Chronic effects of ammonia were studied in juvenile seabass, Dicentrarchus labrax (mean weight=11 g), exposed for 63 days to eight stable ammonia concentrations, ranging from 0.24 to 0.90 mg l−1 unionised ammonia nitrogen (UIA-N), respectively, from 6.1 to 22.3 mg l−1 total ammonia nitrogen (TA-N). Temperature (21.8 °C), pH (8.0), salinity (37.0 ppt), and oxygen concentration (over 80% saturation at the outlet) were maintained constant. Fish were fed using a self-feeder device, and they were starved during the last 8 days. Mortality of 28.9 and 42.6% occurred within the first 8 days at the two highest UIA-N concentrations, respectively, 0.90 and 0.88 mg l−1. From days 0 to 55, a 1.8-fold increase in weight gain was observed under the 0.90-mg l−1 UIA-N condition, compared to a 3.4-fold increase in the control. Weight gains were negatively correlated to ambient ammonia concentrations. Weight loss, or a transient period of growth stagnation, was observed from the onset of ammonia exposure to day 13 in seabass exposed to concentrations above 0.43 mg l−1 UIA-N. After day 13, weight gains were observed in all groups, indicating that the fish were able to adapt to increased ambient ammonia concentrations over time. By the end of the experiment, plasma ammonia levels were positively related to ambient ammonia concentrations, and oxygen consumption recorded in fasting fish was significantly dependent on ammonia concentrations. In seabass juveniles, the 0.26-mg l−1 UIA-N concentration, under an average pH of 8.0, can be considered as a safe long-term limit conditions in seawater.

The chronic effects of exposing sea bass (average initial weight 100 g) to ammonia in water at 22 degreesC were first evaluated over a 61-day period (period 1, P1) during which nine different groups were submitted to nine ambient ammonia levels ranging from 0.014 to 0.493 mg 1(-1) NH3-N (0.53-16.11 mg 1(-1) total ammonia nitrogen (TA-N)) and fed using self-feeders. At the end of P1, the fish were starved for 10 days (P2). Their recovery capacity was tested over 43 days (P3) after which the exogenous ammonia supply was stopped in all treatments and the fish were allowed to feed. After 20 days of exposure a highly significant effect of ammonia was evident from the decrease in feeding activity, voluntary feed intake (VFI) and specific growth rate (SGR), and the increase in the feed conversion ratio (FCR). Ammonia exposure had no effect on circadian feeding rhythm or hourly actuation profiles. At the end of PI, the fish seemed to have adapted to all ambient ammonia concentrations tested since feeding and growth parameters were independent of ammonia levels. But they were unable to compensate for growth losses. Physiological adjustments were observed: plasma TA-N concentrations were positively related to ambient TA-N while there was no major disturbance in plasma urea. Plasma tri-iodo-thyronine concentrations were affected by ambient ammonia concentrations and there were no significant changes in hydromineral balance. During P2, oxygen consumption and urea excretion did appear to have been affected by ambient ammonia. When the exogenous supply of ammonia was stopped (M), fish exhibited hyperphagia and compensatory growth. In fish previously exposed to the highest ammonia levels, SGR and VFI were highest, and their FCR was improved. At the end of the experiment the final average weights were similar in all of the treatments (range 337-396 g). Depending on the concentrations used, ammonia exposure may enhance subsequent fish appetite and growth rate and have a similar effect on growth performances as restricting feeding level. Within the range tested, no detrimental effect of ammonia on the metabolic capacity of the fish, measured by oxygen consumption and urea excretion, or on their physiological status was recorded, and the fish had a good recovery capacity. In the conditions of the experiment, the non-observable effect concentration (NOEC) was 6 mg 1(-1). (C) 2003 Elsevier SAS and Ifremer/MD/Inra/Cemagref. All rights reserved.

