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Traditional animal welfare paradigms have focused on maintaining physical health and mitigating negative impacts to wellbeing. Recently, however, the field has increasingly recognized the importance of positive welfare (i.e., mental and physical states that exceed what is necessary for immediate survival) and accordingly introduced manipulations and indicators of positive welfare for use in agriculture, laboratories, and zoos/aquaria. The creation and monitoring of positive welfare requires an in-depth knowledge of species-specific behavior and biology, which necessitates species-specific or, at a minimum, taxa-specific standards. Research on positive welfare in fish is lagging in this regard and therefore merits further consideration. In this paper, we aim to merge what is already known about positive welfare with the existing fish behavior literature to develop a plan of action for fish welfare research that will ultimately contribute to the development of positive welfare standards and assessment strategies for fish. We begin by exploring the origins of positive welfare research and then outline the physical, psychological and species-specific areas of inquiry that can be investigated in fish. In addition to presenting current findings on fish motivation, emotion, potential sources of positive welfare such as fulfillment of motivational urges (establishing agency, engaging in exploration and learning), and play behavior, we also identify promising areas for future research aimed at developing accurate and appropriate indicators of positive welfare in fish.

Twelve groups of farmed lumpfish and one of wild lumpfish were screened for cataract and sampled for fish muscle tissue, whole heart and both eye lenses to investigate possible relations between cataract and tissue free amino acid concentrations. Cataract prevalence ranged from 20% to 100%, with the highest average score of 7.3 (max 8) and incidences of severe cataract (>5) in all groups. Cataract could not be explained by suboptimal histidine concentrations in the feed. Neither muscle nor cardiac tissues had concentrations of free histidine compounds. The lumpfish lens contained N-acetylhistidine ( NAH), of which low concentrations were strongly related to cataract severity. However, no correlation between lens NAH and cataract severity was found in the present sample set. Wild lumpfish had higher levels compared to farmed lumpfish, suggesting that the farmed lumpfish may have been deficient in histidine or have a higher utilization of NAH due to osmotic problems. Thus, cataract in farmed lumpfish may be related to primary or secondary disturbed nutrient metabolism or malnutrition, shown by the high levels of specific amino acids in different tissues, which may cause osmotic imbalance and cataract development. This nutritional or environmental-related welfare problem deserves further research. [ABSTRACT FROM AUTHOR]

An excessive and inappropriate handling of live fish is currently performed to obtain its size and weight. As an ameliorative attempt, a system named "IchthyoJHOL" was devised for the measurement of live specimens of fusiform fish species. The system holds the fish ventrally between supports and is mounted upon a scale. An image is captured on camera and processed with a digital measurement system. The system generates biometric data such as length, height and weight. This methodology creates a two-stage biometric process, the first being the capture of the images and the second their digital analysis, allowing for each stage to be carried out at distinct times and places. With this methodology, a reliability of over 96.8% is achieved, similar to that produced by the traditional system, but with a significant reduction in handling time and stress induced on the fish, allowing for large-scale biometric analysis. In addition, the library of fish images can be shared for posterior evaluation, redescription studies or a more extensive morphometric analysis. [ABSTRACT FROM AUTHOR]

Understanding how animals experience stress in a laboratory environment is crucial for improving their welfare. Increasing numbers of fish are being used in scientific studies and further research is required to ensure appropriate conditions are used to promote good conduct and correct housing as well as guaranteeing scientifically valid results. As zebrafish are a gregarious species, social enrichment is particularly important, with individuals separated from a group experiencing isolation stress. The present study aimed to determine the effects of social context on recovery from common laboratory procedures. Additionally, we investigated whether the noninvasive measure of water-borne cortisol can be utilized to gauge physiological stress by comparing it to an invasive measure, whole-body cortisol concentration. Zebrafish (AB strain, male) were housed in differing social contexts and were randomly assigned to one of three treatment groups: undisturbed, anaesthetized and handled, or anaesthetized and fin clipped. Behavioural and physiological stress indicators were recorded before and after treatment. The results indicated social context, in the form of group housing, was important in enhancing recovery from welfare challenges in zebrafish, since group-housed fish resumed normal behaviours more quickly than pairs or individuals. Moreover, the strong correlation between water-borne and whole-body cortisol suggests that the noninvasive measure is an appropriate ethical alternative as an indicator of physiological stress. These findings represent an important refinement in reducing the severity of stress through housing zebrafish in their original groups and by adopting a noninvasive measure of cortisol which will act to reduce the numbers of individuals required for time series studies on physiological stress. [ABSTRACT FROM AUTHOR]

The present study was undertaken to investigate the effect of biofloc-based aquaculture system on inorganic nitrogen control, and growth and immunological responses of rohu ( Labeo rohita ). Fingerlings of rohu were cultured in indoor biofloc tanks in light-limited condition for 16 weeks with daily addition of molasses as carbon source to maintain the C/N ratio at 15. Different water quality parameters were measured at weekly interval. Growth, survival and welfare indices including stress and immunological status were measured after the end of the experimental period. Factor analysis using water quality variables indicated ammonia immobilization by heterotrophic bacteria, rather than nitrification, as the dominant mechanism influencing ammonia removal in the biofloc system. Ammonia immobilization was concomitant with microbial biomass production, as also revealed through factor analysis. The biofloc culture system enhanced the growth of rohu significantly than the control fish. As a measure of stress, the serum glucose content did not differ significantly between the fish cultured in biofloc and control system. Among the immunological parameters, respiratory burst and alkaline phosphatase activity showed significant enhancement in fish grown in biofloc system than the control. However, myeloperoxidase activity and total serum protein content did not show any statistically significant difference between biofloc and control system. The enhanced growth and welfare of the cultured fish may be attributed to in situ maintenance of water quality within the biofloc system and presence of microbial floc. [ABSTRACT FROM AUTHOR]

