Ambient salinity and osmoregulation,energy metabolism and growth in juvenile yellowtail kingfish (Seriola lalandi Valenciennes 1833) in a recirculating aquaculture system

The effects of salinity on plasma osmolality, branchial chloride cell density, feed consumption and conversion and growth performance of yellowtail kingfish (Seriola lalandi) were evaluated. Fish (11.6 ± 0.6 g) were kept for 29 days at 14, 18, 22, 26 (experimental) and 30 g L^?1 (control) salinity in independent, pilot‐scale recirculation aquaculture systems. No differences in plasma osmolality or chloride cell numbers in gills were observed, pointing to a strong osmoregulatory capacity in the juveniles. Fish at 14, 18 and 22 g L^?1 (7.61 ± 0.19, 7.61 ± 0.01 and 7.61 ± 0.13% day^?1, respectively) had higher growth rates than fish at 26 and 30 g L^?1 (7.10 ± 0.05 and 6.97 ± 0.06% day^?1 respectively). The higher growth rate at lower salinity resulted from increased feed intake; feed conversion was not different. An evaluation of the impact of salinity on growth rate of on‐growing stages (till market size) seems warranted to assess whether the profitable effects of low salinity persist in later stages of this important aquaculture species.     

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).     

Larval development and salinity tolerance of Japanese flounder (Paralichthys olivaceus) from hatching to juvenile settlement

Salinity tolerance and growth of Japanese flounder Paralichthys olivaceus at different developmental stages were evaluated, including newly hatched larvae (nhl), yolk sac larvae (ysl), oil droplet larvae (odl), post oil droplet larvae (podl), premetamorphic larvae (preml) and prometamorphic larvae (proml), at 11 salinities from 5 to 55 g L^?1 for 96 h. The ontogenesis during the early life of P. olivaceus was investigated under hatchery salinity 35 g L^?1. The results showed that suitable salinities for nhl, ysl, odl, podl, preml and proml larvae were 10 to 25 g L^?1, 10 to 30 g L^?1, 20 to 30 g L^?1, 30 g L^?1, 10 to 30 g L^?1, 15 g L^?1, respectively, demonstrating an ontogenetic variation of salinity tolerance. The salinity tolerance of nhl, ysl, preml was higher than that of odl, podl and proml. The ysl and preml larvae displayed wide salinity tolerances. The present findings demonstrate that the suitable salinity for larviculture of P. olivaceus is 20–25 g L^?1 before the depletion of oil droplet; after that, higher salinity (30 g L^?1) should be ensured for the post‐oil droplet larvae; the premetamorphic larvae can be cultured at a wide salinity range (10–30 g L^?1), and the metamorphosed larvae should be reared at salinity about 15 g L^?1.     

Effects of salinity on standard metabolic rate and critical oxygen tension in the giant freshwater prawn (Macrobrachium rosenbergii)

The extensively farmed giant freshwater shrimp, Macrobrachium rosenbergii, can survive salinities up to 26 g L^?1, but the commercially important grow‐out occurs exclusively in freshwater areas. Recent studies suggest the shrimp equally capable of growing in brackish as fresh water and a better understanding of how this species responds to changing salinity could significantly impact freshwater prawn farming in deltas and coastal areas. Here, the effect of salinity (0 and 15 g L^?1) on standard metabolic rate (SMR) and critical oxygen tension (P_crit) was measured in adult M. rosenbergii using intermittent closed respirometry. SMR was 79.8 ± 3.1 and 72.7 ± 2.9 μmol kg^?1 min^?1 in fresh and brackish water, respectively, with no significant difference between the two salinities (P = 0.122). During hypoxia M. rosenbergii maintained oxygen uptake down to a P_crit of 26.3 ± 1.4 mmHg in fresh and 27.2 ± 2.0 mmHg in brackish water (P = 0.682), showing that salinity had no overall effect on oxygen conductance in the animals. These findings are in agreement with recent growth studies and provide further evidence that grow‐out phase could be accomplished in brackish water areas. Thus, the predicted intrusions of brackish water in tropical deltas as a consequence of future global warming may not impact this important production.     