Brown trout and rainbow trout (average weight 100 g) were reared in fresh water at 12 degrees C under the same conditions before transfer of brown trout to sea water, in order to compare nitrogen utilisation in the two species. Apparent protein digestibility (ADC), nitrogen (ammonia and urea) excretion, protein productive value (PPV) and actual observed nitro en mass balance were determined. Rainbow trout raised in fresh water had a higher growth rate (1.1 vs 0.8%. d(-1)), better food conversion ratio (0.7 vs 1.0), better ADC (91 vs 85%) and PPV (45 vs 35%) and lower ammonia excretion rates than brown trout reared in fresh water. Transferring brown trout to sea water induced lower PPV (30%) and ammonia and urea excretion. Salinity did not modify metabolic efficiency in brown trout. Fat content was higher in brown trout (7.7-8.9% ww) than in rainbow trout (5.7-7.6% ww). Nitrogen mass balance indicated that compounds other than ammonia and urea were produced in higher quantities by seawater brown trout. Behaviour, less domestication and specific ability to utilise protein could explain the differences between the two species.

Pollack or lythe, Polluchius pollachius (Gadidae) is a carnivorous fish. Pollack occurs on the Atlantic Coast, from Portugal to the north of Norway. Annual landings are 16 200 tonnes (FAO, 1993). Data recorded on pollack biology are scarce. Large aggregations are observed during the spawning period. Spawning begins between February in Spain and May in Norway, in depths less than 150 m, at water temperature close to lO0C (Moreau, 1964). Mean egg diameter is 1.16 mm (Hislop and Bell, 1987). Hatched larvae are pelagic and their length ranges from 3 to 4 mm. Larvae occur near the surface, at depth lower than 10 m (Russel, 1976). Pollack growth is rapid: a weight of 0.9 kg was recorded by Dupouy et al. (1990) for three-year-old fish. Adult food is primarily composed of fish but also cephalopods and crustaceans (Du Buit, 1982). Aquaculture of pollack has not been reported in the literature. Because its growth is rapid, the captivity performances of pollack were tested in order to select new candidates for aquaculture. The purpose of this work was to examine the performances of this species during reproduction, larval rearing, weaning and ongrowing phases.

Three species of marine fish, sea bass, sea bream and turbot, and two salmonids, rainbow trout and brown trout, were raised under similar feeding and environmental conditions in order to compare their nitrogen utilisation. Apparent digestibility of protein, ammonia and urea excretion patterns, plasma ammonia and urea concentrations and liver arginase activities were measured.No differences in protein digestibility were noticed among the five species. Ammonia and urea losses were quantitatively similar in all species, except in turbot, in which ammonia production was significantly lower, Ammonia excretion patterns were linked to ingested nitrogen and showed no inter-species differences. Conversely, urea excretion patterns were specific in turbot and sea bream. Plasma urea levels were higher in marine fish than in salmonids, with the highest values being reached in turbot. Some specificity with regard to liver arginase was also detected in the turbot.Turbot demonstrated some metabolic characteristics that could be connected with its elevated position in the phylogeny of fish.

Laboratory experiments on juvenile turbot. Scophthalmus maximus (L.), were carried out under controlled temperature (12°C) and feeding regimes in a flow-through system. Monitoring of total ammonia nitrogen (TAN) and urea nitrogen (Urea-N) was performed through continuous sampling of the effluent sea water. The effects of ingested nitrogen levels on TAN and Urea-N daily and hourly excretion rates were studied. Strong relationships were found between ingested nitrogen, and both TAN and Urea-N excretion for both daily and hourly maximum excretion rates. Turbot showed a low metabolic activity, confirmed by low excretion levels. Daily patterns for TAN and Urea-N production were different, suggesting specific physiological phenomenon for urea excretion mechanisms. The question of whether turbot are partly ureotelic or ureogenic is underlined.

A wide range of methodologies is used for measurement of excretion by fish and for the evaluation of dissolved nutrient loadings from fish farms. In order to improve the accuracy and utility of monitoring devices, two specific apparatus have been adapted and improved: continuous sampling for simultaneous analysis and pooled samples continuous collection for later analysis. Provided that pooled samples are stabilized with 10 ml litre−1 chloroform at ambient temperature after phytoplankton and organic matter removal, these methods give similar and accurate results when applied to nitrogen-dissolved compounds, and are applicable to nutritional studies. Applied to soluble phosphate, the pooled method gives, after 72 h storage, results within 5% of the exact value. It is suitable for statistical evaluation of nutrient loadings on large-scale field studies.