5.3.1. Introduction
5.3.2. Types of mesocosms
5.3.3. Mesocosm protocol
5.3.4. Comparison to intensive methods
Mesocosm systems are culture systems for fish larvae with a water volume ranging from 1 to 10,000 m³. In these large enclosures a pelagic ecosystem is developed, consisting of a multispecies, natural food chain of phytoplankton (diatoms, flagellates, Nannochloris,...), zooplankton (tintinnid ciliates, Synchaeta and Brachionus rotifers, copepods,...) and predators (fish larvae). Intensification of mesocosms is determined by the initial load and by the level of exogenous compounds (fertilizer,...). Fish larvae are stocked in the mesocosms when prey densities have reached appropriate levels, or the organisms cultured in a mesocosm system are harvested from time to time and supplied to fish larvae held in separate tanks. Environmental conditions of mesocosm systems are fully related to the local climate. The production output of such mesocosms can be improved by rearing different species during one year cycle. The production season can be started with the rearing of one cohort of cold water species (halibut or cod) from February to May, and followed by three cohorts of species that do better in warmer water (turbot, seabream, seabass).
5.3.2.1. Pold system (2-60 m³)
5.3.2.2. Bag system (50-200 m³)
5.3.2.3. Pond system
5.3.2.4. Tank system
There are two methods to obtain a mesocosm system which offers natural live food during the rearing of the fish larvae, provided that the fish larvae are the sole top predators in the system. In the first method the water in the system is continuously renewed at a high rate. An example of such a system is an isolated tidal pond in which the inflowing water is filtered from predators allowing phyto- and zooplankton to flow into the system, while the outflowing water is filtered to retain the fish larvae in the enclosure. Such a system is called “advective” since it depends on external, rather than internal processes. The other method consists of a semi-enclosed or closed system, which is dominated by internal processes. These systems require less technical backing and are thus more convenient for aquacultural applications.
(Semi-) closed mesocosm systems are small enclosures, which consist of water masses retained:
· by dams in isolated bays, branches of a fjord or lagoons: pold system
· in bags hung up in the sea or lakes: bag system
· in man made ponds on land: pond system
· in tanks: tank system
In these systems either zooplankton is developing in the mesocosm system (with or without fertilization), or is additionally pumped in from the surrounding waters.
The pold system is an isolated water volume, such as an isolated bay, or a branch of a fjord or a lagoon. Before each production cycle the enclosed water volume is treated with chemicals (rotenone) to make the enclosure free from predators, including fish larvae. Predators can also be removed from the pold system by emptying, drying and refilling the enclosure with filtered seawater (200-500 µm). The copepod resting eggs can resist the rotenone treatment and will ensure a zooplankton bloom in the mesocosm. After the treatment of the pold system, and fertilization of the enclosure or lagoon, inoculation with microalgae should be carried out to promote a phytoplankton bloom. When needed, zooplankton harvested from nature can be introduced into the system. When a sufficient density of copepod nauplii is reached (50-200.l-1), the pold system is ready for stocking with fish larvae at stocking densities of 1-2 larvae per litre (i.e. for turbot or cod). Each day the zooplankton density must be checked and in case of zooplankton depletion, fresh (filtered) zooplankton, Artemia nauplii or artificial feeds (at later stages) should be added to the mesocosm. When sufficiently old, the fry can be concentrated, caught and transported to nursery or grow-out systems.
The bag system (Fig. 5.8.) is a simplification of the former system, since the isolation of a large water volume is easier achieved: black or transparant polyethylene or PVC bags are used tied to a floating wharf. These bags have a conical bottom with an outward hose from the bottom to the surface for water renewal. Two internal flexible hoses with plankton filter maintain the water level in the bags (Fig. 5.8.); the bags having been filled with filtered (100-200 µm) seawater and inoculated with microalgae. The enclosed water is then fertilized with an agricultural fertilizer to promote algal bloom, after which the screened zooplankton (copepods) can be introduced. When sufficient zooplankton production is achieved (50-200 copepod nauplii.l-1 or 100-500 microzooplankters.l-1), fish larvae can be released into the bags at a stocking density of 1-3 larvae.l-1.
Figure 5.8. Plastic bag system for larval rearing (modified from Tilseth et al., 1992).
As before the daily control of the zooplankton density is advisable and should be between 50-500 zooplankters.
