An Investigation of the Biology and Culture of an East African Oyster Crassostrce cucullata.
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Date
1961
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CrassostrtZa cucullata (Born) is the most important edible oyster of the East African coast, and is .a purely littoral species occurring intertidally on open reefs, mangrove roots and creek rock exposures. Heavy commercial exploitation ofoyster beds on the Kenya coast has led to almost complete destruction of these beds, and in all areas the rate of natural regeneration and recovery of beds appears practically negligible, or at the best, an extremely slow process. Exploitation of beds, hitherto unlicensed, is now strictly controlled by Government. A survey of the biology and ecology ofoysters was carried out between September 1953 and August 1954, with further brief visits in October 1954 and September 1956, with particular reference to conditions in Mida Creek, one ofthe largest natural beds of oysters on mangroves. Here two forms of oyster occur; one, a spiked form growing on mangrove roots at higher tidal levels, and the other a flat form, growing at the lower levels. These appear to be distinct species, the latter being recognizable as C. cucullata; no attempt 4as been made to determine the identity ofthe former, which is referred to as Species A. The tidal and certain physico-chemical conditions in Mida are described in detail, these being different from those of the open shore, owing to the geographical formation of the Creek. The particular differences in each oyster zone are well marked. The oysters themselves are confined only to the eastern fringes of the mangrove forests, a spatial distribution which limits their abundance, and which may be due to the habit ofthe settling spat. They occur also on old reef rock expanses, and are phoretic on the abundant mollusc Terebralia palustris Brug. Zonation on the mangrove roots is distinct, and bears a strict relation to tidal levels. Temperatures, salinity and rainfall figures were noted, and their seasonal and local variations are described. Weekly dissections of adult oysters were made to determine spawning rhythms. While spent oysters may be found in armost any week ofthe year, and a low level ofspawning is continuous, nevertheless spawning sho~s two maximal periods coinciding with the onset of the rainy seasons in the year. The environmental factors related to spawning are complex, and it is suggested that (a) maximal water temperatures oYer 85"F (29'S°C), (b) the immediate onset of rain and (c) a fall in salinity are jointly responsihle for major spawning activity. While natural spat is very rare on all Kenya oyster beds, nevertheless a variety of spat collecting devices tried at Mida showed that spat would settle readily on Mangalore tile collectors, and shell culch strung on wires. The importance of cleanliness of spat collectors is not yet fully established for Kenya conditions and further trials are required both in this connection and that of the most suitable method in which to arrange the collector and the best type of collector. Itis, however, certain that some form of cultural method, using spat collectors, is fully practicable. While spat may settle at almost any time of the year, the heaviest spatfalls occurred at the beginning of, and during, the rainy seasons of November/December and April/May, the latter season providing maximal spatfall. The duration of free swimming larval life is not, however, known. All spat collected settled within a few inches ofground level, and appeared to be that of C. cuculla!a. Under natural conditions spatfall is fractional only, and the evidence suggests that the spat settle by preference on hard surfaces, mangrove bark being a second "choice". The conditions influencing settlement of spat are discussed. Growth of spat collected at Mida is very fast, averaging about 0·6 em a month; at only seven months the young oysters have reached the average adult length, but not thickness; thickness is added during the second year of life, and artificially grown oysters reach marketable size in about 2! to 3 years. Spat from elsewhere, grown at Mida, does not grow so fast, nor does Mida spat at levels above their normal tidal zone. Growth or survival of spat under pennanent submerged conditions is not known• Natural mortality of spat is not particularly heavy, nor apparently are there many serious natural predators. The growth of spat may vary according to season, but once adult, the oysters apparently cease to grow. Size at first maturity is not known. The preliminary results indicate that artificial culture of these Kenya C. cucul/ata oysters on shell culch strung on wires, and on tile collectors, would be practicable and economic, and indeed essential if exploitation is to continue at its present level. Possible cultural methods are discussed, and in places such as Mida or Ngomeni reef, there are large potentially productive areas which could be utilised for such culture.Pages
37ppPublisher or University
Her Majesty's Stationary OfficeSeries : Nr
Colonial Office Fishery Publications;14Collections