The Gulf of Guayaquil and the Guayas River Estuary, Ecuador
259
Shrimp production increased from 5,000 tons in 1979 (US$ 56.9 million)
to more than 100,000 tons in 1991 (US$ 482 million; Olsen and Arriaga
1989; Aiken 1990). An infectious disease (identified as a virus), known as
Taura Syndrome (TS), affected juvenile shrimp (Penaeus vannamei) in
densely stocked shrimp ponds, causing an estimated loss of US$ 100 million in the mid 1990s and threatening the Ecuadorian market (and several
other markets in Latin America) . The elevated biological productivity and
economic value of the Gulf and Guayas River estuary and the impact
shrimp pond construction and operation have on ecological processes and
on environmental quality have led to a controversy on coastal resource
management.
17.8 Preliminary Models and Ecosystem Management
A dynamic box model for the Guayas River estuary considers scenarios
of mangrove to shrimp pond conversion in three regions of the estuary
and different rates of river discharge ( 100, 50 and 10% based on the 1989
flow) after construction of a dam (Twilley et al. 1998). Seasonally high
river flow and tidal exchange with low water residence time (11 days)
maintain good water quality. After a 90% reduction of mangrove forests
caused by shrimp pond construction, total nitrogen concentration would
increase fivefold. However, if river discharge decreases to 10 % at the same
time, nitrogen concentrations would increase 60-fold. Increases in nitrogen concentration are higher in the upper than in the lower estuary, thus
the sensitivity of the environment to changes in intertidal and upland
land use is site-specific and linked to the hydrography of the estuary. It
thus appears that shrimp pond farming in the Guayas River estuary has
managed to sustain high productivity and profits largely as a consequence of high river discharge (Twilley et al. 1998). However, the construction of the Daule-Peripa Dam threatens the industry by substantially
diminishing river flow, with water quality problems leading to reduced,
though spatially selective, profitability of the shrimp industry. If shrimp
pond effluents were introduced into nearby mangrove forests, the natural
potential of mangroves to remove excessive nutrient loads (Rivera-Monroy et al. 1999) could benefit shrimp pond management, enhance environmental quality in the estuary, and also possibly increase mangrove productivity.
The conceptual model argues that ecological and socioeconomic forcing functions, such as international markets, political processes, exchange rates, and monetary and trade policies, control coastal zone
259
Shrimp production increased from 5,000 tons in 1979 (US$ 56.9 million)
to more than 100,000 tons in 1991 (US$ 482 million; Olsen and Arriaga
1989; Aiken 1990). An infectious disease (identified as a virus), known as
Taura Syndrome (TS), affected juvenile shrimp (Penaeus vannamei) in
densely stocked shrimp ponds, causing an estimated loss of US$ 100 million in the mid 1990s and threatening the Ecuadorian market (and several
other markets in Latin America) . The elevated biological productivity and
economic value of the Gulf and Guayas River estuary and the impact
shrimp pond construction and operation have on ecological processes and
on environmental quality have led to a controversy on coastal resource
management.
17.8 Preliminary Models and Ecosystem Management
A dynamic box model for the Guayas River estuary considers scenarios
of mangrove to shrimp pond conversion in three regions of the estuary
and different rates of river discharge ( 100, 50 and 10% based on the 1989
flow) after construction of a dam (Twilley et al. 1998). Seasonally high
river flow and tidal exchange with low water residence time (11 days)
maintain good water quality. After a 90% reduction of mangrove forests
caused by shrimp pond construction, total nitrogen concentration would
increase fivefold. However, if river discharge decreases to 10 % at the same
time, nitrogen concentrations would increase 60-fold. Increases in nitrogen concentration are higher in the upper than in the lower estuary, thus
the sensitivity of the environment to changes in intertidal and upland
land use is site-specific and linked to the hydrography of the estuary. It
thus appears that shrimp pond farming in the Guayas River estuary has
managed to sustain high productivity and profits largely as a consequence of high river discharge (Twilley et al. 1998). However, the construction of the Daule-Peripa Dam threatens the industry by substantially
diminishing river flow, with water quality problems leading to reduced,
though spatially selective, profitability of the shrimp industry. If shrimp
pond effluents were introduced into nearby mangrove forests, the natural
potential of mangroves to remove excessive nutrient loads (Rivera-Monroy et al. 1999) could benefit shrimp pond management, enhance environmental quality in the estuary, and also possibly increase mangrove productivity.
The conceptual model argues that ecological and socioeconomic forcing functions, such as international markets, political processes, exchange rates, and monetary and trade policies, control coastal zone