The benefits of intensive farming of the giant tiger prawn Penaeus monodon in the Philippines are discussed in relation to the environmental costs. Ecological effects include mangrove conversion into ponds; use of antibiotics and chemicals leading to drug resistance; dumping of pond effluents which affect neighboring ecosystems; and pumping of groundwater that causes saltwater intrusion and vulnerability to floods. In addition, these effects lead to social costs in the form of reduction in domestic and agricultural water supplies; decreases in the production of foodfish and other food crops; further marginalization of coastal fishermen; displacement of labor; and credit monopoly by big businessmen. Comparative economic analyses of three prawn-farming systems showed that, compared to extensive and intensive culture, semi-intensive farms give the best performance using undiscounted (payback period, return on investment) and discounted (net present value, internal rate of return) economic indicators. With a 20% fluctuation in inputs or selling price intensive farming will no longer be profitable because of the high variable cost. The paper concludes with recommendations for strict enforcement of existing government guidelines (e.g. ban on mangrove conversion); institution of new policies on the use of groundwater, seawater and public credit; diversification of cultured species; and emphasis on semi-intensive farming parallel with brakes on further intensification of prawn farming.

The potential use of recirculating systems with facilities for waste removal for prawn (Penaeus monodon) hatchery and broodstock maturation tanks was investigated. Recirculating systems evaluated for larval culture tanks were biological filtration and zigzag stream sedimentation. Systems using airlift and airstones as aeration devices with partial water change but with no provision for recirculation served as control. Results showed successful rearing of Penaeus monodon (nauplii-postlarvae) in systems with facilities for wastewater recirculation. Aeration systems became ineffective in maintaining acceptable water quality as shown by increases in 5-day biochemical oxygen demand (BOD₅), ammonia (NH₄-N) and nitrite (NO₂-N) nitrogen, and decreases in dissolved oxygen (DO). For broodstock maturation, a recirculating system with a preconditioned biological filter was compared with a flow-through system. Biological filtration proved to be useful in meeting water quality requirements. Nitrification was satisfactory with ammonia levels lower than 1·0 mg liter⁻¹. The pH values in the biofilter effluent were maintained at 7·8–8·3. BOD₅ levels were below 10 mg liter⁻¹ while DO tended to drop due to the nitrifying activity of the filter. Better reproductive performance of Penaeus monodon broodstock was obtained in the recirculating system than in the flow-through system.

A preliminary study was conducted to determine growth and survival of Clarias macrocephalus fed live zooplankton (Artemia or Moina macrocopa ) and/or a dry artificial diet. The specific growth rate over a 14-day rearing period was higher for fish fed Artemia plus a dry diet than for other treatment groups fed either live zooplankton or a dry artificial diet alone. C. macrocephalus larvae can directly take dry diet during the early days of exogenous feeding, however, continued feeding on dry artificial diet resulted in poor fish growth and survival. Mortality due to observed cannibalism in fish fed exclusively an artificial diet increased from 4% to 18% during the last two days of the rearing period. A significantly lower proportion of mortality due to cannibalism was observed in those fed an artificial diet after a 7-day period of feeding on Artemia . The survival rate was generally high, except for the fish fed a dry diet alone.

Fifteen day old sea bass larvae were acclimated to 10‰ S and fed ad libitum levels of Moina, Artemia or Moina + Artemia (at 1:1 ratio). Fish kept at 32 ‰ S and fed Artemia served as a control. After 20 days, specific growth rates of fish ranged from 8.8 ± 2.5%/ day to 12.9 ± 1.4%/day and did not differ significantly in any of the treatments (P > 0.05). Survival rates of fish fed Artemia in either 10 (26.9 ± 8.4%) or 32 (30.2 ± 12.8 %)‰ S and Moina+Artemia (7.7 ± 2.8%) were similar and higher than the Moina-fed group (2.6 ± 1.4%). Results seemed to relate to poor utilization of Moina by younger sea bass (15-day-old). In another experiment, the amount of Moina ingested in the presence or absence of Artemia by 15-, 20-, 25-, 30-, and 35-day-old sea bass was determined. Thirty-day-old sea bass fry ingested the highest number of Moina, whether with (17.19 ± 1.96) or without (25.29 ± 2.93) the addition of Artemia, than did the other age-groups (P < 0.05). There was a low feeding incidence of Moina by 15-day-old sea bass. The number of ingested Moina was positively correlated (r = 0.6358) to the fish body length. The amount of Moina ingested by sea bass of the same age group was not influenced by the length of time after feed introduction. Moina can be used as partial or complete substitutes for Artemia, provided they are sieved beforehand for efficient utilization by the younger sea bass larvae.

Male and female gulf killifish, Fundulus grandis, were kept in large cages submerged in their natural habitat in the Gulf of Mexico. Gonadosomatic indices (GSI: 100 × gonad weight/body weight) and blood samples were obtained from male and female fish every 4 hours (beginning at 1600) throughout a day on 3 separate days of the semilunar cycle: at the GSI and spawning peak (July 27–28), at midcycle (August 2–3), and on a day prior to the probable spawning peak (August 9–10). Blood samples of females were assayed for cortisol, estradiol-17β (E₂), progesterone, testosterone, thyroxine (T₄), and triiodothyronine (T₃). The daily rhythms of hormones at each of the three days during the cycle differed markedly from one another. At the spawning peak, E₂ and T₃ peaks occurred at 1200, testosterone and progesterone peaks were found at 2400 and the GSI and cortisol peaks were at 0400. The GSI decreased sharply at 0800 indicating a period of probable spawning. At midcycle, most hormone levels were low throughout the day and the amplitudes of the hormone rhythms were suppressed. One day prior to the spawning peak, testosterone and progesterone variations (peaks at 2400) were similiar to those found on the day of the first semilunar GSI and spawning peaks. Daily variations of other hormones, especially E₂, were prominent on the day prior to peak spawning but different from those found on the day of peak GSI/spawning. In males, plasma concentrations of cortisol, testosterone, T₃ and T₄ also underwent daily variations that differed from one another on the 3 days of the cycle. The results clearly demonstrate that there are dramatic changes in the daily variations of plasma hormone concentrations during the semilunar spawning cycle.