Sagittal otoliths are essential components of the sensory organs that enable all teleost fish to hear and maintain balance, and are primarily composed of calcium carbonate. A deformity, where aragonite (the normal crystal form) is replaced with vaterite, was first noted over 50 years ago but its underlying cause is unresolved. We evaluated the prevalence of vateritic otoliths from two captive rearing studies which suggested that fast growth, due to environmental rather than genetic control, led to vaterite development. We then tested this by varying light and temperature to create phenotypes with different growth rates, which resulted in fast growers (5 times larger) having 3 times more vaterite than slow growers. A decrease in either the ratio of otolith matrix proteins (otolin-1/OMM-64) or [Ca2+]/[CO3 2-] may explain why fast growth causes vaterite deposition. As vaterite decreases hearing sensitivity, reducing growth rates in hatcheries may improve the welfare of farmed fish and increase the success of conservation efforts. [ABSTRACT FROM AUTHOR]

Spontaneous mortality of seemingly healthy, farmed Atlantic salmon ( Salmo salar L) is an increasing problem in Norwegian aquaculture. In this study, we present a morphological study of the previously undescribed syndrome of arteriosclerosis of the ventral aorta and epicarditis of the adjacent bulbus arteriosus found in farmed Atlantic salmon, with wild-captured fish as a control group. Both the ventral aorta and epicardium are vital for correct arterial compliance and vascular resistance in the respiratory capillaries of the gills. We discuss the possible implications of ventral aorta arteriosclerosis and epicarditis for blood vascular health and in particular for the increasing frequency of spontaneous gill bleeding in farmed salmon. As both these conditions primarily occur in farmed salmon, we suggest that they should be considered pathological. [ABSTRACT FROM AUTHOR]

Behavioral responses, blood markers, and fillet properties were investigated after silver catfish exposure to different electric field strengths, frequency, and duration of electric current in the preslaughter stunning. All combinations of electric fields and frequencies were able to stun fish. Longer apparent stun was obtained at the intermediate electrical frequencies. High electric field strength detracted the texture of fillets. At least 5 s was required to stun fish, and longer exposure to electrical current did not prolong the apparent stun or damage fillet properties. Results indicate that silver catfish are relatively resistant to electronarcosis. [ABSTRACT FROM PUBLISHER]

Peracetic acid (PAA) products are being introduced to aquaculture as sustainable disinfectants. Two strategies are used to apply PAA: high dose pulse applications, or low dose continuous application. In the present study, their impacts on fish health and water quality were investigated in triplicate flow-through tanks stocked with rainbow trout. The gentler and shorter water cortisol increase measured along twice-per-week pulse applications of 1 mg L −1 PAA indicated a progressive adaptation of fish. In contrast, the continuous application of 0.2 mg L −1 PAA caused no stress to fish. Meanwhile, no mortality and no impact on growth or innate cellular immunity were observed. The pulse applications restricted biofilm formation, and partially inhibited nitrification. Additionally, the highest oxygen concentration and stable pH were observed. In contrast, the continuous application promoted biofilm formation, and caused a pH increase and intermediate oxygen concentration. The contrast was probably due to different susceptibility of microbes to PAA-induced oxidative stress. To summarize, pulse PAA applications cause minor stress in fish, but have advantages over continuous application by ensuring better water quality. [ABSTRACT FROM AUTHOR]

Fish health under live haul transportation on wellboats and in other recirculating aquaculture systems depends on frequent and reliable measurements of water quality, that may change in response to changes in the environmental conditions and treatment processes. While reliable technologies exist for sensors measuring some water quality parameters such as Oxygen concentration and salinity, there is a lack of sensor technologies for the measurement of other critical water quality parameters such as water pCO 2 and NH 3 concentrations. Presented is the theory of operation and performance of prototype optical sensors for the measurement of pCO 2 and NH 3 , as well as the methods used for their calibration and characterization. In order to evaluate the sensor’s suitability for aquaculture applications we temporarily installed the sensors on a wellboat and monitored the water quality during the transportation of live mature Atlantic salmon between fish farms and slaughter facilities on the Norwegian coast. Under transportation with different stocking weights and recirculation conditions, the CO 2 optode prototypes recorded measurements that coincided with measurements from standard laboratory analysis of spot samples. The pCO 2 concentration in the well throughout the testing was shown to vary between approximately 300 μatm and 20 000 μatm. Compared to the pH derived pCO 2 estimation method widely applied in aquaculture, the CO 2 optode was more sensitive to small fluctuations in CO 2 concentration, and gave measurements closer to the CO 2 values recorded by laboratory analysis of spot samples. The NH 3 optode reported measurements that were consistent with spot samples analyzed using the salicylate–hypochlorite reaction. In contrast to pCO 2 , the concentration of NH 3 during the observed wellboat missions was found to be very low and stable at 0.8 ± 0.3 μg L −1 . Once fully developed, the CO 2 and NH 3 optodes could contribute to better water quality control and efficiency during well boat transportations and other recirculating aquaculture systems. [ABSTRACT FROM AUTHOR]

