Development of a reference diet for use in indispensable amino acid requirement studies of gilthead seabream Sparus aurata L.

The development of a test diet supporting good fish growth to be used in gilthead seabream indispensable amino acid (IAA) requirement studies based on dose–response relationship was the objective of the present study. Four isonitrogenous diets, one serving as control (diet C) based on fish meal LT, and three others in which the fish meal component was reduced to 45% and the rest of the nitrogen was provided by mixtures of crystalline AA were formulated. Diet S resembled the whole body IAA profile of wild seabream. Diet D was made deficient in all IAA and a 10% excess of IAA compared with diet S was used in diet E. Triplicate groups of fish were fed the diets to satiation for 6 weeks. Growth parameters and nutrient utilization efficiencies among diets C and S were similar, while excessive supplementation of IAA negatively affected most of them, compared with the control. Whole body composition was differentiated in relation to the diet fed. No significant differences were observed among groups fed the control and diet S. As shown by the results, the presence of crystalline AA in diet S does not significantly affect growth, and could therefore serve as control diet in IAA requirement studies of gilthead seabream.     

Apparent digestibility coefficients of feed ingredients and their predictability in compound diets for gilthead seabream, Sparus aurata L.

Apparent digestibility of crude protein, amino acids, lipid, carbohydrate and energy was measured for a range of feed ingredients fed to gilthead seabream, Sparus aurata L. — fish meal, poultry meal, meat meal, blood meal, squid meal, extracted soyabean and wheat flour. Chromic oxide was used as a non-absorbed reference substance and faeces were collected by stripping. Diets compounded from mixtures of these ingredients were then used to examine the possibility of predicting the digestibility of formulated diets.
Apparent digestibility of crude protein ranged from 79% to 90%, lipids from 83% to 95% and energy from 72% to 88% in the different ingredients. Apparent digestibility of carbohydrates was lower and ranged from 49% to 77%. Apparent digestibility of amino acids was higher than that of crude protein and differences were found among digestibilities of individual amino acids.
Tests conducted using five compound diets indicated that ingredient digestibility was additive for protein, amino acids, lipids and energy, whereas the digestibility of carbohydrates in the compound feeds was slightly lower than predicted.
Diets for Sparus aurata may thus be formulated on the basis of digestibility of individual ingredients.     

Effects of the dietary protein : lipid ratio on growth and nutrient utilization in gilthead seabream (Sparus aurata L.)

Gilthead seabream Sparus aurata L. (initial mean body weight: 42.5 g) were fed four experimental diets containing either 47 or 51% of dry matter (DM) as protein and either 15 or 21% as lipid for 12 weeks. Each diet was hand-distributed to triplicate groups of 60 fish, three times a day until satiation. The digestibility coefficients of the dietary components were determined using chromic oxide as a marker. The levels of protein or lipid in the diets did not affect the digestibility. Fish regulated their feed intake and attained the same weight at the end of the experiment. However, feed efficiency varied between diets, with best values obtained with both diets containing 21% lipid. When diets contained only 15% lipid, feed efficiency increased with dietary protein level. Nitrogen retention was significantly higher with high fat diets regardless of dietary protein level. Neutral lipid deposition was significantly higher in liver for diets rich in lipids. It was elevated in muscle only in fish fed the diet containing 47% protein and 21% lipid and this deposition in muscle contributed to a significant increase in body fat content. Phosphorus load to the environment, measured as percentage retention of ingested or digestible phosphorus, was significantly lower with both diets higher in lipids.     

Towards an inert diet for first-feeding gilthead seabream Sparus aurata L. larvae

The development of an inert food to replace live prey during the early stages of marine fish larvae requires research in different fields and therefore a precise work strategy. Our research on this subject has been carried out in successive steps using the gilthead seabream Sparus aurata . The first step was the design of a food particle that would be well accepted and ingested by free-swimming marine larval fish during the first developmental stages. We chose microencapsulation by polymerization of the dietary protein as the most appropriate method for making the particles; different types of microcapsules were made using a basic diet containing only the major dietary components. In the second step, our aim was to keep the larvae alive in a routine rearing system in 300-L tanks, using exclusively this kind of food, long enough to detect any changes in growth, survival, or anatomical and histological status of the larvae, in order to verify whether the technological changes were positive. The third step focused on diet formulation and searching for clues to inefficient assimilation and growth. The use of ' in vitro ' digestibility techniques allowed us to detect the inhibitory effect of some diet ingredients on larval proteases and to determine more suitable sources of protein. We now have a microcapsule able to efficiently support growth and development of S. aurata larvae , at least during the first 2 weeks of life, although the larvae still need to feed on rotifers during the first 2–4 days of exogenous feeding. This microcapsule will make it possible to make advances in determining the specific nutritional requirements of larval fish.     