Two peaks in biomass were recorded from natural beds of Gracilaria sp. at Leganes, Iloilo and Batan, Aklan study sites. The major peak occurred in February 1989 for both areas. The minor peak occurred in September 1988 at Batan and October 1988 at Leganes. Highest biomass at Ivisan, Capiz occurred in May 1988. Lowest biomass was recorded in June at Batan and December 1988 at Leganes. At Ivisan, no biomass was recorded from November 1988 to February 1989. Correlation analysis showed no relationship between biomass and temperature or pH at all study sites. However, salinity was negatively correlated with biomass at Leganes and Batan. Rainfall was inversely correlated with biomass. Based on salinity data gathered from the three study sites, Gracilaria sp. is euryhaline and can tolerate a wide range of salinity. The results show a marked seasonality in the biomass of Gracilaria sp.

This paper reviews studies on mercury and selenium interaction. It includes the effects of selenium on mercury toxicity on the organism, organ/tissue, and subcellular levels. The paper also touches on possible mechanisms for the "protective action" of selenium against mercury toxicity and deals briefly with the synergism between the two elements.

Soybean oil, cod liver oil and coconut oil were tested singly or in combination (1:1) as a source of dietary lipid for sea bass fry. Growth and survival rates of fry fed a diet containing cod liver oil and soybean oil (1:1 ratio) were highest, followed by those fed cod liver oil alone and soybean oil alone. Fry growth and survival rates were low in the coconut oil diet and lowest in the diet containing no lipid supplement. The fatty acid profiles of the sea bass fry were influenced by the fatty acid composition of the dietary lipid sources they were fed. Sea bass fry appear to favor lipid over carbohydrate as a major source of energy and a blend of cod liver oil and soybean oil was an effective dietary lipid source. Thus, a substantial savings in feed costs can be achieved if soybean oil is substituted in part for fish oil as a dietary lipid source for sea bass fry.

The effects of three selected algal species, Tetraselmis tetrahele, Isochrysis galbana and marine Chlorella sp. on the population growth of Brachionus plicatilis was evaluated after 3, 5 and 7 days of culture. The rotifers fed on T. tetrahele showed superior growth with mean peak density of 92.5 individuals per ml to those fed on I. galbana (48.2 individuals per ml) and Chlorella sp. (47.2 individuals per ml) in 5 days. In another experiment, milkfish. Chanos chanos Forsskal, fry with initial mean body weight and standard length of 6.26 ± 1.07mg and 13.04 ± 1.40mm, respectively, were fed B. plicatilis reared on the three algal species for 30 days. Results showed that growth, measured in terms of gain in weight and length, was significantly different (P > 0.05) among treatments. Fastest growth was obtained in fry fed B. plicatilis cultured on T. tetrahele with mean gains in weight and length of 61.48 ± 8.40mg and 9.37 ± 1.68 mm, respectively, followed by fry fed B. plicatilis reared on I. galbana (43.56 ± 8.35mg and 6.25 ± 0.48mm). Milkfish fry fed B. plicatilis cultured on Chlorella sp. gave the poorest growth with mean gains in weight and length of 11.05 ± 1.82mg and 2.33 ± 0.29mm, respectively. Percentage survival ranged from 93.1 to 95.3 percent and was not significantly different between treatments (P < 0.05). The differences in population growth of B. plicatilis and consequently growth of milkfish fry was almost certainly attributed to the differences observed in the biological values of their algal diet, and in particular their protein and lipid contents. T. tetrahele and I. galbana improved the food value of B. plicatilis as live food for the milkfish fry. Their high dietary value was related to the higher algal protein and fat levels and contents of n-3 HUFA particularly 20:5n-3 and 22:6n-3 compared to B. plicatilis cultured on Chlorella sp. The results of the present study point to T. tetrahele as the most suitable food for the mass culture of the rotifer, B. plicatilis.

The salinity tolerance of freshwater-spawned and reared Oreochromis mossambicus, O. niloticus and their F₁ hybrids of various ages was studied. Several tests were conducted using three indices as practical measures of salinity tolerance: (1) mean survival time (MST); (2) median survival time (ST₅₀); and (3) median lethal salinity-96 hours (MLS-96). Interspecific and age-specific differences (P<0.01) in salinity tolerance were observed in these species and their F₁ hybrids on the basis of MST and ST₅₀ indices, with salinity tolerance generally increasing with age of brood. No significant age-specific differences (P > 0.05) in salinity tolerance were observed in all four groups on the basis of MLS-96 index. At the same salinity O. niloticus fry at ages from 15 to 90 days post-hatch exhibited significantly lower (P<0.05) salinity tolerance than O. mossambicus and F₁ hybrids. Changes in salinity tolerance were determined to be more closely related to body size than age.