Little attention has been paid to the environmental impacts of clam cultivation, and although analogies can be made to other cultured species that have been more extensively researched, it should be noted that there are likely to be differences and that such analogies can only suggest possible impacts, and the actual effects should become the focus of future research. The anticipated environmental impacts caused by the cultivation of clams can be separated into three main areas;
- The effects of col lecting spat from wild populations.
- The effect of stocking clams at high densities on the local ecosystem.
- The effect caused by harvesting the clams.
There is little research into the effect of removing spat from the natural populations to suppor t clam cultivation, although evidence from mussel studies suggest that if too much of the natural spatfall is removed natural populations fail, causing increased mortality in organisms which predate in them. An example of this is the increased mortalities of eider ducks witnessed in the Wadden Sea, the Netherlands, in 1990 and 1991, following the removal of the natural spatfall (Kaiser et al, 1998). The removal of clam seed by hydraulic dredges could potentially damage the benthic ecosystem and the communities residing there, although it should be noted that there has as yet been no research into such activities. In the UK the majority of clam seed is produced by hatcheries and thus removes the problems associated with wild spat collection. The use of hatchery seed will allow the spat procurement phase to comply with Principle 2. Further research is needed in order to assess whether wild collection of spat could comply with the criteria for Principle 2.
The on-growing of clams occurs either in or on the bottom , and as such results in a
lower stocking density per m 2 of seabed than that of mussel culture. It is possible however, that similar effects associated with sedimentation and removal of phytoplankton (and nutrients) could occur. A study by Mojica and Nelson (1993) investigated the effect of Mercinaria mercinaria culture (in growbags) in a lagoon in Florida. Results indicated that, as with mussel culture, the sediment below the culture system had a lower mean grain size than the control sites, with an increased percentage silt/clay component. The redox layer was also much shallower within the
farm site at 0.5mm, compared to 24.33mm and 16.5mm at the control sites. The study showed significant variation in nutrient and plankton concentrations betwee n the sites, although no trend could be detected. No significant effect on the benthic community was detected although there was a reduction in seagrass es within the culture site. The authors concluded that, although the significant change in sediment ch aracteristics could potentially alter the ecosystem dynamics, such alterations were not evident and that there were no negative impacts on the environment from the culture site. It should be noted that no stocking densities were given and that an increase in intensity could result in negative impacts. A negative impact on seagrass beds could potentially lead to a failure to meet the second criteria of Principle 2, as many seagrass beds are endangered or protected habitats. The seed for this operation were produced in a commercial hatchery and harvest was by hand, by removal of the growbags. Such activities are likely to cause minimal environmental impacts, if at all.
With regards to growbag culture, impacts from harvesting are likely to be minimal as
bag s are removed by hand, with the possible assistance of motorised vehicles to carry the bags to shore. Such activities limit the effects of trampling to small tracks between the rows of growbags. With harvesting likely to be limited to a yearly activity at its most frequent, recovery of these tracks is likely to be rapid. Such production systems will comply with the Principle 2 criteria and could be seen as sustainable.
Harvesting of cultured clams has received little attention from researchers, although the effects of the different harvesting methods can be inferred from other harvesting
activities. This is especially true for the hydraulic dredging of clams from sediment, which has received considerable attention regarding the wild capture fisheries. The hydraulic dredge works by blasting water infront of the dredge, which liquefies the sediment allowing a blade to cut through it and harvest the clams (Messieh et al , 1991) . The action of the hydraulic dredge has a significant impact on the physical appearance of the sediment and on the benthos. The sediment is left scarred and the
scars, which can be detected with side-scan sonar, can remain there for long periods of time (over 3 months [Thrush et al , 1995; Curry & Parry, 1996] ) and if harvesting is repetitive on a short timescale, full recovery may never be achieved (Jones, 1992).
Other impacts that can occur include; increased mortality and damage to non- target
species, increased predation of infaunal species due to increased exposure, changes to the chemical and physical characteristics of the sediment (Messieh et al, 1991). Such dramatic changes to the sediment or the benthos are unlikely to be covered by the criteria of P rinciple 2, therefore harvesting by hydraulic dredge could result in a
fail ure to comply with Principle 2.
A further cause of impacts worth mentioning is the structures associated with the culture of clams. When clams are re-laid into the sediment they are vulnerable to predation. Netting can be placed above the clams to reduce mortality from predation, but such activities can potentially affect sedimentation and local hydrography. A study by Munroe and McKinley (2007) demonstrated that although the netting appeared to increase the population of clams there were no significant effects on sediment composition. An increase in organic carbon was detected beneath the netted areas, but this was attributed to the increased density of clams beneath the netting, rather than increased sedimentation caused by the structure itself. They concluded that the netting had little effect on the environment and that increases in sedimentation noted in other studies are likely to be due to the individual oceanographic characteristics of each site. As the netting is not having an effect on the ecosystem or the benthos the use of netting could comply with the criteria for Principle 2.