Reporting period: July 31, 2003 – November 30, 2003 (Final Report) funding level
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- Evaluate growth and survival of hard clams (Objective 2) and oysters (Objective 3)
- American Oyster ( Crassostrea virginica )
| PROJECT TITLE: Evaluation of the Natural Occurrence and Stock Enhancement Potential of Hard Clams (Mercenaria mercenaria) and Oysters (Crassostrea virginica) in Little Assawoman Bay, Delaware
REPORTING PERIOD: July 31, 2003 – November 30, 2003 (Final Report) FUNDING LEVEL: $9,500 CIB Grant #: EPA CE 99399005 UD Code: 4-3-25-3502-15 PARTICIPANTS: John W. Ewart, Aquaculture Specialist (Project Manager) Delaware Sea Grant Marine Advisory Service Graduate College of Marine Studies, University of Delaware 700 Pilottown Road, Lewes, DE 19958 Phone: 302‑645‑4060; E-mail: EJ Chalabala, Delaware Center for the Inland Bays (CIB) Morgan Pitts, Delaware Center for the Inland Bays (CIB) Jack Pingree, Delaware Department of Natural Resources and Environmental Control (DNREC) PROJECT OBJECTIVES: 1) Conduct a qualitative assessment of the natural occurrence of hard clams, oysters and other commercially important bivalves in Little Assawoman Bay 2) Evaluate the growth and survival of in-bottom plantings of hard clam seed 3) Evaluate the growth and survival of off-bottom plantings of seed oysters ANTICIPATED BENEFITS: Episodic fluctuations in salinity, poor water exchange rates, extensive zones of soft mucky sediments high in silts and clays, incompatible plankton assemblages and low broodstock biomass or the absence of viable broodstock are considered to be factors limiting natural bivalve populations in Little Assawoman Bay. Developing current information on natural bivalve distribution and abundance in the Little Assawoman Bay and determining the technical feasibility and potential for using aquaculture methods for shellfish stock enhancement and replenishment will improve management options for state resource managers. PROGRESS OF THE WORK AND PRINCIPAL ACCOMPLISHMENTS: Conduct a qualitative assessment of the natural occurrence of hard clams, oysters and other commercially important bivalves in Little Assawoman Bay (Objective 1) Anecdotal reports suggest that hard clams occur naturally in some areas of the Little Assawoman Bay where suitable bottom habitat exists but not in sufficient quantities to support either recreational or commercial fisheries. During fall 2002 and summer/fall 2003 bivalve shellfish surveys were conducted at thirteen stations in the Little Assawoman Bay (Figure 1.). The surveys revisited the same thirteen field stations originally established in 1991 by the Delaware Department of Natural Resources and Environmental Control (DNREC). The 1991 DNREC survey of hard clams in Little Assawoman Bay yielded no live animals, but did produce dead shells of hard clams, soft shell clams and oysters (Ullman et al., 1991). During this project, the fall 2002 survey conducted yielded no live commercially important bivalves or evidence of the previous occurrence of hard clams, soft shell clams and oysters. The summer/fall 2003 field survey of the thirteen stations plus additional locations around Little Assawoman Bay also produced no live examples of commercially important bivalves but did yield evidence of prior occurrence. A small bed of dead oyster shell (Figure 2) was located at Station 6 near the mouth of Dirickson Creek. A single specimen of old and weathered dead shell of the hard clam (Mercenaria mercenaria) was found at Station 3 and numerous dead shells of the Stout Razor Clam (Tagelus plebius) were collected at Stations 1 and 3. The Atlantic Ribbed Mussel (Geukensia demissa), while not considered to have commercial importance, was the only naturally occurring living bivalve observed in the Little Assawoman Bay. The mussels were commonly found in the high intertidal zone along banks of marsh grass especially in the southern portion of the bay. Evaluate growth and survival of hard clams (Objective 2) and oysters (Objective 3)The adequacy of local water quality and environmental conditions in Little Assawoman Bay to support introduced plantings of hard clams and oysters has not been previously evaluated. The advent of a citizens volunteer Inland Bays Oyster Gardeners program, organized and managed during summer/fall 2003 by the Center for the Inland Bays and Delaware Sea Grant Marine Advisory Service, provided an opportunity to deploy seed oysters and hard clams at various locations around the bay to assess shellfish growth and survival. Hard clam (Mercenaria mercenaria) One hundred hard clams (25 mm average Shell Length) were deployed in off-bottom Taylor Floats at each of three oyster gardening locations in the Little Assawoman Bay – Bay View Park (northern), Coastal Kayaks beach site Between South Bethany and Fenwick Island (central) and Sharks Cove Marina (southern) (green asterisks, Figure 1). Salinities ranged from 22 parts per thousand (ppt) in the upper bay to 24 ppt in the lower bay. Clam growth at all locations was low (3-4 mm) during eleven weeks of deployment. Because off-bottom (vs. in-bottom) culture is not considered optimal for larger sized clam seed, the low growth observed was anticipated. However, hard clam survival (98-100%) was high at all three locations. During the summer/fall 2003 field survey, two locations with firmer bottom characteristics (sandy-mud sediments) suitable for in-bottom plantings of clams were identified at Stations 2 and 11 (blue asterisks, Figure 1). In addition to sampling the general area around the stations, shallow sub-tidal areas closer to shore were examined using hand rakes but no evidence hard clams (or other bivalves) were found at either location. Because bottom (sediment) conditions were otherwise suitable for hard clams, small recreational scale plantings of two hundred and fifty clams were deployed at each location at a density of 1-2 clams per square foot. Hard clam growth and recovery will be assessed during summer 2004 and the project report will be amended with this additional information. American Oyster (Crassostrea virginica)Hatchery produced oyster spat (on shell) with an initial average shell height of 10 millimeters were deployed in Taylor Floats (Figure 3) during the first two weeks of August 2003 at nine locations in Little Assawoman Bay (Figure 1). Table 1 summarizes average oyster growth (shell height) observed at each location during fall 2003 after an Table 1. Final oyster growth measurements (October 23-30, 2003)
eleven week deployment. The average increase in oyster growth, which ranged from 23 to 34 millimeters, was considered to be very good to excellent. Good performance was observed at all locations especially in the southern portion of Little Assawoman Bay. Oyster mortality was negligible at all locations. While bottom (sediment) conditions in most of Little Assawoman Bay are typically too soft to support natural aggregates of benthic oyster populations, local water quality conditions will support rapid oyster growth when oysters are deployed using off-bottom gear such as Taylor Floats or similar. Growth rates observed during the initial deployment of oysters in Little Assawoman Bay contradict anecdotal estimates that bay water quality conditions are generally unsuitable for bivalves. These results also demonstrate the high potential for oyster aquaculture in the bay using off-bottom gear. PUBLICATIONS AND OTHER OUTREACH ACTIVITIES: Outreach activities include presentation of results on the Delaware Aquaculture Resource Center Web site Download 4.77 Mb. Share with your friends:
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