The effects of three levels of stocking density (0.5, 1.5 or 3.0 m⁻²) and monosex culture (male or female) on the growth, apparent feed conversion ratio (FCR), survival, and production of mixed species of mud crabs, Scylla serrata and S. tranquebarica, were investigated. The juvenile crabs were stocked in 150 m² earthen ponds with Gracilariopsis bailinae as shelter and fed a mixed diet of 75% fresh mussel flesh and 25% fish bycatch. There was no interaction between stocking density levels and monosex culture (P<0.05) so that data were pooled for each sex or stocking density treatment. Highest survival and the most efficient FCR were obtained from stocking density of 0.5 m⁻² (P<0.05). Crab growth rates in different stocking densities were not significantly different (P>0.05). Male crabs attained significantly higher (P<0.05) final weight and specific growth rate (SGR) than female crabs. However, final crab length, width, FCR, survival, and production were not significantly different between male and female crabs (P>0.05). Highest return on investment (ROI) and lowest production cost were obtained from 0.5 m⁻². Partial budgeting analysis showed that no net benefit accrued from stocking beyond 1.5 m⁻². Both male or female monosex cultures gave high net revenue and ROI of more than 100%. Results suggest that the culture of male or female mud crabs at 0.5–1.5 m⁻² is economically viable but male monosex culture is more profitable.

Intensified production of fish involves stocking at high densities and the use of artificial feeds. These practices result in eutrophication and environmental degradation mainly because of feed wastage and fish excreta. To minimize waste, the maximum amount of feed consumed by fish must be known. Food consumption and utilization, however, may vary with the size and physical condition of the fish. Milkfish Chanos chanos Forsskal and some hatchery-produced fish, such as seabass Dicentrarchus labrax L. have been observed to have morphological defects that could affect normal food intake and utilization. Jaw abnormalities in hatchery-bred milkfish interfere with feeding and result in very slow growth. In the Philippines, milkfish fry for production in ponds and cages are caught from the wild or produced through artificial spawning. Studies have been conducted to estimate the feed ration for milkfish reared in brackish water ponds where natural food contributes significantly to the nutrition of the fish. In ponds and marine cages, where fish are largely dependent on artificial feeds, daily feed ration has to be estimated. This study determined the maximum feed ration for different sizes of wild and hatchery-bred milkfish based on assimilation of energy.

The yield (%) and viscosity (cps) of alginate from Sargassum carpophyllum, S. ilicifolium and S. siliquosum collected along the inter- and subtidal zones of Currimao, Ilocos Norte were determined monthly for a period of one year. Results show that each species demonstrated an individual pattern of alginate characteristics which is significantly influenced by the collecting zone, fertility state, and collecting month (P < 0.05). Positive correlations were observed in alginate yield and viscosity with species and fertility states. Among the three species, S. ilicifolium is the best species for alginate production for the food industry based on viscosity characteristics, followed by S. siliquosum and S. carpophyllum.

The process required and options for water treatment in recirculating systems have been clearly identified. The four most critical processes are: (1) solids removal, to take out fecal waste, uneaten feed, excess bacterial biomass, and other solids also reducing biochemical oxygen demand (BOD) in the process; (2) biological filtration or biofiltration, primarily nitrification to convert toxic ammonia and intermediate from nitrite to nitrate; (3) gas exchange, which includes aeration to ensure sufficient oxygen supply for fish and biofiltration, and degasification to stripe carbon dioxide; and what may be called (4) ion balance, primarily to maintain pH and alkalinity and prevent the buildup of nitrate, dissolved organic, and other substance. System with extended hydraulic retention times must generally have an alkalinity replenishment regime to compensate for the alkalinity-consuming nitrification process. Additional treatment processes include denitrification (nitrate removal, which helps to augment alkalinity), ozonation (to remove color, organics and BOD; also sterilizes), ultraviolet (UV) sterilization (for disease/pathogen control), and foam fractionation (to remove dissolved organics surfactants). Heaters and/or chillers may be provided for temperature control. It must be emphasized that while the processes and equipment are provided for specific purposes, a complex interrelationship exists in recirculating systems. Recirculating system components and sizing criteria vary widely, and are mainly provided to comply with specific production needs. A combined upflow and fluidized sand filter design that is applicable and easily adaptable to any system that may be recirculated is presented. Design and operational criteria are provided, as applied to a milkfish broodstock system. In general, for better management and control of water quality, to conserve water and energy and reduce wastewater, and to essentially provide for complete treatment of wastes and mitigate its impact on the environment, a high degree of water recirculation is recommended.

Penaeus monodon postlarvae (PL1, PL5, PL10, and PL15) and juveniles (0.2 and 2 g mean weights) were acclimated gradually within 6 h to test temperatures of 22°, 28°, and 33°C. After 24 h, the postlarvae were abruptly transferred to salinity levels of 8, 16, 24, 32, 40, or 50 ppt, and juveniles to 0, 4, 8, 16, 20, 24, 28, 32, 40, 50, or 60 ppt. All test animals were previously reared in 32 ppt sea water at a temperature of 27-29°C. Survival rates were compared after four days for PL1, PL5, and PL 10, and 10 days for PL 15. Tests for juveniles lasted seven days. Length and total (TPI) or mean (MSI) population stage indices were compared for postlarvae. Survival rates of postlarvae and juveniles were generally lower at 33°C. Length and MSI of postlarvae were higher at 33°C than at 22°C. TPI at PL10 and PL15 were not significantly affected by salinity. Optimal salinity ranges for postlarvae and juveniles were identified based on the different parameters measured.

