Effect of rearing density on the growth and welfare indices of juvenile spotted wolffish,Anarhichas minor (Olafsen)

The goal of this study was to determine the optimal stocking density for rearing juvenile spotted wolffish, Anarhichas minor (Olafsen), at two different sizes and assess the welfare status in relation to density. No major growth impairment was observed, although smaller fish (50–100 g) were significantly affected by density during the 120 days of the experiment, with final mean weights of 119.6 ± 11.6, 118.0 ± 5.8 and 88.7 ± 0.6 g for initial rearing densities of 10, 20 and 40 kg m^?2 respectively. No effect of rearing density was seen for larger fish (100–160 g) during the 90 days of the experiment, with final mean weights of 160.2 ± 5.9, 159.7 ± 3.7 and 163.7 ± 11.5 g at fixed rearing densities of 20, 30 and 40 kg m^?2 respectively. Our results suggest that the optimal rearing density of juvenile spotted wolffish is below 40 kg m^?2 for smaller size fish (～50–100 g) and probably ≥40 kg m^?2 for the larger fish (100–160 g). Furthermore, it appears that the range of rearing density used did not have a significant effect on a selection of stress indicators (Na⁺ and K⁺ concentration, haematocrit, hepatosomatic index, total amount of plasma proteins and liver and muscle water content) and immunity response (plasma lysozyme activity) of juvenile spotted wolffish, making it a very tolerant species to crowding.     

Plasmatic levels of cortisol in the response to acute stress in Nile tilapia, Oreochromis niloticus (L.), previously exposed to chronic stress

Many studies have been made about the physio-logical effects of isolated chronic or acute stress. However, few studies have been made to assess the combination of both responses. The fish submitted to chronic stress may be subjected to an additional acute stressor. The aim of the present work was to evaluate the acute stress response in Nile tilapia Oreochromis niloticus (L.) previously subjected to chronic stress. For this, two experiments were performed. In the first experiment, the fish were subjected to chronic stress followed by an additional acute stress. In the second experiment, the fish were submitted only to an acute stress. The data showed that Nile tilapia fingerlings can adapt to chronic stress situations, and this decreases, but does not eliminate, their capacity to respond to an additional acute stressor. In both experiments, plasma cortisol levels reached a peak 1 h after administration of the acute stressor. In fish previously submitted to chronic stress, the highest concentration of plasma cortisol measured was 196 ng mL^–1. This value was significantly different from the cortisol concentration obtained in the second experiment (267 ng mL^–1) with non-chronically stressed fish. The data also suggest that the chronic stress response can provoke a reduction in performance and growth rates compared with non-stressed fish.     

Stress responses of Atlantic salmon (Salmo salar L.) elicited by water level reduction in rearing tanks

The Atlantic salmon (Salmo salar) stress response was examined by measuring plasma cortisol, glucose and chloride in fish after water level reduction within rearing tanks. Maximum plasma cortisol levels (366±43 (SD) nM and 534±280 nM for Groups 1 and 2, respectively) were observed 20 min after application of the stressor. Cortisol levels were down to control levels 24 h later. The pattern for cortisol changes observed within these two groups were comparable. Plasma glucose and chloride concentrations did not change significantly in either of the experimental groups. The fish in group 2 were then repeatedly subjected to the same stressor every third day. After the fifth exposure to the stressor, blood was obtained from fish at times corresponding to sampling after the first exposure. Their maximum plasma cortisol level only reached 223 (±96) nM, and was down to prestress levels within 2 h. Plasma glucose and chloride concentrations did not change significantly in this second experiment. This challenge test revealed an acute primary response in Atlantic salmon without any apparent harmful secondary responses, that may thus serve as a standardized reference stressor using other fish groups under comparable conditions.     