Growth differences of Japanese eels Anguilla japonica between fresh and brackish water habitats in relation to annual food consumption in the Kojima Bay‐Asahi River system,Japan

To learn about the relationships between feeding and growth of temperate eels in freshwater and brackish water habitats, we analysed 533 yellow‐phase Japanese eels Anguilla japonica collected in both types of habitats in southeastern Japan. Because male eels were very rare in each habitat (FW, N = 1; BW, N = 20), characteristics of female eels were compared between the different habitats. Annual food consumption was evaluated with the consideration of instantaneous food consumption and annual activity period. Stomach fullness index (stomach content weight/body weight) was used as an indicator of instantaneous food consumption. The ratios of number of months with eel catch to those when eel sampling was conducted were used as an indicator of activity period. Female yellow eels tended to be older and slower growing in fresh water (N = 78; age, mean ± SD = 7.9 ± 2.4 years; growth rate, 59.8 ± 14.0 mm year^?1) than in brackish water (N = 229; age, 5.5 ± 1.8 years; growth rate, 90.1 ± 24.4 mm year^?1). Irrespective of sex, yellow eels in brackish water had a higher stomach fullness index and a greater ratio of months with eel catches, indicating greater annual food consumption by brackish water eels. These results indicate that greater annual food consumption contributes to the greater growth rates of Japanese eels in brackish water habitats.     

Effects of salinity on standard metabolic rate of juvenile marbled spinefoot (Siganus rivulatus)

Marbled spinefoot, Siganus rivulatus, is a herbivorous euryhaline teleost widely distributed in the Eastern Mediterranean. It is an economically valuable species and a suitable candidate for warm water aquaculture. Accordingly, understanding the effects of environmental factors on fish metabolism is important to optimize culture conditions. Two experiments were performed to establish standard metabolic rate and study the effect of salinity on metabolism of marbled spinefoot. In the first experiment, a series of flow‐through respirometry experiments was performed at 27°C and 35 g L^?1. The standard metabolic rate of marbled spinefoot juveniles was calculated as 0.57 ± 0.02 mg O₂ g^?1 h^?1 (mean ± SE). In the second experiment, fish were maintained at salinities of 25, 30, 35 and 40 g L^?1 for 2 weeks. Flow‐through respirometry was performed to measure respiration rates at the various salinities. Respiration rates were similar among fish in salinities of 30, 35 and 40 g L^?1 but increased significantly at 25 g L^?1. Results suggest that despite the euryhalinity of marbled spinefoot, farmers should maintain salinity within the optimal range of 30–40 g L^?1 in order to improve productivity.     

Isolation of a novel polyomavirus,related to Japanese eel endothelial cell‐infecting virus,from marbled eels,Anguilla marmorata (Quoy & Gaimard)

Marbled eels, Anguilla marmorata (Quoy & Gaimard), cultured in Taiwan exhibited haemorrhage and mortality in January 2012. The severely diseased eels bled from the gills and showed congestion of the central venous sinus of the gill filaments and haemorrhage throughout the body similar to viral endothelial cell necrosis of eel. In this study, a novel polyomavirus (AmPyV) was isolated from the diseased eels using the AMPF cell line established from the pectoral fin of healthy marbled eels. AmPyV was found to encode a long T‐antigen orthologous gene. Phylogenetic analysis showed that AmPyV was closely related to Japanese eel endothelial cell‐infecting virus. PCR assays revealed AmPyV infection throughout the systemic organs. AmPyV proliferated in the AMPF, EK‐1 and EO‐2 cells at temperatures 25–30 °C, and the progeny virus yields were 10^7.0, 10^7.4 and 10^7.7 TCID₅₀ mL^?1, respectively. The purified virions were icosahedral particles, 70–80 nm in diameter. No clinical signs or mortality was observed among the eels injected with the virus; however, the virus was reisolated from the brain, eyes, kidneys, fins and gills of infected eels 2 month after injection. Our results suggest that AmPyV exhibits a latent infection. Pathogen of the disease needs to study further.     

