Enhancing hard clam (Mercenaria spp.) population density in the Indian River Lagoon, Florida:: A comparison of strategies to maintain the commercial fishery

Hard clams of the genus Mercenaria support an important commercial fishery in the Indian River Lagoon on the east central coast of Florida. The fishery is relatively young but has proven to be quite sporadic, with two periods of exceptional landings (mid-1980s and mid-1990s) bounded by periods of almost complete fishery collapse. In response to a request from commercial fishery participants, three strategies for enhancing the abundance of harvestable hard clams in the lagoon were compared. The first strategy involved harvesting adult clams from a low-density population and transplanting them at high density in a concentrated area, in an effort to increase fertilization efficiency and thereby maximize reproductive success. That approach does not appear to be viable in the Indian River Lagoon because mortality of transplanted clams approached 100% and because Indian River hard clams display a remarkably high incidence (>80%) of gonadal neoplasia. Neoplasia reduces the life span of Indian River hard clams relative to their northern congeners and probably reduces fecundity of those clams that do survive. The second strategy involved seeding juvenile clams at very high densities (843-7165 m(-2) depending upon seed size), again with the intent of maximizing fertilization efficiency but with the additional goal of maximizing residual reproductive value. Various planting treatments were tested in an effort to minimize mortality of seed clams, but losses were still high (generally >70%), and the yield did not appear to justify the cost. The final strategy involved spawning large numbers of hard clams in the laboratory, fertilizing the resultant eggs, and immediately releasing the larvae at a predetermined location in the lagoon. Large numbers of larvae did appear to survive the 8-day larval life span, but it remains to be seen whether those larvae will translate into harvestable hard clams. In any event, enhancement of hard clam populations appears to be feasible only when the density of naturally occurring clams is so low that reproductive viability is compromised. Otherwise, natural reproductive potential will swamp any artificial efforts at population enhancement.