8 Externalities in Aquaculture
257
The best documented example of this is the conversion of mangrove swamps to
shrimp farming. Barbier and Strand (1998) estimate the effects of changes in mangrove area on the shrimp fisheries of Campeche State (Mexico) based on a
production function methodology, and using data for the decade 1980–1990 find
that a 1 km
2 decline in mangrove area leads to a loss of 14.4 t of commercial shrimp
harvest worth some US $144,000. Though mangrove deforestation over that period
was comparatively small, the authors point out that in the future this is likely to
worsen due to urban expansion and mariculture development. In other words,
shrimp aquaculture has the potential to impose significant external costs on society
by indirectly causing a decline on the harvest of capture fisheries. Studies of coastal
wetland systems in Thailand, where the area of mangrove has been severely
reduced over the last 30 years, have focussed more specifically on the link with
shrimp farming. Research based on a small local community in southern Thailand
(Sathirathai 1998; Sathirathai and Barbier 2001) derived values for several of the
functions supported by mangrove. Using field surveys the value of direct use of
wood as well as other resources collected from the mangroves was estimated to be
$88 per ha. Benefits in terms of off-shore fishery linkages were estimated using a
production function approach and were in the range of $21–$69 per ha. The value
of coastline protection was estimated using a replacement cost method (based on
the cost of replacing mangroves with breakwaters to prevent erosion) and amounted
to $3,679 per ha. Over a 20-year period, the total present value of the mangrove
system to the local community was estimated to be as much as $27,264–$35,921.
Of particular interest was the evidence showing the discrepancy between the private
and social returns from converting mangrove wetland to commercial shrimp farms.
Private returns were substantial, a result which clearly explains the incentive that
has driven the expansion of the aquaculture industry hitherto. Once the external
costs are accounted for, however, the returns to society from mangrove conversion
are shown to be negative (Sathirathai 1998). A more recent study by Barbier and
Cox (2004) looks more closely at the economic drivers behind mangrove deforestation in Thailand, and offers very clear evidence of the role played by shrimp farm
profitability.
Habitat degradation caused by mariculture may also result from farming species
other than shrimp, and we would point to the evidence which reveals the growing
threat that cage aquaculture poses for seagrass meadows in the Mediterranean
(Holmer et al. 2003). Sedimentation of waste products from fish farms has been
shown to adversely impact the growth of seagrass (Posidonia oceanica) in the
vicinity of sea cages, and given that seagrass is an important source of food and
habitat for several varieties of marine organisms it seems likely that fish farms
located inappropriately close to P. oceanica meadows may have indirect negative
effects on the biodiversity and productive capacity of the marine environment. This
is likely to translate into an external cost, most obviously a reduction in the harvest
of capture fisheries, though the magnitude of this loss is as yet unknown. Evidence
from the US, however, suggests that the economic value of seagrass habitat is substantial, and in the State of Florida it is recognised as being a key resource in supporting fisheries as well as having an important nutrient recycling function. The
257
The best documented example of this is the conversion of mangrove swamps to
shrimp farming. Barbier and Strand (1998) estimate the effects of changes in mangrove area on the shrimp fisheries of Campeche State (Mexico) based on a
production function methodology, and using data for the decade 1980–1990 find
that a 1 km
2 decline in mangrove area leads to a loss of 14.4 t of commercial shrimp
harvest worth some US $144,000. Though mangrove deforestation over that period
was comparatively small, the authors point out that in the future this is likely to
worsen due to urban expansion and mariculture development. In other words,
shrimp aquaculture has the potential to impose significant external costs on society
by indirectly causing a decline on the harvest of capture fisheries. Studies of coastal
wetland systems in Thailand, where the area of mangrove has been severely
reduced over the last 30 years, have focussed more specifically on the link with
shrimp farming. Research based on a small local community in southern Thailand
(Sathirathai 1998; Sathirathai and Barbier 2001) derived values for several of the
functions supported by mangrove. Using field surveys the value of direct use of
wood as well as other resources collected from the mangroves was estimated to be
$88 per ha. Benefits in terms of off-shore fishery linkages were estimated using a
production function approach and were in the range of $21–$69 per ha. The value
of coastline protection was estimated using a replacement cost method (based on
the cost of replacing mangroves with breakwaters to prevent erosion) and amounted
to $3,679 per ha. Over a 20-year period, the total present value of the mangrove
system to the local community was estimated to be as much as $27,264–$35,921.
Of particular interest was the evidence showing the discrepancy between the private
and social returns from converting mangrove wetland to commercial shrimp farms.
Private returns were substantial, a result which clearly explains the incentive that
has driven the expansion of the aquaculture industry hitherto. Once the external
costs are accounted for, however, the returns to society from mangrove conversion
are shown to be negative (Sathirathai 1998). A more recent study by Barbier and
Cox (2004) looks more closely at the economic drivers behind mangrove deforestation in Thailand, and offers very clear evidence of the role played by shrimp farm
profitability.
Habitat degradation caused by mariculture may also result from farming species
other than shrimp, and we would point to the evidence which reveals the growing
threat that cage aquaculture poses for seagrass meadows in the Mediterranean
(Holmer et al. 2003). Sedimentation of waste products from fish farms has been
shown to adversely impact the growth of seagrass (Posidonia oceanica) in the
vicinity of sea cages, and given that seagrass is an important source of food and
habitat for several varieties of marine organisms it seems likely that fish farms
located inappropriately close to P. oceanica meadows may have indirect negative
effects on the biodiversity and productive capacity of the marine environment. This
is likely to translate into an external cost, most obviously a reduction in the harvest
of capture fisheries, though the magnitude of this loss is as yet unknown. Evidence
from the US, however, suggests that the economic value of seagrass habitat is substantial, and in the State of Florida it is recognised as being a key resource in supporting fisheries as well as having an important nutrient recycling function. The