Effects of acute change in salinity and moulting on the infection of white leg shrimp (Penaeus vannamei) with white spot syndrome virus upon immersion challenge

In the field, moulting and salinity drop in the water due to excessive rainfall have been mentioned to be risk factors for WSSV outbreaks. Therefore, in this study, the effect of an acute change in environmental salinity and shedding of the old cuticle shell on the susceptibility of Penaeus vannamei to WSSV was evaluated by immersion challenge. For testing the effect of abrupt salinity stress, early premoult shrimp that were acclimated to 35 g L^?1 were subjected to salinities of 50 g L^?1, 35 g L^?1, 20 g L^?1, 10 g L^?1 and 7 g L^?1 or 5 g L^?1 and simultaneously exposed to 10^5.5 SID₅₀ mL^?1 of WSSV for 5 h, after which the salinity was brought back to 35 g L^?1. Shrimp that were transferred from 35 g L^?1 to 50 g L^?1, 35 g L^?1 and 20 g L^?1 did not become infected with WSSV. Shrimp became infected with WSSV after an acute salinity drop from 35 g L^?1 to 10 g L^?1 and lower. The mortality in shrimp, subjected to a salinity change to 10 g L^?1, 7 g L^?1 and 5 g L^?1, was 6.7%, 46.7% and 53.3%, respectively (P < 0.05)_. For testing the effect of moulting, shrimp in early premoult, moulting and post‐moult were immersed in sea water containing 10^5.5 SID₅₀ mL^?1 of WSSV. The resulting mortality due to WSSV infection in shrimp inoculated during early premoult (0%), ecdysis (53.3%) and post‐moult (26.72%) demonstrated that a significant difference exists in susceptibility of shrimp during the short moulting process (P < 0.05). The findings of this study indicate that during a drop in environmental salinity lower than 10 g L^?1 and ecdysis, shrimp are at risk for a WSSV infection. These findings have important implications for WSSV control measures.     

Quantitative dietary leucine requirement of juvenile Pacific white shrimp,Litopenaeus vannamei (Boone) reared in low‐salinity water

The experiment was conducted to determine the leucine requirement of juvenile Pacific white shrimp Litopenaeus vannamei (Boone) in low‐salinity water (0.50–1.20 g L^?1). Six diets were formulated to contain 410 g kg^?1 crude protein with fish meal, peanut meal and precoated crystalline amino acids with different concentration of l ‐leucine (16.72, 19.60, 22.06, 24.79, 27.28 and 30.16 g kg^?1 dry diet). Each diet was randomly assigned to triplicate groups of 30 shrimps (0.38 ± 0.002 g), and the feed trial lasted for 8 weeks. The results indicated that the maximum weight gain was observed at 24.95 g kg^?1 dietary leucine group, whereas the diets containing higher leucine concentration conversely reduced the growth performance (P < 0.05). Moreover, the highest body protein content and body protein deposition and the lowest haemolymph AST and ALT activities were also found at 24.95 g kg^?1 dietary leucine group. With the increase in leucine in diets, a dose‐dependent increase was found in body lipid content and haemolymph urea concentration. The polynomial regression calculated using weight gain, feed efficiency and body protein deposition indicated that the optimal dietary leucine requirement for L. vannamei reared in low‐salinity water was 23.73 g kg^?1 leucine of dry diet, correspondingly 57.88 g kg^?1 of dietary protein.     

Immunostimulatory effects of dietary fructooligosaccharides on red swamp crayfish,Procambarus clarkii (Girard)

Fructooligosaccharides (FOS) are widely researched as prebiotics and immunostimulants in humans and terrestrial animals. However, limited researches on immune functions of FOS were reported in crustacean. In the present study, red swamp crayfish Procambarus clarkii were fed with different concentrations of FOS (2.0, 5.0, 8.0, 10.0 g kg^?1 diet), and then the immunostimulatory effects of FOS were evaluated by measuring immune parameters and examining immune‐related gene expression. PO and SOD activities were significantly increased in groups fed with 8.0 or 10.0 g kg^?1 of FOS compared with those in control group. In addition, significant elevations of SOD and phagocytic activities were also observed in group fed with 5.0 g kg^?1 of FOS. Among examined immune‐related genes, expressions of crustin1, lysozyme, SOD and pro‐PO were significantly up‐regulated by feeding with 8.0 or 10.0 g kg^?1 of dietary FOS. Survival rates of crayfish challenged with Aeromonas hydrophila were significantly improved by feeding with 5.0, 8.0 or 10.0 g kg^?1 of dietary FOS for 30 days. Phagocytic activities and bacterial clearance efficiencies were also significantly increased in groups fed with 8.0 or 10.0 g kg^?1 of FOS. The results indicated that dietary FOS effectively induced innate immune responses of red swamp crayfish in a dose‐dependent way and could be used as potential immunostimulant in crustacean.     

