Effects of acute and chronic toxicity of unionized ammonia on mud crab, Scylla serrata (Forsskål, 1755) larvae

While the effects of ammonia on fish and prawn larvae are well documented, little is known of its effect on mud crab (Scylla serrata) (Forsskål, 1755) larvae. Two experiments were conducted in 5 L hemispherical plastic bowls, containing 3 L of ultra‐filtered and settled seawater and various larval stages of mud crab to (1) determine the acute median lethal concentration (LC₅₀) of unionized ammonia and (2) to determine the chronic effects of unionized ammonia on survival and percentage moulting to zoea and megalop stages. The larval stages that exhibited the highest tolerance to ammonia over 24 h were zoea 1 (LC₅₀ of 4.05 mg L^?1 of unionized ammonia) and zoea 5 (LC₅₀ of 6.64 mg L^?1 of unionized ammonia). The megalop stage had the lowest total ammonia LC₅₀ at both 24 and 48 h, making it the larval stage most susceptible to total ammonia. Exposure to 6.54 mg L^?1 of unionized ammonia resulted in 100% death of all larvae within 24 h. The tolerance of S. serrata larvae to total ammonia did not appear to increase with ontogenetic development. The results indicate that the concentrations at which total ammonia produces an acute or chronic response in mud crab larvae are far higher than those experienced in current larval production systems (0–0.5 mg L^?1 of total ammonia) used as industry standards in Australia.     

Ammonia tolerance of Litopenaeus vannamei (Boone) larvae

The tolerance of Litopenaeus vannamei larvae to increasing concentrations of total ammonia nitrogen (TAN) using a short‐term static renewal method at 26°C, 34 g L^?1 salinity and pH 8.5 was assessed. The median lethal concentration (24 h LC₅₀) for TAN in zoea (1‐2‐3), mysis (1‐2‐3) and postlarvae 1 were, respectively, 4.2‐9.9‐16.0; 19.0‐17.3‐17.5 and 13.2 mg L^?1TAN (0.6‐1.5‐2.4; 2.8‐2.5‐2.6 and 1.9 mg L^?1 NH₃‐N). The LC₅₀ values obtained in this study suggest that zoeal and post‐larval stages are more sensitive to 24 h ammonia exposure than the mysis stage of L. vannamei larvae. On the basis of the ammonia toxicity level (24 h LC₅₀) at zoea 1, we recommend that this level does not exceed 0.42 mg L^?1 TAN – equivalent to 0.06 mg L^?1 NH₃‐N – to reduce ammonia toxicity during the rearing of L. vannamei larvae.     

Acute toxicity of nitrite on white shrimp Litopenaeus vannamei (Boone) juveniles in low‐salinity water

The nitrite toxicity was estimated in juveniles of L. vannamei. The 24, 48, 72 and 96 h LC₅₀ of nitrite‐N on juveniles were 8.1, 7.9, 6.8 and 5.7 mg L^?1 at 0.6 g L^?1; 14.4, 9.6 8.3 and 7.0 mg L^?1 at 1.0 g L^?1; 19.4, 15.4, 13.4 and 12.4 mg L^?1 at 2.0 g L^?1 of salinity respectively. The tolerance of juveniles to nitrite decreased at 96 h of exposure by 18.6% and 54.0%, when salinity declined from 1.0 to 0.6 g L^?1 and from 2.0 to 0.6 g L^?1 respectively. The safe concentrations at salinities of 0.6, 1.0 and 2.0 g L^?1 were 0.28, 0.35 and 0.62 mg L^?1 nitrite‐N respectively. The relationship between LC₅₀ (mg L^?1), salinity (S) (g L^?1) and exposure time (T) (h) was LC₅₀ = 8.4688 + 5.6764S – 0.0762T for salinities from 0.6 to 2.0 g L^?1 and for exposure times from 24 to 96 h; the relationship between survival (%) and nitrite‐N concentration (C) for salinity of 0.6–2.0 g L^?1, nitrite‐N concentrations of 0–40 mg L^?1 and exposure times from 0 to 96 h was as follows: survival (%) = 0.8442 + 0.1909S – 0.0038T – 0.0277C + 0.0008ST + 0.0001CT–0.0029SC, and the tentative equation for predicting the 96‐h LC₅₀ to salinities from 0.6 to 35 g L^?1 in L. vannamei juveniles (3.9–4.4 g) was 96‐h LC₅₀ = 0.2127 S² + 1.558S + 5.9868. For nitrite toxicity, it is shown that a small change in salinity of waters from 2.0 to 0.6 g L^?1 is more critical for L. vannamei than when wider differences in salinity occur in brackish and marine waters (15–35 g L^?1).     

