Fig. 4. Annual mean weight increase for sea bass at different weights stocked in sea cages.
Table 3 Recycling system for fish nursery culture
System characteristics Rearing characteristics
Mesh filter Final average body weight: 10–150 g
Initial stocking density: 10 kg m
− 3
UV disinfection Biofilter
Final stocking density: 50–80 kg m
− 3
Packed column CO
2
Duration: 1–6 months Make up water \100 of rearing volume per day
Heating and cooling Oxygenation
Cleaning every cycle pH control
3. Developing recirculating on-growing systems
Fish production in recirculating systems usually lasts several years without any seasonal or complete interruption of the production, and the fish are usually grown
at high density within large production units. These features have several important consequences for the design and operation of the rearing system: 1 the need to
design and manage large water treatment units, 2 the need to take into account the high oxygen requirement and the rapid changes in oxygen demand, and 3 the
need to control the levels of nitrate and various other compounds tea-colored water when the water renewal rate is low Table 4. A diagram of such a system
is presented on Fig. 5. Although efficient seawater denitrification processes function at a pilot scale, no commercial farm uses it at the present time for Mediterranean
fish production, and nitrate limitation is operated through water renewal. Until now, fish farms using recirculating systems for Mediterranean fish production have
always been economic failures. This might be explained either by faults in the technical design of the production system or by incorrect operation over the
long-term not stocked and managed at optimal production level.
The promising results obtained recently with laboratory size units encouraged the European Union to contribute to the development of industrial farms by financing
pilot units on an commercial scale through the DG 13 and Eureka programs. Iceland, Sweden and France are all involved in the DG 13 project, referred to as
Table 4 Recycling system for Mediterranean fish growth
Rearing characteristics System characteristics
Final average body weight: 0.2–1.5 kg Mesh filter
Disinfection UV, O
3
Initial stocking density: \25 kg m
− 3
Final stocking density: \100 kg m
− 3
Large nitrifying biofilter Duration: 1–2 years
Denitrifying biofilter optional Packed column CO
2
Replacement water: 100 of total water volume per day Cleaning without complete stop
Heating and cooling Oxygenation
pH control
Fig. 5. Recirculating system for growth of fish at a high stocking density.
Mari-Tech. The project focuses on the recirculating loop of two pilot commercial size units, which are designed to produce 50 – 70 tons of sea bass per year. The
completion date for the project is at the beginning of 2000 and the final result will provide a pool of practical and theoretical knowledge on the design, management
and cost of the pilot commercial farms.
The Eureka project involves Iceland and France and concerns aspects of fish rearing at high density in large production units such as the scaling-up of demand
feeding systems, the swimming activity of fish living in high density populations and the improvement of feed for fish waste reduction. Additionally the project will
investigate combining UV radiation and ozone for disinfection, optimization of the fish environment for fish growth, optimization of the replacement waterrecircu-
lated water ratio, the acidbase equilibrium of fish in recirculating water, and the supervision of the production units. These two projects should provide much of the
information necessary for the design of efficient commercial production units.
4. Some particular questions related to fish rearing in recirculating systems
