 FC per 100 grams (g) was found in 92 Rangia samples collected over the 2 year study. A seasonal pattern in clam FC was shown. February through April had a median MPN of less than 20 FC/100g id.th no samples exceeding the federal and state standard of 230 FC/lOOg. Standard plate counts showed greater variability and ranged from mean monthly values of 30,000 SPC/g in January to 610,000 SPC/g in December. A statistical analysis of clam SPC versus environmental factors gave a significant negative linear correlation between clam SPC and rainfall. Multivariate regressional analyses showed that 26.5 percent of the variation in clam SPC can be explained by variations in turbidity. river index, water temperature, and salinity. The interaction of the observed environmental factors aids in understanding the large difference in mean Rangia SPC found at the 4 collection stations. A field experiment conducted to assess the influence of sediment on clam SPC demonstrated the greater importance of factors associated with the water in determining SPC than with the sediment. High turbidity conditions caused by dredging did not significantly affect clam SPC. However, a large degree of variation among composite clam samples was found. Analyses of SPC variance among individual clams revealed a similarly large degree of variation. High variance in SPC complicates the effort to market Rangia as a fresh seafood product. More frequent sampling of clams from open, high saline shellfishing waters supporting Rangia during the months of February through April is recommended. Failure to demonstrate a pattern in Rangia SPC may lead to alternative methods of marketing the clam such as depuration or pasteurization.