Short-term effects of commercial clam (Mya arenaria L.) and worm (Glycera dibranchiata Ehlers) harvesting on survival and growth of juveniles of the soft-shell clam

摘要

In Maine, USA, commercial fisheries for soft-shell clams, Mya arenaria L., and blood worms, Glycera dibranchiata Ehlers, occur simultaneously on muddy intertidal flats. Local and state clam managers frequently close flats to shellfishing for conservation purposes, but have no jurisdiction over wormers who are legally permitted to harvest G. dibranchiata on any intertidal flat. This sometimes causes conflicts, especially when wormers dig in clam conservation areas where clammers have enhanced stockswith wild or cultured "seed" clams (<1 cm shell length. SL). Clammers believe wormers kill or injure small clams directly or indirectly while commercially harvesting G. dibranchiata. To help resolve these long-standing conflicts, we worked collaboratively with clammers and wormers and used an experimental approach to test the short-term interactive effects of clam and worm harvesting, harvesting intensity, time of harvest after seeding, and predator exclusion on the fate of small wild and cultured M. arenaria at an intertidal mud flat in Brunswick, ME. We added 50 cultured juveniles of M. arenaria (SL = 12.5 mm) to 120 l-m~2 plots, 40 of which were undisturbed controls (20 protected with plastic netting--6.4 mm aperture; 20 unprotected) from May to August 1996. The remaining 80 plots were assigned to one of 16 treatments. One half of the plots were protected from predators with the same plastic netting used in the undisturbed control plots. One half of the plots were harvested by a professional wormer or clammer who searched each plot for commercial size blood worms and soft-shell clams, respectively. Plots were harvested either once (after two weeks or four weeks) or twice (two weeks + two weeks, or four weeks + four weeks). Any effect due to clamming or worming on cultured clams or wild individuals of similar size was masked by clam losses exceeding 95 percent in the unprotected control plots. Intense predation by horseshoe crabs, Limulus polyphemus L. and the nemertean worm. Cerebratuhts lacteusLeidy, are blamed for the high mortalities among clams. Only in protected plots was any effect detected and this depended on clam origin. Compared to the fate of cultured clams in protected controls, worming had no effect, but clamming contributed an additional 15 percent loss. Both types of commercial harvesting reduced wild clam numbers significantly Compared to controls, but effects due to worming were more benign than effects due to clamming probably because wormers excavate less volume of sediments than clammers do as commercial size G. dibranchiata are shallow burrowers compared to commercial size M. arenaria. Unless clam managers actively take steps to deter predators by using netting or other means, blood wormers should continue to harvest commercially from areas closed to shellfishing without reprisal or fear that they are causing damage to populations of juvenile soft-shell clams.