Physiological stress responses in spotted wolffish (Anarhichas minor) subjected to acute disturbance and progressive hypoxia

The spotted wolffish (Anarhichas minor) is considered a promising species for coldwater aquaculture. It is a sedentary, bottom-dwelling fish which exhibits a calm and “non-stressed” behaviour in captivity. There are, however, no reports on the physiological responses to stressors in this species. In the present study we investigated primary (cortisol secretion) and secondary (glucose mobilization) responses to common aquaculture stressors like disturbance and hypoxia. Pre-stress plasma cortisol levels were within those generally considered representative for unstressed fish (~ 10 ng ml^− 1), whereas basal glucose levels were unusually low (0.3–0.4 mM). After exposure to a short, but severe disturbance challenge (emptying the tank of water for 10 min), the increase in plasma cortisol level was slow and relatively weak, reaching a peak level of 25 ng ml^− 1 4 to 8 h after disturbance. When the fish were exposed to a gradually decreasing oxygen level in the tank, a significantly elevated plasma cortisol level (35 ng ml^− 1) was seen in the fish that remained in the tank until oxygen saturation had decreased to 20% oxygen saturation after 2.5 h. However, a two-fold higher plasma cortisol level (~ 70 ng ml^− 1) was seen in all fish exposed to reduced oxygen levels (60, 40 and 20% oxygen saturation) after 3.5 h recovery in normoxic water. Plasma glucose levels showed only moderate increases (~ 70%) following disturbance and hypoxia challenges. An in vivo injection of ACTH caused a strong elevation of plasma cortisol (peak level ~ 170 ng ml^− 1), demonstrating a high capacity for interrenal steroidogenesis in the spotted wolffish. The slow and relatively weak cortisol response to stressors, and low plasma glucose levels, may relate to the sedentary lifestyle of the spotted wolffish. The stress-response is characterised by a passive (reactive) coping style, which is considered adaptive for farming of this species.     

Whole-body corticosteroid and plasma cortisol concentrations in larval and juvenile Atlantic cod Gadus morhua L. following acute stress

Methods were developed to assess whole‐body immunoreactive corticosteroid concentrations (IRC) and plasma levels of cortisol in Atlantic cod subjected to several common, acute stressors. A measurable corticosteroid stress response was observed at the first sampling in whole bodies of 8‐day post‐hatch (dph) larvae. Two groups of juveniles (～5 and 30 g) were subjected to a 30 s net stressor and whole‐body IRC and plasma cortisol levels were determined. Post‐stressor IRC in smaller fish rose approximately 14‐fold, peaked at 1 h, were sustained for 3–6 h and returned to pre‐stressor levels within 24 h. Post‐stressor plasma cortisol levels in larger fish rose approximately 18‐fold, peaked at 0.5–1 h, were sustained for 1–3 h and then returned to near pre‐stressor levels after 24 h. Immunoreactive corticosteroid concentrations appeared to remain elevated longer than plasma cortisol levels suggesting that steroids other than cortisol were contributing to total immunoreactivity in homogenates. Juveniles exposed to either a grading procedure or high density transport had maximal IRC and plasma cortisol levels within 90 min which returned to pre‐stressor levels within 24–72 h. A reduction in water salinity (20 g L^?1) did not moderate the corticosteroid response in juveniles. The results show that Atlantic cod respond to common, acute stressors in a manner similar to other teleosts. Whole‐body homogenates can be used to identify changes in IRC in response to acute stress in cod with the caveat that recovery IRC may differ from plasma cortisol concentrations.     

Diel changes in plasma cortisol and effects of size and stress duration on the cortisol response in European sea bass (Dicentrarchus labrax)

European sea bass (Dicentrarchus labrax), one of the most economically important fish in Mediterranean mariculture, shows high basal cortisol concentrations compared with other teleosts. The present study aims (a) to identify cortisol diel variation in fish held under a 12L:12D cycle and minimum handling stress, and (b) to establish the effect of fish size and stressor duration on the cortisol response. The results indicate high intrapopulation variability in plasma cortisol and a significant diel fluctuation with a peak value at dusk (18 h). Stressors of different intensity and/or duration affected the cortisol stress response in a differential manner according to fish size (and/or age). Maximum cortisol values in small-size fish were found at 1 and 2 h post-stress, depending on the duration of the stressor, while at 0.5 h post-stress in large fish regardless stress duration.     

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.     

Increased stocking density influences the acute physiological stress response of common carp Cyprinus carpio (L.)

