Factors influencing the recruitment and abundance of Didemnum in Narragansett Bay, Rhode Island

The non-indigenous colonial tunicate Didemnum sp. A has been observed in Narragansett Bay, Rhode Island, since 2000. We compared weekly recruitment of the species and environmental parameters (i.e. temperature, salinity, chlorophyll a, dissolved oxygen, pH, and nutrient concentrations) over a 6-month period among three sites: (i) the University of Rhode Island Graduate School of Oceanography dock (GSO), (ii) the Department of Environmental Management pier at Fort Wetherill (FW), and (iii) the Prudence Island T-wharf. At the GSO and FW, divers surveyed the sites for percentage cover of Didemnum. To assess the spread of Didemnum in the bay and what factors may predict the tunicate's presence, we also surveyed intertidal sites in October and November 2005, noting Didemnum presence, salinity, number of boats and moorings, and distances to major ports at each site. GSO had the highest percentage cover of adults and the highest recruitment of the tunicate (p<0.01), reaching average peak values of 319 individuals per 100 cm(2) in September 2005. Temperature and salinity demonstrated the best correlation with recruitment, and higher boat and mooring numbers may be a reliable predictor of tunicate presence. Further monitoring is needed to assess the potential spread of Didemnum throughout Narragansett Bay.