Cyclical fluctuations in ammonia and nitrite-nitrogen resulting from the feeding of turbot, scophthalmus maximus (L.), in recirculating systems

Daily and weekly cyclical fluctuations in ammonia (NH₄---N μg litre⁻¹) and nitrite---nitrogen (NO₂---N μg litre⁻¹) were shown to occur in marine recirculating systems containing juvenile turbot, Scophthalmus maximus (L.). A stepwise multivariant regression analysis, computed for the prefeeding morning base levels, showed that ammonia---nitrogen concentration in the fish tanks was related to inflow ammonia---nitrogen concentration, water flow rate, water temperature, mean fish weight and fish biomass. Influent ammonia concentration and water flow rate were by far the most important variables accounting for 68·0% and 16·7% of the variation in the regression respectively.

The fluctuations, which occurred at concentrations thought to be tolerated by the fish, were attributed to the amount and timing of feeding. The significance of the existence of such fluctuations was discussed together with the importance of their amplitude and frequency.     

The oxygen consumption rate of Atlantic salmon (Salmo salar L.) reared in a single pass landbased seawater system

The oxygen consumption rates at 15.00 h of Atlantic salmon postsmolt (200–850 g) reared under normal production conditions in a 450-m³ landbased circular tank were measured and multiple regression analysis applied to obtain a model for predicting metabolic rates directly applicable to landbased fish farms. The water temperature was in the range 5–9°C.

The oxygen consumption rate was found to be a function of fish size and temperature in agreement with the results of earlier investigations on salmonids. However, on average the rates of oxygen consumption were 1·4–2·3 times higher than those given by an earlier model for Pacific salmon and the 95% confidence intervals for the predictions were broad. This is an important consideration when a landbased system is in the planning phase.     

Ammonia and pH effects on some metabolic parameters and gill histology of silver catfish, Rhamdia quelen (Heptapteridae)

The aim of the present study was to assess the effect of ammonia exposure at different pH on survivorship and metabolic parameters in the liver, muscle and gill histology of silver catfish (Rhamdia quelen). The 96 h-LC50 of un-ionized ammonia (mg L^− 1) at pH 6.0, 7.5 and 8.2 were: 0.44 (C.I. 0.38–0.49), 1.45 (C.I. 1.25–1.65) and 2.09 (C.I. 1.85–2.36), respectively. Survival of juveniles exposed to different ammonia levels was altered by pH, and fish exposed to all ammonia levels and different pH showed muscle glucose, muscle and liver glycogen reduction. Liver glucose and muscle and liver lactate levels increased in all fish exposed to ammonia as compared to the control. Exposure to waterborne ammonia increased total ammonia levels in both tissues and also induced gill epithelium damages such as lamellar fusion and edema as compared with controls at different pH. Silver catfish exposed to pH 6.0 and different NH₃ levels presented significantly higher hepatic glucose and protein levels when compared to those maintained at low NH₃ levels. Juveniles exposed to NH₃ levels at pH 7.5 and 8.2 showed lower hepatic protein levels compared to those maintained at low NH₃ levels. These parameters are indicative of pH dependence on ammonia toxicity in silver catfish. The metabolic parameters and gill histology may be used as early indicators of ammonia toxicity in silver catfish.     

Measurements of oxygen consumption and ammonia excretion of Atlantic salmon (Salmo salar L.) in commercial-scale, single-pass freshwater and seawater landbased culture systems

The oxygen consumption of Atlantic salmon was measured in large culture tanks for a period of 20 months from the parr to the adult stage. In addition, diurnal sampling was conducted for estimation of both oxygen consumption and ammonia excretion. The oxygen consumption was affected especially by temperature, season and smoltification. For parr the oxygen consumption rate was 1–6 mg O₂/kg min and the ammonia excretion rate was 0·037–0·13 mg N/kg min from autumn to spring. The corresponding rates for adult salmon during the period October to July were 1·5–4·5 mg O₂/kg min and 0·075–0·13 mg N/kg min.     

Comparison of three fish-loading systems to harvest food-size channel catfish (Ictalurus punctatus)

Three fish-loading systems were used to harvest food-size channel catfish (Ictalurus punctatus). Loading rates of equipment and their effect on post-harvest fish survival and weight gain during cool and warm weather were studied.

