RECENT ADVANCES IN AQUACULTURE IN MAURITIUS

M Bhikajee

University of Mauritius

ABSTRACT

Aquaculture in Mauritius started in the 1800’s but formal aquaculture research dates back to 1950 with the introduction of the tilapia. Presently aquaculture is based on a variety of species, namely, Macrobrachium rosenbergii, Penaeus monodon, Rhabdosargus sarba, Siganus sutor, Oreochromis spp and the recently introduced fresh water lobster Cherax quadricarinatus. Research in aquaculture is carried out at the Albion Fisheries Research Centre and at La Ferme Fish Farm by the Ministry of Fisheries and Marine Resources, at the University of Mauritius and by some of the larger aquaculture companies attached to the sugar industry. Recent research and consultancy projects of the Ministry of Fisheries and Marine Resources and the University of Mauritius are listed and details are discussed. Aquaculture in barachois and the various problems associated with it are discussed. Recent production ,based on official figures of the Ministry of Fisheries and Marine Resources, are given.

INTRODUCTION

Historical Background

Formal aquaculture research in Mauritius started in the 1950's with the introduction of the tilapia. In 1971, an F.A.O expert, Mr. Fujimora studied the possibilities of aquaculture development in Mauritius (SCET 1975). Following this study, the fresh water prawn Macrobrachium rosenbergii was introduced in 1972. After an initial trial period, a pilot hatchery was set up at Trou d'Eau Douce as a UNDP/FAO project with the aim of producing post larvae for sale to commercial enterprises. In the first year of its operation, the hatchery produced 43 000 juveniles and in 1973/74, the production was 1 million juveniles (SCET 1975). In 1982, a new hatchery was constructed at Albion and all research on Macrobrachium rosenbergii was transferred there. In the same year, a ten hectare farm with a total water spread area of 4 hectares was constructed at La Ferme. In 1975-76, six species of major Indian and Chinese carps were introduced in Mauritius and were cultured at the Curepipe Livestock Breeding Station. These cultures were subsequently transferred to the La Ferme Fish Farm Experimental Station. Induced breeding of the carps in captivity were successful using hormone injection. However, because carps were not easily accepted by consumers, production was stopped and research efforts were concentrated on the fresh water prawn. With the production of Macrobrachium by the private sector, La Ferme discontinued further development in this field so as to allocate more resources to the diversification of other species for aquaculture development. In 1988, the Marine Shrimp Culture Experimental Station was constructed at Albion and its activities included: Penaeus monodon culture, breeding and culture of a few marine fishes, fresh water fish culture and plankton culture as live feed for hatchery use. In November 1990, a triple cross hybrid of three species of tilapia (Oreochromis niloticus, O. mosambicus and O. aureus) was introduced from Malaysia. These were bred in fresh water at La Ferme and in sea water at Albion Fisheries Research Centre.

PRESENT WORK

Marine shrimp culture

In order to prevent the contamination of local stocks of marine shrimps from infections from abroad, the Ministry of Fisheries and Marine Resources uses locally available spawners for its broodstock. The broodstock obtained from the wild are kept in maturation tanks and are fed on special diets. When fully mature, they are induced to spawn by eye stalk ablation.

Production of post larvae was started in 1989 with a small output of 19 972 units. Production in subsequent years increased to reach a peak of 806195 individuals in 1993 due to the timely availability of good spawners from the wild. In 1995, eighty two adult Penaeus monodon (34 of which were females) were obtained from the wild by Albion Fisheries Research Centre and the production of post larvae reached 303 189. Out of this, 295 228 were stocked in the nursery ponds and later transferred to grow-out ponds for culture trials. In the same year, 3 tonnes of Penaeus monodon were harvested with a survival rate of nearly 70% (Ministry of Fisheries & Marine Resources 1996). The reduced production in 1994/95 was attributed to the fact that pond-reared females developed a high percentage of resorbed ovaries and poor quality of eggs; persistent fungal and bacterial infections also gave low overall survival rates of the larvae. Table 1 lists the production of post larvae from 1989 to 1995.

Table 1 Annual production of Penaeus monodon

In December 1995, 7961 post larvae were also stocked in cages in the following barachois: Bocambous (Bambous virieux), Beau Rivage (FUEL), Choisy (Constance La Gaiete) and Montagu (FUEL) for experimental purposes.

Marine fish breeding

Rhabdosargus sarba

In 1989, the marine fish Rhabdosargus sarba, locally called "Gueule Pavé", was induced to spawn in captivity by hormone injection and 5000 juveniles were produced in that year. In 1995, Rhabdosargus sarba broodstock were collected from 3 barachois, namely, Melville, Montagu and Beau Rivage and were kept under controlled conditions. Spontaneous spawning took place in captivity and 8575 fingerlings were produced and these were stocked in floating net cages in barachois for culture trials. Table 2 shows the production of Rhabdosargus sarba fingerlings from 1989 to 1995. The drop in fingerling production is attributed to recurrent fungal and protozoan infections.

Polyculture of Siganus sutor and Rhabdosargus sarba.