Effect of ammonia and nitrite on vigour,survival rate,moulting rate of the blue swimming crab <Emphasis Type="Italic">Portunus pelagicus</Emphasis> zoea

The effects of ammonia and nitrite on vigour, survival rate, moulting rate of zoea of blue swimming crab, Portunus pelagicus, were studied. A total of five nitrite-N treatments (26.67, 53.34, 106.68, 213.36, 426.72 mg/l) and a control (no nitrite-N added) were set up for the acute nitrite-N toxicity experiment; a total of five ammonia-N treatments (8.43, 16.86, 33.72, 67.44, 134.88 mg/l) and a control (no ammonia-N added) were set up for the acute ammonia-N toxicity experiment. The results showed that the vigour, survival rate and moulting rate of zoea of the blue swimming crabs exposed to over 53.34 mg/l were significantly different (P < 0.05) from the control group. The zoea LC₅₀ values (mg/l) of nitrite-N were 179.47, 76.56, 66.70, 37.49, 25.01, 25.35, 25.34 mg/l for 24, 36, 48, 60, 72, 84, 96 h, respectively. The vigour, survival rate and moulting rate of zoea of the blue swimming crabs exposed to over 16.86 mg/l were significantly different (P < 0.05) from the control group. The zoea LC₅₀ values (mg/l) of ammonia-N were 51.04, 39.62, 38.72, 24.43, 16.90, 13.42, 11.16 mg/l for 24, 36, 48, 60, 72, 84, 96 h, respectively. The zoeae are highly sensitive to ammonia and nitrite, and the toxicity of ammonia and nitrite on Portunus pelagicus decrease with development of this crab.     

Acute Toxicity of Ammonia and Nitrite to Painted River Prawn,Macrobrachium carcinus,Larvae

This study evaluated the toxicity of ammonia and nitrite to different larval stages of Macrobrachium carcinus. Three replicated groups of larvae in the zoea stages II, V, and VIII (hence named Z₂, Z_5, and Z₈, respectively) were exposed separately to five ammonia (5, 10, 20, 40, and 80 mg total ammonia nitrogen [TAN]/L) and six nitrite concentrations (5, 10, 20, 40, 80, and 160 mg NO₂‐N/L), plus a control treatment with no addition of ammonia and nitrite, at a salinity of 20 g/L. The ammonia LC₅₀ values at 96 h for Z₂, Z₅, and Z₈ were 8.34, 13.84, and 15.03 mg TAN/L (0.50, 0.71, and 0.92 mg NH₃‐N/L), respectively, and the nitrite LC₅₀ values at 96 h for Z₂, Z₅, and Z₈ were 3.28, 9.73, and 34.00 mg NO₂‐N/L, respectively. The estimated LC₅₀ values for NO₂‐N were lower than those for TAN in most of the stages evaluated. This observation suggests that M. carcinus larvae are more tolerant to ammonia, except at Z₈, in which larvae had a higher tolerance to nitrite. Based on the lethal concentrations at 96 h, it may be concluded that the tolerance of M. carcinus to ammonia and nitrite increases with larval development. Safe levels were estimated to be 0.834 mg TAN/L (0.05 mg NH₃‐N/L) and 0.328 mg NO₂‐N/L; therefore, efforts should be made to maintain lower concentrations of these compounds throughout the larval rearing of M. carcinus.     