Amino acid retention of gilthead sea bream (Sparus aurata,L.) fed with pea protein concentrate

A trial of 218 days of duration was carried out to assess the use of pea protein concentrate (PPC) as a substitute for fish meal in diets for juvenile gilthead sea bream (52 g average initial weight), using four diets (0, 16, 32 and 48) with PPC inclusion levels of 0, 162, 325 and 487 g kg^?1, respectively. At the end of the trial, the fish reached weights of 397, 385, 383 and 355 g for 0, 16, 32 and 48 diets, respectively; diet 48 gave the lowest specific growth rate (SGR, 0.88% per day) but 0%, 16% and 32% PPC did not present statistical differences. Feed conversion ratio (FCR), protein efficiency ratio (PER) and body composition were not affected by the diets. Digestible protein retention was significantly higher with the 0% diet, but no energy retention differences were observed. Retention rates of ingested and digestible amino acids were very similar; in general, the retention of essential amino acids decreased as dietary PPC increased, and lysine retention was the highest and phenylalanine retention was the lowest. From the results of this experiment, it may be concluded that fish meal can be replaced up to 32% by PPC in sea bream without negative effects on fish weight gain, FCR and PER.     

Mono- and duoculture of juvenile sharpsnout seabream Diplodus puntazzo (Cetti) and gilthead seabream Sparus aurata L. in a recirculated water system

Although the growth of intensively reared sharpsnout seabream Diplodus puntazzo (DP) has been suggested to improve in duoculture with gilthead seabream Sparus aurata (SA), the stocking ratio for most effective duoculture rearing has not been investigated. For this purpose, juvenile D. puntazzo (1.68±0.011 g) and S. aurata (1.43±0.007 g) were reared in duplicated groups of 165 specimens each, in two monoculture (100% DP, 100% SA) and four duoculture combinations (80% DP–20% SA, 60% DP–40% SA, 40% DP–60% SA, 20% DP–80% SA) for 96 days under recirculated water system conditions. Both species exhibited the highest growth when stocked at 20% and the poorest growth when stocked at 80%. For each species, no major differences for coefficient of weight variation and carcass proximate composition were detected. As the percentage of stocked S. aurata increased, D. puntazzo aggressiveness and weight variability of whole population were reduced, while total biomass, food consumption and food conversion ratio were increased. The results obtained are discussed in relation to fish behaviour and social interactions. From the fish farming point of view, it is indicated that under the present experimental conditions, juvenile S. aurata should be reared in monoculture and D. puntazzo in duoculture 40% DP–60% SA.     

Effects of dietary dehydrated lemon peel on some biochemical markers related to general metabolism,welfare and stress in gilthead seabream (Sparus aurata L.)

Dehydrated lemon peel (DLP) at two levels (1.5% and 3%) was included for 30 days in the diet of gilthead seabream (Sparus aurata L.) and after 15 and 30 days the serum and skin mucus of fish were analysed. In serum, both experimental diets led to an initial decrease in glucose and lactate after which the levels became comparable to the control. Dietary DLP modulated the activity of transaminases, alanine aminotransferase and aspartate aminotransferase. The total antioxidant status showed a progressive increase in relation to levels of inclusion of DLP and time of administration (p < .05). In skin mucus, some biomolecular markers related to general stress, oxidative stress and apoptosis exhibited variations that suggest an adaptive response of fish to the new metabolic situation, resulting from the experimental diet. The inclusion of DLP in the diet seems to positively modulate the central metabolism and welfare of gilthead seabream.     

Pharmacokinetics of oxolinic acid in gilthead sea bream, Sparus aurata L.