An experimental culture of hatchery produced fingerlings of Rhabdosargus sarba and Siganus sutor fingerlings from a wild stock were reared together. The experiment lasted 16 months and the average weight attained was 450 g for Rhabdosargus sarba and 635 g for Siganus sutor. The Rhabdosargus sarba reached maturity at the age of 12 months with an average weight of 275 g. During the experiment, 3640 kg of supplemental feed were applied which gave a Feed Conversion Ratio of 6.5:1 and a fish production of 2.24 kg m^-3. Table 3 summarises the results of the experimental culture.

Table 2 Rhabdosargus sarba fingerlings production from 1989 to 1995

Source: Ministry of Fisheries and Marine Resources 1996

Table 3 Results of polyculture experiment

Source: Ministry of Fisheries and Marine Resources 1996

Tilapia culture

The triple cross hybrid of three species of tilapia (Oreochromis niloticus, O. mosambicus and O .aureus) was introduced from Malaysia in 1990 and experimental culture started at La Ferme Fish Farm in 1991. In order to benefit from the high growth rate of males compared to females, fry were collected from broodstock ponds and were sex-reversed by feeding them on a diet containing methyl testosterone for four weeks. In 1995 a total of 440 684 tilapia fingerlings were produced, most of which were distributed free to 330 fish farmers at sixty three different sites. The total area under land-based tilapia culture in Mauritius is 110 585 m². Tilapia are also acclimated to sea water and are cultured in barachois. Table 4 gives the annual production of tilapia fingerlings by the Ministry of Fisheries and Marine Resources. In addition to these figures, fingerlings were also produced by the private sector for sale to entrepreneurs.

Table 4 Annual production of tilapia fingerlings by the Ministry of Fisheries.

Fresh water lobster

The Red Claw fresh water lobster, Cherax quadricarinatus, has recently been introduced in Mauritius from Australia. These lobsters are highly adaptable and both males and females attain maturity within 9 months after hatching. Their growth is relatively fast over a wide range of temperature (from 23^oC to 31^oC) and they can reach 60 to 120 g in a year They are detrivores and can be cultured on simple diets. Pre-release experiments are still in progress on this species.

Other productions at La Ferme Fish Farm

La Ferme Fish Farm has been phasing out the culture of Macrobrachium rosenbergii and carps in order to allow more space for the introduction of new species. The production of these 2 species are given in Table 5.

Table 5 Production of La Ferme Fish Farm kg

Barachois Culture

Barachois culture dates back to 1800’s with the impounding of several bays, creeks and estuaries to form pens. A barachois is a coastal indentation cut off from the sea by an artificial embankments, several sluice openings closed by gratings, enable the water within to be renewed twice daily by tidal action, but prevent the fish from escaping. The barachois are therefore partly enclosed water bodies that retain a free communication with the open sea. There are 33 barachois in Mauritius, out of which 24 are operational. The areas of the barachois vary from 0.5 to 45 ha (mean area 13.4 ha; total area 268 ha).

The tidal range being quite small (less than 0.6 m) there is only limited water exchange between the enclosed area and the open sea. Fresh water seepage into the sea occurs in most of the wide lagoons. In some of the areas, springs and small streams further reduce the salinity. With few exceptions, the water in the barachois is brackish and salinity varies from 0 to 35 parts per thousand depending on the amount of fresh water run-off from the land or from streams. Barachois are used for the culture of finfish, crabs and oysters. Traditional management practice is restricted to stocking, some control on predation and harvesting. Table 6 gives the official production figures for barachois culture

Problems of culture in Barachois

The major problem derives from the fact that the barachois are natural water bodies which are lined by mangroves . As a result, all the stocked fish seek refuge in the Acropora roots and cannot be approached for monitoring or harvesting. In addition the irregular bottom of the barachois does not allow the use of any seine net for sampling or harvesting. Predators such as Barracuda (Sphyraena barracuda) and Carangids (Caranx spp) enter the barachois through the gridded walls as larvae and greatly reduce the number of stocked fish.

Table 6 Barachois production from 1991 to 1995

Source: Ministry of Fisheries & Marine Resources 1996

Historically, barachois owners practised extensive culture and relied entirely on primary productivity for feeding their fish. With the introduction of the red tilapia, they started using supplemental feed in the form of commercial pellets. However the dispersion of the fish in the wide barachois casts doubts on the availability of the pellets to the entire stock being given that locally produced commercial pellets sink and crumble very fast in water. In the absence of any local cereal industry, low cost by-products for feed manufacture like rice-husk or wheat bran are not available, and have to be imported adding considerably to the cost of the feed and hence to the cost of production.. Although the land surrounding a barachois can belong to the entrepreneur exploiting the barachois, the water body itself is legally considered as public domain. Any intruder found fishing inside a barachois can only be sued for trespassing over the land belonging to the owner. The fine is minimal and does not discourage poaching.