Improved techniques for rearing mud crab Scylla paramamosain (Estampador 1949) larvae

A series of rearing trials in small 1 L cones and large tanks of 30–100 L were carried out to develop optimal rearing techniques for mud crab (Scylla paramamosain) larvae. Using water exchange (discontinuous partial water renewal or continuous treatment through biofiltration) and micro‐algae (Chlorella or Chaetoceros) supplementation (daily supplementation at 0.1–0.2 million cells mL⁻¹ or maintenance at 1–2 millions cells mL⁻¹), six different types of rearing systems were tried. The combination of a green‐water batch system for early stages and a recirculating system with micro‐algae supplementation for later stages resulted in the best overall performance of the crab larvae. No clear effects of crab stocking density (50–200 larvae L⁻¹) and rotifer (30–60 rotifers mL⁻¹) and Artemia density (10–20 L⁻¹) were observed. A stocking density of 100–150 zoea 1 (Z1) L⁻¹, combined with rotifer of 30–45 mL⁻¹ for early stages and Artemia feeding at 10–15 nauplii mL⁻¹ for Z3–Z5 seemed to produce the best performance of S. paramamosain larvae. Optimal rations for crab larvae should, however, be adjusted depending on the species, larval stage, larval status, prey size, rearing system and techniques. A practical feeding schedule could be to increase live food density from 30 to 45 rotifers mL⁻¹ from Z1 to Z2 and increase the number of Artemia nauplii mL⁻¹ from 10 to 15 from Z3 to Z5. Bacterial disease remains one of the key factors underlying the high mortality in the zoea stages. Further research to develop safe prophylactic treatments is therefore warranted. Combined with proper live food enrichment techniques, application of these findings has sustained a survival rate from Z1 to crab 1–2 stages in large rearing tanks of 10–15% (maximum 30%).     

Nitrite Toxicity and Methemoglobin Changes in Southern Flounder,Paralichthys lethostigma,in Brackish Water

This study was performed to estimate the nitrite toxicity to southern flounder, Paralichthys lethostigma, in brackish water (7.5 ppt of salinity). For a LC₅₀ test, 20 fingerlings (5.7 ± 0.4 cm) in each aquarium (15 L) were exposed to the concentrations of 0, 1, 5, 10, 15, 30, 60, 120, and 240 mg NO₂^?‐N/L in duplication for 10 d. Median lethal concentration at 96 h (96‐h LC₅₀) was calculated as 81.6 mg NO₂^?‐N/L. For a verification test, young flounder (164.2 ± 9.1 g) were exposed to a simulated culture condition in recirculating systems (1000 L). Sodium nitrite was not added to control system, whereas it was added to Treatment system 1 (TS 1) and Treatment system 2 (TS 2) to maintain nitrite concentrations of 20 and 30 mg NO₂^?‐N/L, respectively. The plasma nitrite concentrations of the young flounder in TS 1 and TS 2 were 4.5 and 6.6 mg NO₂^?‐N/L, respectively, after 2 wk. At this time, the methemoglobin percentages in TS 1 and TS 2 reached 85.8 and 89.7%, and survival rates were 37.5 and 25.0%, respectively. The results of these tests indicate that southern flounder do not concentrate nitrite in blood from the environment, but they seem to be more sensitive to nitrite compared with other species that do not concentrate nitrite.     

Effects of temperature and salinity on the survival and development of mud crab, Scylla serrata (Forsskål), larvae

