Reproductive characteristics and variations in the biochemical composition of Neptunea arthritica cumingii crosse through embryonic development

The copulation, egg laying, embryonic development and changes in amino acids and fatty acids in Neptunea arthritica cumingii during embryogenesis were studied to understand the embryo development process and nutritional requirements in the early life phase. The results showed that N. arthritica cumingii has direct development within the egg capsule and the development of embryos was classified into five stages: cleavage, egg swallowing, protoconch forming, shell development and juvenile. Embryos develop through the provision of nurse eggs as an extra‐embryonic source of nutrition. As development continued, the body of the embryo began to coil. After about 70–80 days, young N.arthritica cumingii started to emerge through a hole underneath the capsule. Biochemical results showed that the total amount of amino acids showed a decreasing trend as embryonic development progressed. The content of all nine essential amino acids decreased significantly from the egg‐swallowing stage to the post‐larva stage (p < .05). Concentrations of five of the seven nonessential amino acids also showed a decreasing trend from the egg‐swallowing stage to the post‐larva stage; the exceptions were Ala and Gly. Gly is the only amino acid that consistently increased in concentration during the development process. Most fatty acids increased after the eggs hatched, except for C20:1, C20:2, C22:5 and C22:6 (docosahexaenoic acid, DHA). The data in this study may provide a starting point for the formulation of well‐balanced early‐stage larval diets, although N.arthritica cumingii is still in the exploration stage.     

The first success of glass eel production in the world: basic biology on fish reproduction advances new applied technology in aquaculture

The eel has long been esteemed as an important food fish in the world, especially in Japan, and has been used as an experimental fish for many fields of fish physiology. However, the decreases in eel resources have been a serious concern in recent years. The catches of glass eels as seedlings for aquaculture have shown a long-term decrease in both Europe and East Asia. To increase eel resources, the development of techniques for artificial induction of maturation and spawning and rearing their larvae have been eagerly desired. Recent progress of reproductive physiology of fish, especially mechanisms of oocyte maturation and ovulation in female and of spermatozoa maturation in male, facilitate to establish techniques for hormonal induction of maturation and spawning in sexually immature eels. With persistent effort to development of rearing techniques of larvae, we have first succeeded to produce glass eel. These applied techniques are may contribute to understand the basic reproductive physiology of the eel.     

A first step towards improving copepod cultivation using modelling: the effects of density,crowding, cannibalism,tank design and strain selection on copepod egg production yields

Copepods are the optimal live feed for hatcheries and improvement of cultivation techniques, to provide a constant food source, is crucial for the expansion of the industry. However, studies based on experimental work and real observations can be labour intensive and expensive. A simple model was developed based on the well‐known life history traits of Acartia tonsa to describe batch cultures and their productivity. Model results were compared to observations from real cultures. For maximizing egg production yields, the optimal stocking density of copepods should be adapted to the design (depth) of the culture tanks. At high densities, stress due to encountering conspecifics, as well as cannibalism of eggs by adults, limits egg production yields. Using this model, the potential selection efficacy of copepod strains was also evaluated in order to increase production yields. Selecting larger copepods increases the egg production per litre of culture, but decreases the optimal stocking density and the range of densities at which egg production yield is high, and vice‐versa. Selecting copepods that are less affected by stress due to conspecifics only affect production yields at very high adult densities. However, selecting copepods with a high Specific Growth Rate (SGR), or improving their SGR, was found to be an alternative which did not affect the optimal cultivation densities but improved egg production yields.     

Effects of stocking density and substratum on the survival,growth, burrowing behaviour and shell morphology of juvenile basket cockle,Clinocardium nuttallii: implications for nursery seed production and field outplanting

The basket cockle, Clinocardium nuttallii, is a candidate species for aquaculture in British Columbia, Canada. Previous research on broodstock conditioning, embryonic development and larval rearing of C. nuttallii demonstrated the potential for reliable hatchery production of cockle seed. In this paper, we investigated the effects of culture density (50% and 150% bottom cover in a monolayer) and substratum (none and fine sand) on cockle seed survival, growth, behaviour and shell morphology to improve the efficiency of the nursery production phase. Low stocking density (50% cover) yielded a shell length increase from 3 to 7 mm over a 4‐week period. High stocking density (150% cover) negatively impacted some of the growth and condition parameters studied, but did not have a statistically significant effect on seed survival. Growing seed without substratum did not significantly affect seed survival or growth, but led to shell deformities (at shell length >15 mm) and lower burrowing rates when cockles were subsequently placed on sand. We recommend planting seed for grow‐out at a shell length not exceeding 15 mm. Acclimating seed to the substratum before planting may increase the burrowing rates, thereby reducing the risks of seed dislocation and mortality. Additional studies are required to determine optimal substratum acclimation times and techniques.     

Effect of varying density and water level on the spawning response of African catfish Clarias gariepinus: Implications for seed production

The development of a reliable methodology for spawning of African catfish, Clarias gariepinus without the use of hormone injections would greatly improve the prospects of aquaculture in Africa. Earlier work has shown that it is possible to produce C. gariepinus fingerlings by subjecting the broodfish to a physical stress of reduced water depth and/or increased temperature. The hypothesis that C. gariepinus could be induced to spawn through a combined physical stress of lowered water level and increased stocking density was tested in concrete tanks. Three water levels (25, 50 and 75 cm) and three stocking densities (2, 4 and 6 pairs of broodfish at a 1:1 sex ratio in each hapa) were tested. Water depth in the tanks and brood fish density in the hapas affected spawning success. The percentage of spawning females was significantly higher when broodfish were stocked at 2 and 4 pairs in each hapa at water levels of 25 cm or 50 cm. There was no significant difference in spawning response between the 25 and 50 cm depths while a significant difference was seen between the 75 cm and both 25 and 50 cm depths. The results indicate optimum levels and densities for enhancing spawning success in C. gariepinus.