Introduction Recirculating systems in commercial scale

parameters. In parallel, Ifremer is developing a research program in four main areas : 1 the influence of the rearing environment on fish; 2 the influence of fish on the rearing environment; 3 reducing the environmental impact of the farms, and 4 the optimization of the primary water treatment system, and its integrated management. The most promising foreseeable improvement in recirculating systems used for growing Mediterranean fish are: 1 an increase in mechanical filtration efficiency; 2 a better understanding of the standard optimal rearing medium parameters; 3 more information on the bacterial populations and how to control them, and 4 the incorporation and recycling of waste. © 2000 Elsevier Science B.V. All rights reserved. Keywords : Recirculating systems; Mediterranean fish

1. Introduction

It is now widely acknowledged that fish supplies from the world fisheries are unlikely to increase substantially and that the expansion of the aquaculture sector will probably provide the solution to the problem of the projected shortfalls Chamberlain and Rosenthal, 1995. The potential of aquaculture has been clearly demonstrated by the rapid expansion of this sector: it is the fastest growing food-producing sector in the world, with an annual growth rate of almost 10 between 1984 and 1995, compared with 3 for livestock meat and 1.6 for capture fisheries production FAO, 1997. Although Mediterranean fish make up only a small percentage of the worldwide aquaculture production, the growth in this sector is following a similar pattern of expansion. Most sea bass fingerlings reared in farms are produced in recirculating systems, which minimizes land requirements and the impact on the environment, thanks to the high level of control over the environment in which the fish are grown. Studies are in progress to define economically efficient recirculating systems for the on-growing phase from several grams to the market size of the production of Mediterranean fish. In the first part of this paper, commercial scale recirculating systems that are now being used to produce various species of Mediterranean fish from breeders to fingerlings will be presented, and the method used to define efficient commercial scale production units will also be described. In the second part, we will look at various aspects of fish rearing in recirculating systems, concerning particularly the fish and their behavior, the bacteria present within the rearing systems, and technical problems.

2. Recirculating systems in commercial scale

Since 1987, sea bass fry production in French hatcheries has increased steadily from 1 million to around 15 million fingerlings per year Fig. 1. At the same time, the average sale price, on a real time cost basis, has fallen by more than 60, and now averages 1.5 French fr. per fish De la Pomelie, pers. commun.. The two main explanations for the fact that production has increased by 15 times in the past 1o years are: 1 a steady improvement in feed composition and feeding procedures, and 2 the use of water recirculation technology. Flow through hatcheries on marine water supplies are subject to large fluctuations in water quality that are difficult to control. However, recirculation systems provide a rearing medium that is constant and adjustable, showing only slight and slow variations. Moreover, there is minimal heat loss in recirculated water systems, which normally operate above ambient water temperature. In a recirculating system, the saving in heating energy required for the production of 1 g fingerlings corresponds to approximately 50 of the production cost of fingerlings from a heated flow through hatchery Fig. 2. The type of recirculating system that seems most promising for sea bass and bream, the specific treatment processes and their impact on water quality were extensively described Blancheton and Coves, 1993; Blancheton et al., 1996, 1997. The water quality parameters levels that are being targeted as safe levels in fry, fingerlings and adult fish production systems are summarised in Table 1. A diagram of a recirculating rearing system for the production of larvae and fingerlings is presented in Fig. 3, and the main characteristics of fish rearing and of the system are summarized in Table 2. During the 1990s, an increasing number of French fish farms that used sea cages needed to increase their production to meet an expanding market demand. Given the inherent difficulties in physically enlarging their rearing systems, they decided to Fig. 1. Sea bass fry production and its cost in France. Fig. 2. Direct production cost of 100 000 1-g sea bass fingerlings in flow through and recirculation systems. Table 1 Safe levels of water quality parameters for sea bass and bream rearing Adults Parameter Larvae Fingerlings 22–24 16–20 Temperature °C 22–24 \ 90 Dissolved oxygen saturation \ 90 \ 90 B 2 B 0.5 Total ammonia nitrogen mg l − 1 B 0.2 Nitrite nitrogen mg l − 1 B 0.5 B 2 B 0.2 B 100 – – Nitrate nitrogen mg l − 1 6.5–8.3 7.5–8.3 6.5–8.3 pH Carbon dioxide mg l − 1 B 40 B 40 – increase the productivity of their existing facilities. On the French Mediterranean coast, the annual mean weight of fish stocked in cages increases with the average weight at stocking Fig. 4. Therefore, the cage productivity per unit volume increases and one solution was to stock the cages with fish that had been pre-grown in a recirculating system. The general rearing and system characteristics for fish nursery or pre-growing in recirculating systems are summarized in Table 3. The basic organization of the recirculating system for this rearing phase is the same as for larval rearing, with the differences that: 1 the sand filter is replaced by a mesh filter, which is situated as close as possible to the rearing tanks in order to avoid waste particle degradation by circulation pumps or by bacterial activity in the sand filter; 2 water is superoxygenated in order to meet the oxygen requirements of the large fish biomass; 3 the packed column is adapted to efficiently remove the dissolved carbon dioxide produced by fish respiration; and 4 the pH is maintained at around 7 by injection of sodium hydroxide NaOH. Fig. 3. Recirculating system used for larval rearing of Mediterranean species. Table 2 Recycling system for larval rearing and fingerling production System characteristics Rearing characteristics Fingerlings Larvae Larvae Fingerlings Mesh filter Final average Sand filter body weight 40 mg 1–5 g UV disinfec- Initial stocking tion density 0.4 kg m − 3 0.4 kg m − 3 Biological filter Final stocking Packed column density 30 kg m − 3 Heating cool- 4 kg m − 3 ing Duration Oxygenation Aeration 90 days 40 days Cleaning every 40 days 90 days 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