The combined effects of temperature and salinity on larval survival and development of the mud crab, Scylla serrata, were investigated in the laboratory. Newly hatched larvae were reared under 20 °C temperature and salinity combinations (i.e. combinations of four temperatures 25, 28, 31, 34 °C with five salinities 15, 20, 25, 30, 35 g L⁻¹). The results showed that temperature and salinity as well as the interaction of the two parameters significantly affected the survival of zoeal larvae. Salinity at 15 g L⁻¹ resulted in no larval survival to the first crab stage, suggesting that the lower salinity tolerance limit for mud crab larvae lies somewhere between salinity 15 and 20 g L⁻¹. However, within the salinity range of 20–35 g L⁻¹, no significant effects on survival of zoeal larvae were detected (P>0.05). The combined effects of temperature and salinity on larval survival were also evident as at low salinities, both high and low temperature led to mass mortality of newly hatched larvae (e.g. 34 °C/15 g L⁻¹, 34 °C/20 g L⁻¹ and 25 °C/15 g L⁻¹ combinations). In contrast, the low temperature and high salinity combination of 25 °C/35 g L⁻¹ resulted in one of the highest survival to the megalopal stage. It was also shown that at optimal 28 °C, larvae could withstand broader salinity conditions. Temperature, salinity and their interaction also significantly affected larval development. At 34 °C, the mean larval development time to megalopa under different salinity conditions ranged from 13.5 to 18.5 days. It increased to between 20.6 and 22.6 days at 25 °C. The effects of salinity on larval development were demonstrated by the fact that for all the temperatures tested, the fastest mean development to megalopa was always recorded at the salinity of 25 g L⁻¹. However, a different trend of salinity effects was shown for megalopae as their duration consistently increased with an increase in salinity from 20 to 35 g L⁻¹. In summary, S. serrata larvae tolerate a broad range of salinity and temperature conditions. Rearing temperature 25–30 °C and salinity 20–35 g L⁻¹ generally result in reasonable survival. However, from an aquaculture point of view, a higher temperature range of 28–30 °C and a salinity range of 20–30 g L⁻¹ are recommended as it shortens the culture cycle.     

Acute Toxicity of Ammonia and Nitrite to Sea Bream,Sparus aurata (Linnaeus, 1758), in Relation to Salinity

Sea bream, Sparus aurata, is one of the most important fish species that is commonly cultured in the Mediterranean and the eastern coasts of the Atlantic Ocean. The life cycle of sea bream in its natural habitat passes through hyposaline and hypersaline lagoons. It is important to determine the tolerance of the fish to nitrogenous compounds for aquaculture at maximum stocking densities. In the present study, a series of acute experiments were performed to evaluate the effect of salinity on ammonia and nitrite toxicity to sea bream. The fish were exposed to different ammonia and nitrite concentrations according to the static renewal methodology at three different salinities (10, 20, and 30 ppt) and at a temperature of 20 C and a pH of 8.2. The toxic effect of total ammonia nitrogen (TAN) and nitrite nitrogen (NO₂‐N) decreased with increasing salinity levels (P < 0.001). Acute toxicity (96‐h lethal concentration 50 [LC₅₀]) values of TAN were determined to be 5.93, 11.72, and 19.38 mg/L at 10, 20, and 30 ppt salinity, respectively. The 96‐h LC₅₀ values of NO₂‐N were determined to be 370.80, 619.47, and 806.33 mg/L at 10, 20, and 30 ppt salinity, respectively. Results indicate that sea bream is less tolerant to ammonia but more tolerant to nitrite compared with some other fish species.     

Acute tolerance of juvenile Florida pompano, Trachinotus carolinus L., to ammonia and nitrite at various salinities

The acute tolerance of juvenile Florida pompano Trachinotus carolinus L. (mean weight±SE=8.1±0.5 g) to environmental unionized ammonia‐nitrogen (NH₃‐N) and nitrite‐nitrogen (NO₂‐N) at various salinities was determined via a series of static exposure trials. Median‐lethal concentrations (LC₅₀ values) of NH₃‐N and NO₂‐N at 24, 48, and 96 h of exposure were calculated at salinities of 6.3, 12.5 and 25.0 g L^?1 at 28 °C (pH=8.23–8.36). Tolerance of pompano to acute NH₃‐N exposure was not affected by salinity, with 24, 48 and 96 h LC₅₀ values ranging from 1.05 to 1.12, 1.00 to 1.08 and 0.95 to 1.01 mg NH₃‐N L^?1 respectively. Regarding NO₂‐N, tolerance of pompano to this environmental toxicant was compromised at reduced salinities. Median‐lethal concentrations of NO₂‐N to pompano at 24, 48 and 96 h of exposure ranged from 67.4 to 220.1, 56.9 to 140.7 and 16.7 to 34.2 mg NO₂‐N L^?1 respectively. The results of this study indicate that juvenile Florida pompano are relatively sensitive to acute NH₃‐N and NO₂‐N exposure, and in the case of the latter, especially at lower salinities.     

