Evaluation of spatfall in the Cape Fear estuary

 

 

As part of an ongoing Fisheries Resource Grant (project number 00-AM-07 “Evaluation of Stock differentiation…”) the primary investigators been following the growth and survivorship of five different oyster stocks in four southeastern North Carolina watersheds.  This project has sites in the Cape Fear River, New River, and White Oak River as well as one site located on a commercial bottom lease in Stump Sound. Last spring juvenile oysters (10-20mm) from each site (plus a hatchery stock from Virginia) were collected, measured and deployed in 1m by 1/2m bottom cages in each of the four sites.  Growth and survival is being determined at 10-12 week intervals.  At the completion of the September/October measurement cycle oysters placed at the Cape Fear River site exhibited high mortality for all stocks, exceeding that observed in the other estuaries.  This mortality at the Cape Fear site was accompanied by an extremely high overspatting event.  The caged experimental oysters had been free from overspat at the previous observation date (oysters were scrubbed to improve measurement accuracy), suggesting that the settlement event took place in late July.  The caged experimental oysters in Cape Fear River measured ~ 36-74mm in shell height  (~21-43mm shell length) at the September/October sampling and averaged ~22 spat per individual (range: minimum 7- maximum 42).  Spatfall over the same time period in the other systems was significantly less, averaging fewer than 1 per individual.

 

The substantial overspatting in the Cape Fear estuary was unanticipated as the system is generally considered marginal oyster habitat.  It lacks the extensive oyster reefs found in other estuarine systems in North Carolina.  This lack could be due in part to the dynamic nature of the Cape Fear River (relatively high flushing rates) and the absence of suitable settlement substrate (cultch).  However, insufficient larval supply could be a factor since areas in the estuary with seemingly suitable hard substrates (upstream of the rock walls at Fort Fisher and the Southport Ferry terminal) did not support significant oyster populations. Communication with local fishermen supported observations that oysters simply did not settle beyond the mouth of the Cape Fear estuary.  Recent observations, however, indicate that these assumptions may not be well founded and that the absence of substantial oyster populations in the Cape Fear could be the results of other yet-to-be-determined post-settlement processes that result in mass mortality.  In order to address these questions, we propose to 1) to evaluate spatfall in the Cape Fear River and 2) determine viability of the larvae that do settle.  This study will provide much needed data, which is critical for guiding future efforts at habitat restoration and enhancement in the region of the Cape Fear.  This data will also provide valuable information concerning the potential for targeting the Cape Fear as a source of much needed recruits that could then be relayed to more suitable grow out areas

 

 

 

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