Effects of secondary crops on bacterial growth and nitrogen removal in shrimp farm effluent treatment systems

Secondary crops provide a means of assimilating some effluent nitrogen from eutrophic shrimp farm settlement ponds. However, a more important role may be their stimulation of beneficial bacterial nitrogen removal processes. In this study, bacterial biomass, growth and nitrogen removal capacity were quantified in shrimp farm effluent treatment systems containing vertical artificial substrates and either the banana shrimp Penaeus merguiensis (de Man) or the grey mullet, Mugil cephalus L. Banana shrimp were found to actively graze biofilm on the artificial substrates and significantly reduced bacterial biomass relative to a control (24.5±5.6 mg C m⁻² and 39.2±8.7 mg C m⁻², respectively). Bacterial volumetric growth rates, however, were significantly increased in the presence of the shrimp relative to the control (45.2±11.3 mg C m⁻² per day and 22.0±4.3 mg C m⁻² per day, respectively). Specific growth rate, or growth rate per cell, of bacteria was therefore appreciably stimulated by the banana shrimp. Nitrate assimilation was found to be significantly higher on grazed substrate biofilm relative to the control (223±54 mg N m⁻² per day and 126±36 mg N m⁻² per day, respectively), suggesting that increased bacterial growth rate does relate to enhanced nitrogen uptake. Regulated banana shrimp feeding activity therefore can increase the rate of new bacterial biomass production and also the capacity for bacterial effluent nitrogen assimilation. Mullet had a negligible influence on the biofilm associated with the artificial substrate but reduced sediment bacterial biomass (224±92 mg C m⁻²) relative to undisturbed sediment (650±254 mg C m⁻²). Net, or volumetric bacterial growth in the sediment was similar in treatments with and without mullet, suggesting that the growth rate per cell of bacteria in grazed sediments was enhanced. Similar rates of dissolved nitrogen mineralisation were found in sediments with and without mullet but nitrification was reduced. Presence of mullet increased water column suspended solids concentrations, water column bacterial growth and dissolved nutrient uptake. This study has shown that secondary crops, particularly banana shrimp, can play a stimulatory role in the bacterial processing of effluent nitrogen in eutrophic shrimp effluent treatment systems.     

Effect of ammonia on the growth rate and oxygen consumption of juvenile greenlip abalone, Haliotis laevigata Donovan

Juvenile greenlip abalone, Haliotis laevigata, (mean whole weight 4.48±1.9 g, mean±s.d., n=953) were highly sensitive to ammonia as indicated by depressed growth rate and food consumption measured over 2–3 months in bioassay tanks. For growth rate expressed on a whole weight basis, the EC₅ and EC₅₀ values (5 and 50% growth reductions) were 0.041 mg FAN l⁻¹ (Free Ammonia–Nitrogen) and 0.158 mg FAN l⁻¹, respectively. Shell growth rates declined over the entire experimental range (0.006–0.188 mg FAN l⁻¹). At the end of the bioassay, groups of abalone were transferred to respiratory chambers. Oxygen consumption rate increased to a maximum of 188% of control values at 0.235 mg FAN l⁻¹ and decreased slightly at the highest concentration of 0.418 mg FAN l⁻¹.     

Metabolic scope, swimming performance and the effects of hypoxia in the mulloway, Argyrosomus japonicus (Pisces: Sciaenidae)