Acute ammonia toxicity during early ontogeny of ide Leuciscus idus (Cyprinidae)

Acute ammonia toxicity was investigated in four developmental stages of the juvenile ide, Leuciscus idus: 1, 10, 20 and 30 days after the first feeding. Mean (±SD) total length of the larvae was 8.5 ± 0.3, 15.7 ± 0.7, 23.0 ± 2.0 and 29.7 ± 2.0 mm, and standard weight was 1.6 ± 0.3, 9.2 ± 5.5, 94.9 ± 31.0 and 196.0 ± 31.7 mg, respectively. The larvae used for toxicity tests were reared in the experimental, closed recirculating system. Groups of fishes (n from 7 to 10; in respect of fish size) were exposed to the ammonium chloride solutions in 1-L glass units. Water temperature was 25 ± 0.1 °C for both the rearing and the toxicity tests. pH was not adjusted and ranged between 8.4 and 8.7. The ammonium chloride solutions were renewed every 12 h. At the same time, dead larvae were counted and removed, and the pH and temperature measurements were taken. Each acute toxicity test duration was 96 h, and lethal concentration LC₁, LC₅₀ and LC₉₉ values were calculated for 24, 48, 72 and 96 h. The susceptibility of the ide larvae to ammonia decreased linearly with age up to 20th day and surprisingly increased during the next 10 days. The LC₅₀ (48 h) values ranged from 0.27 mg L^?1 of unionized ammonia nitrogen for 1 day after the first feeding (AFF) larvae to 1.42 mg L^?1 at day 20 after first feeding. The LC₅₀ (48 h) for 30 days AFF was as high as 0.67 mg L^?1. The critical level of the unionized ammonia nitrogen for ide larvae was suggested as 0.21 mg L^?1.     

Effects of 2-phenoxyethanol on survival of normal juveniles and malformed juveniles having lordosis or nonfunctional swimbladders of European sea bass (Dicentrarchus labrax L., 1758)

The objectives of this study were to evaluate the effects of 2‐phenoxyethanol (2‐PE), which is an anaesthetic, on survival rates of normal juveniles and malformed juveniles having lordosis or nonfunctional swim bladders of European sea bass (Dicentrarchus labrax L., 1758) and to establish the LC₅₀ (the concentration lethal to 50% of test animals at concentrations of 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4 and 4.5 mL L⁻¹) and LT₅₀ (the time lethal to 50% of test animals after 10‐, 20‐, 30‐, 40‐, 50‐ and 60‐min time periods) of 2‐PE at 19±0.5°C, salinity 38 g L⁻¹, pH 7.4–7.8 and dissolved oxygen >8 mg L⁻¹. Between concentrations of 0.05 and 0.25 mL L⁻¹, 2‐PE did not cause any mortality or toxicity on normal, lordosis and nonfunctional swimbladder juveniles of sea bass during the 60‐min exposure period. On the other hand, significance in each group fish in their mortality rates between concentrations of 0.30 and 0.45 mL L⁻¹ was observed (P<0.05). The nonfunctional swimbladder juveniles showed lower LC₅₀ than normal and lordosis juveniles respectively. Also, nonfunctional swimbladders juveniles showed lower LT₅₀ than normal and lordosis juveniles respectively. At concentrations of 0.30, 0.35, 0.40 and 0.45 mL L⁻¹, induction times were found to be significantly different among the three groups (P<0.05). Recovery times were not found to be significantly different in two groups at concentrations of 0.30 and 0.40 mL L⁻¹ (P>0.05). The toxic effect of 2‐PE on sea bass juveniles increased depending on the exposure times (P<0.05). The most suitable concentrations of 2‐PE were 0.30–0.35 mL L⁻¹ between minutes 10 and 30, although the normal juveniles can resist to 0.45 mL L⁻¹ of 2‐PE concentration for 20 min. The 2‐PE showed toxicity in relation to the concentrations and exposure time combinations among the three groups in the order; nonfunctional swimbladder fish >lordosis fish >normal fish.     

