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Objectives
The
specific objective is to ascertain tentatively the hypothetical existence
of a quantifiable relationship between shrimps production and ecological
indicators (see Fuchs et al.,
1998). This will allow to select ecological indicators in order (1) to
improve site selection methodology (2) to assess the potential production,
and, (3) to improve the methodology of site monitoring bound to prevent
any decrease in the sustainability
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Innovative
approach
In
order to determine and select efficient ecological indicators, we propose
to link the rearing performances and actual production of shrimp, in
selected farms, to ecological quantifiable indicators, with special
attention to the trophic state of the surrounding coastal water based on
former results gained during the STD3 program (1994-1997). This makes for
the original approach of activity 1.
Methods
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Background
and methodological approach
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In
the Mekong delta, aquaculture is either fully extensive (1 or 2 shrimps
per m2) or “ extensive + ” (up to 5 shrimps per
m2 with addition of small amounts of feed pellets). This
means that the whole production in the first case, or most of it in the
second, is supported, through food web, by the natural primary
production. This kind of production can be compared to that of molluscs
(oysters and mussels) whose growth and reared biomass are fully
dependant on the “ available food ” (mainly phytoplankton)
in the ecosystem (Bacher et al.,
1998; Kashiwai, 1995; Ferreira et
al., 1998), which constitutes the major component studied within the
concept of carrying capacity developed for molluscs farming (Dame and
Prins, 1998).
One
may suppose that the production of shrimp in extensive or “ extensive
+ ” way can be efficient only in environment with a sufficient
“ available food ” to allow the development of sufficient
“ in pond ” secondary productivity. The latter consists
mainly in meiofauna and zooplankton, which constitutes the natural food
for shrimps (Hunter et al., 1987; Reymond and Lagardère, 1990, Cam et al., 1991; Focken et al.,
1998), even if shrimps can ingest plant material to a certain extent,
depending on the species (Rothlisberg, 1998). Accordingly, oligotrophic
waters, or waters with low Primary Production (PP) capacity can support
very limited extensive shrimp aquaculture. The same situation is found
in dystrophic environments, with hypertrophic and/or heterotrophic type
productivity. As an example, almost nil or very low shrimp production
(few kg per pond ha) was observed in extensive aquaculture types (Nioc
Hien area, south Mekong Delta), due to farm positioning in the ecosystem
and the nature of the water related to it. Only heterotrophic and
cyanobacterial mat production were observed in the ecosystem (Fuchs et
al., 1998). Figure 5 depicts the hypothetical production of
extensive and “ extensive + ” shrimp aquaculture with
respect to the trophic state of the surrounding environment.
Water entering shrimp farm systems, through pumping or inlet channel,
reflects the “ history ” of the water in the coastal
ecosystem: residence time, confinement, inputs of organic matter (from
river, mangrove, aquaculture pollution...) (Folke and Kautsky, 1992;
Guelorget et al., 1996).
The ecosystem features
(confinement, high or low seawater renewal rate,....) will determine its
capacity to assimilate or evacuate the wastes issued from the
aquaculture activity, that is to say to maintain its original
characteristics and so, its capacity to sustain the activity. Concerning
that last point, it is to be noticed that most collapses observed all
over the world occurred after several years of production and were
mostly related to a change of the surrounding environment (Philipps et
al., 1993). This means that the capacity of production of a given
ecosystem varies with the evolution of this ecosystem.
Figure
5.
The hypothetic relationship between shrimp production (extensive and /or
extensive+) and the trophic state of the ecosystem.
Our
hypothesis is that for extensive and “ extensive + ”
rearing, the establishment of quantified relationships between the
actual production in selected farms and environmental indicators related
to the trophic state of the surrounding water can give evidence of the
capacity of a potential site to support shrimp production. These
quantified relationships will allow to assess
the potential capacity of production for
given characteristics of water. Furthermore, these indicators
will be useful to monitor the evolution of the hosting site and
consequently the evolution of its production capacity.
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Methods
: |
In
order to determine the relationships between shrimp production and
environmental parameters, correlations studies will be carried out
between the rearing performances (growth, survival) and actual
production of selected farms, and quantified parameters related to the
trophic state of the water in the surrounding environment. These
correlations will allow to select the environmental parametres most
representative of the shrimp production.
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Selection
of study sites and sampling strategy |
Field
studies will be carried out on 3 geographically different areas (see
figure 4). In each area, several farms (6
to 12) representative of all possible ecological situations
will be selected (from seashore to the upper part of shrimp activities).
The pilot sites will be monitored, during 2 periods per year
corresponding to rearing cycles,
for two years, with a special attention to water sampled at
high tide ±
2-2.5 h (that is to say the period during which ponds and farm systems
are being filled up).
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Ecological
data collection |
The
studied parameters will reflect the trophic state of the water: (1)
Primary Production (2) concentration of Total Suspended Matter,
Particulate Organic Matter, Chl-a + pheopigments (and ratio),
particulate nitrogen and Particulate Organic Carbon (C/N ratio), (3)
the planktonic (phytoplankton + bacteria) community structure, the
sulphate reducing bacteria. Hydrobiological characteristics (O2, T°,
S‰)
will be monitored. Concerning these parameters the methods of analysis
have been described in Fuchs et
al. (1998). Concerning the planktonic cells, a special attention
will be given to picoplankton. As a matter of fact, picoplankton is
made of procaryotes (cyanobacteria and heterotrophic bacteria) and
eukaryotes with a size less than 3 µm. In the inter-tropical non
polluted areas, picoplankton contributes to a large percentage to
phytoplankton biomass and productivity. Phytoplanktonic
and bacterial community structure will be assessed by the use of a
flow cytometer according to Blanchot & Rodier (1996)
methodology. The contributions of the
picoplankton taxa to phytoplankton biomass will be calculated using
size fractionated chlorophyll measurements and in
vivo fluorescence data obtained with the flow cytometer according
to the method described in Charpy & Blanchot (1998).
Expected
outputs and benefits
Establishment
of quantified relationships between the actual production in selected
farms and environmental indicators
Duration
30
months
Participants
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Vietnam
: IO Nha Trang, Fisheries Authorities in the Mekong delta provinces
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EU:
IFREMER, IRD
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