The culture of the mulloway (Argyrosomus japonicus), like many other Sciaenidae fishes, is rapidly growing. However there is no information on their metabolic physiology. In this study, the effects of various hypoxia levels on the swimming performance and metabolic scope of juvenile mulloway (0.34 ± 0.01 kg, mean ± SE, n = 30) was investigated (water temperature = 22 °C). In normoxic conditions (dissolved oxygen = 6.85 mg l^− 1), mulloway oxygen consumption rate (M·o₂) increased exponentially with swimming speed to a maximum velocity (U_crit) of 1.7 ± < 0.1 body lengths s^− 1 (BL s^− 1) (n = 6). Mulloway standard metabolic rate (SMR) was typical for non-tuna fishes (73 ± 8 mg kg^− 1 h^− 1) and they had a moderate scope for aerobic metabolism (5 times the SMR). Mulloway minimum gross cost of transport (GCOT_min, 0.14 ± 0.01 mg kg^− 1 m^− 1) and optimum swimming velocity (U_opt, 1.3 ± 0.2 BL s^− 1) were comparable to many other body and caudal fin swimming fish species. Energy expenditure was minimum when swimming between 0.3 and 0.5 BL s^− 1. The critical dissolved oxygen level was 1.80 mg l^− 1 for mulloway swimming at 0.9 BL s^− 1. This reveals that mulloway are well adapted to hypoxia, which is probably adaptive from their natural early life history within estuaries. In all levels of hypoxia (75% saturation = 5.23, 50% = 3.64, and 25% = 1 .86 mg l^− 1), M·o₂ increased linearly with swimming speed and active metabolic rate (AMR) was reduced (218 ± 17, 202 ± 14 and 175 ± 10 mg kg^− 1 h^− 1 for 75%, 50% and 25% saturation respectively). However, U_crit was only reduced at 50% and 25% saturation (1.4 ± < 0.1 and 1.4 ± < 0.1 BL s^− 1 respectively). This demonstrates that although the metabolic capacity of mulloway is reduced in mild hypoxia (75% saturation) they are able to compensate to maintain swimming performance. GCOT_min (0.09 ± 0.01 mg kg^− 1 m^− 1) and U_opt (0.8 ± 0.1 BL s^− 1) were significantly reduced at 25% dissolved oxygen saturation. As mulloway metabolic scope was significantly reduced at all hypoxia levels, it suggests that even mild hypoxia may reduce growth productivity.     

Stocking density and shelter type for the optimal growth and survival of western rock lobster Panulirus cygnus (George)

The growth and survival of three size classes of wild caught western rock lobster, Panulirus cygnus (post-pueruli: mean 2.14 ± 0.07 g, 13.2 ± 0.1 mm CL; year 1: post-settlement juveniles, 57.1 ± 1.1 g, 38.7 ± 0.28 mm CL; and year 2 post-settlement juveniles, mean 138.2 ± 2.26 g, 51.9 ± 0.25 mm CL) were examined at combinations of two stocking densities (post-pueruli: 50 and 100 m^− 2; year 1: 11 and 23 m^− 2; year 2: 10 and 19 m^− 2) and two shelter types (a novel rigid plastic mesh shelter or bricks) over a period of 6 months. Survival of lobsters held at the lower densities (90–95%) was significantly greater than for lobsters held at higher densities (post-pueruli = 78%, year 1 = 86%, year 2 = 88%). Post-pueruli survival was significantly higher in tanks with mesh shelters (91.7%) than brick shelters (75.8%) with a similar trend exhibited by year 1 and year 2 lobsters. Densities tested did not significantly affect lobster growth for any size class. Growth of post-pueruli was considerably higher in tanks with mesh shelters (641.7% weight gain; specific growth rate 1.07 BW day^− 1) (p < 0.05) but there was no difference in the growth of year 1 and year 2 lobsters between mesh and brick shelters. Feed intake (g pellet dry matter lobster^− 1 day^− 1) was not significantly different between densities. This study has shown that P. cygnus is well suited for aquaculture based on the collection and ongrowing of wild caught pueruli, as this species exhibits good survival at high densities (up to 100 m^− 2) without adverse effects on growth, and shows no captivity-related health problems. We recommend mesh shelters, with stocking densities of 50 m^− 2 for post-pueruli and between 20 and 25 m^− 2 for year 1 and year 2 juveniles, to maximise survival and production.     

Marine shrimp aquaculture: a novel waste treatment system

Ecuadorian Penaeus vannamei were cultured in dirt ponds (each of approximately 163 m²) at four different stocking densities, i.e. 5 shrimp m⁻², 10 shrimp m⁻², 15 shrimp m⁻² and 20 shrimp m⁻². Experiments were carried out over three different periods during the year. Each experiment lasted for 11–14 weeks. No commercial feed was given to the shrimp. The only input to the ponds was about 30 kg of cattle manure per pond per week. Chemical composition of the cattle manure was analyzed. Water quality parameters such as temperature, pH, DO and turbidity were recorded twice daily for each experiment; nutrients (nitrite, nitrate, ammonium and phosphate), water ATP, sediment ATP, H₂S and chlorophyll were measured twice weekly for each experiment. Shrimp were sampled either weekly or bi-weekly for body weight measurements.