The Atlantic salmon aquaculture industry is expanding to more exposed locations that often are characterized by occasional very strong currents. This necessitates new guidelines concerning acceptable animal welfare and a need for fundamental ethological knowledge in these high energy environments. To assess the critical current velocity in growing Atlantic salmon during commercially relevant conditions, a push-cage setup was constructed, which allowed for the exposure of an entire stocked sea cage to controlled current velocities generated by a fixated ship. Three replicates of a critical swim speed trial were made each containing approximately 1500 novel adult Atlantic salmon (3.4 kg). At 125 cm s − 1 (1.97 BL s − 1 ) fish would start to become fatigued meaning that short durations of currents at or above this magnitude in an exposed setting would be detrimental to animal welfare. Furthermore, the normal circular schooling pattern started to become disrupted at current velocities of 30–35 cm s − 1 , and above 45–65 cm s − 1 all fish would stand on current evenly spread out in the entire sea cage. This change from a voluntary cruising speed at low currents to a swimming speed solely dictated by the environment at intermediate to high currents could become a significant stress factor if chronically exposed. Regarding welfare guidelines in exposed aquaculture, it is therefore important to both consider the magnitude and duration of current velocities when moving to new locations. Technological solutions in current damping through sea cages could potentially mitigate these challenges. [ABSTRACT FROM AUTHOR]

The organic farming of sea bass and sea bream was investigated in a commercial farm by assessing environmental impact, growth performance, fish welfare and product quality in a multidisciplinary approach. Organic and conventional juveniles were reared in four circular 3.800 m 3 floating sea cages at a maximum final stocking density of 15 kg/m 3 and fed on organic or conventional commercial feeds. Conventional farming was started simultaneously and monitored as the reference condition. Environmental and biological samplings were performed after 16 months at the first fish harvesting. No environmental impact was observed in the organic farming area, whereas a significant increase in total phosphorus concentration in the sediment and some changes in the structure and species composition of benthic assemblages were detected just beneath the conventional sea cages with no impact 25 m from the cages. Organic fish showed a better growth performance consistent with the lower feed conversion ratio and higher metabolic status, as evidenced from the protein and energy profiles, compared with conventional fish. No dietary influence on stress and immune response was observed. A higher incidence of fin splitting occurred, probably due to nutritional factors and/or to the larger size of organic fish. Similar good morphological, physical and sensory traits between organic and conventional fish were observed, as denoted by the high fillet yield, low flesh lipid content, long freshness and shelf life both in sea bream and in sea bass. Conversely, the fillet fatty acid composition was different, being poorer in n-3 PUFA and richer in linoleic acid (C18:2 n-6) in organic versus conventional fish, mirroring the fatty acid profile of the diet. The organic farming of sea bass and sea bream exhibited an overall good performance. Husbandry and feeding practices did not produce any evident environmental impact although long-period investigations covering several full production cycles are needed. Organic fish grew well, displayed an overall state of well-being and had good quality features despite a lower EPA and DHA content. Improvement of organic feed formulation and quality of feed ingredients emerge as important issues to be addressed to optimize the nutritional quality of organic sea bass and sea bream. [ABSTRACT FROM AUTHOR]

For the development of commercial scale semi-closed sea systems for farming post-smolt Atlantic salmon ( Salmo salar ), further knowledge is required on the interaction between fish density, farming conditions and fish welfare. In this experiment post-smolts (115.0 g ± 13.6) were stocked at 5 different densities (25, 50, 75, 100 and 125 kg m − 3 ), and kept at these densities for 8 weeks. All treatments received an equal specific flow rate of 0.6 L kg fish − 1 min − 1 of flow-through seawater (fully oxygenated, salinity 34‰ and temp. 9.3 °C) and water oxygen (O 2 ), pH, carbon dioxide (CO 2 ) and total ammonia nitrogen (TAN) levels were monitored in the outlet and kept within recommended limits. Over the 8 week period, specific growth rate (SGR %) was significantly reduced in stocking densities of 50 kg m − 3 and above. Increasing density from 100 kg m − 3 to 125 kg m − 3 lead to a 42% decrease in SGR. Between 50 kg m − 3 and 125 kg m − 3 there was a correlation between reduced feed intake and increased stocking density and there was a linear increase in feed conversion ratio (FCR) with stocking density (25 kg m − 3 to 125 kg m − 3 ). At the end of the 8 week period primary and secondary stress responses such as elevated plasma levels of cortisol, sodium, p CO 2 and decreased plasma pH were observed in the highest density treatment compared to other treatments. In combination with the reduced SGR in the highest density treatments these results indicate an allostatic overload i.e. the environment has exceeded the adaptive ability of the fish with chronic adverse effects on fish welfare. Stocking densities of 100 kg m − 3 or more also increased pelvic fin damage and the prevalence of cataracts was higher in the 125 kg m − 3 treatment. In conclusion, our results suggest that at this temperature and fish size it is feasible to rear Atlantic salmon post-smolts in densities up to 75 kg m − 3 without compromising performance and welfare. Statement of relevance The data presented here are highly relevant for Aquaculture as further knowledge is required on the interaction between fish density, and fish welfare for the development of commercial scale semi-closed sea systems for post-smolt Atlantic salmon farming. [ABSTRACT FROM AUTHOR]

The performance of Clarias gariepinus (Burchell, 1822) fingerlings reared in white, green, blue, black and red coloured tanks was investigated. Each colour was tested in triplicate 1 m³ tanks with an initial stocking density of 100 fingerlings per tank (1.07 g). Fish were fed a commercial diet at 5% body weight per day for eight weeks. Fish weights were sampled every 2 weeks to adjust feeding rates. Tank colour had a significant effect (P < 0.05) on the overall performance of the fingerlings. Rearing in black tanks resulted in a higher daily feed intake (0.44 g) and better growth performance when compared to other tank colours, while the poorest performance was observed in blue and green tanks. Carcass protein and fat content at 8 weeks revealed a trend similar to that observed for growth. Also, serum glucose tests showed higher levels in light background tanks and were thought to be an indication of stress. Survival was not affected by tank colour. It was concluded that dark-coloured tanks such as black or red were better in comparison to light-coloured tanks for the rearing of African catfish fingerlings. [ABSTRACT FROM AUTHOR]