The turbine pump loading rate (300·7 kg/min) was greater than the lift net (158·00 kg/min) and vacuum pump (54·1 kg/min), while the lift net was greater than the vacuum pump (p < 0·05).

Mean survivals (± SD) for catfish loaded by lift net, turbine pump and vacuum pump from earthen ponds during cool and warm weather were 96·6% (±2·8), 93·9% (±7·4) and 97·7% (±1·1) and 94·0% (±5·7), 85·3% (±8·6) and 89·0% (±12·5), respectively. Mean weight change (± SD) per catfish loaded by the same equipment during cool-and warm-weather tests were −8·2 g/fish (±31·1), −12·4 g/fish (±15·2) and −17·5 g/fish (±19·9), and 67·6 g/fish (±60·6), 33·9 g/fish (±137·4) and 32·4 g/fish (±142·1), respectivley. There were no significant differences among loading systems for survival or mean weight change per fish. The turbine pump is a possible alternative to the traditional life net as a means of loading food-size channel catfish. Further testing on commercial catfish farms is required.     

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.     

Shipping studies with juvenile and adult Malaysian prawns Macrobrachium rosenbergii (de Man)

Four studies examined shipping factors of packing technique, density, duration, type of water and use of habitat material for shipping juvenile and adult prawns Macrobrachium rosenbergii. Prawns were shipped in double polyethylene lined 38 × 38 × 20 cm deep styrofoam boxes containing oxygenated water. At temperatures of 19–20°C, 17 g prawns could be shipped safely for 42 h at a density of 10–12 prawns per box (12–15 g liter⁻¹ shipping water). Juveniles, mean size about 6 g, could be shipped at a density of 40 per box (18 g liter⁻¹) for 24 h or 20–25 per box (9–11 g liter⁻¹) for 48 h. Use of mesh material to increase surface area in the box did not appear beneficial nor did shipping in brackish water (salinity8‰). Adults packed unrestricted resulted in survival rates substantially higher than those obtained from immobilized prawns wrapped in mesh.

During the shipment, pH and dissolved oxygen concentrations decreased whereas ammonia concentrations increased. The decreased pH levels may have reduced the ammonia toxicity by decreasing the amount of toxic unionized ammonia (NH₃) in solution. In general, dissolved oxygen concentrations appeared more closely related to survival rates than did other water quality parameters which were measured.     

Calculation of pH in fresh and sea water aquaculture systems

A procedure for the calculation of pH in fresh and salt waters has been developed. The method is based on a fourth-order polynomial relationship between hydrogen ion concentration and other (conservative) water quality parameters. The method avoids trial and error estimations and results in a direct calculation procedure that can be implemented in models developed in various modeling environments, such as spreadsheets, conventional programming languages (BASIC, C, FORTRAN, PASCAL, etc.) or specialized modeling languages (Extend^™, Stella^™).

The method developed is based on the solution of the full alkalinity-pH equation. Because of the need for simplification of the equations to yield explicitly solvable polynomial equations, the accuracy of the solutions depends on the simplification made and varies with water properties. Three simplifications are tested based on a second-, a third-and a fourth-order polynomial equation for hydrogen ion concentrations. the equations have been tested for salinities ranging from 0 to 35‰ (fresh to sea water), for temperatures ranging from 0 to 35°C, for total carbonate carbon concentrations of 0·1 and 5·0 mmol/liter, and for total ammonia nitrogen concentrations of 0 and 10 mg/liter. Approximations are most accurate in waters of high total carbonate carbon and low ammonia concentrations, where the fourth-order approximation yields results that are within 0.05 pH units for the full range of pH values tested (5–10).     

Information on Selected Water Quality Characteristics for the Production of Black Sea Bass,Centropristis striata,Juveniles

Abstract

Recent interest in the culture of black sea bass, Centropristis striata, has led to questions regarding the environmental requirements of this species for economically-viable production. Here, we present the results of short-term laboratory studies on ammonia toxicity, nitrite toxicity and oxygen consumption rates. Fifty percent of the juveniles died when exposed to 0.7-0.8 mg/L un-ionized ammonia-N for 24 hours (25°C, 23 g/L salinity). All exposed fish survived exposure to ≤ 0.6 mg/L un-ionized ammonia-N for 10 days. Black sea bass juveniles survived 24-hours exposure to 250 mg/L nitrite-N when exposed in 12 or 20 g/L salinity. Fish exposed for 24 hours to nitrite in 35 g/L salinity died when exposed to 250 mg/L nitrite-N, but survived exposure to 100 g/L nitrite-N. All fish exposed to 50 mg/L nitrite-N for 10 days survived (salinities of 12, 20 and 35 g/L). Mean oxygen consumption rate was 0.09 mg/hour/g (25°C). Fish weight significantly affected oxygen consumption rate with larger fish consuming less oxygen per unit time on a weight-specific basis. No treatment effect was observed due to salinity or dissolved oxygen concentration. The information reported here in combination with previous reports provides insight into the environmental requirements of black sea bass culture.     