The results showed a negative correlation between stocking density and growth. Weekly growth ranged from 0·44 to 1·58 g week⁻¹. Survival was over 50% in all treatments and averaged at 70·8%. Under these stocking densities, shrimp production ranged from 4·4 to 18·8 kg ha⁻¹ day⁻¹. The stocking density of 15 shrimps m⁻² provides better production than the other stocking densities.

Water quality data did not relate to any shrimp growth. Water nutrient levels in pond discharge water were less than or equal to the nutrients in the incoming water in spite of the weekly addition of cattle manure and did not increase with the addition of cattle manure. No coliform bacteria were detected in any pond water samples through the study period. This indicates digestion of cattle manure in marine shrimp ponds would not pollute the environment with high concentrations of dissolved nutrients.

Thus, a marine shrimp pond can be considered a dissolved nutrient marine treatment plant converting unwanted cattle manure (1841 kg cattle manure ha⁻¹ week⁻¹ in this study) into a valuable commodity — shrimp.     

Graded environmental hypercapnia in juvenile spotted wolffish (Anarhichas minor Olafsen): effects on growth, food conversion efficiency and nephrocalcinosis

Growth performance and food conversion efficiency (FCE) were investigated in juvenile spotted wolffish (Anarhichas minor Olafsen), mean (S.D.) initial weight 15.7 (4.8) g, reared at four levels of carbon dioxide (CO_2(aq)) for 10 weeks at 6 °C and 33‰. CO₂ levels averaged 1.1 (control), 18.1 (low), 33.5 (medium) and 59.4 (high) mg l⁻¹, with corresponding pH values of 8.10, 6.98, 6.71 and 6.45, respectively. In addition, kidneys from sampled fish were examined macroscopically for gross signs of calcareous deposits, i.e. nephrocalcinosis, at the start and end of the experiment. Growth was significantly reduced at the highest concentration (P<0.0001), as compared to all other groups, while no overall differences in growth rate or mean weight were seen in the range of 1.1–33.5 mg CO₂ l⁻¹ at the end of the experiment. Daily feeding rates and total food consumption were reduced at the highest concentration (P<0.001), whereas food conversion efficiency did not vary significantly between groups. Plasma chloride levels displayed a significant decrease with increasing CO₂ levels, from 151.3 mmol l⁻¹ (1.1 mg CO₂ l⁻¹) to 128.3 mmol l⁻¹ (59.4 mg CO₂ l⁻¹) at the end of the experiment, whereas plasma osmolality in the high CO₂ group was significantly higher compared to the control group at the end of the experiment (371.4 and 350.8 mOsmol kg⁻¹, respectively). Nephrocalcinosis was observed in all groups at the end of the experiment, but was most pronounced in the medium and high CO₂ group.     

Filtration of king scallops (Pecten maximus)

Filtration rates of hatchery-reared king scallop (Pecten maximus L.) juveniles, fed a single species alga diet (Pavlova lutheri (Droop) Green), were measured at a range of temperatures (6–21 °C). Weight specific filtration rate (ml min⁻¹ g⁻¹ (live weight)) of juveniles of a selected size range of 17–19 mm shell height (0.26–0.36 g live weight) increased with temperature above 16 °C and decreased below 11 °C, but was not significantly different between these two temperatures. Measurements at 16 °C using juveniles with a wider size range of 10–25 mm shell height (0.05–0.8 g live weight) gave the allometric equation: filtration rate (ml min⁻¹)=12.19×weight (g)^0.887. Filtration rate decreased significantly when the cell concentration was greater than 200 cells μl⁻¹ (4.25 mg (organic weight) l⁻¹). With six other algae food species, filtration rates similar to those with P. lutheri were only achieved with Chaetoceros calcitrans (Paulsen) Takano. All other algae species tested were cleared from suspension at significantly lower rates. Experiments with diet mixtures of P. lutheri and these other algae suggested that this was usually a reflection of lowered filtration activity, rather than pre-ingestive rejection of cells. In experimental outdoor nursery rearing systems, the filtration rate was inversely proportional to the concentration of cells in the inflow, in the range 5–210 cells μl⁻¹. It was not affected by flow rate (2–130 l h⁻¹, equivalent to 0.12–28.38 l h⁻¹ g⁻¹ (live weight)) with scallop juveniles stocked from 2 to 62 g l⁻¹. The results are discussed in relation to on-growing scallops at field sites.     

