WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2014 53 concentrations ranged from 5.5 to 5.9 mg/L in the dry season and from 5.8 to 6.5 mg/L in the rainy season. Chlorophyll a in the rainy season ranged from 5-14 μg/L and in the dry season from 1-2 μg/L. Total suspended solids in the dry season ranged from 21 to 353 mg/L and in the rainy season from 10 to 39 mg/L. The pH was stable across seasons, ranging from 7.8-7.9 in the dry season and 8.0-8.1 in the rainy season. Sexual Maturity and Annual Spawning Peaks Samples of 30 adult oysters were collected monthly from 2003- 2005. In Crassostrea lugubis, sexes are separate, but gonads appear similar and have a cream color, making it difficult to visually differentiate between sexes. Smears of oyster gonads were analyzed to determine the stage of gonadal development as proposed by Quayle and Newkirk (1989) and Gosling (2003). Stage 0: Difficult to determine sex. No follicles present, mainly Leydig tissue. Stage 1: Beginning of gametogenesis, with the appearance of follicles; primary sex cells on the follicle walls. Stage 2- Follicles enlarged and gametes well formed, but still not ripe. Stage 3- Sexually mature with ova capable of fertilization and sperm active. The estimated average diameter of egg was 58 μm. Under the microscope at a 400× magnification, active sperm could be observed easily in clean water conditions. Stage 4: Recently spawned with follicles partially collapsed with no or only a few relict ova or sperm. This is followed by replacement with Leydig tissue and a return to stage 1. Analysis of the development stages of gonads indicated that sexual maturity occurred at about eight months of age, at a shell length of 59 mm (Fig. 4). However, some individuals smaller than this were sexually mature. Oysters in Nha Phu spawn year-round and histology of gonadal sections indicated ripe spermatozoa and eggs in all months. However, the proportion of mature specimens with stage 2 to 3 gonads over 50 percent occurred in two peaks, from March to June and August to October (Fig. 5). Collection of wild spat for aquaculture of oysters should be based on the breeding season. Oyster Growth Oyster seed were collected as natural spat from Nha Phu Lagoon, supplemented by commercial seed (Nguyen 2006). Seed (7-8 mm) were placed in black plastic cages with 4-5 mm mesh polyethylene fiber netting. After 2 months, oysters were transferred to cages with larger-size mesh. Cages were suspended on racks (Fig. 6) at two water levels—the intertidal and subtidal— on the same string to compare oyster growth rate. Cages were 50 cm apart and 50 cm from the bottom. The density in each cage was 500 individuals. Cages and oysters were cleaned as needed, depending on location and season. Oysters grew well in Nha Phu Lagoon, with the greatest relative growth rates in April (Fig. 7). After three months, shell length was 26 ± 4 mm, equivalent to 0.53 ± 0.09 g. Oysters of four months of age were 6 ± 2 g and began to grow rapidly. Oysters were 56 ± 4 g after 12 months of culture in Nha Phu Lagoon, equivalent to a shell length of 80 ± 4 mm (Fig. 7). This species, in Thailand, is 86 mm at one year of age (Angell 1986). The average growth rate of Crassostrea lugubris in the intertidal zone (9.0 ± 0.5 mm/mo) was slightly greater than the growth rate in the subtidal zone (7.6 ± 0.6 mm/mo) (Fig. 8). Survival rate in the intertidal zone (92 percent) was significantly greater than in the subtidal zone (82 percent). This may have been the result of elevated turbidity near the lagoon bottom. TOP, FIGURE 4. Size at sexual maturity was determined when shell length reached 59 mm. BOTTOM, FIGURE 5. Reproductive cycle and spawning season of Crassostrea lugubris in Nha Phu Lagoon. FIGURE 6. The rack system in Nha Phu Lagoon used to hang monitoring plates and cages. (Photo: Cao Van Nguyen) (CONTINUED ON PAGE 54)
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