Capture-based aquaculture ( CBA) of Atlantic cod ( Gadus morhua) has become increasingly important in recent years, and increased attention is being paid to animal welfare issues linked to these activities. Earlier studies showed that some cod develop secondary exophthalmia in captivity. This study investigated the development of secondary exophthalmia in two groups of wild-caught cod, one of which was exposed to rapid decompression causing acute barotrauma (treatment group) while the other was not (control group). Photographs and radiographs before and up to 33 days after barotrauma revealed a significant increase in overall eye protrusion caused by an accumulation of gas in the orbita in the treatment group, first observed on day 9 after decompression, while no protrusions were observed in the control group. Barotrauma was thus identified as an important trigger for the development of secondary uni- or bilateral exophthalmia of wild-caught cod. Two underlying mechanisms are suggested, where the more likely is residual swim bladder gas taking the route of least resistance, while the less likely is the exsolution of gas from the blood. Our results have implications for a wide range of contexts in which cod are rapidly brought to the surface from great depth. [ABSTRACT FROM AUTHOR]

Aquaponics is the combination of aquaculture (fish) and hydroponic cultivation of plants. This review examines fish welfare in relation to rearing water quality, fish feed and fish waste and faeces to develop a sustainable aquaponic system where the co-cultured organisms, fish, bacteria in biofilters and plants, should be considered holistically in all aquaponics operations. Water quality parameters are the primary environmental consideration for optimizing aquaponic production and for directly impacting fish welfare/health issues and plant needs. In aquaponic systems, the uptake of nutrients should be maximised for the healthy production of the plant biomass but without neglecting the best welfare conditions for the fish in terms of water quality. Measures to reduce the risks of the introduction or spread of diseases or infection and to increase biosecurity in aquaponics are also important. In addition, the possible impacts of allelochemicals, i.e., chemicals released by the plants, should be taken into account. Moreover, the effect of diet digestibility, faeces particle size and settling ratio on water quality should be carefully considered. As available information is very limited, research should be undertaken to better elucidate the relationship between appropriate levels of minerals needed by plants, and fish metabolism, health and welfare. It remains to be investigated whether and to what extent the concentrations of suspended solids that can be found in aquaponic systems can compromise the health of fish. Water quality, which directly affects fish health and well-being, is the key factor to be considered in all aquaponic systems. [ABSTRACT FROM AUTHOR]

The primary aim of this study was to comprehensively evaluate the welfare of Nile tilapia (Oreochromis niloticus) throughout their entire life cycle within aquaculture, spanning from reproduction to slaughter. The methodology was structured to identify welfare indicators closely aligned with the principles of animal freedoms defined by the Farm Animal Council, encompassing environmental, health, nutritional, behavioral, and psychological freedom. Notably, psychological freedom was inherently considered within the behavioral and physical analyses of the animals. To accomplish this, an integrative systematic literature review was conducted to define precise indicators and their corresponding reference values for each stage of tilapia cultivation. These reference values were subsequently categorized using a scoring system that assessed the deviation of each indicator from established ideal (score 1), tolerable (score 2), and critical (score 3) ranges for the welfare of the target species. Subsequently, a laboratory experiment was executed to validate the pre-selected health indicators, specifically tailored for the early life stages of tilapia. This test facilitated an assessment of the applicability of these indicators under operational conditions. Building on the insights gained from this experimentation, partial welfare indices (PWIs) were computed for each assessed freedom, culminating in the derivation of a general welfare index (GWI). Mathematical equations were employed to calculate these indices, offering a quantitative and standardized measure of welfare. This approach equips tilapia farmers and processors with the tools necessary for the continuous monitoring and enhancement of their production systems and stimulate the adoption of more sustainable and ethical practices within the tilapia farming., Competing Interests: AP was employed by Wai Ora Aquaculture and Environmental Technology Ltd. MQ was employed by FAI Farms from Brazil. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Pedrazzani, Cozer, Quintiliano, Tavares, Biernaski and Ostrensky.)

The aim of this study was to investigate the physiological response of rainbow trout ( Oncorhynchus mykiss ) before slaughtering in the last phase of farming analyzing skin mucus and plasma. Two groups of rainbow trout were considered: Group UN ("unstressed"), represented by fish randomly captured from raceways, in the last phase of a standard fattening cycle; Group S ("stressed"), collected at the end of the pre-slaughtering tank, soon after slaughtering. The fish skin mucus was swabbed from head to tail using a sterile plastic spatula and the blood was collected through an endocardial puncture. qRT-PCR was used to study the gene expression in skin mucus. The mRNA expression levels of the IL-6 and IgD genes were higher in the S than in the Group UN. The plasma analysis showed an only a decrease in the glucose plasma levels in the Group S when compared to the Group UN. The present results indicated that the procedures adopted after slaughtering only affected changes in plasma glucose and skin mucus activity in rainbow trout suggesting that management protocol was compatible with non-stressful farming conditions., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Fiordelmondo, Magi, Friedl, El-Matbouli, Roncarati and Saleh.)