Modeling industry-wide sediment oxygen demand and estimation of the contribution of sediment to total respiration in commercial channel catfish ponds

Data collected from 45 commercial channel catfish, Ictalurus punctatus, ponds were used to develop empirical models predicting sediment oxygen demand (SOD). Seven acceptable models were combined with a Monte-Carlo sampling distribution to predict industry-wide sediment oxygen demand (SOD_i). The SOD_i values obtained from the best equation were used in simulations to assess the effect of diurnally varying water column dissolved oxygen (DO) concentrations on SOD and the effect of pond water depth on the contribution of SOD to overall pond respiration. Estimated SOD_i ranged from 62 to 962 mg m⁻² h⁻¹, with a mean of 478 mg m⁻² h⁻¹. There was a 95% probability of mean SOD_i being ≥700 mg m⁻² h⁻¹. The effects of diurnal variation in DO concentration in the water column on expression of SOD was modeled by combining maximum SOD_i, an empirical relationship between DO and SOD, and simulated pond DO concentrations. At DO concentrations >15 mg l⁻¹, diel SOD in catfish ponds exceeded 20 g O₂ m⁻² day⁻¹. But when average diel DO was <4 mg l⁻¹ and the range of DO concentration was 6–8 mg l⁻¹, SOD decreased to 13 g O₂ m⁻² day⁻¹ because DO availability limited the full expression of potential SOD. Respiration totals for sediment (average SOD_i), plankton, and fish respiration were calculated for pond water depths ranging from 0.25 to 4 m. Although whole-pond respiration increases as pond depth increases, the proportion of total respiration represented by sediment decreased from 48 to 10% by increasing water depth over this range. The results of these studies show that SOD is a major component of total pond respiration and that certain management practices can affect the impact of SOD on pond oxygen budgets. Mixing ponds during daylight hours, either mechanically or by orienting ponds for maximum wind fetch, will increase oxygen supply to sediments, thereby allowing maximum expression of SOD and maximum mineralization of sediment organic matter. Given a mixed condition caused by wind or other artificial means, the construction of deeper ponds increases the total mass of DO available for all respiration, causing nighttime DO concentrations to decline at a slower rate, reducing the need for supplemental aeration. Because a pond’s water volume decreases over time from sediment accumulation, annual aeration costs will increase with pond age. Constructing ponds with greater initial depth will therefore reduce long-term cost of aeration, allow more flexible management of pond water budget, and reduce the long-term expense associated with pond reconstruction.     

Using a macroalgae Ulva pertusa biofilter in a recirculating system for production of juvenile sea cucumber Apostichopus japonicus

A simple indoor recirculating system for production of juvenile sea cucumber (Apostichopus japonicus) was operated on a commercial scale for 90 days during winter. The system consists of three 70 m³ sea cucumber rearing tanks and one biofilter tank where macroalgae (Ulva pertusa) was used as a biofilter in order to reduce water requirements. Effluent from the sea cucumber tanks drained into the macroalgae biofilter tank and were then returned to the sea cucumber tanks by a discontinuous-flow recirculation system. Survival and growth rates in the sea cucumber culture tanks were similar to those in the control tank (with one water exchange per day). The survival rate averaged about 87%. The average body weight increased from 3.5 ± 0.3 g to 8.1 ± 0.8 g and total sea cucumber biomass production over the experimental period was 745 g m⁻² after initial stocking densities of 375 g m⁻². The growth rate of U. pertusa was 3.3% day⁻¹. U. pertusa was efficient in removing toxic ammonia and in maintaining the water quality within acceptable levels for sea cucumber culture; there were only small daily variations of temperature, pH and DO. The U. pertusa tank removed 68% of the TAN (total ammonia-nitrogen) and 26% of the orthophosphate from the sea cucumber culture effluent; the macroalgae biofilter removed ammonia at an average rate of 0.459 g N m⁻² day⁻¹. It would be efficient to use the U. pertusa biofilter in a recirculating system for production of A. japonicus juveniles in winter.     