Formalin as an alternative to trifluralin as prophylaxis against fungal infection in mud crab Scylla serrata (Forsskål) larvae

The toxicity of formalin and trifluralin to the larval stages of the mud crab Scylla serrata was compared in a static bioassay. Prophylactic doses of 5, 10, 15, 20 and 25 μg L⁻¹ formalin and 0.05, 0.1, 0.2, 0.4 and 0.8 μg L⁻¹ trifluralin were used. Toxicity was assessed on the basis of survival of larvae after 24, 48, 72 and 96 h exposure to the test chemicals and metamorphosis to the next larval stage. Result shows that larval survival in all stages was significantly reduced at concentrations of 20 and 25 μg L⁻¹ formalin whereas larvae were able to tolerate all trifluralin treatments. However, larvae became more tolerant to high formalin concentrations as the larval stage progressed. Survival was better at 5, 10 and 15 μg L⁻¹ formalin and in all trifluralin treatments than the control in almost all the larval stages. Faster metamorphosis was observed at 5 and 10 μg L⁻¹ formalin and 0.05, 0.1 and 0.2 μg L⁻¹ trifluralin concentrations. Doses of formalin and trifluralin obtained from the toxicity experiments were applied as prophylaxis to newly hatched larvae in white plastic basins. Prophylactic doses of 5 and 10 μg L⁻¹ formalin and 0.05 and 0.1 μg L⁻¹ trifluralin applied every other day were found to be effective in enhancing survival and larval development to megalopa compared with control. However, no megalopae survived to crab instar in all formalin treatments. Although the use of fungicides in rearing systems resulted in higher survival compared with controls, other strategies (i.e. maintenance of good water quality and hygienic practices in the hatchery) should be further investigated as an alternative to the use of chemicals in hatcheries.     

Lethal doses of ammonia on the late-stage larvae of Chinese mitten-handed crab, Eriocheir sinensis (H. Milne-Edwards), (Decapoda: Grapsidae) reared in the laboratory

Four successive life stages (zoea-III, zoea-IV, zoea-V and megalopa) of the Chinese mitten-handed crab, Eriocheir sinensis (H. Milne-Edwards), were exposed to ammonia in a series of short-term bioassays with the static-renewal method at 22°C, pH 8.0 and 25%o salinity. The greatest sensitivity was observed in the zoea-III stage. The 24-h LC₅₀ values for zoea-III, zoea-IV, zoea-V and megalopa were 32.8, 73.1, 84.0 and 90.1 mg L^?1 for NH₃+ NH₄⁺, and 1.11, 2.36, 2.77 and 3.18 mg L^?1 for NH₃, respectively. The 72-h LC₅₀ values for zoea-III, zoea-IV and zoea-V were 11.9, 23.6 and 38.2 mg L^?1 for NH₃+ NH₄⁺, and 0.40, 0.76 and 1.26 mg L^?1 for NH₃, respectively. The 96-h LC₅₀ values for megalopa were 37.3 mg L^?1 for NH₃+ NH₄⁺ and 1.31 mg L^?1 for NH₃. It was found that ammonia tolerance increased with larval development from zoea-III to megalopa, especially from zoea-III to zoea-IV and from zoea-IV to zoea-V. A comparison of safe levels of ammonia among the different life stages indicated that all stages were significantly different with respect to safe levels of ammonia (P < 0.05) except zoea-V and megalopa, which had the highest safe levels. In general, both the larvae and juveniles of E. sinensis are less resistant to ammonia than those of other crustacean species studied so far.     