This is the first study on the pharmacokinetic parameters of oxolinic acid (OA) in gilthead sea bream, Sparus aurata L. The kinetic profile of OA was studied after a single intravascular injection (20 mg kg⁻¹) in 100 g fish at 20 °C. The distribution half-life ( t _1/2α) and the elimination half-life ( t _1/2β) of the drug were found to be short (0.51 and 12.60 h, respectively). The drug penetration from the plasma to the tissues was adequate as the apparent volume of distribution of the drug at steady-state ( V _d(ss)) was found to be 2.11 L kg⁻¹. The mean residence time ( MRT ) of OA was short (14.25 h) and the total clearance rate ( Cl _T) of the drug was low (0.15 L kg⁻¹ h⁻¹). The bioavailability ( F %) of OA following oral administration (30 mg kg⁻¹) was also low (14%). Maximum values were observed for muscle at 0.5 h after injection, with levels declining as with subsequent sampling. At the first two time points (0.5 and 1 h) plasma levels of OA were higher than muscle, however, the reverse was evident for subsequent samples. Following oral administration, highest muscle levels were found at 16 h and, with the exception of the 24-h sampling, muscle OA concentrations were higher than plasma at all time points. The fast elimination of OA suggests short withdrawal times with reference to human consumption of treated fish.     

Energy and protein requirements for maintenance and growth in gilthead seabream (Sparus aurata L.)

Factorial determinations of energy and protein requirements in growing Sparus aurata were carried out at 23–24°C. The energy content in the whole fish was dependent on fish weight and ranged from 5 to 11 MJ kg⁻¹ body mass for 1–250 g fish, whereas the protein content remained constant at 179 g kg⁻¹.
During starvation the fish lost 42.5 kJ body weight (BW) (kg)^−0.83 day⁻¹ and 0.42 g protein BW (kg)^−0.70 day⁻¹. The maintenance requirement for energy was calculated to be 55.8 kJ BW (kg)^−0.83 day⁻¹ and for protein 0.86 g BW (kg)^−0.70 day⁻¹. Utilization of digestible energy and digestible crude protein below and at maintenance was determined as 0.72 and 0.51, respectively. Utilization of digestible energy and digestible crude protein for growth above maintenance was determined as 0.46 and 0.28, respectively.
These values allow estimation of requirements for growing Sparus aurata .     

Pigmentation of gilthead seabream, Sparus aurata (L. 1875), using Chlorella vulgaris (Chlorophyta, Volvocales) microalga

A feeding experiment was conducted over 9 weeks with seven groups of 30 (fish per group) unpigmented gilthead seabream, Sparus aurata (L. 1875) (initial mean weight = 145.2 ± 12.3 g). Three experimental diets were prepared by adding to a basal diet free of carotenoid (final pigment content of around 40 mg per kg feed): (i) a biomass of the carotenogenic Chlorella vulgaris (Chlorophyta, Volvocales); (ii) a synthetic astaxanthin; and (iii) a mixture (1:1) of microalgal biomass and synthetic astaxanthin. At 3‐week intervals, five fish were sampled from each tank for total carotenoids analysis in skin and muscle. The carotenoid pigments (total amount = 0.4%) identified in the carotenogenic alga were lutein (0.3%), β‐carotene (1.2%), canthaxanthin (36.2%), astaxanthin, free and esterified forms (55.0%), and other pigments (7.3%). Carotenoid pigments were significantly deposited in the four skin zones studied during the feeding trial: the forefront between the eyes, the opercule, along the dorsal fin and in the abdominal area. In the muscle, regardless of the astaxanthin source, the amount of carotenoids measured was very low (less than 1 mg kg^?1) and differences not significant. Moreover, no muscle pigmentation was evident, and there was no variation in the amount of carotenoid analysed in skin tissue, through the trial, for each treatment. It was concluded that supplementing the feed with C. vulgaris would be an acceptable practice in aquaculture to improve the market appeal of the gilthead seabream.     

Cottonseed oil as a complementary lipid source in diets for gilthead seabream Sparus aurata juveniles

The efficacy of using cottonseed oil (CSO) as a fish oil (FO) substitute in gilthead seabream (Sparus aurata) juveniles feed was evaluated. Fish (BW_i 4.0 ± 2.9 g) were fed one of four isoproteic (~48% CP) and isolipidic (~18% L) diets for 9 weeks. Added oil was either FO (control diet, CTRL) or CSO, replacing 50% (CSO50 diet), 60% (CSO60 diet) and 70% (CSO70 diet) of dietary FO. Results indicated that FO replacement by CSO up to 60% level had no detrimental effects on growth or nutritive utilization and composition in fish muscles. Higher CSO intake (CSO70 diet, 56 g kg^?1) led to a 16% reduction in weight gain, 14% in feed utilization (FCR) and 57% in muscle n‐3 long‐chain polyunsaturated fatty acids (lc PUFA) as compared with CTRL and to abundant accumulation of lipid within the hepatocytes. Use of CSO altered fatty acid (FA) profiles of muscle and liver. Data suggested utilization of linoleic acid (LOA) by fish and retain of docosahexaenoic acid (DHA) in muscles. Therefore, limits of CSO inclusion as the main source of supplementary dietary lipid, with no negative effects on fish performance or nutritive composition and utilization in muscles, are: 40–48 g kg^?1 feed for gilthead seabream juveniles.     