A simple test to estimate the salinity resistance of fish with specific application to O. niloticus and S. melanotheron

In order to develop a simple and accurate index of the salinity resistance of tilapia, batches of 10 juveniles (5 to 20 g) of two different species Oreochromis niloticus and Sarotherodon melanotheron reared in freshwater were subjected to gradual increases in salinity until 100% mortality. Seven daily increments of salinity were tested with 4 replicates: 2, 4, 6, 8, 10, 12 and 14 g l⁻¹ day⁻¹, while control batches were kept in fresh water. The temperature was maintained at 27 °C. The concentration of oxygen, ammonia and the pH were not limiting factors. The mortality, monitored on a daily basis, appeared after 2–51 days and was spread out over 1–20 days, depending on the increment of salinity. The higher the daily rate in salinity increase, then the shorter the time lapse before total mortality occurred. The cumulative mortality as a function of salinity fit well with simple linear regressions. The criterion of the resistance to salinity was the index MLS (median lethal salinity) defined at each daily rate as the salinity at which 50% of fish died. For S. melanotheron, the mean MLS was 123.7±3.5 g l⁻¹ whatever the daily rate in salinity. For O. niloticus, the MLS was 46.3±3.4 g l⁻¹ for daily increases in salinity ranging from 2 to 8 g l⁻¹ day⁻¹ and decreased significantly (P<0.05) above this level. The MLS-8 g l⁻¹ day⁻¹ ,which takes into account the full capacity of the fish to adapt to the increasing salinity, appeared to be a simple, optimized and efficient criterion for assessing the resistance to salinity for O. niloticus and S. melanotheron. This criterion can be a useful tool for ranking the different parental strains and hybrids of different genus and species of tilapia used in programmes of genetic selection for growth and salinity tolerance.     

Characterization of water quality factors during intensive raceway production of juvenile Litopenaeus vannamei using limited discharge and biosecure management tools

Disease epizootics have negatively affected production and expansion of the shrimp culture industry. This, along with environmental concerns regarding limited water resources and contamination of receiving streams, has caused the industry to investigate more sustainable and biosecure management practices. A study was conducted to evaluate the effect of limited water exchange on water quality, growth and survival of the Pacific white shrimp Litopenaeus vannamei postlarvae (PL) in greenhouse-enclosed raceways. Concentrations of NH₄-N did not exceed 2.0 mg l⁻¹ during this period; whereas, NO₂-N exceeded 26.4 mg l⁻¹, indicating assimilation of primary amines by primary productivity. Periodic removal of suspended solids by a common pressurized sand filter and injection of oxygen into culture water resulted in high-survival rates for both raceways (97.5 and 106.0%) with an average biomass yield of 4.29 ± 0.06 kg m⁻³. Shrimp samples collected during the nursery trial and at harvest showed no signs of bacterial or viral pathogen infections.     

Outdoor phytoplankton continuous culture in a marine fish–phytoplankton–bivalve integrated system: combined effects of dilution rate and ambient conditions on growth rate, biomass and nutrient cycling