Introduction of cages

Rearing fish in cages solves most of the problems encountered in open barachois culture. The cages are placed in the barachois itself because of the protection it offers and the advantages are multi-fold. The fish do not disperse in the barachois or hide in the mangroves; this solves the problem of feeding as all the fish are aggregated in one place; it also greatly facilitates harvesting. The usual way to harvest is to tow the cage ashore or to a shallow region and to scoop out the fish by means of nets. Once the tilapia are reared in cages, they are not accessible to the predators which already exist in the barachois.

Persistent Problems

Rearing fish in cages solved several problems linked to the Mauritian aquaculture industry; however, there are still some problems which await a solution. The non-availability of a proper floating extruded feed still hinders the development of aquaculture; locally produced feed sink and crumble within a minute; if not eaten immediately, it passes through the mesh of the cages and is lost to the fish. Accumulation of uneaten feed under the cages gives rise to water quality problems. The low tidal range prevents adequate water exchange in the barachois; this is not improved by the small mesh of the gridded gates which also impede water circulation; the result is a lowering of the salinity of the water in the barachois during heavy rains or an increase in salinity in case of drought. Furthermore, accumulation of detrital matter increases the oxygen consumption by organic matter. Moving from open culture in the barachois to cage culture has been an important step in improving management of the cultured stock. However, a number of measures need to be taken to ensure survival of the industry, formulation and manufacture of extruded floating pellets and engineering of proper water flow systems, require immediate attention. Several problems hinder the proper development of aquaculture in Mauritius. As a result the total aquaculture production has been increasing only slowly in the recent years. Table 7 shows aquaculture production figures from 1991 to 1995 (excluding oysters).

Table 7 Aquaculture production in Mauritius tons

Aquaculture research at the University of Mauritius

At the University of Mauritius, because of the distance from the sea, most of the research projects have been on fresh water aquaculture. The involvement of the University of Mauritius in aquaculture research started in 1991 with very limited resources and it gained momentum only after 1994 with the addition of a wet laboratory.

Research activity in aquaculture at the University of Mauritius can be broadly divided into 2 types:

Research projects initiated at the University

Consultancy projects

Research Projects initiated at the University of Mauritius.

The following is a list of completed research projects in aquaculture.

& 1991. A comparative study of two culture media for the culture of Skeletonema costatum, a live prawn feed at different temperatures.

& 1994. Food selection and feeding behaviour of an Oreochromis hybrid.

& 1994. Gastric emptying and the return of appetite in an Oreochromis hybrid.

& 1995. Influence of increase in food fibre on cellulolytic microflora and growth performance of an Oreochromis hybrid.

& 1996. Effect of temperature on the ingestion rate and growth of an Oreochromis hybrid.

& 1997. Change in water parameters following a power failure simulation in a closed aquaculture system.

& 1997. Digestibility of commercial feeds by an Oreochromis hybrid.

& 1997. Studies on an experimental fouling-resistant floating cage for mariculture.

& 1997. The aquaculture industry in Mauritius- a diagnostic study.

All these projects were planned after consultation with the Ministry of Fisheries or with other people who are actively involved in commercial aquaculture. The reports of all these research projects are available in the library of the University of Mauritius.

Consultancy Projects

Consultancy work in aquaculture by the University was initiated in 1994 through the Mauritius Chamber of Agriculture and the Mauritius Meat Producers' Association. The Association groups a number of barachois owners; problems encountered in the management of water bodies are due to the absence of baseline data concerning the depth of barachois, water quality and potential for fish and shell fish production.

1 1994. Belle Vue Mauricia/Beau Plan Sugar Estate Mauritius. (Bassin Saint François, Cap Malheureux).

1 1994 & 1996. Constance Sugar Estate, Mauritius.(Barachois Choisy and Bassin Nozaic).

1 1995. Flacq United Estate Ltd., Mauritius (Barachois Beau Rivage and Bassin Poulailler).

1 1995. Les Beaux Multipliants Ltd, Mauritius (Bassin Montagu, Trou d'eau Douce).

The University surveyed six barachois, listed above to establish their potential for aquaculture.

CONCLUSION

Aquaculture development in Mauritius is progressing quite slowly due to several problems linked to production costs and to natural conditions. However, it is expected that some of these problems will be solved in the coming years and with the increase in the expertise available, aquaculture research will no doubt progress at a faster pace.

REFERENCES

MINISTRY OF FISHERIES & MARINE RESOURCES. 1996. Annual Report 1995. Albion Fisheries Research Centre., Mauritius, 57 p.

SCET see under Sociéte Centrale pour l’Equipement du Territoire .

SOCIETE CENTRALE POUR L’EQUIPEMENT DU TERRITOIRE. 1975. Problèmes posés par le développement et l’organisation des pêches mauriciennes. France : Societé Centrale pour l’ Equipement du Territoire International, 268 p.

COMMENTS

Q. In one of your graphs you have shown a decrease in the production of fingerlings for marine shrimps. Any particular reason for that?
A. In 1993, the Fisheries Department was lucky to get females with eggs from fishermen; this is why there has been an increase in this year as compared to other years

Q. Is there any interest in the rearing of tilapia?
A.Tilapia can be reared easily but the only problem is that of marketing and consumer acceptance.