Example of intensive cage rainbow trout production assessment for a hypothetical natural lake in Europe (see Section 4.3.3.2).

__Site:__

Surface Area of Lake | = | 100ha (calculated from map). |

Mean depth, Z, | = | 10m (from hydrographical survey). |

Flushing coefficient, , | = | 1 yr^{-1} (determined from sampling outflows). |

__Method__

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} |

Since[P] | = |

L_{fish} | = |

R_{fish} 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 10^{6} m^{2}, the total acceptable
loading = 0.833 x 10^{6} 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 O_{2} levels.

Example of extensive cage tilapia production for a hypothetical tropical reservoir (see Section 4.4.5).

__Site:__

Surface Area = 100ha.

__Method:__

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 10^{6}
fingerlings.

= 780 x 10^{3} fingerlings per crop.

Example of semi-intensive cage tilapia production assessment for a hypothetical tropical lake (see Section 4.5).

__Site:__

Surface area = 100 ha

mean depth, Z, = 10 m

flushing coefficient, ρ, = 1 yr^{-1}

__Method:__

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/10^{6} m^{2} = 996.24 mg m^{-2}); R is
derived from Larsen and Mercier (1976) (Table 23) (1/1 +
0.747ρ^{0.507} = 0.54):-

Using the formula: -

ΣPP_{fish} = 31.1 [P]^{0.54} (OECD, 1982) to relate increase in
[P] to primary production,

ΣPP_{fish} = 31.1 x 45.8^{0.54} = 50.5 g C m^{-2} y^{-1} increase.

Step 5. Fish yields due to ΣPP_{fish} can be calculated using the
conversion efficiencies in Table 27: -

ΣPP_{fish} → 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 O_{2}.

__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 SD_{MAX}, as governed by food supply:-

, where SD_{MAX} = fish m^{-3}; |

V_{i} = 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.

V_{i}, 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 m^{3})

L | = | 5m |

V_{i} | = | 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 O _{2} requirements__

If the current velocity through the cages is computed, and the O_{2} concentration
of the water known, then the supply of O_{2} to the fish cage can
be calculated. If the O_{2} 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 m^{2} |

L | = | 5m |

V_{i} | = | 0.001 m s^{-1} |

Temp. | = | 30°C |

∴ O_{2} content of water, assuming 100% saturation at sea level = 7.6 mg 1^{-1}

∴ O_{2} supply to cage = | V_{i} × A × 1000 × 7.6 | = 152 mg O_{2} s^{-1} |

= 5.47 x 10^{5} mg O_{2} h^{-1} |

Assume O_{2} content of water leaving cage = 3 mg 1^{-1}

Total O_{2} leaving cage each hour | = | V_{i} × A × 1000 × 3600 × 3 |

= | 2.16 x 10^{5} mg O_{2} h^{-1} |

O_{2} available to fish = 3.31 x 10^{5} mg O_{2} h^{-1}

Assuming cages stocked with 50g tilapia, O_{2} requirements following a meal
(2% body weight per day) = 328 mg O_{2} 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.