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Ecological consequences of aquaculture on foreshore ecology; can we
sustainably develop the shellfishery based on lessons learned from other
primary resource industries?
L.I. Bendell-Young
Center for Coastal Studies, Simon Fraser University, 8888 University Ave, Burnaby, B.C.
With the serious conservation concerns associated with the wild fishery and the downturn
in the forestry industry, Canadian federal and provincial governments are under increasing
pressure to diversify coastal community economies that previously relied on these two primary
resources. The consequence is tremendous pressure to exploit yet another primary resource, the
shellfishery. Common sense would suggest given our track record which demonstrates our poor
ability to properly care for the natural environment that sustains these resources, new resource
development should proceed cautiously. However, on the west coast of Canada, this is not the
case. The shellfishery is being developed at an alarming rate with an astonishing lack of respect
for the environment and without heed or thought given to the ecological consequences of such
rapid development.
With the development of shellfish aquaculture at its initial stages, we have the
opportunity to manage this resource in an ecologically sustainable way. Lessons learnt from the
over exploitation of wild fish stocks and the extensive logging of forests in the absence of sound
ecosystem practices should provide us insight as to how to develop foreshore aquaculture
sustainably.
The intertidal is “fished” for shellfish in two distinct ways; 1) the actual leasing and
farming of the intertidal for the economically preferred Manila clam and Pacific oyster; i.e., the
shellfish industry and 2) the commercial or wild clam fishery or harvest. The former is under
provincial, the latter under federal jurisdiction. The shellfish industry. In the last decade, the
Manila clam (Venerupis philippinarum) has been farmed commercially in British Columbia,
Canada. Farming involves seeding, netting, and harvesting (Kraeuter and Castagna 1989). In
the seeding process, hatchery raised clams, of lengths 4-10mm, are spread on the beach at low
tide. These seed clams are then covered with plastic netting secured to the beach to provide
protection from predators (Spencer et al. 1992; Manzi 1985). Following a three-four year growout period, the beach is harvested by removing nets and hand-raking to recover clams of legal
sizes (minimum 38mm in length). The clam fishery. The history of the wild clam fishery dates
to before the turn of the century. Four species of clams are harvested; Manila, native littleneck
(Protothaca staminea), butter (Saxidomus giganteus) and razor (Siliqua patula). All are
harvested from unleased regions of the intertidal given tide determined availability. There was a
rapid escalation of the clam fishery during the 1980s with peak landings occurring in the late
1988s. Predictably, this peak was followed by a decline due to the over removal of bivalves
from the beaches and more restrictive management measures. Future plans for the
shellfishery. The federal Department of Western Economic Diversification recently concluded
that shellfish farming had the potential to become a $100 million dollar industry that could create
more than 1,000 person years of employment in British Columbia coastal communities over the
next ten years. Hence, in November 1998, the provincial government announced an initiative to
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double the Crown land (land under Canadian federal control) available for shellfish
aquaculture at a rate of 10% per year over 10 years. However, although a considerable amount of
the intertidal area in British Columbia has been leased for shellfish aquaculture (and more is
being proposed), with unleased portions being used intensively for the wild shellfish harvest,
little is known about the ecosystem effects of such practices (Mojica and Nelson 1993; Spencer
et al. 1996, 1997).
The majority of impact assessments of intertidal fisheries and aquaculture focus solely
on commercially valuable species such as mussels (Mytilus edulis) and oysters (Crassostrea
gigas). This lack of knowledge makes it impossible to predict the impact of industry expansion
or even long term shellfish farming at current levels (Pearse 1994). Limited studies that have
attempted to assess the impact of the shellfishery on the intertidal ecosystem structure and
function have found that use of the foreshore for the shellfishery; 1) lowers intertidal species
richness 2) alters intertidal community composition from one represented by surface and
subsurface species, to one represented only by sub-surface species and 3) affects the cycling of
major nutrients such as carbon.
However, of the various farming practices used to increase bivalve production, the use of
anti-predator nets could prove to be the most ecologically damaging especially to higher trophic
levels that rely on the intertidal for some aspect of their life history. Currently, there are no
regulations as to the extent of the beach that can be netted or the length of time the nets are left in
place. As the factors that give the area its shellfishery potential are likely also those that make it
attractive to wildlife such as birds, the potential for negative effects must be taken seriously. The
direct removal of potentially marketable clams by avian and other predators is without question
of greatest immediate interest to the shellfishery. But looked at the other way, the extensive
harvest of potentially consumable clams, and the exclusion of birds by nets and other methods,
are potential threats to the health of the avian populations that winter in these waters.
One main lesson of a half-century of ecological research is that interrelations in a food
web such as the foreshore are likely to be intricate. It would be no surprise to find that practices
that alter foreshore ecology could reduce the productivity of benthic invertebrates impacting its
suitability for both shellfish growers and wildlife. Detailed studies of the feeding ecology of sea
ducks and other wildlife in regions currently used and proposed for the intertidal fisheries are
required. Information on how anti-predator nets influence the geochemical cycles of essential
nutrients such as carbon, nitrogen and phosphorous, key elements required for the normal
functioning of an ecosystem are needed. The effect of long-term use of anti-predators nets on a
foreshore is information that not only ecologists would be interested in, but as well the shellfish
growers. It could be that prolonged coverage could in fact reduce clam growth rates and hence
biomass yield, rather than lead to an increase in product. Indeed, it would seem prudent that this
industry develop carefully and make some investment in uncovering and understanding these
relations, before discovering too late that the changes induced cannot be easily reversed.
Krauter, J.N., and Castagna, M. 1989. Factors affecting the growth and survival of clam seed
planted in the natural environment. In Clam mariculture in North America.
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Developments in aquaculture and fisheries science, Vol. 19. Edited by J.J. Manzi and
M. Castagna. Elsevier Science Publishers, New York, N.Y. pp. 149-200.
Manzi, J.J. 1985. Clam Aquaculture. In Crustacean and Mollusk Aquaculture in the United
States. Edited by J.V. Huner and E. Evan. Avi Publishing Company, Inc., Westport,
Connecticut. pp. 275-310.
Mojica, R., and Nelson, W.G. 1993. Environmental effects of a hard clam (Mercenaria
mercenaria) aquaculture site in the Indian River Lagoon, Florida. Aquaculture 113:
313-329.
Pearse, G. 1994. A determination of the current economic value of bivalve aquaculture in
Baynes Sound, Vancouver Island and an estimation of the potential value of the
industry under improved conditions. M.Sc. Thesis, Simon Fraser University, Burnaby,
BC.
Spencer, B.E., Edwards, D.B., and Millican, P.F. 1992. Protecting Manila clam (Tapes
philippinarum) beds with plastic netting. Aquaculture. 105: 251-268.
Spencer, B.E., Kaiser, M.J., and Edwards, D.B. 1996. The effect of Manila clam cultivation on
an intertidal benthic community: the early cultivation phase. Aquaculture Research 27:
261-276.
Spencer, B.E., Kaiser, M.J., and Edwards, D.B. 1997. Ecological effects of intertidal Manila
clam cultivation: observations at the end of the cultivation phase. Journal of Applied
Ecology 34: 444-452.