Natural phytoplankton populations were cultured in outdoor continuous cultures using fish-farm effluents as the source of nutrients. The dilution rate was assumed to be the integrating factor of phytoplankton growth and biomass development (flux and stock). In this context, the combined effects of (i) dilution rates of the outdoor culture and (ii) ambient conditions were tested on phytoplankton growth, biomass and cycling of the major nutrient elements (C, N and P). Experiments were carried out in outdoor polyester tanks (0.7 m deep), homogenised by gentle aeration. Si/P ratio was balanced at around 5 in the inflow in order to induce diatom domination while maintaining high N and P assimilation by phytoplankton. Nutrient cycling was assessed through analyses of the different forms of particulate and dissolved nutrients in the inflow and the outflow. Culture dilution rates determined the longevity of the culture and the assimilation efficiency of nutrients. Dissolved phosphorus was the most limiting nutrient. The optimal dilution rate was approximately 0.5 day⁻¹ at 10 °C and 1.5 day⁻¹ at 20 °C with a mean diatom biomass of 9 μM P. Under these conditions, 80% of the dissolved nutrients provided to the tanks were transformed, a production of 8 g C m⁻² day⁻¹ and an assimilation rate of 0.3 g P m⁻² day⁻¹ were recorded. Assimilation by diatoms was the major pathway of nutrient cycling. During the experiment, a bottom sediment developed progressively and this also played an important role in denitrifying the excess dissolved nitrogen in the fish-farm effluent. However, the results showed that diatom biomass can collapse and we hypothesize that this was the consequence of an increase in cellular sinking rates due to cell aggregation under nutrient or light stress. Modelling approaches are needed in future research in order to determine optimal dilution rates taking into account phytoplankton growth rates, nutrient inputs and ambient conditions (e.g. light and temperature).     

Physiological changes in rainbow trout held under crowded conditions and fed diets with different levels of vitamins E and C and highly unsaturated fatty acids (HUFA)

Rainbow trout (Oncorhynchus mykiss) maintained in crowded (100 kg m^− 3) and uncrowded (20 kg m^− 3) conditions were fed 42 days with five experimental diets having different levels of vitamin E (25.6 and 275.6 mg kg diet^− 1), C (0 and 1000 mg kg diet^− 1) and HUFA (highly unsaturated fatty acids, 12.5 and 320.5 g kg diet^− 1): −E−HUFA, −E+HUFA, +E−HUFA, +E+HUFA, −C+E+HUFA. Cortisol, plasma metabolites, tissue glycogen, fish composition, and tissue fatty-acid profile were evaluated at the end of the experimental period. In general, no changes in cortisol levels were associated with crowding, although +E+HUFA and −C+E+HUFA fish showed higher levels (mean ± SE, 55.5 ± 11.1 and 78.0 ± 11.3 ng ml^− 1) as a consequence of a possible interaction between chronic crowding and diet composition. Protein and glucose con-centration in plasma displayed no effect of crowding, but liver glycogen showed a general tendency to decrease in −E−HUFA, −E+HUFA, +E−HUFA, +E+HUFA, −C+E+HUFA crowded groups (70.2 ± 2.1, 52.1 ± 2.5, 73.4 ± 7.4, 91.7 ± 3.3, 74.2 ± 8.4 mg g^− 1 tissue, respectively) compared to uncrowded groups (108.9 ± 14.2, 82.7 ± 8.8, 92.4 ± 10.7, 99.1 ± 10.0, 103.5 ± 15.6 mg g^− 1 tissue, respectively), thus proving significant in −E+HUFA fish. Variations in total lipids, triglycerides, total cholesterol and HDL as well as LDL cholesterol in plasma were manifested under crowding conditions, displaying a certain influence of vitamin E and HUFA dietary content. Final body composition, in general, showed no change attributable to fish density, but some differences associated with diet composition were found in lipid and moisture percentages of crowded fish. Liver and muscle fatty-acid profile revealed a clear effect of the dietary lipid source that was more evident in muscle than in liver at normal fish density, and in some cases this effect was modulated by dietary vitamin E and C content and fish-culture conditions.     

Performance of packed column aerator at low hydraulic loading

A laboratory-scale packed column aerator filled with ceramic Raschig rings was tested for its performance with hydraulic loadings in the range of 16–86·3 m³ m⁻² h⁻¹. Two columns of 0·19 m and 0·24 m inside diameter and packing sizes of 15, 25 and 36 mm were used. The system equation developed by previous workers for trickling filters in waste-water treatment was not generally applicable across the lower hydraulic loadings. The system coefficient incorporating the oxygen absorption coefficient (k_La) was found to vary within the range of hydraulic loadings studied. The oxygen transfer rate equation developed for surface and submerged aerators was used for estimating standard oxygen transfer efficiency of the packed column aerator, which, in this case, ranged from 6·2 to 22·6 kg O₂ kWh⁻¹.     