The aims of this study were to determine the prevalence of fin damage in farmed rainbow trout and to see whether the level of damage differed between different fish categories and farms. The study was field based and included the fin damage analysis and clinical description of the damaged fins. Fins were analyzed in two categories of fish (weight below 30g (min. 5g) and over 100g (max. 250g)). Thirty fish per category were randomly selected, netted and each rayed fin was assessed and photographed (total of 5880 fins were analyzed in 840 fish from seven rainbow trout farms). The prevalence of fin damage was 100% and there was a large range in the level of damage which was mainly characterized by surface abrasions. Worst affected fins in both fish categories were dorsal and pectoral fins. Fin damage was present to a lesser degree in the smaller categories, but there was fin damage in the smallest fish examined. Pattern of damage was Dorsal > Pectoral > Abdominal > Anal > Tail fin. Differences in fin damage in all surveyed farms indicate that some factor or group of factors specific to each farm influence the extent of damage. Fin damage is operational welfare indicator and future research should identify and explore the impact of the factors affecting fin damage and propose management practices that can minimize the level of fin damage. Additional knowledge is needed to identify whether fin damage is etiologically connected to different production system, handling procedures or another background.

Zebrafish, a highly-social species of freshwater fish, are widely studied across many fields of laboratory science including developmental biology, neuroscience, and genomics. Nevertheless, as standard housing for zebrafish typically consists of small and simplistic environments, less is known about their social behavioral repertoire in more naturalistic settings. In particular, spontaneously occurring, socio-positive affiliative behaviors (e.g., social coordination and cohesion) that may be indicative of positive emotional experiences have rarely been reported or studied deliberately in zebrafish. Housing adult zebrafish (10 fish/tank) in large semi-natural tanks (110 L; n = 6) with sloping gravel substrate, rocks, and artificial plants, we observed a previously undescribed behavior: Distinct periods of spontaneous, synchronized, compact aggregations, what we call “heightened-shoaling”. This project aimed to quantify the characteristics of this distinctive behavior and compare parameters of heightened-shoaling to baseline periods (normal behavior) and pre-feed periods (feed-anticipatory behavior). First, across 4 days, we selected video-clips (100 s each) from within (i) instances of heightened-shoaling (n = 9), (ii) baseline periods (n = 18), and (iii) pre-feed periods (n = 18). For each of these video clips, we scan sampled every 10 s to determine fish orientations and location within the tank and agonistic behavior. Next, we used an all-occurrence sampling method to record the timing and duration of all episodes of heightened-shoaling behavior when tank-lights were on (8:00 h to 18:00 h) across 10 days. From the scan-sampling data, we found that compared to baseline periods, heightened-shoaling was characterized by increased shoal cohesion (p < 0.0001), increased adherence to the horizontal plane (p < 0.0001), decreased agonism (p < 0.0001), and no diving behavior (lower positions within the water column signal negative effect in zebrafish, p > 0.1). From the all-occurrence sampling data, we found 31 episodes of heightened-shoaling with instances observed in all six tanks and only a single case in which heightened-shoaling occurred in two tanks at the same time. The median episode duration was 7.6 min (Range 1.3–28.6). As the first systematic description of heightened-shoaling behavior, this research contributes to our knowledge of the range of zebrafish social dynamics living in naturalistic environments. Moreover, as a spontaneously occurring, protracted, affiliative behavior, heightened-shoaling appears to be a good candidate for future research into positive emotional behavior in zebrafish.

The present study investigates the suitability of a machine vision-based method to detect deviations in the Shannon entropy (SE) of a European seabass (Dicentrarchus labrax) biological system fed with different selenium:mercury (Se:Hg) molar ratios. Four groups of fish were fed during 14 days with commercial feed (control) and with the same feed spiked with 0.5, 5 and 10 mg of MeHg per kg, giving Se:Hg molar ratios of 29.5 (control-C1); 6.6, 0.8 and 0.4 (C2, C3 and C4). The basal SE of C1 and C2 (Se:Hg > 1) tended to increase during the experimental period, while that of C3 and C4 (Se:Hg < 1) tended to decrease. In addition, the differences in the SE of the four systems in response to a stochastic event minus that of the respective basal states were less pronounced in the systems fed with Se:Hg molar ratios lower than one (C3 and C4). These results indicate that the SE may be a suitable indicator for the prediction of seafood safety and fish health (i.e., the Se:Hg molar ratio and not the Hg concentration alone) prior to the displaying of pathological symptoms. We hope that this work can serve as a first step for further investigations to confirm and validate the present results prior to their potential implementation in practical settings.

• Interactive effects on crowding stress, refrigerated seawater holding and frozen storage. • Quality metrics with K-value, WHC, cathepsin activity, lipid oxidation, texture, pH, colour. • Frozen storage with the largest and most consistent direct effects on muscle quality. • Crowding stress affected quality through interactions with frozen storage and RSW treatment. • Further studies with larger sample sizes necessary to elucidate complex effects and interactions. Crowded (stressed) and unstressed Atlantic mackerel with or without pre-freezing holding in refrigerated sea water (RSW) were stored at −19 °C for ∼12 months and analysed for nucleotide degradation (K value), muscle pH, water holding capacity (WHC), fillet firmness, cathepsin B/L like activity, lipid oxidation and fillet colour. The frozen storage showed the largest and most consistent direct effects on the quality metrics leading to increased lipid oxidation, discolouration (yellowing) and reduction on WHC and cathepsin activity. RSW treatment promoted nucleotide degradation and reduced WHC and fillet firmness in interaction with frozen storage and affected fillet colour lightness and saturation. Although showing only marginal main effects, crowding stress modified WHC, cathepsin activity and fillet firmness and colour through significant interactions with the frozen storage and RSW treatment. Further studies with larger sample sizes would be needed to elucidate their complex effects and interactions on the quality and storage stability of mackerel. [ABSTRACT FROM AUTHOR]