Effect of dietary vegetable oils on health and liver histology of gilthead seabream (Sparus aurata) growers

There is an increasing trend towards the replacement of fish oil (FO) by vegetable oils in feeds for farmed fish, particularly marine carnivores. Previous researches on partial or complete replacement of FO by alternate vegetable oils for young gilthead seabream (Sparus aurata) were successful. The aim of the present study was to investigate the effects of different dietary vegetable oils on seabream growers' health and liver structure. Two dietary vegetable oil blends (Mix 1 and Mix 2), namely sunflower oil, cottonseed oil and either linseed oil (LO) or soybean oil (BO), were tested as 60% FO substitutes versus the 100% FO control (CTR) diet. Three isonitrogenous (∼47% CP) and isolipidic (18.2% L) diets were fed, twice a day, 6 days a week to apparent satiation, to triplicate groups of seabream growers (mean IW, 130.85 g) in PVC tanks. The growth trial lasted for 20 weeks at an average ambient water temperature of 27.0±1.8°C. At the end of the experiment, fish attained market size (300–400 g) and their health status was evaluated by blood analyses, head kidney prints and liver histology. Haemoglobin and haematocrit contents, erythrocyte (RBC) and leucocyte (WBC) count remained significantly unaffected by any dietary treatments. Despite the apparent relatively higher thrombocyte counts for fish fed vegetable oil diets, especially Mix 2 as compared with those of CTR fish, variations were found to be insignificant (P>0.05). However, a slight deformation in erythrocyte shape was noticed in the blood films of fish fed the vegetable oil diets. Inclusion of either dietary vegetable oil blend had also elevated the number of both developing myelocytes and phagocytes in fish head kidney prints examined compared with those of CTR fish particularly for Mix 2 group. Liver from seabream fed vegetable oils showed slightly swelled hepatocytes due to accumulation of oil droplets in the cytoplasm. In the liver sections investigated ultra‐structurally, nuclei and cytoplasmic organelles were shifted towards the cell periphery. Hepatocytes of fish fed Mix 2 diet showed relative shrinkage and picnotic nuclei, compared with those of CTR fish, suggesting an effect on lipid transport and/or metabolism in fish. The comparative hepatic features between experimental fish groups were described. Accordingly, Mix 1 diet (with LO) was considered better than Mix 2 (with BO) in terms of influence on seabream growers' health status.     

金头鲷的形态学及框架结构的研究

从初孵仔鱼培育到17月龄鱼种中,检测50尾鱼(鱼龄17个月)的体重(g)、体长(cm)、全长(cm)、尾叉长(cm)、体高(cm)、体宽(cm)、头长(cm)、吻长(cm)、眼后头长(cm)、眼径(cm)、尾柄长(cm)和尾柄高(cm)等的平均值及标准差分别为241·11±49·56、18·72±1·30、24·15±1·39、22·02±1·23、8·20±0·63、3·18±0·31、5·56±0·36、1·98±0·28、2·47±0·21、1·52±0·09、2·56±0·45、1·96±0·17;体长/体高、体长/头长、体长/尾柄长、体长/眼径、头长/眼径、头长/尾柄长、头长/尾柄高、尾柄长/尾柄高、头长/眼后头长和体高/体厚等的平均值分别为2·29±0·10、3·33±0·18、7·49±1·25、12·37±1·12、3·72±0·30、2·26±0·43、2·88±0·25、1·31±0·21、2·29±0·16和2·59±0·22。其可数性状:背鳍、胸鳍、腹鳍、臀鳍等的鳍式为D.Ⅺ-12~14、P1.5 (9~10)、P2.Ⅰ-1 4、A.Ⅲ-11;侧线鳞为56~69枚、侧线上鳞为6~8枚;脊椎骨数为9~10(胸骨) 13~14(尾骨)根;上颌齿式为:1~5 7~10、2~13、4~9、0~9/0~7、3~10、3~15、4~11 1~4;下颌齿式为:1~2 6~15、4~10、0~8/0~7、4~9、6~12 1~2。通过观察,运用传统测量法对其外观的可数、可量性状进行测量与分析,并结合Humphsies,Bookstein和Bookstein等提出的框架法(Truss)思路来构建金头鲷的框架结构图,几何地描述金头鲷的体形特征,可为研究金头鲷的种质提供科学依据。     

