| || || Prasad, Rajesh.|
| || || Reproductive biology and growth of Saccostrea commercialis (Iredale and Roughley) in tropical Queensland |
Institution: James Cook University.
Subject: Oysters -- Australia -- Queensland -- Reproduction , Oysters -- Australia -- Queensland -- Cultural control , Oyster culture -- Australia -- Queensland
Call No.: pac SH 367 .A8 P727 1997
Copyright:Under 10% of this thesis may be copied without the authors written permission
Abstract: The aims of this study were to determine the reproductive and spatfall seasonality, and the growth and survival rates under different culture conditions, of an oyster species that is cultured on a small-scale at White Lady Bay on Magnetic Island, north Queensland, Australia. This oyster, known locally as the "milky" oyster, was identified as the Sydney rock oyster, Saccostrea commercialis (Iredale and Roughley, 1933), which is generally regarded as a temperate/subtropical species. Reproductive seasonality of this oyster was studied via monthly measurements of condition index and histological examination of gonads between December 1993 and March 1995. Sample sizes of 20 and 15 adult oysters of 13 g mean weight were used for condition index and histological examination, respectively. Gametogenic activity and spawning of S. commercialis at White Lady Bay were continuous throughout the year, with at least 7% of oysters with spawned gonads and at least 14% with ripe gonads in each month. However, spawning activity may be intensified during the wanner months, in summer and autumn, when between 2 0 - 4 0 % of oysters had spawned gonads. The condition indices during the warmer months were between 8 -10, compared to 10 - 1 3 at other times of the year. Gametogenic activity and spawning levels differed during the two summers of this study. The reproductive seasonality described for S. commercialis in this study is similar to the reproductive seasonality of other tropical oysters. It differs from the reproductive seasonality of S. commercialis in temperate Australia, because the reproductive season becomes more defined with increase in latitude. vii -------------------------------------------------------------------------------- In the sample of oysters used for histological examination there were 67% females, a biased sex ratio in reproducing oysters, which has been previously reported in this species. Temperature, salinity and chlorophyll a were measured to investigate their influence on the reproductive activity of oysters at White Lady Bay. Water temperatures ranged from 18°C to 33.5°C. There was a significant negative correlation between monthly temperatures and monthly condition indices. Although temperatures remain suitable throughout the year for gametogenesis, the temperature rise after winter probably provides a cue for the majority of the oysters to increase their rate of gametogenesis or spawn. Salinity at this site was stable, between 35 - 36 ppt, and unlikely to act as a cue for spawning. Chlorophyll a levels in the water were between 0.19 -1,02 pig/L, which is low compared to chlorophyll a levels in temperate waters. S, commercialis spat were collected on PVC collectors at four locations in the intertidal zone at White Lady Bay from March 1994 to April 1995. Spatfall also reflected the continuous reproduction of this oyster, since spat were found throughout the year. Intensive spawning during the wanner months was also confirmed by significantly increased spat numbers. Spat numbers ranged from means of 1 - 4 spat/collector/month (1 collector =700 cm1) between May and October and 24 - 36 spat/collector/month between December and March. Spat numbers did not differ significantly at four locations within a small area in the intertidal zone at White Lady Bay. There was also no significant difference in spatfall at intertidal heights of 1.24 m, 1.45 m and 1.67 m above datum, While 52% of the total spat settled on the lower surfaces of collectors, there was no significant preference for the lower surface. This result was similar to that observed in temperate viii -------------------------------------------------------------------------------- Australia with the same type of spat collectors. Growth rates were determined by monthly weight measurements of oysters under four different culture conditions. Oysters were cultured in the intertidal and subtidal zones at White Lady Bay, between April 1994 and April 1995, and in a reservoir pond and prawn ponds at a prawn farm, between May 1994 and May 1995. Oysters were cultured in the reservoir pond and in prawn ponds, under monoculture and poly culture, respectively, to investigate the feasibility of oyster cultivation in ponds. At all sites the oysters were cultured in mesh trays with lids. Mean initial weights of oysters were 5.0 - 5.7 g. Oysters in the prawn ponds did not grow and none survived beyond 10 months due to fouling by barnacles. Over a year, the mean weight increments of oysters in the reservoir pond, subtidal zone and intertidal zone were 6 . 6 ± 0 . 2 g , 3 . 5 ± O . l g and 1,3 ± 0.1 g, respectively. Oysters in the reservoir pond and subtidal zone gained in dry tissue weight by 210% and 120% respectively, but oysters in the intertidal site lost 23 % dry tissue weight. The survival rates in the reservoir pond, subtidal and intertidal zones were similar at 70 - 72%. The better growth rates obtained in the reservoir pond compared to the growth rates in the subtidal and intertidal zones indicate that monoculture of oysters in ponds is feasible. However, an aquaculture operation based entirely on pond cultivation has to be considered in respect of investment costs involved. Natural ponds or ponds on existing aquaculture farms could probably be used for small-scale cultivation of oysters with minimal cost. Polyculture of oysters with prawns or other aquaculture animals needs further investigation. Control of fouling should be a major consideration. Overall growth rates obtained in this study were low compared to growth rates of this species in temperate Australia. The levels of food available to the oysters appear to be the limiting factor for growth, especially at White Lady Bay, On the bases of size and growth rates, this oyster has little potential for an aquaculture industry in tropical Queensland, If these oysters are cultured in this region they may have to be marketed at around 15 g, as opposed to 40 - 50 g, the size marketed in New South Wales. However, as indicated by growth rates obtained in the reservoir pond in this study, growth rates can be improved slightly of this species in temperate Australia. The levels of food available to the oysters appear to be the limiting factor for growth, especially at White Lady Bay, On the bases of size and growth rates, this oyster has little potential for an aquaculture industry in tropical Queensland. If these oysters are cultured in this region they may have to be marketed at around 15 g, as opposed to 40 - 50 g, the size marketed in New South Wales. However, as indicated by growth rates obtained in the reservoir pond in this study, growth rates can be improved slightly.