Effect of increasing salinity on physiological response in juvenile scallops Argopecten purpuratus at two rearing temperatures

Argopecten purpuratus can be cultivated using Recirculating Aquaculture Systems (RAS) as a method to increase production. In order to determine physiological response of A. purpuratus under different salinities and temperature conditions, two groups of juvenile scallops (small: h = 6.5 mm and large: h = 25.5 mm) were acclimated and close-cultured at salinities of 34, 38, and 42 g/l, at 16 and 22 °C and fed on Isochrysis galbana and Chaetoceros calcitrans. Survival, shell growth and scope for growth were determined at the end of the trials. Survival showed an inverse relationship with temperature and ammonia levels. In small scallops an increase in salinity at 16 °C increased survival. However, this relationship was not evident at 22 °C. On the other hand, salinity did not affect survival of large juveniles. Small juveniles had a lower survival (approximately 40%) than larger scallops (up to 85%) throughout the trials. Oxygen consumption was not affected by salinity. Small scallops showed similar oxygen consumption at 16 and 22 °C but in large juveniles higher values were registered at 22 °C. In large juveniles routine consumption at 16 °C was higher (up to 35%) than standard consumption. This pattern was not evident at 22 °C, suggesting that oxygen demand is higher regardless of feeding condition. NH₄⁺–N excretion rate is inversely related to salinity. Only small juveniles showed a higher NH₄⁺–N excretion rate at 22 °C. Scope for growth was positive in all treatments, although the upper limit of salinity should not be based only in this index. Higher scope for growth values at 38 and 42 g/l was related with a reduction in ammonia excretion and high absorption efficiency. In addition, an increase in salinity produced a reduction in NH₃–N proportion and under hypersaline conditions scallops tended to decrease excretion as a way of osmoconformation. This explains our findings of higher survival rates at higher salinities. Even though the scope for growth is positive at 42 g/l, the osmotic stress reduces the survival chances. The data obtained can be considered useful information for A. purpuratus culture under controlled conditions.     

A partial-reuse system for coldwater aquaculture

A model partial-reuse system is described that provides an alternative to salmonid production in serial-reuse raceway systems and has potential application in other fish-culture situations. The partial-reuse system contained three 10 m³ circular ‘Cornell-type’ dual-drain culture tanks. The side-wall discharge from the culture tanks was treated across a microscreen drum filter, then the water was pumped to the head of the system where dissolved carbon dioxide (CO₂) stripping and pure oxygen (O₂) supplementation took place before the water returned to the culture tanks. Dilution with make-up water controlled accumulations of total ammonia nitrogen (TAN). An automatic pH control system that modulated the stripping column fan ‘on’ and ‘off’ was used to limit the fractions of CO₂ and unionized ammonia nitrogen (NH₃---N). The partial-reuse system was evaluated during the culture of eight separate cohorts of advanced fingerlings, i.e., Arctic char, rainbow trout, and an all female brook trout × Arctic char hybrid. The fish performed well, even under intensive conditions, which were indicated by dissolved O₂ consumption across the culture tank that went as high as 13 mg/L and fish-culture densities that were often between 100 and 148 kg/m³. Over all cohorts, feed conversion rates ranged from 1.0 to 1.3, specific growth rates (SGR) ranged from 1.32 to 2.45% body weight per day, and thermal growth coefficients ranged from 0.00132 to 0.00218. The partial-reuse system maintained safe water quality in all cases except for the first cohort—when the stripping column fan failed. The ‘Cornell-type’ dual-drain tank was found to rapidly (within only 1–2 min) and gently concentrate and flush approximately 68–88% (79% overall average) of the TSS produced daily within only 12–18% of the tank’s total water flow. Mean TSS concentrations discharged through the three culture tanks’ bottom-center drains (average of 17.1 mg/L) was 8.7 times greater than the TSS concentration discharged through the three culture tanks’ side-wall drains (average of 2.2 mg/L). Overall, approximately 82% of the TSS produced in the partial-reuse system was captured in an off-line settling tank, which is better TSS removal than others have estimated for serial-reuse systems (approximately 25–50%). For the two cohorts of rainbow trout, the partial-reuse system sustained a production level of 35–45 kg per year of fish for every 1 L/min of make-up water, which is approximately six to seven times greater than the typical 6 kg per year of trout produced for every 1 L/min of water in Idaho serial-reuse raceway systems.     

