A Study of the Effects of a Commercial Hydraulic Clam Dredge on Benthic Communities in Estuarine Areas

Godcharles, Mark

A Maryland soft-shell escalator clam dredge, the R/V Venus, was used in a systematic sampling program to study its effects on representative bottom types (habitats) in Tampa Bay and to conduct clam exploration in Tampa and Boca Ciega Bays, the Cedar Keys area, and Tarpon Springs vicinity. Six experimental stations established in Tampa Bay were visually inspected and sampled with trynet before dredging and at various intervals after dredging. Benthic plug samples were taken at the final sampling. Sediment samples were also taken to assess textural changes by particle size analyses. Collected fauna were identified, counted, and in most instances, measured. After more than a year no recolonization of sea grasses, Thalassia testudinum and Syringodium filiforme, occurred in any dredged area. Some regrowth of Caulerpa prolifera was observed 86 days after dredging. No increase of clam set was detected during the study. Analyses of trynet hauls showed no faunal variations between dredge and control plots at any time after dredging and benthic plug samples revealed marked faunal differences at only one station. Redredging with the R/V Venus revealed no faunal declines except for a marked decrease in quahogs, Mercenaria campechiensis, at one station. Dredgehead water jets penetrated the substrate 18 inches and uprooted all vegetation. Dredge tracks remained visible from one to 86 days and some spots remained soft for over 500 days. Two stations showed a decrease of silt/clay particles immediately after dredging but only one showed a sustained decrease. The greatest density of clams, Mercenaria campechiensis, was found in Tampa Bay and considerable numbers of surf clams, Spisula raveneli, were found on the Gulf side of Mullet Key. The greatest production of clams (M. campechiensis) in the Cedar Key survey occurred at the Suwannee Reef and in the Tarpon Springs survey north of Honeymoon Island. Two modifications to the harvester are recommended to increase efficiency and retard substrate damage. It is also recommended that these harvesters be permitted to operate in Florida on a permit basis and be prohibited in grassy areas.