Tracking Larvae of the Hard Clam (Mercenaria) in the Indian River Lagoon, Florida, USA

Hard clams of the genus Mercenaria support an important commercial fishery throughout the eastern seaboard of the United States. The clam populations that support those fisheries are characterized by drastic interannual fluctuations in density, which creates considerable hardship for the fishery participants. In Florida, we are testing various strategies to enhance clam abundance in an attempt to maintain employment for a limited number of licensed fishery participants. Those strategies include spawner transplants, seeding, and direct larval injection. Direct larval injection consists of fertilizing clam eggs in an upland hatchery and immediately releasing those larvae into suitable and predetermined waters.

In the Indian River, a shallow, microtidal lagoon on the east central coast of Florida, we have conducted larval releases and tracked the resultant larval mass to determine rates of advection and diffusion. We use shallow-water drifters to track the water mass, and we use sulfur hexafluoride to estimate rates of diffusion. We have also developed a genetic probe that is quantitative and Mercenaria-specific; it allows us to map the distribution of the larval mass relative to the water mass. Preliminary results suggest that, in the enclosed waters of the Indian River, diffusive processes may outweigh advective processes in the dispersal of the Mercenaria larval mass. Thus, larval dispersal is localized. The possibility exists to release large numbers of fertilized eggs to create localized but high-density patches of harvestable clams sufficient to maintain employment of clammers during periods of low "natural" clam abundance.

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