Development of a microsatellite multiplex genotyping tool for the fish Gilthead seabream (Sparus aurata): applicability in population genetics and pedigree analysis

In spite of the numerous studies performed in the aquacultured fish gilthead seabream, the overall structure of wild and cultivated stocks is rather confusing. In this study, we developed a 10 microsatellite genotyping tool SaGT(6+4), conformed by two polymerase chain reaction‐multiplex reactions (SaGT6 and SaGT4), which can be simultaneously combined in automatic sequencers. The utility of this tool was proven through the following applications: (i) characterization and differentiation of wild and cultivated populations; (ii) pedigree reconstruction and estimation of the effective size in a cultivated stock; (iii) ability for pedigree reconstruction under different simulated situations; and (iv) determination of genetic relationships in the absence of data from parents. Based on our results, some recommendations have been provided on the management of the screened stocks. Our results also support the use of this tool in a standardized way, to understand the actual status of gilthead seabream from both wild and cultivated populations.     

Effect of diet composition on nutrient digestibility and digestive enzyme levels of gilthead sea bream (Sparus aurata L.)

An experiment was conducted to evaluate the effects of dietary composition on apparent digestibility coefficients of protein, fat and carbohydrate of gilthead sea bream. In addition α‐amylase and proteolytic activities were measured in the digestive tract of fish, held at 20±1°C. Six experimental diets were formulated containing approximately 40%, 45% and 50% protein, 11% and 21% fat and a starch level which fluctuated from 14% to 36%. Fish having an average weight of 100–130 g were used, fed at 1.5% of their body weight daily, and digestibility was measured using an indirect method. Enzyme activities were measured in the digestive tract of fish, fed diets containing a combination of 40%, 50% protein with 11%, 21% fat at 0.5, 5, 10, 24 h after a single meal. Starch digestibility was reduced with its level in the diet. It also negatively affected fat digestibility as well as protein digestibility, the last only at the high fat level. Dietary fat level had a negative effect on starch and protein digestibility. Fat affected also strongly α‐amylase levels in the digestive tract, while its effect on protease activity was of smaller magnitude. These results indicate that digestive enzyme activities and nutrient digestibility values are affected by dietary composition, carbohydrates and fat indicating the strongest effect.     

Virulence factors and pathogenicity of Hafnia alvei for gilthead seabream, Sparus aurata L

Virulence factors (eae gene, haemolytic capacity, fimbriae, resistance to the bactericidal effect of serum, siderophore production) and pathogenicity for gilthead seabream, Sparus aurata L., were analysed for 23 Hafnia alvei strains. None of the strains used in LD50 studies were lethal for seabream at doses as high as >10(8) cfu mL(-1). In chronic challenge studies differences in severity of the inflammatory response were observed between strains. On the basis of correlation of the inflammatory response to different strains of H. alvei in seabream with those virulence factors studied, it was only possible to establish a positive correlation between pathogenicity and resistance to the bactericidal effect of fish serum. Gilthead seabream is thus a species with considerable resistance to experimental infection with H. alvei. The bacterium does, however, have the capacity to remain viable in seabream for up to 3 months, without any clinical signs. Hafnia alvei is a well-recognized human and animal pathogen. Thus, as the pathogen can coexist with aquaculture operations, cultured gilthead seabream could represent a risk to human health as a carrier in some circumstances.     

