Ayu (Plecoglossus altivelis) were infected with Vibrio anguillarum by a water-born method. At 6, 12, 18, 36, 38-45 (moribund stage) and 48 h (dead) after infection, fish were sampled to determine the fate and location of the bacterium in various tissues by viable cell count and the enzyme-labeled antibody technique (ELAT). V. anguillarum was first detected in the skin at 12 h by bacterial isolation. It appeared in the muscle, spleen and liver at 24 h, but was not isolated from the gills or intestine until 36 h or 38-45 h. The same trend in the fate of the pathogen was confirmed by ELAT, and the cells were found in dermal layer of the skin from the early stage (12h) of infection. Based on these observations it was concluded that the first colonization site of V. anguillarum in ayu was the skin.

Wild immature Penaeus indicus females (11.5 ± 3.1 g body weight) were ablated by pinching, cautery or tying of one eyestalk, and stocked with control (unablated) females and males (9.2 ± 1.5 g) in a 12-m³ maturation tank. Full ovarian maturation and spawning were attained 4 days after ablation/stocking in all treatments, with a peak at 5–6 days. Seventy-five percent of ablated and unablated females spawned during the study period. Average egg numbers from complete spawns increased with size of females for all treatments. There was no significant difference in fecundity of complete spawns from the various treatments. However, hatch rates of unablated P. indicus were significantly higher than eyestalk-pinched females but not those ablated by cautery and tying. Similarly, survival after the 15-day period was lowest among pinched females.

An investigation was conducted to establish water hardness concentration for optimum hatching of silver carp (Hypophthalmichthys molitrix) eggs. Eggs were incubated for 19 h at 26.5°C at six levels of water hardness: 100, 200, 300, 400, 500 and 600 mg/l CaCO₃. Water absorption at 100–200 mg/l CaCO₃ caused eggs to burst prematurely and minimal water absorption occurred at 600 mg/l CaCO₃. Chloride concentration at 0 and 6 h post-fertilization was significantly related to egg hatchability. Total ammonia-nitrogen at 6 h, pH at 6 h, magnesium hardness at 18 h and chlorides at 18 h significantly influence viability of larvae. A water hardness of 300–500 mg/l CaCO₃ is recommended for the successful hatching of silver carp eggs.

The effect of molting on osmotic and chloride concentrations in the blood of the prawn Penaeus monodon Fabricius (20±3 g) at various salinities was investigated. Prawns were obtained from ponds in Iloilo, Philippines, in 1984. They were stocked in salinities of 8, 20, 32 and 44‰, and their hemolymph was sampled during molt (Time 0), and then 0.125, 0.25, 0.5, 1, 2, 4, 6, 10 and 14 d after molting. Prawns during and immediately after molt tended to conform to the environmental osmolality. Subsequent postmolt (≧0.5 d) stages displayed more divergence from external salinity. The isosmotic point was higher (940±30 mOsm kg-1) during molt than during intermolt (663±8 mOsm/kg-1), suggesting different osmotic requirements in early molt. Hyperregulation of hemolymph chloride below 20‰ S, as well as isoionic point (301±6 mM), were independent of molting stage. At 20‰ S and above, newly molted (0 to 0.25 d post-molt) individuals tended to conform to the external chloride concentration while intermolt (≧0.5 d) post-molt individuals did not. Contribution of hemolymph chloride to hemolymph osmolality was greater during intermolt than during ecdysis, suggesting an important role for other negatively charged ions during molt. When molt occurred in 20‰ S (the test salinity most similar to the isoionic salinity), there was little or no change in hemolymph osmolality or chloride concentration from 0 to 14 d postmolt. At 8, 32 and 44‰ S, the change from molt to intermolt values in hemolymph osmotic and chloride concentrations was hyperbolic. Non-linear least-squares regression showed that prawns generally achieved intermolt values within 1 d after molting. Prawns at intermolt regulated hemolymph osmolality (620 to 820 mOsm kg-1) and chloride concentration (300 to 450 mM) at a much narrower range than during molt (520 to 1 170 mOsm kg-1 and 250 to 520 mM, respectively). Hemolymph osmolality was a more sensitive indicator of physiological response than hemolymph chloride concentration. Distribution and culture of P. monodon might be limited in low salinities by its ability to maintain a hemolymph osmolality ≧500 mOsm kg-1 during molt and ≧600 mOsm kg-1 in intermolt, and in high salinities by its capacity to reduce the hemolymph osmolality from values at molt to those in intermolt. Osmotic and chloride concentrations in the blood of P. monodon clearly varied with both molt stage and salinity of the medium. Dependence on external factors, however, gradually declined in older molt stages, suggesting a reduction in integument permeability and greater development of ion absorption/secretion mechanisms as the exoskeleton hardened.