Effects of incubation water hardness and salinity on egg hatch and fry survival of Nile tilapia Oreochromis niloticus (Linnaeus)

This study examined the effects of water hardness and salinity on yolk sac larvae and swim‐up fry survival of Nile tilapia, Oreochromis niloticus (Chitralada strain), eggs during artificial incubation. Four experiments were conducted to evaluate the effects of hardness, salinity and the sources of saline incubation water. High water hardness treatments (500–4200 mg L^?1 as CaCO₃) resulted in higher yolk sac larvae and swim‐up fry survival than low water hardness treatments (50.0 and 132 mg L^?1 as CaCO₃); although yolk sac larvae and swim‐up fry survival did not differ among the high or low hardness treatments. Salinity of 4.0 g L^?1 using seawater, and 4.0 and 8.0 g L^?1 using unprocessed common salt resulted in the higher survival rate of yolk sac larvae and swim‐up fry than other salinity treatments. Yolk sac larvae and swim‐up fry survival was found to decrease with the increase in salinity and increase with the increase in water hardness. The present study demonstrated the positive effects of increased water hardness level (>132 mg L^?1) on yolk sac larvae and swim‐up fry survival. The study also showed that seawater salinity of 4 g L^?1 was the most appropriate salinity level for incubating Nile tilapia eggs.     

The effects of salinity on reproductive performance and plasma levels of sex steroids in Caspian kutum Rutilus frisii kutum

Sexually mature kutum, Rutilus frisii kutum, captured from its natural habits, the Caspian Sea and the Khoushkrood River, reared at 0.5 g L^?1 and 8–13 g L^?1 for approximately 1 year in experimental condition, for assessing the effect of salinity on reproduction. Plasma concentrations of sex steroid hormones (17β‐estradiol, testosterone and 17α‐hydroxyprogestrone) were measured in the three stages of gametogenesis. Female kutum held at <0.5 g L^?1 or 8–13 g L^?1 had no ovulated oocytes in their ovaries. In contrast, males held in captivity were spermiated, similar to their wild counterparts. The average sperm volume of males held at <0.5 g L^?1 (2.36 ± 0.46 mL) was lower than males held at 8–13 g L^?1 (3.65 ± 0.73 mL) at the end of the experimental period. The highest concentration of testosterone was observed in mid‐gametogenesis in wild fish that was significantly higher than the concentration seen in fish held in either <0.5 g L^?1 or 8–13 g L^?1. Female kutum showed suppressed steroid hormones in captivity, resulting in failure in the gonad development. However, male kutum adapt well to captivity and showed synchrony in steroid hormone variations with the wild fish, resulting in the testicular development. Results of this study also indicate that salinity plays a minor, but vital, role in reproduction of kutum, a factor that needs to be considered for keeping broodstock of brackish water fish species like kutum.     

Rhodovulum sulfidophilum,a phototrophic bacterium,grown in palm oil mill effluent improves the larval survival of marble goby Oxyeleotris marmorata (Bleeker)

Survival of marble goby larvae fed either Rhodovulum sulfidophilum, a phototrophic bacterium cultured from palm oil mill effluent (pPB), or microalgae ( Nannochloropsis sp.) was evaluated at two salinities. Larvae directly fed pPB had survival of 0–29% at 5 g L^?1 salinity and 0–19% at 10 g L^?1 salinity, whereas larvae directly fed microalgae suffered complete mortality after 20 days of culture at both salinities. However, larvae indirectly fed pPB or microalgae, i.e. via rotifers (Days 1–30) and Artemia nauplii (Days 21–30) cultured solely from pPB or microalgae, showed improved survival of 35–55% or 44–49% at 5 g L^?1 salinity respectively. In all experiments, fish larvae reared at 5 g L^?1 salinity showed significantly higher (P < 0.01) mean survival than those reared at 10 g L^?1 salinity. The survival of larvae fed the bacterial‐based diet was higher compared with microalgal diet used in previous studies. The pPB had higher total polyunsaturated fatty acids and docosahexaenoic acid (DHA) than the microalgae, which had very high eicosapentaenoic acid (EPA). Larvae with very high ratios of DHA/EPA (>11) or/and ARA (arachidonic acid)/EPA (>5), attributable to their given diet, however suffered the highest mortality.     