Six semi-purified casein based diets were formulated to contain either a complete mineral mixture (control) or mineral premixes from which a specific test mineral was deleted to obtain phosphorus(P)-free, magnesium(Mg)-free, iron(Fe)-free, zinc(Zn)-free, or manganese(Mn)-free diets. These diets were fed to juvenile milkfish (mean initial weight 2.60±0.08g) for a 22-week experimental period. Final mean percent weight gain ranged from 1022 to 1379% with P-free (1022%) and Fe-free (1066%) diets obtaining a significantly lower weight gain (p<0.01) than the control diet (1270%). Survival was greater than 90% and did not differ significantly among treatments. Upon termination of the growth experiment, milkfish flesh, bones, and combined samples of head, skin, and scales were dissected and analyzed for ash, P, Ca, Mg, Fe, Zn, and Mn content. The deletion of P or Fe from mineral mixture lowered P content in flesh and bone. Zn content in bone of fish was also lowered by exclusion of Zn, Mn, Mg or Fe. The result of this study demonstrated that it is necessary to supplement P and Fe even to semi-purified casein based diets.

The distribution of phytoplankton and zooplankton around three sandbanks (Gootebank, Westhinder and Buitenratel sandbank) off the Belgian Coast was investigated in February 1994. The abundance of phytoplankton taxa was significantly different between the sandbanks. Community analysis using TWINSPAN resulted in a clear separation of clusters corresponding to the different sandbanks. The zooplankton community analysis, on the contrary, showed a rather indistinctive division of the sandbank stations. This was due to the omnipresence of three dominant copepod species (Temora longicornis, Pseudocalanus elongatus and Centropages hamatus). When these species were excluded from the analysis, a clearer distinction between the different sandbanks was found. The observed differences in phyto- and zooplankton species distribution could be explained by the position of the sandbanks. Westhinder is positioned further from the coast than Buitenratel, while Gootebank has an intermediate position. Buitenratel and Gootebank harbour typical coastal plankton communities, while the plankton community over Westhinder is clearly influenced by the Atlantic current penetrating the southern North Sea from the English channel. More phyto-benthic species were found at Buitenratel than at Gootebank, probably because of its limited depth. Thus, the Belgian coastal zone, which is considered as one box in most spatial descriptions of the North Sea plankton, in fact harbours heterogeneous plankton communities at the end of winter.

Major disease problems particularly viral and bacterial in origin have resulted in significant decline in the production of farmed shrimps worldwide. The major diseases of viral and bacterial origins are listed and described. Innovations being applied to prevent and/or minimize their effects are enumerated. The paper suggests a broad outlook of shrimp disease by considering not only the etiological causes but also the epidemiological approach. Attempts to produce disease-free fry are still expensive and cannot be applied widely in commercial farms. The aim is to attain sustainable shrimp farming.

Growth hormone (GH, 0.0025 and 0.025 I.U. ml⁻¹) and γ-aminobutyric acid (GABA, 50 μg ml⁻¹) enhance rotifer population growth in batch cultures. In order to further understand the mechanism of their actions, we conducted experiments culturing isolated females at low food and high free ammonia levels. At an optimum food level of 7×10⁶Nannochloropsis oculata cells ml⁻¹ or at low free ammonia level of 2.4 μg ml⁻¹, the F₁ offspring of rotifers treated with GH at 0.0025 I.U. ml⁻¹ had significantly higher population growth rate (r) and net reproduction rate (Ro), and shorter generation time than untreated rotifers. At a lower food level of 7×10⁵ cells ml⁻¹ or at high free ammonia level of 3.1 μg ml⁻¹, rotifers treated with GABA at 50 μg ml⁻¹ had significantly higher r and Ro, and shorter generation time. These results indicate that GABA is effective in enhancing rotifer reproduction when rotifers are cultured under stress whereas GH enhances rotifer reproduction when culture conditions are optimal. Significant effects were also observed in F¹ and F² generations which were not treated with hormones. These data may be useful for treating rotifer mass cultures to mitigate the effects of stress caused by high population densities.

The effects of different stocking densities on the growth, feed conversion ratio and survival of two size groups of the tropical abalone Haliotis asinina were determined. Three culture trials were conducted in net cages installed in a sheltered cove, Guimaras Province, Philippines. Trials 1 and 2 were conducted using 15–20 mm abalone juveniles for 150 days, while trial 3 was conducted using 35–40 mm abalone for 180 days. The animals were fed sufficient amounts of the red alga, Gracilariopsis bailinae (=G. heteroclada), throughout the experiment. There was an inverse relationship between growth (length and weight) and stocking density. Feed conversion ratio was not influenced by density, but was observed to be higher for larger animals. Survival was not significantly affected by density. Net cages are appropriate for culture of H. asinina. This study showed that H. asinina can reach commercial size of about 60 mm in one year. It also showed that growth of H. asinina can be sustained on a single-species diet. An economic analysis will be important in choosing the best stocking density for commercial production.