Food ration requirements of 0 yellowtail flounder Limanda ferruginea (Storer) juveniles

Several studies have shown that food ration can affect the growth of cultured fish. Determining the optimal food ration would help to achieve better growth and also provide direct economic benefits due to reduced food wastage, which would lead to commercial success. Therefore, we studied the effects of ration levels on growth performance of 0⁺ juvenile yellowtail flounder to determine the optimal food ration. Two experiments were conducted; the first experiment as a preliminary using ration levels of 1%, 2%, 4%, 6% body weight per day (% bw day⁻¹) held at 7.0 °C with a stocking density of 0.95 kg m⁻² (45% bottom coverage). Results of this preliminary experiment indicated that fish fed with 1% bw day⁻¹ had significantly lower growth (weight, length, body depth and specific growth rates (SGR)) than those fed with 2%, 4% and 6% ration. However, fish fed with rations of 1% and 2% showed significantly lower gross food conversion ratios (GFCR) than fish fed with 4% and 6% rations. Survival was not significantly affected by different ration levels. Based on these preliminary results, we used ration levels of 1%, 1.5%, 2% and 3% for the main experiment. Fish were held at 10 °C with a stocking density of 1.45 kg m⁻² (34% bottom coverage). Results indicated that fish fed with 1%, 1.5% and 2% bw day⁻¹ had significantly lower growth than fish fed with 3% bw day⁻¹. GFCR was significantly different for all four rations. It was lower for 1% than 1.5%, 2% and 3% rations. Survival was not significantly different between any treatments. We discuss our results with emphasis on growth and economics (i.e., feed wastage) and stress the need to balance both components in a commercial operation.     

Effects of body weight, water temperature and ration size on ammonia excretion by the areolated grouper (Epinephelus areolatus) and mangrove snapper (Lutjanus argentimaculatus)

The effects of body weight, water temperature and ration size on ammonia excretion rates of the areolated grouper Epinephelus areolatus and the mangrove snapper Lutjanus argentimaculatus were investigated. Under given experimental conditions, L. argentimaculatus had a higher weight-specific ammonia excretion rate than E. areolatus. Weight-specific ammonia excretion rates of fasted individuals of both species showed an inverse relationship with body weight (W, g wet wt.), but a positive relationship with water temperature (t, °C). The relationships for total ammonia nitrogen (TAN) were: E. areolatus: TAN (mg N kg⁻¹ d⁻¹)=21.4·exp^0.11t·W^−0.43 (r²=0.919, n=60); L. argentimaculatus: TAN (mg N kg⁻¹ d⁻¹)=121.5·exp^0.12t·W^−0.55 (r²=0.931, n=60). Following feeding, the weight-specific ammonia excretion rate of E. areolatus increased, peaked at 2 to 12 h (depending on temperature), and returned to pre-feeding levels within 24 h. A similar pattern was observed for L. argentimaculatus, with a peak of TAN excretion being found 6 to 12 h after feeding. Stepwise multiple regression analysis indicated that weight-specific TAN excretion rates of both species increased with increasing temperature and ration (R, percent body wt. d⁻¹): E. areolatus: TAN (mg N kg⁻¹ d⁻¹)=22.8·t−28.8·R−378.2 (r²=0.832, n=24); L. argentimaculatus: TAN (mg N kg⁻¹ d⁻¹)=22.9·t−25.4·R−216.4 (r²=0.611, n=24). The effect of body weight on weight-specific postprandial TAN excretion was not significant in either species (p>0.05). This study provides empirical data for estimating ammonia excretion of these two species under varying conditions. This has application for culture management.     

The effects of density, light and shelter on the growth and survival of African catfish (Clarias gariepinus Burchell, 1822) fingerlings

The effects of stocking density, light and shelter on the growth and survival of Clarias gariepinus fingerings was evaluated. In this experiment African catfish with initial individual mean weight 0.79±0.1 g were reared at two different stocking densities—5 fish l⁻¹ and 10 fish l⁻¹ in either sheltered or unsheltered tanks with reduced and normal light condition. In all conditions growth rate was significantly affected by stocking density. The growth rate was significantly higher at low densities and in reduced light conditions where shelter was provided. Survival rate was in excess of 79% in all treatments and was not affected by treatment.     