Influence of ammonia on growth of Penaeus monodon Fabricius post-larvae

Abstract. Tiger shrimp, Penaeus monodon Fabricius, post-larvae (32·0±3mg, 1·43±0·03 cm) were exposed to control, 0·12, 0·60, 1·20 and 2·40mg/l ammonia-N (un-ionized plus ionized ammonia as nitrogen) which is equivalent to control, 6,32,63 and 126μg/1 NH₃-N (un-ionized ammonia as nitrogen) for 8 weeks in 25 ppt, as pH of 7·85–8·18 and 26–28°C by static renewal method. Growth in weight and length of the shrimps exposed to 1·20 and 2·40 mg/l ammonia-N were significantly lower (P < 0·05) than those exposed to control. The EC₅₀ (concentration that reduced growth by 50% of that of the controls) was 1·33 mg/l ammonia-N, 70 μg/l NH₃-N for weight gain, and 2·35 mg/1 ammonia-N, 123μg/l NH₃-N for length gain of P. monodon post-larvae. The'maximum acceptable toxicant concentration'(MATC) of ammonia-N and NH₃-N for P. monodon post-larvae was 0·60mg/l and 32μg/l, respectively after 6 weeks of exposure.     

Histopathology of rainbow trout, Salmo gairdneri Richardson, exposed to diurnally fluctuating un-ionized ammonia levels in static-water ponds

Abstract. Rainbow trout, Salmo gairdneri Richardson, were reared intensively in nine aerated static-water ponds. Dissolved oxygen concentrations were maintained above 5 mg/l and un-ionized ammonia (NH₃) levels were monitored in all ponds. Fish were removed periodically and gill, liver and trunk kidney samples were removed for histological examination. Average NH₃ concentrations did not exceed a previously reported chronically toxic level, but average daily maximum concentrations exceeded this level in all but two ponds. Gills exhibited lesions characteristic of those attributed to ammonia exposure in all ponds and fish with the most damaged gills had livers demonstrating reduced glycogen vacuolation. No histological changes were noted in kidney tissue. Because gills were damaged even though average NH₃ levels were quite low, it is recommended that research in static water carefully consider the cyclical nature of NH₃ concentrations.     

Acute Effect of Ammonia and Nitrite on Respiration of Penaeus setiferus Postlarvae under Different Oxygen Levels

The white shrimp Penaeus setiferus is a potential species for culture in the Gulf of Mexico. It has been demonstrated that postlarval P. setiferus is a species sensitive to ammonia and nitrite. In this study, the effect of ammonia and nitrite on the oxygen consumption of Penaeus setiferus postlarvae subjected to different dissolved oxygen concentrations was investigated. Un-ionized ammonia tended to reduce the oxygen consumption of P. setiferus postlarvae. The addition of 0.4 mg/L un-ionized ammonia-N modified the response of the postlarvae to lower dissolved oxygen levels. Exposure to 0.7 mg/L un-ionized ammonia-N caused severe effects on the respiratory rate and it was lethal at low dissolved oxygen concentrations. Nitrite exposure decreased the respiratory rate of post-larvae in high dissolved oxygen concentrations, whereas postlarvae exposed to nitrite under hypoxic conditions showed higher oxygen consumption than unexposed animals. The decreases of the oxygen consumption induced by the interactive effect of ammonia or nitrite and low dissolved oxygen concentration indicate a harmful condition. The results indicate that P. setiferus postlarvae is a sensitive organism to the combined effect of un-ionized ammonia or nitrite and low dissolved oxygen concentrations.     