The physiological response of common carp, Cyprinus carpio (L.) to increased stocking density and an additional acute net confinement stressor was investigated. Stocking densities were increased from 28.4 to 56.8 or 113.6 kg m^?3 by the use of crowding screens and fish were sampled from the crowded groups after 15, 39 and 87 hours of crowding (hc). A transient elevation of plasma cortisol was found in the higher density group after 15 hc before values returned to control levels. Increased stocking density also increased plasma levels of glucose, free fatty acids (FFA) and lactate during the experimental period. No effect was found on oxygen radical production in the blood, haemoglobin or haematocrit levels, but leucocrit values were lower after 39 hc in both groups. Net confinement resulted in a significant increase in plasma cortisol levels in all groups. However, after 15 hc, cortisol and FFA levels in both crowded plus confinement groups were higher than in the control and confined groups. Confinement resulted in increased oxygen radical production in the crowded plus confined groups at all times. Results indicate that although carp responded with a mild stress response to increased stocking density and adaptation occurred by 87 hc, the crowded fish were more sensitive to an additional acute stressor.     

Selection for improved stress tolerance in rainbow trout (Oncorhynchus mykiss) leads to reduced feed waste

The magnitude of the cortisol response to a standardised stressor is a heritable trait in salmonid fishes, and selection for stress responsiveness induces differences in both behaviour and neuroendocrine function. For instance, in laboratory studies, fish with a high cortisol response show a greater propensity for stress induced anorexia. Some authors have, however, commented that controlled studies encompassing relatively small groups of fish hold little or no relevance to practical aquaculture. This notion may be flawed, since understanding the mechanisms behind the behaviour of individuals is a proviso to predict behaviour in groups even with the caveat that some behaviors may be modified by group size. As an example, optimal feeding regimes should be easier to predict in a population consisting of individuals whose appetite is relatively less affected by external stressors. In a fluctuating and potentially stressful environment, such a population should also generate less feed waste, if kept on fixed rations. In the present experiment, we tested this hypothesis by monitoring feed waste and feed conversion efficiency in lines of rainbow trout selected for a low (LR) or high (HR) cortisol response to stress. The study was carried out after fish had been transported between rearing sites in the United Kingdom and Norway. There was significantly more feed waste from rearing units containing HR fish, and these fish also showed lower feed efficiency (growth per unit feed consumed). The difference in feed waste became more apparent with increasing time after transport, as rations increased. Simultaneously, size was more variable and growth was slower in HR rearing units. These results suggest that there are several potential benefits of selection for low stress responsiveness in aquaculture.     

High stocking density induces crowding stress and affects amino acid metabolism in Senegalese sole Solea senegalensis (Kaup 1858) juveniles

Fish held at high stocking densities are generally exposed to chronic stress situations that impose severe energy demands and may predispose the fish to infection. Senegalese sole Solea senegalensis (Kaup) juveniles (78.8±18.9 g body weight) were maintained at low (LSD; 4 kg m^?2 at the end of the experiment), medium (MSD; 9 kg m^?2) and high (HSD; 14 kg m^?2) stocking densities during a period of 63 days. Although disease outbreaks were observed in fish reared at HSD, growth and food consumption did not vary among different treatments. Results from plasma cortisol and free amino acids (FAAs) showed significant differences among different rearing densities pointing to HSD as stressful rearing condition. However, higher plasma glucose and osmolality levels indicated that fish held at MSD may also be under stress. The higher usage of FAAs from the HSD group may be due to the higher demand for energy production in order to cope with stressful rearing conditions, higher rate of protein synthesis or due to synthesis of other important metabolites related to stress response. Therefore, results from the present study point out to HSD as a stressful rearing condition and suggest that crowding stress may affect amino acid requirements.     

Plasma growth hormone levels in fed and fasted rainbow trout (Oncorhynchus mykiss) are decreased following handling stress