Two experiments were conducted to measure the economic viability of retarding milkfish growth (stunting) in brackishwater ponds. In the first experiment, 2-month-old fish were reared for 6 months in ponds with initial stocking densities of 15, 20, 25, and 30 fish/m². The second experiment had a common stocking density of 20 fish/m² with rearing periods of 6, 9, and 12 months. All experiments followed the lab-lab method of growing natural food plus additional substrates. Supplemental feeding using rice bran mixed with ground trash fish started 60 days after initial stocking. Under Philippine conditions stunting milkfish fingerlings at 20 fish/m² for 6–9 months is most cost effective; this permits the production of milkfish fingerlings for lower cost than they can be purchased.

The purpose of this study was to find out how yolk and oil globule absorption in Siganus guttatus proceed as the digestive tract develops, in order to determine the probable causes of early larval mortality. Yolk and oil globule absorption in the rabbitfish were compared with the same processes in the more sturdy seaperch larvae during the first 10 days of larval life under identical rearing conditions in 32‰ sea water at 27°–30°C. The rapid decline of yolk in both species coincided with the rapid development of the digestive system within 24 h from hatching, indicating that most of the yolk was used for organogenesis. Whereas yolk was depleted in both fish in 3 days, the oil globule persisted in the rabbitfish only for 4 days and in the seaperch for 7 days. Oil globule depletion in the rabbitfish coincided with a negative mean length increment, implying an energy deficit even when the larvae had already started to feed. Ultrastructural observations of the gut epithelia of the rabbitfish revealed pinocytosis in the hindgut cells immediately after ingestion of rotifers, well in advance of complete yolk and oil globule absorption. Therefore, starvation due to exhaustion of the endogenous energy reserves in addition to the physical inability to feed were ruled out as major causes of larval mortality in rabbitfish.

Nitrite was about 55 times more toxic to milkfish juveniles in fresh water than in 16% brackish water: the 48-h median lethal concentrations were 12 mg NO_2-N/l (95% confidence limit=7.4–19.6) and 675 mg NO_2-N/l (95% confidence limit = 435.8–1,045.4) respectively. Methemoglobin levels were higher for a given concentration of nitrite in milkfish kept in fresh water than in the brackish water. Methemoglobin decreased to a normal level within 24–26 hours of the removal of nitrite.

The effects of loading density, length of transit time, temperature and salinity on milkfish (Chanos chanos Forsskal) eggs during simulated transport were examined. Rocking motion approximating conditions of transport of eggs collected from milkfish broodstock floating net cages to a hatchery was simulated using a laboratory orbit shaker. Loading densities of more than 7000 eggs/l in shipping bags resulted in decreased rates of survival and correspondingly lower hatching rates. Prolonged shaking simulating extended periods of egg transport also resulted in low egg survival and hatching rates compared to fertilized eggs not subjected to simulated transport. Egg survival after simulated transport at 20°C was lower than at 28°C, except at 20 ppt salinity, where survival was equal. Egg survival at 20°C progressively increased with declining salinity levels whereas high egg survival rates were observed after 2 h of simulated egg transport at 28°C and at the three salinities tested. Hatching rates of fertilized eggs after simulated transport were higher at 28°C than at 20°C regardless of salinity. Neither salinity nor its interaction with temperature affected hatching rates of eggs after simulated transport. These results indicate that survival and hatching of fertilized milkfish eggs after simulated transport is influenced by loading density, transport time, temperature and, to some degree, the salinity of the water. Based on these results, guidelines for handling and transporting milkfish eggs are given.

The ability of luteinizing hormone-releasing hormone analogue [(D-Ala⁶, Pro⁹-NEt)LHRH], 17α-methyltestosterone (MT), and their combination to advance gonadal maturation and spawning in captive sea bass (Lates calcarifer Bloch) broodstock was tested. Several hormonal therapies were tested including monthly implantations of a low dose (100 μg/kg body weight) of each hormone and a high dose (200 μg/kg) implanted to sexually-quiescent sea bass at 45-day intervals. A high dose of LHRHa alone or in combination with MT induced a significant number of mature females (43–71%) in April, 45 days after a single implantation in early March. A lower dose of pelleted LHRHa and LHRHa plus MT was found effective (78–80%) only in May following three monthly implantations. Two and three monthly implantations of a low dose of MT failed to stimulate oocyte growth in April and May whereas a less frequent mode of application of a high dose of the androgen inhibited ovarian growth in April only. A low dose of MT alone significantly increased the number of mature males in April (90%) and May (100%) after two and three monthly implantations. All male sea bass which received three monthly implantations of a low dose of LHRHa and LHRHa plus MT were sexually mature in May. Sexually mature sea bass obtained from these experiments spawned in early May after a single intramuscular injection of LHRHa. These results demonstrate the potential use of pelleted LHRHa and MT to significantly advance gonadal development and spawning in sea bass earlier than the annual breeding season.

An opercular tag for marking adult milkfish (Chanos chanos Forsskal) and seabass (Lates calcarifer Bloch) is described. High tag retention and relatively low mortality rates were observed in adult fish handled two to ten times during 14-to 60-day tests. The features and advantages of the tag for marking large-sized fish in short-term studies are discussed.