Acute Toxicity of Ammonia and Nitrite to White Shrimp Penaeus setiferus Postlarvae

Penaeus setiferus postlarvae were exposed to acute levels of ammonia, nitrite, and to a mixture of both by a short-term static method, The 24h, 48-h and 72-h LC50 values for un-ionized ammonia were 1.49, 1.21 and 1.12 mg/L NH₃-N (un-ionized ammonia as nitrogen), and 11.55, 9.38 and 8.69 mg/L ammonia-N (un-ionized plus ionized ammonia as nitrogen). The 24-h, 48-h and 72-h LC50 values for nitrite were 268.06, 248.84 and 167.33 mg/L nitrite-N (nitrite as nitrogen). Nitrite was much less toxic than ammonia. The joint effect of ammonia and nitrite on the postlarvae was synergistic at 48-h exposure and antagonistic after 72 h. Postlarvae of P. setiferus may be considered as organisms sensitive to ammonia and nitrite.     

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.     

Energy and nitrogen partitioning in 250 g Atlantic cod (Gadus morhua L.) given graded levels of feed with different protein and lipid content

The effects of feed intake level on energy and nitrogen partitioning were studied in juvenile Atlantic cod (250 g) fed two fish meal based diets differing in protein and lipid content (54:31 and 65:16) at 10 °C. Replicate groups of cod were feed deprived for 32 days or fed one of the two diets at 25, 50, 75 or 100% of group satiation for 60 days. Feed intake and oxygen consumption were measured daily and weights and chemical composition of carcass, liver, viscera and whole body were measured at start and end. Diet digestibilities were assessed in a separate experiment.

The whole body and carcass growth rates at a given feed intake did not differ between dietary groups, but the liver grew faster in the fish fed the low protein diet, resulting in higher hepatosomatic indices at the end of the experiment in the groups fed this diet.

The efficiency of utilisation of digestible nitrogen for growth (k_DNg) was higher for the low protein diet (0.73 ± 0.02) than for the high protein diet (0.53 ± 0.05), resulting in higher nitrogen retention at a given nitrogen intake. No difference in percentage nitrogen retention was seen in full-fed fish however (31.2 ± 2.5 and 28.4 ± 1.6% for the low protein and high protein diets, respectively). This can be explained by higher nitrogen intake in the fish fed the high protein diet, resulting in a smaller proportion of the intake being used for maintenance.

There was no difference in energy utilisation between dietary groups. The digestible energy requirement for maintenance (DE_maint) was 53.8 ± 0.9 kJ kg^− 1 d^− 1 (42.3 ± 0.7 kJ kg^− 0.8 d^− 1) and the utilisation efficiency for growth (k_DEg) was 0.80 ± 0.02. The energy retention in full-fed fish was 31.3 ± 3.5 and 31.7 ± 1.0% for the low protein and high protein diets, respectively. The deposited energy was distributed in approximately equal proportions in the liver and carcass, whereas viscera accounted for a minor proportion. At a given energy intake, the fish fed the high protein diet deposited more energy in the carcass and less in the liver than did those fed the low protein diet.     

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.     

The pathology of chronic ammonia toxicity in rainbow trout, Salmo gairdneri Richardson

Abstract. Two groups of rainbow trout fingerlings were exposed for 90 days to 0.2 and 0.4 mg/1 of un-ionized ammonia, respectively. Several fish exposed to the higher concentration soon developed clinical signs suggestive of a neurological dysfunction but subsequently recovered. No lesion attributable to ammonia was seen in the gills of any of the fish. This raises questions about the precise role of ammonia in the production of gill diseases in intensively cultured trout.     

Fish mortality due to acute ammonia exposure

Abstract. Fish mortality in the River Umia, near Villagarcia de Arosa, Pontevedra, Spain, which occurred after a discharge from a food sewage works, is studied. Physico-chemical parameters of water samples from sewage and from the River Umia were studied and concentrations of ammonia sufficiently high to cause toxic effects in fish (702 mg/1 and 302 mg/1 as total ammonia) were found. The toxicity of two concentrations of ammonia was investigated on goldfish, Carassius auratus (L.), using water quality values the same as found in the River Umia, to obtain the same concentrations of un-ionized ammonia (2·13 mg/1 and 0·91 mg/1). Fish exposed to high concentrations died within 24 h and three of six, which were exposed to the lowest concentration died within 96h. All fish at first showed symptoms of hyper excitability and hyperventilation, and at times of rest showed a decrease in respiration. Post-mortem findings generally included gill congestion and haemorrhage.