Work Package 2: Broodstock and Larval Quality

VIETNAM 

Histological and biochemical analyses

During the ovarian maturation of wild caught Scylla paramamosain (Keenan et al, 1998), changes both in color and size of ovary was obvious from the first to the last stage of development. Gonadosomatic index (GSI) increased initially increased slowly during the first stages but rapidly at stage V. Histological analysis suggested the 6 different stages of gonad development can be distinguished based on oocyte size and occurrence of follicle cells in accord with the findings at SEAFDEC. 

Proximal analysis and fatty acid profiles of the 6 stages of gonad development were analyzed using FAME (Fatty Acid Methyl Ester) method. Proximal analysis of the gonad revealed an increase in total lipid at later stages. Similarly, essential fatty acids (HUFA) also tend to increase from early to later stages.

Reproductive performance of wild caught and hatchery-raised S. paramamosain fed natural and supplemented artificial diets

Hatchery-reared and wild mature crabs were fed either a natural diet (a mixture of hard clam, squid, shrimp, hermit crab given alternatively daily) or an artificial diet (consisting of DHA milk (5g), Squid (45g), beef liver (5g), meat of marine clam (30g), Vitamin C (1g), Antibio (2g), DHA-selco (5g), Aginate (7g), clorua canxi (5-7g/100ml freshwater)) or a combination of the previous two diets. 

The effectiveness of the diets was assessed on the food intake, number of spawning crabs, fecundity, egg fertilization, hatching rate, quantity and quality of larvae and broodstock survival. Quality of larvae was determined by starvation tests.

Preliminary results (after 40 days) showed that the food intake was highest in the combination treatment (1.65%/body weight as compared to 0.48 and 0.66 for natural and artificial diets, respectively). The weight gain of crabs was also higher in the combination treatment. 

Influence of the content of highly unsaturated fatty acids in the live feed on larviculture success of S. paramamosain

Different levels of docosahexaenoic acid (DHA), eicosapentaenoinc acid (EPA) and arachidonic acid (ARA) were used in live feed enrichment emulsions to investigate the effects on the survival, growth and metamorphosis success of Scylla paramamosain larvae.
Larval development rate and metamorphosis success were more strongly affected by the dietary treatments than survival. The DHA/EPA ratio in the live feed appears to be a key factor. Enrichment emulsions with very high (50 %) total HUFA content but low DHA/EPA ratio (0.6) or zero total HUFA content caused growth retardation and/or metamorphosis failure. An emulsion with moderate total HUFA (30 %) and high DHA/EPA ratio (4) was the best in terms of larval development rate during the zoeal stages and resulted in good metamorphosis. The optimal DHA/EPA ratio of live feed enrichment emulsions for early stages (Z1 - Z2) could however be lower than 4. Dietary arachidonic acid seemed to improve first metamorphosis, but its exact role needs further clarification. 

For the larval rearing of Scylla paramamosain, it is recommended to use enrichment media with a total n-3 HUFA content of approximately 30 %, with a minimum DHA/EPA ratio of 1.

Effect of light intensity on larval growth of S. paramamosain

Light intensity may be important in prey perception. Increasing the light intensity from 1000 to 4000 lux resulted in an incremental increase in growth of S. paramamosain larvae. Although after 12 days a significant difference in LSI could only be detected between larvae grown at the two highest light levels and those raised at the lowest light intensities. 
There was no significant difference in survival, suggesting that growth may be more dependent on increase in nutrition caused by the elevated light levels than survival where disease and water quality may also be equally important. 

Effect of pH on larval growth and survival

Especially in recirculation systems and zero water exchange rearing systems pH can fall. Larvae of S. paramamosain were raise at 3 different pH levels (pH: 6.5, 7.5 and 8.5). Lower pH levels resulted in lower concentrations of ammonia and nitrite. However, LSI suggested that growth was slower at pH 6.5 and 15 days after hatching 100% mortality. In contrast by day 18, larvae in pH 8.5 had significantly higher LSI than those raised in pH 7.5. Following high mortality rates at megalopa, no significant differences in survival could be detected by day 22.
The increased LSI at higher pH levels suggests that pH 8.5 may be beneficial to larval growth.

References

Keenan, C. P., Davie, P. J. F. and Mann, D. L. (1998). A revision of the genus Scylla de Haan, 1833 (Crustacea : Decapoda : Brachyura : Portunidae). Raffles Bulletin of Zoology 46, 217-245.

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