Osmoregulatory ability and stress responses during freshwater adaptation of black porgy (Acanthopagrus schlegeli) treated with exogenous prolactin

The effects of ovine prolactin (oPRL) on osmoregulatory ability (electrolyte balance, plasma osmolality and activity of gill chloride cells and gill Na⁺/K⁺‐ATPase) and stress responses (plasma cortisol, glucose, aspartate aminotransferase: AST and alanine aminotransferase: ALT) were investigated in black porgy transferred to freshwater (FW). Fish in seawater (SW) were injected twice at a 24 h interval with oPRL (at 1, 3, or 5 μg g^–1 body weight) or vehicle (0.9% NaCl) and then transferred to FW. They were sampled 3 days after the transfer. With oPRL at 5 μg g^–1, levels of plasma Na⁺ and Cl^? and osmolality were significantly higher than in saline‐treated fish, whereas gill CCs number and Na⁺/K⁺‐ATPase activity were lower. Also, the 5 μg g^–1oPRL treatment led to significantly lower plasma cortisol levels than did saline treatment. However, there were no significant differences in plasma AST and ALT between groups. These results support the positive osmoregulatory role of PRL in black porgy during FW adaptation.     

Dietary vitamin C mitigates monocrotophos toxicity in the freshwater fish Clarias batrachus

The amelioration effect of balanced diet with supplementation of vitamin C against stress was evaluated in freshwater paddy field fish, Clarias batrachus, exposed to organophosphate insecticide, monocrotophos (MCP). Three groups of fishes, that is, (E₀) Control, (E₁) MCP (2.14 mg L^?1) exposure and E₂) Vitamin C, MCP (0.2 g kg^?1 body weight + 2.144 mg L^?1) were selected for the sublethal study. The effect of MCP on protein metabolism in physiological sensitive tissues, namely gills, kidney, liver and muscle of the fish has been determined. Stress on Clarias batrachus was evident that the total protein, amino acid and ammonia contents were decreased, and urea and glutamine levels were elevated in all tissues for 30 days. Similarly, the activities of protease, aminotransferases and phosphatases were elevated significantly in all the tissues during the same period. Fish fed with a diet supplemented by vitamin C (0.2 g kg^?1 body weight) could significantly counter these effects.     

Supplementation of sodium chloride in diets to improve the meat quality of Pacific white shrimp,Litopenaeus vannamei,reared in low‐salinity water

To investigate the effect of dietary sodium chloride (NaCl) on meat quality of white shrimp (Litopenaeus vannamei) reared in low‐salinity (2 g L^?1) water, shrimp were distributed into four groups (treatments T‐1, T‐2, T‐3 and control) with three replicates. All shrimps were completely randomised stocked into 12 tanks at an initial density of 40 shrimps per tank. Diets for the control, T‐1, T‐2 and T‐3 groups consisted of the basal diet supplemented with 0 g kg^?1, 10 g kg^?1, 20 g kg^?1 and 40 g kg^?1 of NaCl respectively. After 50 days, shrimps in T‐3 showed significantly better (P < 0.05) moisture, crude protein and ash than those of the control and T‐1. Higher muscle Na content was observed (P < 0.05) in T‐3 than that of the control. Significant increases (P < 0.05) in contents of inosinic acid, total free amino acid (TFAA) and essential free amino acid (EFAA) were also found in T‐3. Texture assays showed significant differences (P < 0.05) in hardness, adhesiveness and springiness between group T‐3 as compared with those of T‐1 and control. It indicated that dietary supplementation of NaCl appeared to be a promising practice to improve meat quality of white shrimp reared in low‐salinity waters.     

Recovery from transportation by road of farmed European eel (Anguilla anguilla)

The objective of this study was to assess the effects of transportation of marketable eel (0.15 kg) in the Netherlands with respect to welfare. Eels (Anguilla anguilla) were obtained from a commercial farm and acclimatized for 7 weeks at the laboratory. Fish were transported according to regular commercial procedures. The animals were placed in water‐filled transport tanks on the trailer. Fish density increased from 72 kg m^?3 (husbandry) to 206 kg m^?3 (fasting) and was further increased to 270–290 kg m^?3 during transport. Fish transport lasted 3 h after which the eels were returned to laboratory recirculation systems to measure parameters indicative of stress load, i.e. mortality, plasma cortisol, lactate and non‐esterified fatty acids (NEFA) as well as gill morphology. Samples were taken at 0, 6, 24, 48 and 72 h after transport in transported fish and non‐transported counterparts (controls). Transportation affected water quality within known tolerable limits. No mortality during or after transport was observed. After 6 h, plasma cortisol levels had returned to baseline. However, energy metabolism had increased suggesting that transportation of eels resulted in an increased energy demand that lasted for at least 72 h in the fasted animals. Thus, it is conceivable that exposure to adverse conditions, prior to stunning/killing, in a slaughterhouse may result in allostatic overload in eel.     