Plasma growth hormone concentrations of rainbow trout,Oncorhynchus mykiss, fasted for six weeks, were significantly (p < 0.01) higher than in comparable fed animals; in the fasted fish, the levels fell progressively following acute stress (by displacing the fish within their home aquarium), with significant differences from pre-stressed fish evident between one and thirty-two hours after application of the stressor. Plasma growth hormone concentrations also fell significantly in the fed group, but differences were evident only between two and eight hours after stressor application.Plasma cortisol concentrations in pre- and post-stressed fed and fasted fish were similar. There was a bimodal response to stressor application in both groups, with significantly higher values relative to the pre-stressed sample evident one and eight hours after disturbance, but not after two, four or thirty-two hours. The changes in plasma cortisol levels between the initial (09:00h) sample and the sample taken eight hours later resembles the diet pattern seen in trout given access to self-demand feeders.Plasma glucose concentrations in pre-stressed fed animals were higher than in pre-stressed fasted fish. This relationship was also evident between one and four hours and thirty-two hours after stressor application. The post-stress rise in plasma glucose concentration was evident between one and four hours in the fed group, and between four and eight hours in fasted fish.The diel changes in plasma growth hormone and glucose concentrations could not be attributed to normal circadian patterns, and there was no apparent correlation between changes in plasma growth hormone and cortisol concentrations. There was a significant inverse correlation between plasma glucose and growth hormone concentrations when the total data set were analyzed, but these correlations were not apparent when the treatment groups were analyzed separately.     

Effects of dissolved carbon dioxide on energy metabolism and stress responses in European seabass (Dicentrarchus labrax)

Elevated carbon dioxide concentrations reduce feed intake and growth in several fish species and induce stress responses. In this study, the effects of moderately elevated CO₂ levels on performance, energy partitioning, swimming activity and stress response in European seabass were assessed. European seabass (140.0 g) were reared under two levels of CO₂ (1.6 and 7.0 mg L^?1) and two feeding levels (FLs) (maintenance and satiation) for 60 days, and fish swimming speed was recorded. At the end of the experiment, fish were subjected to an acute stress test. Blood cortisol, glucose and lactate were analysed. Energy and nitrogen balances were quantified based on measurements of body composition and digestibility coefficients. Moderately elevated chronic CO₂ level did neither affect energy requirements for maintenance nor the utilization of digestible energy for growth. However, swimming activity data suggests that FL dependent alterations in energy partitioning took place. Blood cortisol values after the acute stress were affected by additional CO₂ exposure and this effect was also dependent on FL. The elevated CO₂ exposure of 7.0 mg L^?1 appears to act as a chronic stressor as adaptive responses took place, however, this CO₂ exposure seems to be still within the allostatic load of the fish.     

Growth rates of the common wolffish, Anarhichas lupus L., and spotted wolffish, A. minor Olafsen, in captivity

Abstract. Field-caught fry of the common wolfish, Anarhichas lupus L., and the spotted wolffish, A. minor Olafsen, were reared on a commercial dry pellet. Both species experienced the same environmental conditions, including an annual fluctuation in temperature from 5·8 to 13·7°C. The spotted wolffish reached 1·58 kg in 2 years from hatching, more than four times the weight of common wolffish (0·37kg). Analysis showed that for both species the specific growth rate decreased with increasing fish size and increasing temperature. Estimates, based upon optimum conditions, indicate that the spotted wolffish would reach a total wet weight of 5 kg (<7°C) in 2 years from hatching, and the common wolffish (7–9°C) a total wet weight of 2·5kg within that same period. Female common wolffish matured at a weight above 0·5kg, whereas those of the spotted wolffish matured at a weight above 2·5kg. The spotted wolffish had a significantly higher fillet proportion (∼50%) than the common wolffish (∼45%) and a significantly lower hepato-somatic index (3·7% and 5% respectively).     

Effects of photoperiod, stocking density and diet on growth in young spotted wolffish (Anarhichas minor olafsen)

Full scale experiments in tanks were conducted to elucidate the effect of photoperiod regime, dietary fat level and stocking density on growth in spotted wolffish. The study showed that continuous light (LD24:0) did not give a higher final mean weight or faster overall growth rate in spotted wolffish compared to constant 8 h light and 16 h darkness (LD8:16). However, there were indications that the fish needed time to adapt to a new photoperiod regime. The feeding experiment indicated a negative relation between dietary fat level and growth, where fish given 15% fat in the diet had a 13% higher final mean weight compared to fish on a diet with 20% fat. However, no conclusions could be made. The final mean weight was 10% higher at 25 kg/m² compared to 40 kg/m², indicating a negative impact of high stocking density on spotted wolffish.     

