ECOLOGICAL CONSEQUENCES OF MECHANICAL HARVESTING 



OF CLAMS 



Charles H. Peterson, Henry C. Summerson, and Stephen R. Fegleyi 



ABSTRACT 



A field experiment was performed in 1,225 m^ plots in each of two shallow estuarine habitats, a seagrass 

 bed and a sand flat, in Back Sound, North Carolina (USA), to test the impact of clam raking and two 

 different intensities of mechanical harvesting of clams ("clam kicking") for up to 4 years on 1) hard 

 clam, Mercenaria mercenaria, recruitment, 2) seagrass biomass, 3) the density of benthic macroinverte- 

 brates, and 4) the density of bay scallops, Argopecten irradians. The removal of adult hard clams with 

 the contingent sediment disturbance had ambiguous effects on the recruitment of hard clams: in the 

 sand flat recruitment tended to be lower (but not significantly) in intense-clam-kicking matrices than 

 in controls, whereas in seagrass recruitment of hard clams did not not show a clear response to treat- 

 ment. In the raking and light-clam-kicking matrices, seagrass biomass fell immediately by =25% below 

 controls but full recovery occurred within a year. In the intense-clam-kicking matrices, seagrass biomass 

 fell by =65% below levels expected from controls; recovery did not begin until more than 2 years passed, 

 and seagrass biomass was still =35% lower than predicted from controls 4 years later. Clam harvest 

 did not affect either the density or species composition of small benthic macroinvertebrates from sedi- 

 ment cores, probably because of their rapid capacity for recolonization and generally short life spans. 

 In all treatments, densities of benthic macroinvertebrates (mostly polychaetes) were substantially higher 

 in the seagrass than in the sand flat during October samplings but equal during March samplings. Bay 

 scallop density declined with declining seagrass biomass across harvest treatments, but the intense-clam- 

 kicking matrices contained even fewer bay scallops than their seagrass biomass would predict, perhaps 

 because of enhanced patchiness of the remaining seagrass. 



The relative inertia of the change in seagrass biomass following extensive destruction in the intense- 

 ly kicked matrices suggests that seagrass replanting may be an extremely important means of returning 

 disturbed, unvegetated areas to seagrass systems. Emergence during summer of a between-habitat 

 gradient in infaunal densities (higher in seagrass than in sand) supports the hypothesis that seagrass 

 provides a partial prey refuge for infaunal invertebrates. The failure of the benthic macroinvertebrate 

 density to respond to clam harvest treatments in both sand flats and seagrass beds implies that the 

 polychaetes which dominate recover rapidly from disturbance and are probably not adversely affected 

 by clam harvest. The negative and long-lasting impact of intense hard clam harvest on seagrass biomass 

 with its effects on other fisheries, including bay scallops, implies that hard clam fisheries should be 

 managed to minimize the intensity of harvest within seagrass beds. 



Technological innovation is frequently accompanied 

 by an increased risk of harm to various aspects of 

 the natural environment (e.g., Dickie 1974). While 

 such innovation can be considered economically 

 desirable and even inevitable, environmental 

 managers still require ecological inputs to enable 

 them to reach properly informed compromises 

 between uncontrolled application of new technology 

 and unnecessarily cautious protection of natural 

 ecosystems. Because of its inherent lack of general 

 principles and paradigms, ecology is rarely able to 

 provide immediate answers to practical questions 

 of the probable impact of new technology. Conse- 

 quently, careful studies of the ecological impact of 

 the application of each specific new technology are 



'Institute of Marine Sciences, University of North Carolina at 

 Chapel Hill, Morehead City, NC 28557. 



Manuscript accepted January 1987. 

 FISHERY BULLETIN: VOL. 85, NO. 2, 1987. 



often necessary. Such studies can not only provide 

 necessary applied information but also contribute 

 to a better basic understanding of the specific 

 system that is being explored. 



Although fisheries biologists are renowned for 

 managing harvests in a way that will sustain a max- 

 imum yield or maximize yield per recruit (Ricker 

 1975), studies are only occasionally undertaken to 

 compare the environmental damage caused by alter- 

 native fishing gears and technologies (e.g., Caddy 

 1973; Peterson et al. 1983a). Such studies are most 

 common in estuarine and other shallow-water fish- 

 eries, where high coastal productivity of diverse 

 stocks induces intensive exploitation of a common 

 area by multiple, potentially interfering fisheries. 

 As technological advances in fishing gear have been 

 made, this potential for interfishery competition has 

 grown, as has the need for understanding the envi- 



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