Tracking and growth of larvae of the giant scallop, Placopecten magellanicus (Gmelin, 1791) on a scallop farm in Notre Dame Bay, Newfoundland

Crocker, Kevin George (1998) Tracking and growth of larvae of the giant scallop, Placopecten magellanicus (Gmelin, 1791) on a scallop farm in Notre Dame Bay, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Culture of the giant scallop began in Newfoundland in 1986 after research indicated a supply of wild spat was possible. The failure of wild seed production in sufficient quantities had negative consequences, and since 1991 wild scallop spatfall has been very low. It was proposed that the release of millions of D-veliger larvae, raised by hatchery techniques, on a site may increase annual spatfall and collection. Enhancement was attempted in 1993 on a scallop farm in Newfoundland but no increase in spat collection was realized. This study was initiated to investigate the possible fate of these released larvae. -- Three-day-old scallop veligers were stained with calcein (a non-toxic fluorescent dye) by immersion in a solution of 150 mg/L calcein for 16 h at densities of 40 larvae/mL. High mortalities were observed at densities of 250 larvae/mL. Satisfactory fluorescence was observed in preserved samples which subsequently retained the stain for at least two years. -- Two batches of calcein stained scallop larvae were released on a shellfish farm located in Charles Arm, Notre Dame Bay, Newfoundland during the summers of 1994 and 1995. Upon recapture of these larvae by plankton tows, growth rates of 3.57 and 3.85 µm/d in 1994, and 9.72 and 2.52 µm/d, in 1995, were observed. Differences in the growth rates varied seasonally and annually (1994 & 1995), and were related to water temperature and food concentration (chlorophyll-α). Size frequency distribution, current meter data, and current directional maps created by drift bottle drogue surveys showed evidence of possible entrainment of larvae within Charles Arm. As well, evidence was also reported suggesting that bivalve larvae were also transported out of the system. -- Larval abundance and shell height distribution varied with tidal state. Higher numbers and larger size bivalve veligers were observed during the mid to late flood and ebb tides. Mean size of bivalve larvae at four stations sampled on the site differed significantly over the tidal cycle. Variation in larval abundance was also greater during neap tides as compared to spring tides. Diel differences in larval abundance were also observed This has important implications for any sampling regime directed to measuring size and numbers of bivalve larvae over time, for example, larval monitoring programs used in the aquaculture industry -- An enhancement project designed to increase the subsequent spatfall of giant scallop veligers is possible but should be very intensive. Hundreds of millions of greater than 200µm scallop veligers may have to be released within the site to result in an observed increase in spatfall and to counteract losses due to natural mortality and net outward transport from the system.

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