The response of coral trout (Plectropomus leopardus) to capture, handling and transport and shallow water stress

The stress response of coral trout (Plectropomus leopardus ) to wild capture or controlled shallow water stressors was investigated. Stress associated with capture from the wild, handling and transport and shallow water evoked significant changes in circulating levels of cortisol, glucose, lactate, haemoglobin (Hb) and haematocrit (Hct) in coral trout. Plasma glucose, Hb and Hct increased over 30 to 60 min in response to the controlled stressor and then returned toward unstressed levels, even if the stressor persisted. Cortisol increased to maximum levels over the 60 min following the onset of the stressor and remained elevated for 4 h in the case of a single 30 min shallow water event or 3 d in response to the capture, handling and transport. The difference between the responses of blood glucose and cortisol suggests that cortisol does not maintain blood glucose during stress in coral trout. The concentrations of circulating thrombocytes, lymphocytes and granulocytes were also significantly affected by shallow water stress. As lymphocytes and granulocytes are important immune components, reduced concentrations of these cells may explain the increases in disease susceptibility, commonly observed in stressed fish during live transport.     

Cortisol response of golden shiners (Notemigonus crysoleucas) fed diets differing in lipid content

The purpose of this study was to characterize the plasma cortisol response in golden shiners (Notemigonus crysoleucas) under crowding stress, and to determine whether dietary lipid affects the cortisol response. The main differences between the diets were the relative amounts and types of fatty acids of the n-3 and n-6 families. In experiment 1, triplicate groups of golden shiners were fed diets with 4 or 13% menhaden fish oil, or 4 or 13% poultry fat at 3% body weight for 4 weeks before the crowding stress. In experiment 2, fish were fed diets with 8% lipid from soybean oil (SBO), cod liver oil (CLO), (SBO+CLO), or cottonseed oil (CSO) for 6 weeks before the stress test. Plasma cortisol was measured initially, two hours after the stressor was applied, and two hours after the stressor was removed. The overall pattern of the cortisol response was similar to that seen in other warmwater fishes, but the rate and magnitude of the response were not affected by diet in either experiment. Golden shiners have an intermediate cortisol response compared to other fish that have been exposed to similarly stressful conditions.     

Modulation of the response of rainbow trout (Oncorhynchus mykiss Walbaum) to confinement, by an ectoparasitic (Argulus foliaceus L.) infestation and cortisol feeding

The effects of a 21 day infestation with the ectoparasite Argulus foliaceus on the response of rainbow trout Oncorhynchus mykiss to an acute stressor (confinement) were investigated. Fish were divided into a control and two parasitized groups, with one parasitized group having received cortisol feeding 72 and 24 h before infection. Prior to confinement only control fish showed a significant increase in body mass and condition factor. Following 4 h confinement, plasma glucose levels in the normal- fed/infested group were significantly higher than in the controls, while the cortisol-fed/infested group had significantly lower plasma levels of adrenocorticotropic hormone (ACTH) and glucose. Both infested groups, but not controls, had higher plasma ACTH and cortisol levels 48 h after confinement when compared with their pre-confinement levels. Stressor-induced immunomodulation was more evident in the parasitized fish after confinement, as was concluded from decreased lysozyme activity, alternative complement activity and oxygen radical production either immediately after, or 48 h post 4 h confinement. Circulating lymphocyte numbers were significantly reduced in the control and normal-fed/infested fish. Results indicate that low ectoparasitic infestations can, for a prolonged period, lead to a stress experience with effects that impair the response to subsequent stressors. Such effects may be further confounded by a previous elevation of plasma cortisol levels.     

Effects of Temperature on Performance Characteristics and the Cortisol Stress Response of Surubim Pseudoplatystoma sp.

