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
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. Water quality and fish density One of the most significant advantages of recirculating systems is that they allow
the key water quality characteristics to be properly regulated. The technical means for efficient regulation exist, but we still know very little about the right level at
which regulation should be carried out Russo and Thurston, 1991; Handy and Poxton, 1993; Tomasso, 1994. Recent experiments have been carried out in single
parameter conditions in order to determine the optimal level of such basic parame- ters as temperature, TAN or pH Lemarie´ et al., 1996, but almost nothing is
known concerning the effect of carbon dioxide or nitrate, or the possible synergistic effect of some of these substances on Mediterranean fish growth. Moreover, to be
profitable, such systems have to be run with high fish densities. Some very important aspects of fish behavior in high rearing densities such as swimming and
feeding are still poorly understood. Both are closely related to the feeding strategy that should be used in rearing facilities and also to fish welfare, which must always
be taken into account, in as much as it is possible to define it precisely.
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2
. Feed effect Feed composition is a key element in the performances of the fish and the rearing
system Lee and Putnam, 1973; Boujard and Me´dale, 1994. The feed lipid content determines protein utilization by the fish and oxygen consumption within the
rearing system. For instance, if the feed lipid content is increased from 10 to 30 while the protein content is kept stable around 45, the fish reduce their
voluntary feed intake, although their growth rate remains stable and their nitrogen excretion decreases. As a result, fish make better use of the protein available and
the oxygen consumption, both by the fish and within the rearing system, is reduced Cove`s, pers. commun..
Fig. 6. Average cumulative feed intake during the first hours of the photoperiod in relation to the daily exposure to light.
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3
. Effect of photoperiod The photoperiod also has a determining impact on fish feeding behavior,
consequently on the entire rearing system Cove`s, pers. commun., and the maxi- mum level of feed intake and fish growth are obtained with long photoperiod times.
The average cumulative feed intake of fish subjected to varying photoperiod times increases drastically from less than 40 during the first 2 h for fish exposed to 20
h of light per day to more than 80 for fish exposed to only 4 h of light per day Fig. 6. This concentration of the feeding activity in a short period of time results
in a rapid surge in oxygen demand, followed by the excretion of a large quantity of matter into the rearing medium. This has important consequences for the size of the
treatment loop, since a system that is able to cope with such strong variations in water quality would have be very large and probably economically not viable.
Therefore, to limit the daily water quality variations resulting from the concentra- tion of the feeding activity, the farm manager should increase rather than decrease
the light exposure time.
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. Mechanical filtration Mechanical filtration is generally carried out as close as possible to the outlet of
the rearing tanks in order to avoid breaking the larger, more easily removed particles produced by the fish into smaller particles. Despite this, it is estimated that
only 50 – 60 of particulate matter mainly feces and feed dust in the rearing tanks is removed from the water in a mechanical filter equipped with 60- to 80-mm
microscreen panels. Within the recirculation system, fine particulate matter accumu- late and 40 – 70 of the suspended solids concentration on a dry wt. basis are
particles smaller than 20 mm Chen et al., 1993; Heinen et al., 1996. Suspended solids that are not removed from the recirculating flow are partly dissolved and
broken apart, mainly in the pump and their decay in the biological filter increases the ammonia production and the oxygen demand of the rearing system Golz et al.,
1996. One reason for such a modest efficiency of mechanical filters is that the standard drum or disc filters are made with woven materials, which drastically
limits the possibility of using the small pore sizes that provide low porosity. Actually, the void surface percentage of a filter cloth with a 60-mm mesh is around
60, whereas the void surface percentage of a 5-mm mesh is less than 1. The porosity of the microscreen is a key parameter to determine the hydraulic capacity
of the filter, the filter wash frequency and the waste water quantity Cripps, 1995; Kelly et al., 1997. Other kinds of filter cloths are now being developed that could
provide both high porosity and small pore size.
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. Bacterial acti6ity The influence of bacterial activity in a recirculating system is of the utmost
importance. For instance, in high fish density recirculating systems, bacteria and fish consume similar quantities of oxygen per kg of fish produced. However very
little is known about the kind of bacteria that are present within these systems, apart from the nitrifying bacteria. Autotrophic nitrifying bacteria are far less
numerous than heterotrophic bacteria, even when a biological filter, which is usually considered to be a nitrogen transformer, is used. Important questions
remain unanswered, concerning the stability in quantity and composition of the heterotrophic bacteria population within the different components of a recirculating
rearing system. Similarly, the main factors that influence these bacteria are not well understood. According to recent studies Leonard et al., 2000, it appears that the
concentration of free bacteria is stable when the rearing characteristics are stable and that the highest concentration is observed at the outlet of the rearing tanks. Yet
if one examines bacterial production expressed as colony forming units CFU in the main components of the system, the biological filter FB appears to be the
main source of bacteria and is a determining element in the growth of bacteria, when the UV disinfecting unit UV efficiency is 97 on average Fig. 7. Com-
pared to this, the rearing tanks RT and pipes P have little effect on the bacterial population . Models are being developed, to evaluate the evolution of the hetero-
trophic population under various operating conditions of the recirculation system.
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. Treatment and recycling of waste Wastewater treatment is now essential in sustainable aquaculture. It is a costly
process, so recycling fish waste using organisms which feed on the wastes and recycling treated water in the rearing system may help to improve the cost efficiency
of the entire production system. The first step is to quantify the waste and describe it as precisely as possible. This will depend on the way the rearing system is
operated. If we consider nitrogenous waste, it is clear from Fig. 8 that the more closed the system is expressed as a number of the total water volume exchanged
per day, the less dissolved nitrogen expressed as a percentage of the ingested nitrogen will be released into the effluent Pagand et al., 1999, 2000. Some trials
have been carried out with effluents from Mediterranean fish to produce mollusks and algae Shpigel et al., 1993. Another solution is to treat in parallel the
recirculating system effluent first treatment loop in a second treatment loop Fig. 9. Aquaculture waste from the first loop of recirculating systems more closely
resembles urban waste than that produced by flow through systems, and is thus easier to treat using standard techniques. Some recent experiments have shown that
high rate algal ponds fed with seawater effluents produce macroalgae year round
Fig. 7. Heterotrophic bacterial population and production in a recirculating system.
Fig. 8. Nitrogen production of a recirculating system.
Fig. 9. Treatment and reuse of the waste water from a recirculation system.
Fig. 10. High rate algal pond treatment efficiency.
Fig. 11. Sea bass growth in a 100 closed system with high rate algal pond as secondary treatment.
Pagand et al., 1999, 2000. The average yearly nitrogen DIN and phosphorus RP removal efficiency were respectively 60 and 50 with a minimum of below
40, which falls to 20 during the cold period Fig. 10. Moreover, if re-injected into the first loop, water treated in the high rate algal pond seems to have no
detrimental effect on fish growth, at least over a short period of time. The growth of sea bass reared in a completely closed system, with waste water re-injection in the
first recirculation loop after treatment in a high rate algal pond secondary loop, was compared to a control in flow through system over a period of 2 months. In
the completely closed system, the fish growth was comparable to cage culture growth model result, while in the flow through system, it was lower Fig. 11. New
experiments are planned to calibrate and validate a model of the two loops, which will greatly contribute to define the best operating conditions for sea bass produc-
tion in the Mediterranean area.
5. Conclusion