A laboratory scale recycling water unit for tilapia breeding

The technical features of a laboratory scale water recycling unit for experimental small scale tilapia breeding are described. Two units (1 and 2) were operated during a 6 month period, carrying a similar fish load (7·5 kg) and feeding rate (2% fish body weight/day). Unit 1 received natural illumination, while unit 2 was artificially illuminated (14/10 - light/dark cycle). Both units were equipped with a biological filter bed (substrate surface area, 3500 cm²). In unit 1, total ammonium and nitrite concentrations ranged from 0·05 to 0·5 mg liter⁻¹, while nitrate varied between 10–40 mg liter⁻¹. In unit 2 corresponding values were 0·15-3 mg liter⁻¹, 0·05–0·8 mg liter⁻¹ and 10–40 mg liter⁻¹. Temperatures ranged between 20–29°C and pH values between 7·5–6·9 in both units. Dissolved oxygen concentrations decreased gradually from 5·6 to 3·4 mg liter⁻¹ in unit 1 and from 5·6 to 2·6 mg liter⁻¹ in unit 2. Twenty-six spawnings occurred in unit 1 in March and April, while only eight spawnings occurred in unit 2, possibly because of the absence of sunlight. The significance of these results are discussed.     

Protein, carbohydrate, lipid and chlorophyll a content in Isochrysis aff. galbana (clone T-Iso) cultured with a low cost alternative to the f/2 medium

Growth, proximate composition and chlorophyll a content was estimated in static cultures of Isochrysis aff. galbana (clone T-Iso) to compare the quality and quantity of the biomass produced with an agricultural fertilizer medium versus f/2 medium. Culture of microalgae was done with controlled temperature, air flux of 4.5 l min⁻¹, salinity of 33‰ and an irradiance of 110±3 μmol m⁻² s⁻¹. Daily samples from the cultures were taken to estimate cellular density, proximate composition and chlorophyll a content during 7 days. Mean cellular density from samples with f/2 medium and with agricultural fertilizer were similar (no significance at P≤0.05), and mean growth rates of 0.62 and 0.61 doubling per day, respectively. Maximum similar values of protein content of 7.3 and 7.6 pg per cell were estimated on the 5th day in f/2 and agricultural fertilizer medium, respectively. Carbohydrate and lipid content decreased during the first 5 days corresponding to the exponential growth in both experiments. Higher content of carbohydrate (no significance at P≤0.05) was found in both media during the first 2 days, after 4 days the carbohydrates reduced three times theirs concentrations. Generally lipid contents in the cultures with agricultural fertilizer were significantly higher (P≤0.05) than f/2 medium. The chlorophyll a content increased exponentially during the culture, and its concentration was similar in both experiments. In conclusion, the cellular density and the biochemical constituents were similar when using either media. This fertilizer can be used in the production of I. galbana (T-Iso) employed in aquaculture.     

Performance of the amazonian tambaqui, Colossoma macropomum, in pond polyculture

Tambaqui (48%) was reared in polyculture with grass carp (27%) and curimbatá (25%) from 75 g mean weight for a period of 164 days. Ponds of 1200 m² were stocked at a total density of 12720 ha⁻¹. Fish were fed with an experimental diet made from soybean and maize at a daily rate calculated as 3% of the tambaqui biomass. Grass carp received chopped angola grass at a daily rate of 15% of live weight. Ponds were weekly manured with 120 kg of mixed cattle and goat manure. Tambaqui reached a final weight of 492 g. Survival rate was 83%. The experimental polyculture had a mean net yield of 7·5 t ha⁻¹ year⁻¹, with apparent conversion rates of 1·13, 5·5 and 7·7 kg kg⁻¹ of diet, grass and manure, respectively. All three species had a high growth rate (from 1·2 to 2·1 % day⁻¹). Tambaqui's growth was affected by the fall in water temperature in the second part of the experiment. Cost/benefit calculations proved the high profitability of the tested semi-intensive polyculture system.