CARRYING CAPACITY OF TROPICAL RESERVOIRS FOR TILAPIA CAGE CULTURE: CASE STUDIES AT UPPER PARANÁ RIVER BASIN,

SOUTHEASTERN BRAZIL.

David, G. S.*, Carvalho, Vivanco,A, Gerardo D., Dallaglio- Sobrinho, M., Silveira, A. N.

GIANMARCO SILVA DAVIDGERARDO VIVANCO A.

Proc. no. 07/58246-4 and 08/52139-4

Carrying capacity: concepts (Beveridge, 2004)

Cage aquaculture production needs to be sustained by surrounding environment, considering:• What determines the productivity of the environment – in this case, total Phosphorus in water.• What the farmed fish consume/produce, in terms of wastes – here, we consider P loads.• How the environment responds to waste loadings – such a good question!• How much charge is permissible – in this case, Brazilian environmental regulation states the limit for water total P of 30mg/m3 in reservoirs to farm aquatic animals (this is the limit of total P for a lake be classified as eutrophic (Vollenweider, 1975).

Environmental carrying capacity for cage aquaculture for freshwater lakes can be defined as the maximum amount of P that can be loaded keeping total P below the levels assumed to indicate eutrophication

Cage culture of tilapia in large reservoirs is an emergent aquaculture practice in Brazil. Due to the availability of large amounts of suitable quality waters in hydroelectric dams at Grande, Paranapanema and Tietê rivers, there is a large but still undetermined potential for cage aquaculture in the upper Paraná river basin.

Brazilupper Paraná river basin

Sites considered in this study

South American major river basins

Parana river basin

In this work, are presented evaluations of two among 18 suitable sites at Ilha Solteira reservoir, one of the biggest at Brazil (1200 km2) and one among at least 12 at Xavantes reservoir.

Only 1% of the reservoirs area is allowed to be occupied by aquaculture.

Brazilian government will assign part of the Ilha Solteira studied sites to receive small scale, familiar aquaculturists.

Distribution of hydroelectric power plants in Brazil

Multiple uses for reservoirs

Artisanal fisheries

Tourism

Aquaculture

Irrigation

Eletric Power

generation

Former fisheries collapsed after installation of hydroelectric dams, due to ecological constrains for fish production.

Today, fisheries are scarce, resulting on low incomes for fishers.

After many attempts of stock enhancement, tilapia cage culture have became the first ecologically and economically feasible option for fish production in southeastern Brazil hydroelectric dams, since impoundment of rivers in the 60’s.

Model used for tilapia cage aquaculture at upper Paraná river basin.

• Nile tilapia (Oreochromis niloticus) is reared in intensive system• 8-16 m3 cages made of synthetic materials• up to100 kg fish/m3 at the end of cycle• fish weights 500-800 g at the end of cycle• one or two cycles of 4-8 months per year, depending on water temperature.

Ipaussu

Hydrodynamics - in reservoirs with linear morphology, water exchange rate can be obtained from flow measurements of power plants and bathymetry.

METHODS

Ilha Solteira reservoir

Rio Grandinho

Sao Jose dos Dourados

For dentritic reservoirs, water flow must be obtained for each portion by means from current measurements and hydro dynamic modeling for the site named Can Can

ADCP current profile

Bathymetry

Winds

Numerical solution grid

Water circulation pattern

Waves modeling

BathymetryLand use

Carrying capacity was estimated based on the Dillon

& Rigler (1975) mass balance model, considering

limnological data to evaluate area-specific

Phosphorus loads from tilapia cage culture that can

be assimilated in each selected site.

Limit for environmental carrying capacity for P assimilation (L, in grams of Phosphorus/m2/year)

L = (P*Z*θ) / (1-R),

P - is the allowable phosphorus increase, in mg/m3; is the difference of total P from analysis of water samples collected with van Dorn bottles, and the limit stated by environmental agencies (30 mg/m3) and by Vollenweider (1975) as an indicator of eutrophication

Z - is the mean depth obtained from acoustic bathymetry, in meters;

- is the rate of water exchange, i.e., how many times the water is totally exchanged in a year. (this is calculated as 1/residence time); and

R - is the sedimentation coefficient, obtained from direct measurements with sediment traps (desirable), or from numerical modeling based on hydro dynamics data.

θ

Estimation of P loads per ton of tilapia produced

Pe= (Pf x FCR) – Pa

Pe= Phosphorus loads, in kg P/ton tilapia produced

Pf= mean feed Phosphorus content (%), obtained from chemical analysis of samples (N=15) of fish feed used in farms in the study area. (= 27 kg ton-1)

FCR= Food Conversion Rate, obtained from average field farming data for the upper Paraná river basin (= 1,6 ± 1)

Pa= whole fish Phosphorus content (%), specific for Nile tilapia (Dantas & Athayde, 2007) (= 9 kg ton-1)

Pe= 37 kg P ton-1

Parameters used for the estimation of carrying capacity for tilapia cage culture in selected sites on tropical hydroelectric reservoirs at southeastern Brazil.

Parameter unitRio

GrandinhoS.J.

Dourados Ipaussu

Water renovation rate years -1 31,52 9,38 7,80

Area km² 9,62 69,49 25,49

Mean depth m 8,00 11,11 28,89

P water concentration mg m-3 5,34 12,36 16,6

Limit P concentration mg m-3 30,0 30,0 30,0

Sedimentation coefficient - 0,230 0,352 0,373

Area loading rate g m-2 year -1 8,080 2,835 4,814

Total allowable production for the site ton year -1 2106 5340 3326

Area specific production ton km-²year -1 219 77 131

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Ipaussu:comparativelimnological data

Tilapia farm

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• Modeling provided evidence for the importance of feed phosphorus content and availability for determination of total allowable production

• Proper inputs of limnological field data are expansive and demands sophisticated hydrographic studies.

•The availability of sites with similar features for cage culture in southeastern Brazil reservoirs indicates a significant potential for tilapia production at global scale.