The effects of salinity changes (27, 19 and 10%~)) on seawater-adapted juvenile turbot were studied on their plasma osmolarity aiid ion concentrations, on oxygen consumption, on gill Na',K'-ATPase activity after 3 months and on growth parameters. All plasma concentrations (except chloride) were unchanged, suggesting that fish were well adapted to their environment. Oxygen consumption was significantly decreased in the 19 and 10%0 groups, where fish weighed significantly more 105 days after transfer than fish maintained in sea water. These results, and the fact that apparent food conversion rates were iower in a diluted environment, suggest that on a long term schedule growth conditions could be improved by adaptation to brackish waters (salinities between 10 and 19%). The effects of transfer from sea water to 27, 19, 10 and 5% were also followed during the first 3 weeks. With salinity 10% a steady state was reached on day 21 with all plasma values within the same range. The significant differences observed in osmolarity, plasma ion concentrations and Na+,K+-ATPase activity 3 weeks after transfer of juveniles to 5%n salinity, compared with transfers in higher salinities, suggest that there is B threshold of acclimation of turbot to a hypotonic environment. 3: ) 1995 The Fisheries Society uf the British Isles

The effects of constant temperatures on growth, food efficiency, and physiological status were studied in four different batches of juvenile turbot. The growth responses were studied in three experiments lasting 70-85 days under 8-20 C thermal conditions. There was a positive correlation between growth and temperature from 8 to 17 C and a plateau was observed from 17 to 20 C. In fish fed to satiety, specific growth rate was positively correlated to the food intake, which was double at 20 C, compared with 8 C. Minor changes were observed in food efficiency. Body fat deposition decreased as temperature increased (25% lower at 20 C, compared with 8 C). Apparent food conversion, PER (protein efficiency ratio) and PUC (protein utilization coefficient) ranges were 0·8-0·9, 2·1-2·3 and 33-38% respectively. In 70-300 g fish, routine MO 2 increased (2·5-6·5 mol O 2 h 1 g bw 1 ) with temperature up to 20 C, while larger turbot (500-600 g) appeared relatively thermo-independent, with a lower oxygen consumption (1·5 mol h 1 g 1 ). The average daily total ammonia nitrogen (TAN) and urea-N excretion per fish biomass was positively related to temperature. TAN was 30% lower at 8 C, compared with 20 C. Ingested nitrogen was mainly excreted under the final form of TAN, urea-N representing 26% of the total amount. A post-prandial peak in TAN and a delayed peak in urea-N nitrogen were observed. The hydromineral status [osmolarity, sodium, chloride and potassium blood plasma, gill (Na + -K + )-ATPase activity] of turbot was not affected by progressive changes in temperature during the acclimation period. Juvenile turbots show remarkable homeostatic capacities and so they have a relatively thermo-independent physiology within the range of temperature studied. 1996 The Fisheries Society of the British Isles

When juvenile turbot Scophthalmus maximus and sea bass Dicentrarchus labrax were fed to satiation, growth and food intake were depressed under hypoxia (3·2 0·3 and 4·5 0·2 mg O 2 l 1 ). However, no significant difference in growth was observed between fishes maintained in hypoxia and fed to satiation and fishes reared in normoxia (7·4 0·3 mg O 2 l 1 ) and fed restricted rations (same food intake of fishes at 3·2 mg O 2 l 1 ). Routine oxygen consumption of fishes fed to satiation was higher in normoxia than in hypoxia due to the decrease in food intake in the latter. Of the physiological parameters measured, no significant changes were observed in the two species maintained in hypoxia. This study confirms the significant interaction between environmental oxygen concentrations, feeding and growth in fishes. Decrease in food intake could be an indirect mechanism by which prolonged hypoxia reduces growth in turbot and sea bass, and may be a way to reduce energy and thus oxygen demand. 2001 The Fisheries Society of the British Isles

The effects of hypoxia on growth, feed efficiency, nitrogen excretion, oxygen consumption and metabolism of juvenile turbot (120 g) were studied in a 45-day experiment carried out in sea water at 17.0±0.5°C and 34.5 ppt salinity. Fish were fed to satiation at O2-concentrations of 3.5±0.3, 5.0±0.3 mg l−1 (hypoxia) and 7.2±0.3 mg l−1 (normoxia). Both feed intake (FI) and growth were significantly lower under hypoxia than under normoxia, with no significant differences being observed between 3.5 and 5.0 mg O2 l−1. During the first 2 weeks of the experiment, FI was halved under hypoxic conditions, and there were large differences among treatments in feed conversion ratio (FCR), i.e., it was 3.2, 1.5, and 0.9 in turbot exposed to 3.5, 5.0, and 7.2 mg O2 l−1, respectively. Thereafter, FCR was not significantly affected by O2-concentration. Nitrogen excretion and oxygen consumption of feeding fish were significantly higher under normoxia than under hypoxia, but following 7 days of feed deprivation oxygen consumption was similar under normoxia and hypoxia. Plasma osmolarity, ionic balance, and acid-base status were not affected by the two hypoxic conditions tested. Overall, our results indicate that turbot have some capacity to adapt to relatively low ambient O2-concentrations.

An 84-day experiment assessed the combined effects of two fresh water quality levels (H: 1.71 ± 0.15 mg O 2 l −1 and 0.28 ± 0.01 mg l −1 total ammonia nitrogen (T-AN), L: 5.15 ± 0.07 mg O 2 l −1 and 0.54 ± 0.01 mg T-AN l −1 ) and 3 stabilized stocking densities: 24.8 ± 0.2, 74.2 ± 0.5 and 120.0 ± 0.9 kg m −3 ) on rainbow trout. Fish were fed using demand feeders with rewards proportional to stocking density. Mass increase was significantly affected by water quality and stocking density, being highest in H water and the lowest at 120 kg m −3 . There was no significant difference in final weight between 25 and 74 kg m −3 , but at 120 kg m −3 it was 27% and 19% lower in H and L water respectively than at 25 kg m −3 . Feed intake (FI) from day 0-85 was significantly affected by water quality, 1.5% in H compared to 1.1-1.0 in L, but there were no significant differences in apparent feed conversion (AFC). FI was not significantly affected by stocking density but AFC was impaired, it increased with stocking density. Marked changes in fish morphology and composition were related to water quality: Condition K factor and fillet fat content were significantly higher in H than in L groups. Dorsal and pectoral fin condition was affected by stocking density and water quality: fins were significantly longer and less eroded in L groups and at low stocking density. Physiological measures were within the usual ranges, but differed between treatments. Changes in plasma osmolarity, hydromineral balance (Na + ) and acid base balance (HCO −3 ) showed that fish were more affected by water quality than by stocking density. There was no sign of acute stress in acclimated fish as cortisol and glycemia were similar under all experimental conditions. This study highlights the importance of water quality and feeding conditions when considering the effects of stocking density on fish welfare.