Example of intensive cage rainbow trout production assessment for a hypothetical natural lake in Europe (see Section 220.127.116.11).
|Surface Area of Lake||=||100ha (calculated from map).|
|Mean depth, Z,||=||10m (from hydrographical survey).|
|Flushing coefficient, ,||=||1 yr-1 (determined from sampling outflows).|
Step 1: Determine [P]i of lake prior to development. 15 mg m-3 as determined from monitoring programme.
Step 2: Set maximum acceptable [P], [P]f, following the introduction of fish culture. Assuming no other developments or criteria take precedence, then 60 mg m-3 is chosen as target [P]f.
Step 3: Determine Δ[P]
|Δ[P]||= [P]f - [P]i = 45 mg m-3|
Rfish is taken to approximate R calculated from the equation of Larsen and Mercier (1976) (see Table 23)
Step 4: Since the lake has a surface area of 106 m2, the total acceptable loading = 0.833 x 106 g y-1
∴ the tonnage of fish that can be produced, assuming a P loading of 17.7 kg tonne-1 (see Table 16)
This value should be used as a pre-development guide to the carrying capacity of the lake. However a monitoring programme must be implemented, and actual production levels adjusted in the light of information collected on water quality - principally algal biomass and O2 levels.
Example of extensive cage tilapia production for a hypothetical tropical reservoir (see Section 4.4.5).
Surface Area = 100ha.
Step 1. Calculate the annual gross primary production, ΣPP. 1200 g C m-2 y-1, as determined by regular measurement.
Step 2. Convert to annual fish yields, using Table 28.
|i.e.||∼||1.3% ΣPP → fish|
|=||15.6 g fish C m-2 y-1|
|=||156 g fish m-2 y-1|
|=||156 tonnes annual fish production for whole lake.|
Step 3. Assuming 2 crops per year, determine culture periods.
ΣPPcl = ΣPPc2, in order for fish to reach target market size.
ΣPP (Nov. - May) = 570 g C m-2
ΣPP (June - Oct.) = 630 g C m-2
One seven month, and one five month cycle are chosen.
Assume 25g fish stocked
Assume 8 pcs. per kilo target market size (i.e. 125g each)
each fish grows 100g during culture period.
stocking requirements = 156 tonnes/100g = 1.56 x 106 fingerlings.
= 780 x 103 fingerlings per crop.
Example of semi-intensive cage tilapia production assessment for a hypothetical tropical lake (see Section 4.5).
Surface area = 100 ha
mean depth, Z, = 10 m
flushing coefficient, ρ, = 1 yr-1
Step 1. Calculate the annual gross primary production, ΣPP.1200g C m-2 y-1, as determined by regular measurement.
Step 2. Convert to annual fish yields, using Table 28.
|i.e.||1.3% ΣPP → fish|
|= 156 tonnes annual fish production for whole lake.|
Step 3. Assume 100 tonnes of cottonseed meal and 20 tonnes of soya meal is available for feed each year. Using FCR values from Table 30:-
6.6 tonnes can be grown from soya meal and 37.2 tonnes can be grown from cottonseed meal.
Step 4. Total P loadings from fish grown on supplementary food (from Table 30):-
(6.6 x 16.97) + (37.2 x 23.77) = 996.24 kg.
The resultant increase in [P] can be calculated from Dillon and Rigler's (1974) formulation: -
where L is the areal loading from the fish cages; (996.24 kg/106 m2 = 996.24 mg m-2); R is derived from Larsen and Mercier (1976) (Table 23) (1/1 + 0.747ρ0.507 = 0.54):-
Using the formula: -
ΣPPfish = 31.1 [P]0.54 (OECD, 1982) to relate increase in [P] to primary production,
ΣPPfish = 31.1 x 45.80.54 = 50.5 g C m-2 y-1 increase.
Step 5. Fish yields due to ΣPPfish can be calculated using the conversion efficiencies in Table 27: -
|ΣPPfish → fish||= 0.5g fish Cm-2 y-1|
|= 5g fish m-2 y-1|
|= 5 tonnes fish production for whole lake.|
ΣFy, the total fish yield can now be calculated: -
|ΣFy||=||(0.073 x 1200 x 10) + [(100/2.69) + (20/3.04)] + (0.01 x 50.5 x 10)|
|=||205 tonnes fish annum-1|
Calculations of appropriate fish stocking densities for extensive cage culture.
The following stocking density models assume that the growth rate of extensively cultured fishes, such as tilapias, is limited either by food supply or by O2.
Model A Food Supply
If the current velocity through the cage is determined, and the filtering capacity of the fish known, then we can calculate the maximum permissable stocking density SDMAX, as governed by food supply:-
|, where SDMAX = fish m-3;|
Vi = velocity of water inside the cage (m s-1); F = filtering ability of fish (1 s-1); and L = length of cage parallel to the prevailing current.
Vi, L and A can be determined by direct measurement, whilst F can be derived from published data on buccal cavity size, and gill opercular beating rates (see Hoar and Randall, 1976). The following calculations are based on typical values: -
Cage size = 5 x 5 x 4m (100 m3)
|Vi||=||0.1 cm s-1 (0.001 m s-1)|
|F||=||30 ml s-1 fish-1 (data for 18 cm+ S. aureus and S. galilaeus. Drenner et al, 1983).|
This is very much higher than the typical stocking values of 5 – 50 fish m-3 for extensive cage culture. However, the model assumes that the fish themselves do not contribute to the drag forces exerted on currents flowing through cages, or that conversely the movement of fishes in the cages may increase circulation. The relative importance of these two factors remains unknown. Also, it is assumed that the fish fully evacuates its buccal cavity on each occasion, which is unlikely.
Model B O2 requirements
If the current velocity through the cages is computed, and the O2 concentration of the water known, then the supply of O2 to the fish cage can be calculated. If the O2 requirements of the caged fish are computed, assuming worst possible conditions (high temperatures, small fish, requirements following a meal), then we can calculate the appropriate stocking density: -
|Cage size||=||5 x 5 x 4m|
|∴ A||=||20 m2|
|Vi||=||0.001 m s-1|
∴ O2 content of water, assuming 100% saturation at sea level = 7.6 mg 1-1
|∴ O2 supply to cage =||Vi × A × 1000 × 7.6||= 152 mg O2 s-1|
|= 5.47 x 105 mg O2 h-1|
Assume O2 content of water leaving cage = 3 mg 1-1
|Total O2 leaving cage each hour||=||Vi × A × 1000 × 3600 × 3|
|=||2.16 x 105 mg O2 h-1|
O2 available to fish = 3.31 x 105 mg O2 h-1
Assuming cages stocked with 50g tilapia, O2 requirements following a meal (2% body weight per day) = 328 mg O2 kg-1 h-1) data from Ross and Ross, 1983; L.G. Ross, unpublished data).
∴ Sustainable biomass of fish in cage =
∴ Stocking density = 10.1 kg m-3
This value is similar to that typically used in extensive cage culture.