Cobia Rachycentron canadum L. reared in low‐salinity water: does dietary sodium chloride affect growth and osmoregulation?

The effects of NaCl supplementation (0.0%, 2.5%, 5.0%, 7.5% and 10.0% dry weight of a basal diet) on growth, gill histological alterations and osmoregulation of juvenile cobia reared in low‐salinity water (5 g L^?1) were assessed. At the end of the experiment, gills were sampled for Na⁺, K⁺‐ATPase activity determination and histological evaluation. In all treatments, no mortality was observed. Results showed that dietary NaCl supplementation did not alter growth. At the highest supplementations (7.5% and 10.0%), juvenile cobia showed higher feed intake and feed conversion ratio. Na⁺, K⁺‐ATPase activity was higher in fish fed the diet without salt supplementation than in those fed with NaCl‐supplemented diets. The number of chloride cells significantly increased with increasing dietary salt level, being 2.5‐fold higher in fish fed with 10.0% NaCl supplementation (41 cells mm^?2) than in those from the non‐supplemented fed group (16 cells mm^?2). These findings indicate that dietary salt supplementation stimulated chloride cell proliferation paralleled with a reduction in the gill Na⁺, K⁺‐ATPase activity, suggesting a possible decrease in energy consumption associated with osmoregulation. However, the suggested energy sparing did not have a significant impact on juvenile cobia growth.     

An optimized and simplified method for analysing urea and ammonia in freshwater aquaculture systems

This study presents a simple urease method for analysis of ammonia and urea in freshwater aquaculture systems. Urea is hydrolysed into ammonia using urease followed by analysis of released ammonia using the salicylate‐hypochlorite method. The hydrolysis of urea is performed at room temperature and without addition of a buffer. A number of tests were performed on water samples obtained from a commercial rainbow trout farm to determine the optimal urease concentration and time for complete hydrolysis. One mL of water sample was spiked with 1.3 mL urea at three different concentrations: 50 μg L^?1, 100 μg L^?1 and 200 μg L^?1 urea‐N. In addition, five concentrations of urease were tested, ranging from 0.1 U mL^?1 to 4 U mL^?1. Samples were hydrolysed for various time periods ranging from 5 to 120 min. A urease concentration of 0.4 UmL^?1 and a hydrolysis period of 120 min gave the best results, with 99.6–101% recovery of urea‐N in samples spiked with 100 or 200 μg L^?1 urea‐N. The level of accurate quantification of ammonia using the method is 50 μg L^?1 NH₄⁺‐N, and the detection level is 5–10 μg L^?1 NH₄⁺‐N.     

A size‐age model based on bootstrapping and Bayesian approaches to assess population dynamics of Anguilla anguilla L. in semi‐closed lagoons

A size‐age modelling technique is presented for assessing the vital rates, stock and recruitment of eel populations in semi‐closed lagoons with fully monitored migration of silver eels. Data for yellow and silver European eels (Anguilla anguilla L.) were obtained in 2011 from the Comacchio lagoon (Italy). The analysis was performed in three steps: (i) correction of yellow eel data, which are affected by the fyke nets selectivity during samplings, (ii) estimation of survival curve, stock, recruitment and metamorphosis rates of the population (calibration using data from 2011) and (iii) validation of the model using the observed amount of silver eel migrating population of the next year. A bootstrap procedure was used to assess the level of uncertainty for each parameter using the 95% intervals of the highest posterior density distribution HPDD (Bayesian approach). The measured abundance of silver eels was 0.56 ind·ha^?1, while the yellow eel abundance and recruitment were estimated by the model for 2011 at 8.77 ind·ha^?1 and 5.99 ind. ha^?1 respectively. The model performance during validation was satisfactory as the observed total mass of migrating population of 2012 (3777 kg) was inside the 95% HPDD intervals (3197–3839 kg) of model's predictions. The estimated stocks and recruitment were at least ten times lower from the respective estimations of previous studies of 1989 highlighting the crucial conditions of the population. The proposed modelling approach can provide significant information about eel population conditions, facilitating the evaluation of a range of management options in the context of eel conservation plans.