Optimum dietary soybean meal level for maximizing growth and nutrient utilization of on-growing gilthead sea bream (Sparus aurata)

Six isonitrogenous [450 g kg⁻¹ crude protein (CP)] and isoenergetic diets (23 kJ g⁻¹) with six levels of defatted soybean meal inclusion (0, 132, 263, 395, 526 and 658 g kg⁻¹) in substitution of fish meal were evaluated in gilthead sea bream of 242 g initial weight for 134 days. Fish fed diets S0, S13, S26 and S39 had a similar live weight (422, 422, 438 and 422 g, respectively) but fish fed diets S53 and S66 obtained the lowest final weight (385 and 333g, respectively), and similar results were presented in specific growth rate (SGR). Fish fed diets S53 and S66 also obtained the highest feed conversion ratio (FCR). Quadratic multiple regression equations were developed for SGR and FCR which were closely related to dietary soybean level. The optimum dietary soybean levels were 205 g kg⁻¹ for maximum SGR and 10 g kg⁻¹ for minimum FCR. Sensorial differences were appreciated by judges between fish fed S0 and S39 soybean level, but after a re-feeding period of 28 days with diet S0, these differences disappeared.     

Recovery of opercular anomalies in gilthead sea bream, Sparus aurata L.: morphological and morphometric analysis

Opercular anomalies are very frequent in reared gilthead sea bream and these can negatively influence the product value. Field observations have suggested that opercular malformations can recover over time. In order to verify this hypothesis, 140-day-old gilthead sea bream with monolateral opercular anomalies were divided into three groups, according to the type and increasing seriousness of the opercular malformations, and another group was composed of fish with bilateral opercular anomalies. All groups were monitored for 16 months. In the group with monolateral anomalies, the opercular recovery process was documented by morphological (stereomicroscope) and morphometric analysis. For the latter analysis, two relevant areas, A and T, were identified in the cephalic region. The ratio (T - A)/T, tending to 1, represents the recovery index (RI) of anatomical integrity and quantifies the recovery level of opercular complex anomalies. Results suggested that the recovery process was considerable over the 16 months of investigation but should not be considered complete. At the end of the study, 61% of the gilthead sea bream population with monolateral opercular defects recovered external integrity, whereas the population with bilateral defects showed a poorer recovery capability.     

Effects of chronic confinement on physiological responses of juvenile gilthead sea bream, Sparus aurata L., to acute handling

Understanding how gilthead sea bream, Sparus aurata L., an important Mediterranean Sea species for aquaculture, respond physiologically to stressors commonly encountered in intensive rearing is important for effective production, as managing for stress is a major factor in maintaining healthy fish stocks. Our objective was to determine whether holding juvenile gilthead sea bream at a high density (HD), as a chronic stressor, would affect their physiological responses to a subsequent acute handling stressor. After acclimation at a low density (LD) of 6 kg m^?3 in 200‐L circular tanks containing 33–36 g L^?1 recirculating seawater at 19°C under a normal photoperiod, juvenile 37‐g gilthead sea bream were confined for 14 days at a HD of 26 kg m^?3 and then subjected to 30‐s aerial emersion in a dipnet. Plasma levels of cortisol, glucose, lactate, osmolality and chloride were determined in fish held in separate lots during LD (control) and HD confinement at 0, 1, 2, 7 and 14 days, and then after handling at 0, 1, 2, 4 and 8 h. Although plasma cortisol levels were similar in LD and HD fish groups after 14 d of confinement (15 and 23 ng mL^?1, respectively), the cortisol response in fish from the HD treatment at 1 and 2 h following acute handling (70 and 37 ng mL^?1, respectively) was only about half of that measured in the control group (139 and 102 ng mL^?1); plasma cortisol was similar in both groups by 4 and 8 h. In contrast, plasma glucose elevations in response to handling were higher at 4 and 8 h in the HD‐held fish (94 and 72 mg dL^?1, respectively) than in those from the LD treatment (59 and 51 mg dL^?1); glucose responses were similar in both groups at 1 and 2 h after handling and throughout confinement. Plasma lactate levels were higher in LD fish than in the HD group at the beginning of the experiment but were similar after 14 d confinement and responses to handling were similar (e.g. 33 and 35 mg dL^?1 at 1 h). Plasma osmolality showed increases during the first 2 h after acute handling but no differences were evident between the two density treatments at any time during confinement or posthandling. Plasma chloride levels did not change throughout the experiment. The reduced plasma cortisol response to acute handling likely resulted from negative feedback of mildly but chronically elevated circulating cortisol caused by the confinement stressor on the hypothalamic–pituitary–interrenal axis. While other post‐handling physiological changes also showed differences between treatment groups, the suppressed cortisol response in the HD‐held fish suggests a reduction in the gilthead sea bream's normal capacity to respond to an acute stressor.