Toxicity of chlorine to different sizes of black tiger shrimp (Penaeus monodon) in low-salinity shrimp pond water

An experiment was conducted, in a dark room with controlled temperature (27.3–28.4 °C), to determine the acute toxicity of chlorine concentration to black tiger shrimp (Penaeus monodon fabicus) of sizes 0.02 g, 2.75 g, 8.47 g and 23.65 g. Toxicity tests on each of these shrimp sizes were run in triplicate in glass jars under static conditions without media renewal. The concentration of active chlorine that killed 50% of the shrimp of each size after 24‐h exposure (LC₅₀‐24 h) was used as an indicator of acute toxicity. Chlorine concentrations applied in the shrimp toxicity test ranged from 2.0 to 14.5 mg L^?1 in shrimp pond water. As the test water contained total suspended solids of 22.0–85.0 mg L^?1 and total ammonia nitrogen of 0.18–0.40 mg L^?1, the resultant concentrations of combined residual chlorine ranged from 0.6 to 3.5 mg L^?1, which were the effective doses causing shrimp mortality. The test results showed that 24‐h LC₅₀ for average shrimp size at 0.02, 2.75, 8.47 and 23.65 g occurred in water containing combined residual chlorine at a concentration of 0.91, 1.39, 1.74 and 1.98 mg L^?1, for which the original application doses were 6.96, 2.05 11.50 and 13.34 mg L^?1 respectively.     

Acute toxicity of ammonia to the early stage-larvae and juveniles of Eriocheir sinensis H. Milne-Edwards, 1853 (Decapoda: Grapsidae) reared in the laboratory

Static-renewal bioassays were performed to evaluate the acute toxicity of ammonia to Eriocheir sinensis (H. Milne-Edwards) at three growing stages, namely zoea-I, zoea-II, and juvenile (0.06 g wet weight per crab). The 24 h LC₅₀ values were 13.3, 20.2, and 109.3 mg (NH₃+ NH₄⁺) 1^?1 (0.47, 0.71, and 3.10 mg NH₃ I^?1), the 48 h LC₅₀ values being 6.8, 10.3, and 60.9 mg (NH₃⁺ NH₄⁺) 1^?1 (0.24, 0.36, and 1.73 mg NH₃1^?1), while the 72 h LC₅₀ values were 5.7, 7.6, and 45.3 mg (NH₃+ NH₄⁺) 1^?1 (0.20, 0.27, and 1.29 mg NH₃ 1^?1) for zoea-I, zoea-II, and juveniles, respectively. The 96 h LC₅₀ value for juveniles was 31.6 mg (NH₃+ NH₄⁺) 1^?1(0.90 mg NH₃1^?1). It was evident that the tolerance to ammonia increased during the same exposure time as the larvae developed to juveniles and decreased with prolonged exposure time. Compared with larvae, juveniles were more sensitive to the fluctuation of ambient ammonia concentrations in the certain range within which partial kills took place. The ‘safe level’ of ammonia based on the 96 h LC₅₀ value and an application factor of 0.1 was 3.16 mg (NH₃+NH₄⁺)1^?1 (0.09 mg NH₃ 1^?1) for juveniles and those for zoea-I and zoea-II were 0.57 and 0.76 mg (NH₃+ NH₄⁺) 1^?1 (0.02 and 0.03 mg NH₃ 1^?1) based on 72 h LC₅₀ values.     

Effects of ammonia and nitrite on survival, growth and moulting in juvenile tiger crab, Orithyia sinica (Linnaeus)

The effects of ammonia and nitrite on survival, growth and moulting were investigated in juvenile tiger crab, Orithyia sinica (carapace length 3.91±0.15 mm, carapace width 3.84±0.23 mm, n=440), after 30 days exposure to ammonia‐N (0, 20, 50, 100 and 150 mg L^?1) and nitrite‐N (0, 50, 100, 150, 200 and 250 mg L^?1) using a continuous flow system. Survival rates of tiger crab exposed to ammonia and nitrite decreased linearly with the exposure time and concentration. The growth rate of tiger crab exposed to 50, 100 and 150 mg L^?1 ammonia was significantly lower than that of control crabs. The growth rate of tiger crab exposed to nitrite decreased at 150, 200 and 250 mg L^?1 nitrite. During the ammonia and nitrite exposure, the intermoult period of the juveniles of tiger crab O. sinica was shortened between the first and second moult, and the number of moulting of crabs exposed to a higher concentration were significantly higher than that of control crabs.     