In case of depletion, fresh (filtered) zooplankton (Fig. 5.9.), Artemia or artificial feeds (in later stages) should be added. Water exchange is necessary if oxygen saturation drops below 5 mg.l-1 (> 80% saturation) or pH and ammonia reach unfavourable levels. Normally 1-2% of the bag volume is exchanged per day for the first two weeks, and thereafter water exchange increased to 10-100% bag volume per day. These bags are currently being used in Norway to produce turbot and halibut fingerlings (with an overall survival rate of 20% and 40-50%, respectively) and cod fingerlings.
Figure 5.9. Automatic supplementation of zooplankton in bag system. (P): surrounding water with good zooplankton production; (F): filter for concentrating zooplankton; (B): bag system and (T): tank (modified from Tilseth et al., 1992).
Another variation on this prinicipal is to use dug-out land-based ponds. The advantage of such a system is that it is very easy and cheap in construction, maintenance and operation. The ponds are dugged out and covered with plastic liner to prevent leaching. After emptying and cleaning, the ponds are exposed to direct sun light for at least 4 days. The fish can be harvested and transferred to the ongrowing ponds when attaining the appropriate size (sea bream: 10 mm). Before harvesting, the bottom of the tank is carefully cleaned in order to remove sedimentated organic material by siphoning. Afterwards, the water level is lowered and the fish can be fished out using a net. It has been shown that, for instance, larvae of herring, plaice, turbot, goby and cod can easily be grown through metamorphosis in this way. A good review of pond management prior to and during the larval stocking of red drum is described by Sturmer (1987). The number of fry which can be grown per surface unit of pond area determines the efficiency of this method. For carp larvae possible stocking densities of 5 to 600.m-2 have been reported. It is suggested that the quality of zooplankton necessary to ensure the survival of larval carps should be 1.5 to 3.0 food organisms.ml-1 at the beginning. Two to three days later when the larvae have learned to hunt for food more efficiently the concentration may decrease to half of that. These marine systems are currently in use in Norway as well as in Denmark. In China over 95% of the 10 million tonnes of cyprinid fish produced annually are originating from fresh water mesocosm systems.
Cement tanks up to 50 m³ are emptied and cleaned with HCl solution to dissolve calcareous hidings of Serpulidae or shells. Thereafter the tanks are exposed to sun light for at least 4 days and then filled with filtered seawater rich in phyto- and zooplankton. The tanks are then fertilized with N and P to promote phytoplankton blooms. Recommended fertilization rates for gilthead seabream culture in Crete waters being 0.5-2.0 g N.m-3 and a N/P ratio 5-10:1. Fish larvae are generally introduced into the mesocosm tanks after they have absorbed their yolk sac and when the size of the plankton population is adequate to support the fish population. It follows, therefore, that timing of artificial spawning and incubation is of the utmost importance. Stocking densities for gilthead sea bream and European sea bass are generally 0.1-0.5 larvae.l-1 and 1 larva.l-1, respectively. The monitoring of the tank system should include both the measurement of abiotic (temperature, salinity, dissolved oxygen, pH, light intensity and nutrient concentrations) and biotic (plankton concentrations and composition, fish biometrics and condition) parameters.
An example of a super-intensive tank system is the Maximus system (Maximus A/S, Denmark), which produces calanoid copepods in large tanks as the major live feed. The whole system is intensified and therefore requires steady control and continuous re-adjustment by a “Computer Supported Subjective Decision Manipulation Programme” (Fig. 5.10.).
Figure 5.10. Schematic operating model of the intensive tank system (modified from Urup, 1994).
Some of these tanks are stocked with fish larvae, others serve solely for copepod production. The main idea of the Maximus system is to control the abiota (nutrient level, pH, temperature, light intensity,...) and biota (phytoplankton and copepod production, number of predators, bacterial turn-over, regeneration of nutrients from copepods and fish larvae) in such a way that the production of one trophic level matches the predation by the higher trophic level. This makes the management of such a system very difficult and requires automation. The disadvantage of such a system is that it is very expensive to build and operate. In 1992 Maximus A/S produced 700,000 turbot fingerlings with this system, but this can realistically be increased to 1.5 to 2.0 million fingerlings (Urup, 1994).