Electrocardiographic recordings (ECG) provide information on the duration of the action potential and the heart rhythm. Altered ECG may reflect important disturbances to the cardiac function. The aim of this study was to evaluate putative cardiotoxic effects in Colossoma macropomum upon exposure to geraniol (GRL – 70 μL L−1) and citronellol (CTL – 90 μL L−1) in anaesthetics baths. The fish were randomly distributed across the following treatments: a) control (basal recordings), b) vehicle control (ethanol), c) fish exposed to GRL – 70 μL L−1 and subsequent recovery, and d) fish exposed to CTL – 90 μL L−1 and subsequent recovery, and e) washout from ethanol. Five-minute duration recordings were used. The parameters heart rate (HR) (beats min−1), QRS complex amplitude (mV), R-R interval (s) (time between two successive QRS complexes), Q-T interval (s) (ventricular contraction) and QRS duration (s) (ventricular depolarization) were investigated. No mortality was observed and all animals recovered post exposure. The controls and ethanol-exposed fish presented normal patterns in ECG tracings showing sinus rhythm. Some alterations were observed during induction, e.g. , a negative chronotropic effect was observed in fish exposed to GRL, however, with maintenance of the sinus rhythm. Only the Q-T interval was longer than that of the controls. On the other hand, during induction with CTL, marked bradycardia with arrhythmia and prolongation of the Q-T and R-R intervals and QRS complex duration were observed, indicating a potential for atrioventricular blockade. During recovery, albeit slow, all parameters were gradually returning to sinus rhythm, suggesting that although important cardiac changes occurred during induction, mostly with CTL, they did not affect survival. In conclusion, both products induced anaesthesia with full body immobilisation and provoked some cardiac alterations, however, while GRL allowed for a faster recovery, CTL seemed to require a more prolonged time for full resumption of the normal cardiac function. • This is the first report on the cardiac effect of geraniol and citronellol on C. macropomum. • Both anaesthetics promoted a reversible bradycardic response. • Although citronellol led to arrhythmia, it did not result in mortality. • ECG records confirmed the safety of the anaesthetics for short-term exposure. [ABSTRACT FROM AUTHOR]

The present study was designed to characterize the behavior and evaluate the electromyographic and electrocardiographic responses of tambaqui (C. macropomum) exposed to different concentrations of lidocaine hydrochloride in immersion baths and after transfer to anesthetic-free water. For this purpose, fish juveniles (25.38 ± 6.5 g) were acclimated to aquariums for 15 days under controlled environmental and water quality conditions. Four concentrations of lidocaine hydrochloride (LA) (150, 175, 200, and 225 mg/L LA) were used in immersion baths to evaluate behavioral, electromyographic, and electrocardiographic changes during short-term anesthetic induction (5-min exposure) and in a washout. Upon exposure to LA, and irrespective of concentration, fish showed loss of the righting reflex, along with significant myorelaxation, which was partially achieved at concentrations of 150 and 175 mg/L LA and fully attained at 200 and 225 mg/L LA. Notably, a decreased heart rate and alteration of the T wave tracing occurred at higher concentrations. Notwithstanding, although protocols using LA at 225 mg/L or above could require greater care in monitoring, the prompt reversibility of such responses in recovering fish indicates that LA can be safely used in immersion baths for tambaqui C. macropomum as an alternative to other anesthetics, especially those in which important side effects have been documented. • Lidocaine hydrochloride (LA) can be safely used in tambaqui (C. macropomum). • 200 and 225 mg.L−1 of LA showed sedation and deep anesthesia in immersion baths. • QT prolongation observed at 225 mg.L−1 LA was transient and reversible. [ABSTRACT FROM AUTHOR]

Thermal delousing has become the most applied method for treatment against salmon lice. However, the temperature range used is strongly aversive for salmonids, and the method is associated with increased mortality. Treatment temperature × duration combinations should be tailored to maximise delousing efficiency and minimize welfare impacts on the host fish. We tested the detachment rate of sessile, pre-adult and adult male and female salmon lice (Lepeophtheirus salmonis) as a function of ambient temperature (11–16 °C), exposure temperature (28–36 °C), and exposure duration (0–120 s). Dead Atlantic salmon (Salmo salar) hosts were used to avoid negative fish welfare and detachment of lice due to fish behaviour. Within the range tested, higher exposure temperatures were associated with higher detachment rates among pre-adult and adult lice, while no sessile lice detached at any temperature. Moreover, no treatment combination detached 100% of lice of any stage, and at 28 °C, detachment of adult females was negligible. Most detachments occurred within the first 30 s of exposure. We conclude that for a given delousing efficiency, lower temperatures must be compensated for by considerably longer exposure durations. This may be a higher risk for the welfare of the host fish than higher temperatures and shorter exposure durations. • Temperature strongly affected detachment rate of salmon lice from immobile salmon. • At 32–36 °C most of the detachment occurred during the first 30 s of exposure. • Detachment rates of salmon lice at water temperatures of 28–30 °C was low. • Detachment of all lice within 120 s was not found at any temperatures up to 36 °C. [ABSTRACT FROM AUTHOR]