Use of Rotenone as Piscicide: Toxicity Levels in a Few Common Freshwater Predatory and Weed Fishes

Toxicity of rotenone was studied in a few common freshwater predatory and weed fishes through wet laboratory experiments for its use as a piscicide during pond preparation. Cube root powder (CRP) (ENT-133 Rotenone) containing 9% rotenone was used as the toxicant source. Lethal concentration of CRP for these common predatory and weed fishes varied between 0.75–2.70 mg L^?1 (0.068–0.243 mg L^?1 of rotenone). Acute toxicity study revealed Puntius sophore to have more susceptibility to rotenone toxicity with 24 h LC₅₀ value of CRP at 0.50 mg L^?1 (0.045 mg L^?1 rotenone) compared to 1.17 mg L^?1 (0.105 mg L^?1 rotenone) in Anabas testudineus and 1.90 mg L^?1 in Channa punctatus (0.171 mg L^?1 rotenone); while Heteropneustes fossilis showed higher tolerance with 24 h LC₅₀ value at 2.42 mg L^?1 (0.218 mg L^?1 rotenone). Such result suggested rotenone toxicity to depend on the respiratory behavior of fish. The marginal reduction in 48 h LC₅₀ of CRP compared to its 24 h value and no fish mortality beyond 48-h in all tested species suggested faster degradation of the toxicant in water. Since application of the piscicide aims at eradication of all commonly available species of predatory and weed fishes in the culture pond, the study suggested a dose of 2.5 mg L^?1 of CRP (0.225 mg L^?1 rotenone) for pond application.     

Daily variation in short-term static toxicity of unionized ammonia in Litopenaeus vannamei (Boone) postlarvae

Ammonium toxicity of short‐duration alkaline events and their variability, as related to 1–30 day‐old postlarvae whiteleg shrimp Litopenaeus vannamei (Boone), was assessed by determining medium lethal concentration (LC₅₀) of total ammonium‐nitroen (TAN) and NH₃‐N to 4‐h exposures. Exploratory concentrations of TAN were tested at 30°C and pH 9, until mortality from 5% to 95% occurred between 0.9 and 18 mg N L^?1. To determine the daily variation of ammonium toxicity, 64 lots of 20 postlarvae were exposed to eight different ammonium concentrations (0, 0.9, 3, 6, 9, 12, 15 and 18 mg N L^?1), in two different environmental scenarios: α (pH 8, 26°C) and β (pH 9, 30°C). In environmental scenario α, ammonium concentrations up to 18 mg L^?1 pose no short‐term mortality risks for ages 1–30 days. In scenario β, mortality was recorded at all ages. The values of LC₅₀ (4 h) for different postlarvae ages have daily variability, ranging from a minimum of 2.54 to a maximum of 6.02 mg L^?1 of TAN (0.76 and 1.81 mg N L^?1 of NH₃‐N), for PL₃ and PL₁₉, respectively, with a logarithmic linear tendency to increase with age. Postlarvae mortality at 4 h and 3.0 mg N L^?1 TAN exposure was lower and less variable in ages greater than 19 days old.     

Nitrite toxicity to Litopenaeus schmitti (Burkenroad, 1936, Crustacea) at different salinity levels

Litopenaeus schmitti juveniles (total length 15 ± 0.4 cm) were exposed to different concentrations of nitrite using the static renewal method at different salinity levels (5‰, 20‰ and 35‰) at pH 8.0 and 20 °C. The 24, 48, 72 and 96 h LC50 values of nitrite in L. schmitti juveniles were 40.72, 32.63, 24.63, and 19.12 mg L⁻¹ at 5‰; 53.52, 38.60, 27.76, 25.55 mg L⁻¹ at 20‰; 54.32, 47.87, 41.67 and 38.88 mg L⁻¹ at 35‰ salinity. As the salinity decreased from 35‰ to 5‰, susceptibility to nitrite increased by 33.4%, 46.7%, 69.2% and 103.3%, after 24, 48, 72 and 96 h of exposure respectively. Furthermore, we found that exposure of shrimp to nitrite caused an increase in oxygen consumption by 137.3%, 99.2% and 81.4% and an increase in the ammonia excretion level by 112.5%, 87% and 64.3% with respect to the control with decreasing salinity levels.