The mesocosm systems are prepared as follows: they are treated with chemicals to kill predators or they are set dry for at least 4 days, and if needed cleaned with HCl to remove the calcareous cases of various organisms. These culture systems are then filled with adjacent seawater rich in phyto- and zooplankton, using 350-500µm filters, so as to prevent predators from entering the system. The water is then fertilized; recommended quantities are 0.5-2 g N.m-3 and a N/P ratio of 5-10 for seawater systems. For freshwater systems the following procedure can be used: poultry manure (40g.m-3) together with additional fertilization every 3 days with a chemical fertilizer composed of 1.6 g ammonium sulfate, 1.08 g urea, 2.4 g superphosphate of lime.
In the mesocosms different plankton blooms will develop one after the other, and this process is called succession. The first blooming organism will usually be the diatom group, that will soon collapse due to depletion of silicates (only in closed systems: pond and tank system). This bloom is then usually followed by a bloom of nanoflagellates and dinoflagellates, which on their turn is followed by a bloom of ciliates and rotifers. These organisms are important during the first feeding period of fish larvae and also form an additional food source to the copepod nauplii N1. Only when an adequate population of copepods is established can fish larvae be stocked. For Acartia tsuensis maximum values of abundance during the culture can go up to 1,300 nauplii.l-1, 590 copepodites.l-1 with a maximum egg production rate of 350 eggs.l-1.day-1.
The time of introduction of fish larvae into the mesocosms is at startfeeding, but only when adequate plankton populations are established. Syncronization can be carried out by:
· Manipulating the time of artificial spawning· Regulating the rate of development of fish through temperature; i.e. yolksac absorption in sea bream is completed in 3 days at 21°C, 4 days at 18°C and more than 5 days at less than 17°C
· Control over the plankton population growth rate which is related to ambient environmental conditions (temperature, light intensity and nutrients); for example, in Crete plankton populations in the mesocosms reached appropriate densities to stock fish larvae in 12-14 days at 17-22°C and about 20 days at 13-16°C.
The stocking density of the fish larvae depends on the species. For example, for gilthead seabream and the European sea bass low stocking densities are recommended: 0.1-0.5 larva.l-1 and 1 larva.l-1, respectively. The newly-hatched larvae are gently transferred in large containers with sufficient aeration for transportation to the mesocosms. Gradual equalization of the temperature and water salinity in the containers to the mesocosms is needed, after which the larvae can be gently released into the mesocosms.
During the rearing period abiotic and biotic parameters must be frequently monitored. Water analysis is preferentially carried out each day, and the estimation of plankton population growth rate every two days by taking samples and counting under a binocular microscope. As soon as the food consumption of the growing larval biomass exceeds the net zooplankton production, new zooplankton, rotifers, Artemia or artificial feeds are added. Fish larvae are sampled once or twice a week and their length and weight are measured.
The fish can be harvested and transferred to the ongrowing system after they have reached the appropriate size (gilthead seabream: 10 mm). Therefore, the bottom of the system is carefully cleaned by siphoning sedimentated organic material, and afterwards the water level is lowered and the fish readily fished out with a net.
Possible problems or difficulties are:
· Synchronization of mesocosm preparations and fish egg production.· High oxygen concentrations during periods of high light intensity causing mortality due to over-inflation of the swimbladder (gas-bubble disease)
· Formation of a surface lipid film due to excessive phytoplankton production, preventing swimbladder inflation (i.e. need for surface skimmers)
In contrast to mesocosm systems intensive hatcheries require high technology and therefore have a high investment and energy cost. Since the intensive hatchery has to produce sufficient amounts of live food and keep the cultures on during periods of low demand, high functional costs and highly specialized personnel is required. In addition, intensive hatcheries are characterized by the frequent rearing of batches of larvae, with relatively low survival rates (Table 5.8.).
Table 5.8. Comparison of intensive and mesocosm rearing methods.
|
|
Intensive systems |
Mesocosm systems |
|
Installations and equipment |
high technology |
simple technology |
|
Investment cost |
very high |
low |
|
Personnel |
highly specialized |
moderatly specialized |
|
Water volume used |
small |
large |
|
Food used |
Brachionus/Artemia |
natural plankton |
|
Consumables cost |
high |
low |
|
Energy required |
high |
negligible |
|
Operational cost |
very high |
very low |
|
Production |
very high |
low |
|
Survival rate |
low |
moderate high |
|
Growth rate |
moderate |
high |
|
Production quality |
poor to moderate |
good to excellent |
|
Disease control |
moderate |
very low |
|
Subsequent growth rate |
|
20% faster |
|
Risk |
high |
low |
|
Expected profit |
very high |
low |
|
Conformity with wild standards |
low |
high |
