A number of faunistic studies have been made in the Gulf of Aqaba as well as some preliminary attempts at describing the separate benthic habitats (see Fischelson, 1971, for a recent review), but quantitative data on the productivity, or on the standing stock of either the plankton or benthos, is lacking. The very clear water suggests that the standing stock of plankton is low, although this may be counterbalanced by a high rate of production which is quickly consumed by the herbivorous zooplankton. Most of the narrow shallow-water zone is occupied by coral which is either carnivorous or obtains its nourishment from symbiotic algae and typically has a rather low growth rate. Inquiry has failed to reveal any information on the presence of extensive beds of bivalves, ascidians or other filter feeders and there is no evidence of any substantial stocks of fish. All the available evidence suggests that the stock of animals which is supported by natural production in the Gulf is small.
Oysters. Species of oysters represent one of the most common of the larger filter feeders. Natural populations of oysters (probably Crassostrea cucullata) occur on rocky shores in the inter-tidal zone at Elat; they are rarely found where they would be continually submerged. This distribution may be caused by preferential settlement of the larvae in the inter-tidal zone or more probably, by excessive predation in the sub-littoral. They are not present in commercial quantities and generally not in commercial size. The largest individuals have thick, heavy shells and show little sign of recent growth.
Previous studies at Elat. Data on the oyster investigations made at Elat by the staff of OLRC were made available to the consultant. These comprised estimates of condition (May–November 1971) and counts of the spatfall of bivalves on asbestos plates exposed for one-month periods (September 1970–October 1971). These data were mainly collected by Mr. W.L. Hughes-Games.
Oyster condition. The condition index estimates were made on each individual oyster in samples of ten. The samples were drawn at about monthly intervals either from natural populations of C. cucullata or from stocks which had been transplanted to the sub-littoral region. The data have been pooled and the percent frequency in each successive condition index group is given in Table 3. As 10 represents a satisfactory index from the commercial standpoint (Walne, 1969) the summary shows that about 52 per cent of the oysters examined fell below this. The original data suggests that oysters were in better than average condition in the early summer. Culturing for a period below low water mark produced only a slight improvement compared with the original stock in the inter-tidal zone.
The appearance of the oysters during the first part of the consultant's visit to Elat was poor; the meat was shrunken and brown or grey in colour, instead of white and full. The gills, which are markedly pigmented at the ventral end, appeared relatively large, and the overall effect was not appetizing. Towards the end of his visit some samples were seen in which the appearance reached a satisfactory commercial standard.
Spatfall. Data from the square asbestos plates exposed sub-littorally for monthly periods showed a maximum of about 1.5–2.0/cm2 of bivalve spat in December–January, and this declined to negligible numbers of mid-summer. A number of the plates had been allowed to remain in the water for a period of up to a year and these carried spat 20–30 mm in size. A detailed study of these spat by the consultant showed that at least three species of oysters, belonging to the genera Ostrea, Crassostrea and Pycnodonta were present. The desired species (Crassostrea) was in the minority and often absent: the other species had no commercial value. A further oyster species, resembling Lopha cristo-galli was also found in shallow water close to the collectors, although it has not yet been identified on them. This latter species is also present at Elat on black coral at depths of 30–60 metres.
It is clear that this technique is unsuitable for collecting the natural spatfall of the commercially desirable species because of the large numbers of other species which are also collected. Careful study may reveal a method by which the spatfall of C. cucullata can be separated from that of the other species, for example, by seasonal or by special ecological circumstances. It is not easy to separate the species when the spat are small (less than 1 cm in size) and it certainly could not be done on a large scale.
Surface water plankton hauls taken in December 1971 and examined by the consultant showed that mature ‘eyed’ bivalve larvae were common. The consultant also found several specimens of Ostrea sp. brooding larvae while the Pycnodonta sp. was in a ripe condition. The Crassostrea was not ripe during the time of the consultant's visit.
The major factors affecting oyster culture can be summarized under the following three headings: method of culture, stocks of oysters, and productivity of the water. These are discussed in the following paragraphs.
Method of culture. In various parts of the world there are major commercial fisheries for oysters cultured on the sea-bed in the littoral or sub-littoral zone down to a depth of about 10 m, or on racks or trays fixed to posts driven into the sea-bed, or suspended from rafts or long lines anchored in moderate depths of water. The selection of a suitable culture system is particularly influenced by exposure, the bottom soil and the nature of the supply of young oysters that are being cultivated.
It is most unlikely that substantial quantities of oysters can be cultured on the seabed at Elat due to its restricted nature, an unsuitable soil of sand and gravel, exposure to wave action, and the likelihood of human interference. Greater control could be exerted if racks or trays were fixed on to the sea-bed and, providing there was sufficient movement of water containing an adequate food supply, a high production could be obtained from a small area. It should be pointed out that the water temperatures favour the use of divers, although this type of labour is expensive and the number of hours work obtained per day is low. In the opinion of the consultant a technique dependent on divers is not commercially feasible.
Rafts or other floating structures also offer considerable environmental control but possibly the area available (of sufficient depth but free from navigational hazards) is more limited than that for structures fixed to the sea-bed. Nevertheless, it appears to be the most suitable method for Elat.
Stocks of oysters. Stocks of juvenile oysters for transplantation to sites favourable for growth are not present in the area. The supply of suitable juveniles is therefore crucial and there are three possibilities:
Catching the natural spatfall on artificial collectors. The experiments made so far at Elat have not been successful. If more favourable circumstances can be found the collectors can either be small, in which case the spat could remain attached until they reach commercial size, or the collectors could be larger and resemble the French system of limed tiles. Here the juveniles are removed from the collector when they reach about 2 cm in size and are subsequently cultured either on the bottom or in trays.
The purchase of naturally occurring juveniles from more favoured areas is a common commercial practice, and varies between transplantation from regions of the same country to inter-continental movement. For Israel this would mean the importation of oysters, almost certainly of another species, with the attendant risks of disturbing the local ecological balance and the introduction of pest organisms. For these reasons it is recommended that it should not be attempted at this stage.
The production of juveniles in a local hatchery is also possible. The development of commercial hatcheries for oysters and clams is proceeding in the United States, Great Britain and France, and they are likely to become commercially competitive in the near future. A small commercial hatchery needs an initial capital cost of about U.S.$ 50 000 (IL 200 000)1 and, with a minimum staff of three, a running cost of about U.S.$ 25 000 per annum. It is not practical to prepare such an enterprise before a positive decision is made to proceed with the large-scale culture of bivalves in the Gulf of Aqaba.
Productivity. Given a suitable culture technique and an adequate supply of juveniles, the success of oyster culture depends on the natural food (phytoplankton) in the water being adequate for the growth and fattening of the oysters. Growth must be fast enough to ensure a reasonably quick return on capital and the oysters must fatten sufficiently to be commercially acceptable. Formation on both these points is lacking for the Gulf of Aqaba. Pest control may be required but the difficulties of this are likely to be reduced by the necessity at Elat to culture the oysters off the bottom. Although boring gastropods are clearly abundant this is not thought to be a major factor at this stage. The possible incidence of disease (virus, bacteria, protozoa) cannot be assessed at this stage.