Lice shielding skirts are a preventative measure against salmon lice in Atlantic salmon farming. The skirt is wrapped around the top meters of the net cage to divert the current flow around the cage, and thereby keep the salmon lice out. Despite these skirts being used actively in Norwegian aquaculture for the past decade, there is no standardised way of using them, and therefore type, depth and operating procedures vary between sites. The academic literature on the lice shielding efficiency of these skirts is not extensive and reported efficiency varies across studies and sites with some reporting favourable results, while others find none. Some also report of welfare related issues, with dissolved oxygen levels being the most prevalent, but this too varies across sites and through the production cycle. The aim of this paper is to provide a comprehensive overview and summary of relevant academic and grey literature from the last decade to identify knowledge gaps that must be filled to achieve optimal use of lice shielding skirts. This paper focuses on three main topics: lice shielding efficiency, interaction with the current flow and rearing challenges. The positive results from some sites indicate that skirts have potential as a tool against salmon lice, however, to create a best practice recommendation for skirt use, more knowledge is necessary on the interaction between skirt and the environment, and sufficient monitoring procedures and decision-making tools must be established. • A comprehensive review of both academic and grey literature of the use of lice shielding skirts • Most studies report positive lice shielding efficiency, but quality of studies vary, and some see no effect. • Large variations between sites regarding lice shielding efficiency and welfare related issues • More knowledge is needed on the complex interaction between environment and skirt for optimal use. [ABSTRACT FROM AUTHOR]

A 45-day experiment was designed to study the complementary effect of maggot (Musca domestica) meal (MG) in a fishmeal-free soybean meal (SBM)-based diet on the production performance of African catfish hybrids. Four isoproteic (398.78 g/kg ± 3.23) and iso-energetic (20.23 kJ/g ± 0.15) diets were formulated as positive control (FM-based diet), soybean-based diets (FM-free) and soybean-based diets supplemented with 14 % MG and 21 % MG as complementary ingredients, denoted as FM, SBM, MG14 and MG21, respectively. A total of 180 healthy hybrid catfish (average initial body weight: 4.31 ± 0.04 g) were distinctly stocked into 12 trial tanks (60-L capacity) and fed to apparent satiation twice daily with their respective diets. At the end of the feeding trial, fish fed MG21 outperformed those fed other diets in final body weight (FBW), weight gain (WG), specific growth rate (SGR), and daily growth index (DGI) (p < 0.05). However, MG had no influence on feed intake (FI), feed conversion ratio (FCR), or nutrient utilization indices among the groups (p > 0.05). The MG21 supplementation improved the immuno-physiological indices such as white blood cell, lymphocyte count, total protein, and globulin values compared to other groups, while haemoglobin, haematocrit, and red blood cell counts were not affected. Supplementation with MG at 21 % enhanced the antioxidant capacity of hybrid catfish with no detrimental effect on the serum alanine aminotransferase and alkaline phosphatase enzymes activities. However, the aspartate aminotransferase was found higher in MG14 and MG21 fed fish compared to other dietary groups. Based on the results obtained in this study, it can be concluded that maggot meal could be used as a complementary ingredient in place of fishmeal in soybean meal diets for the culture of hybrid African catfish. The data in this article is available upon request from the corresponding author. • Dietary maggot meal at 21 % supplementation in the SBM-based diet improved the growth performance of African catfish hybrids. • Maggot meal supplementation enhanced the haemato-immunological response of African catfish hybrids. • Maggot meal could serve as a complementary ingredient to an SBM-based diet devoid of fishmeal. [ABSTRACT FROM AUTHOR]

As fish welfare becomes a growing concern, it is important to ensure humane treatment during slaughter. This study aimed to assess the onset of unconsciousness in Atlantic halibut immersed in CO 2 -saturated seawater through electroencephalography (EEG). Of the 29 fish studied, 10 exhibited escape attempts, indicating aversion to CO 2 -saturated water despite its oxygenation. EEG signals showed four distinct phases: transitional, excitation (high amplitude-high frequency), suppressed, and iso-electric phases. The onset of the suppressed phase, indicative of unconsciousness, occurred on average 258.8 ± 46.2 s after immersion. The spectral analysis of the EEG signals showed a progressive decrease in median frequency, spectral edge frequency, and high frequency contribution, which corresponded to the gradual loss of consciousness. The study concludes that CO 2 -saturated water is not recommended for pre-slaughter handling of halibut due to the extended time required for the onset of unconsciousness and the observed aversive behaviour. Ensuring humane treatment during slaughter is important for addressing public concern and safeguarding fish welfare in all stages of production.

Sea lice represent a persistent and growing problem, challenging the resilience and growth of the salmon aquaculture industry. In this Norwegian case study, we studied and discuss how the absence of policy instruments directed at stimulating breeding for lice resistance (LR) might be explained. We found well-documented opportunities for selection progress for LR. Hence, breeding on LR appears with an untapped potential. We discuss how market-based, legal, institutional and interest-based factors can explain the absence of policy instruments stimulating LR breeding. Methodologically, we obtained data from document and literature studies and interviews with key players (salmon breeders, farmers, nongovernmental organizations (NGOs) and governmental bodies in Norway). First, LR is a polygenic trait, which makes it poorly suited for patenting. Furthermore, if only a small proportion of fish farmers choose seeds with higher LR, other operators can easily take on the free-rider role because they will not suffer from reduced gain in growth performance as a result of a much stronger emphasis on LR in the breeding goal. The market is thus not expected to stimulate stronger selection for LR in Norwegian salmon breeding. Second, neither genetic engineering (e.g., gene editing), still struggling with consumer acceptance, nor the uncertainty associated with possible changes in the Norwegian Gene Technology Act stimulate investment in LR via, for example, CRISPR technology. Thirdly, public policy instruments in their entirety have targeted other types of innovations against salmon lice, and none have so far been used to stimulate breeding companies to emphasize LR more strongly in their breeding programmes. From a political point of view, it seems that breeding has been left to the market and the private sector. However, neither the NGOs nor the public seem to be aware of, or pay significant attention to, the breeding potential to improve LR and fish welfare. Fragmented management of the aquaculture sector can camouflage the close ties between political and business interests. The industry is hesitant to invest significantly in long-term breeding targets such as significantly higher genetic LR. This may strengthen the assumption that strong economic interests will reduce the role of science in knowledge-based management. As farmed salmon are increasingly being exposed to stressful delousing treatments, mortality and associated welfare problems have increased significantly. For instance, large fish die more often from cardiomyopathy syndrome (CMS), resulting in growing demand for CMS-resistant salmon. This gives rise to a paradoxical situation: increasing treatments with high mortality and fish welfare issues in farmed salmon, while the lice threat to wild salmon persists., (© 2023 NOFIMA and The Authors. Journal of Fish Biology published by John Wiley & Sons Ltd on behalf of Fisheries Society of the British Isles.)