Three experiments were conducted in flow‐through tanks at 24, 27 and 30 C to evaluate growth, food conversion, morbidity, survival, and the cortisol stress response of surubim Pseudoplatystoma sp. at different temperatures. In Experiment 1, fish (mean initial weight, 33.3 ± 7.2 g) reared at 27 and 30 C for 60 d grew significantly faster than fish reared at 24 C (P < 0.05). Fish at the lowest temperature showed the poorest feed conversion ratio (FCR, 5:1), while fish at 27 and 30 C had FCRs of 2.5:1 and 3.5:1, respectively. Mortality and morbidity rates were inversely proportional to the temperatures tested. In Experiment 2, serum cortisol levels following an acute handling stressor peaked at 30 min after stress, returned to near‐resting levels at 1 h after stress, and completely returned to resting levels at 3 h after stress in fish at all temperatures tested. Peak serum cortisol levels were higher in fish at 30 C (321.1 ± 38.8 ng/mL) than those in fish at 27 and 24 C (143.3 ± 37.4 ng/mL and 104.2 ± 37.2 ng/mL, respectively). In Experiment 3, fish (mean initial weight, 67 ± 8.58 g) reared at 27 C for 137 d grew significantly faster than fish at 30 and 24 C. Virtually, all fish reared in 30 C had some sign of morbidity (skin lesions and fin abrasion) compared to only a small percentage of fish in the other two temperatures. Again, the highest cortisol peak level was measured in fish reared at 30 C (117.6 ± 16.7 ng/mL), as compared to fish at 24 and 27 C (99.2 ± 15.5 ng/mL and 80.2 ± 12.8 ng/mL, respectively). Our findings indicate that the optimum temperature for rearing surubim in tanks is 27 C. At 30 C, these fish may suffer from stress, as suggested by the elevated peak of cortisol and morbidity and mortality rates. The relatively rapid cortisol stress response of surubim suggests that they may be more tolerant to handling stressors than other fish species.     

Physiological stress responses to captivity in early developmental stages of the wedge sole Dicologoglossa cuneata (Moreau)

The stress responses in early growth stages of the wedge sole have been studied to determine whether the high cortisol levels described in juvenile fish are present from early developmental stages. Whole‐body cortisol, glucose and lactate contents, as well as biometric parameters in wedge sole larvae were measured at three different stocking densities. Stocking density affected growth‐related variables significantly, and larvae in lower stocking densities grew faster. Survival did not significantly differ among treatments. At hatching, the whole‐body cortisol concentration was 0.33 ± 0.01 ng g^?1 and varied significantly from 0 to 30 days after hatching (DAH) for each stocking density, though values remained stable for the remaining time in the low‐stocking density group. These hormone levels rose significantly (5.17 ± 2.43 to 22.10 ± 4.95 ng g^?1) at the end of the experiment, depending directly on the stocking density. Glucose and lactate‐body concentrations did not vary among treatments. We conclude that the stress responses of wedge sole larvae are detectable from 45 DAH and that stocking density already can be a stressor at that age. As described for juvenile stage, cortisol content values in wedge sole larvae under non‐stressful conditions are one of the highest among those reported in the literature. The captivity conditions could be responsible for this apparently stressful situation, though those values also could be normal in wild specimens.     

Cortisol response to air exposure in <Emphasis Type="Italic">Solea senegalensis</Emphasis> post-larvae is affected by dietary arachidonic acid-to-eicosapentaenoic acid ratio

An experiment was conducted in order to evaluate the effects of feeding frozen Artemia diets differing in arachidonic acid-to-eicosapentaenoic acid ratios (ARA/EPA) on growth, survival and stress coping ability of Senegalese sole post-larvae (19–31 days after hatch). Two experimental diets presenting high (‘High’; 3.0) or low (‘Low’; 0.7) ARA/EPA ratios were tested under two rearing conditions: undisturbed (C) and stressed by a 2-min air exposure every two days (S). Growth, survival and basal cortisol levels were similar between groups indicating that independently of dietary ARA/EPA ratios, fish were able to cope with the repeated stress imposed. Also, cortisol levels at 3 h past air exposure were determined in all groups at the end of the experiment. Among fish fed the ‘Low’ diet, C groups seemed to present a quicker recovery from the acute stress (basal-like levels) than S groups. Repeated stress effects were not apparent in fish fed the ‘High’ diet and, relative to basal levels, twofold higher cortisol concentrations were detected at 3 h, in both C and S groups. This study suggests the importance of ARA in steroidogenesis regulation and the modulatory role of EPA in this process. Despite the tolerance to a wide range of dietary ARA/EPA as indicated by growth and survival results, acute stress coping response may be more efficient in Senegalese sole post-larvae fed low ARA/EPA ratios and, under these particular conditions, a faster recovery of cortisol to basal values could be indicative of rearing conditions (undisturbed vs. repeatedly stressed).