Medicinal plants are powerful antioxidants which can improve well-being and suppress oxidative stress caused by environmental toxins in aquatic animals. In this regard, the present research was designed to show the potential effects of psyllium (Plantago ovata) seed extract (PSE) on the growth, and immune responses of common carp Cyprinus carpio exposed to acute ammonia toxicity. To perform the study, fish were fed with diets containing 0 (T0), 0.25 (T1), 0.5 (T2), and 1% (T3) PSE for 60 days, and then exposed to ammonia (0.5 mg L -1 ) for 3 h. The findings showed that fish given the T1 diet outperformed the T3 and control groups in terms of ultimate weight, weight increase, and food conversion ratio. Additionally, the T1 group showed a significantly higher level of total protein and serum lysozyme activity than the other treatment groups. Moreover, the highest serum total immunoglobulin values were recorded in T1 and T2 groups. The results showed that PSE, especially at moderate levels, could successfully upregulate the transcription of immune-related genes (IFN-γ, Hsp70, TNF-ɑ, IL-1ß, IL-10, and IgE) compared to the control group after exposure to ammonia. Furthermore, improving ammonia-induced down regulations of antioxidant-related gene expressions (CYP1A, SOD, and GPX) was observed in fish fed with PSE-included diets compared to the control one. However, PSE-supplemented diets did not affect the mRNA expression level of CAT. Regarding tight junction-associated genes, the higher mRNA expression level of occludin was observed in the T1 group, whereas the downregulation of CLD3 gene occurred in all experimental groups. Conversely, significant upregulation of osmoregulation-associated gene (NKA) was recorded in all experimental groups compared to the control one. Therefore, the administration of PSE (0.25% of the diet) for 60 days is recommended to increase growth performance, improve health, and increase the resistance of common carp to oxidative stress caused by ammonia., (© 2022. The Author(s), under exclusive licence to Springer Nature B.V.)

In recent years, flathead grey mullets ( Mugil cephalus ) cultured in Eilat (Israel) have been highly affected by Vibrio harveyi , showing neurological signs such as uncoordinated circular swimming followed by high mortality rates. Despite the advances in and different approaches to control vibriosis associated with Vibrio harveyi , including commercial vaccines, most of them have not succeeded in long-term protection. In this study, we evaluated the effectiveness, long-term protection, and antibody production of three vaccine preparations: heat-killed bacteria (HKB), membrane proteins denaturation (BME PROT), and internal proteins (INT PROT) developed specifically against Vibrio harveyi for grey mullets. Our results show that fish immunized with heat-killed bacteria emulsified with adjuvant presented the most effective and long-lasting protection against the bacterium, and a cross-protection against other bacteria from the harveyi clade. The effectiveness of each immunization treatment correlated with the levels of specific antibody production against Vibrio harveyi in the serum of the immunized fish.

The Arctic charr ( Salvelinus alpinus) is a relatively stenothermal fish that displays a high degree of biological plasticity. Although primarily a freshwater fish, there are anadromous populations at the northernmost limits of the distributional range. Developmental plasticity has both advantages and disadvantages for establishment in culture, with variable growth rates and early onset of sexual maturation at small size being distinctly disadvantageous. In addition, a requirement for water of low temperature for egg production (4-7°C) and early development limits the possibilities of farming Arctic charr outside of its natural distributional range. On the other hand, a tolerance of high stocking density (60-150 kg m) makes it a candidate for rearing in recirculation systems where effective use of both water and rearing unit volume are at a premium. It is possible to farm some strains of charr in either fresh water or brackish water (20 ‰) throughout the year, but year-round farming in full-strength seawater (33-35 ‰) does not seem to be feasible. Photo-thermal manipulation can be used to influence feeding, growth, salinity tolerance and the reproductive cycle. Water quality requirements seem to be similar to those of other salmonids. [ABSTRACT FROM AUTHOR]

Prolonged production of Atlantic salmon ( Salmo salar ) post-smolts in closed-containment systems has prompted research into biological requirements under higher production intensities. This study examined the effect of fish density and specific water flow on skin health in post post-smolts particularly focusing on epithelial cell morphology and gene expression. In the density experiment, post-smolts were kept at five different fish densities (25, 50, 75, 100 and 125 kg/m 3 ) at a specific water flow rate of 0.6 l/kg/min. Microscopic examination of fluorescence stained whole-mount skin samples demonstrated differences in epithelial cell morphology with increased spacing between epithelial cells at 50 kg/m 3 and 125 kg/m 3 . Gene expression analysis revealed increased transcription of mucin-like 2 , cathepsins B, -D, -L, matrix metalloproteinase 9 and claudin 10 in fish reared at a density of 125 kg/m 3 , while only matrix metalloproteinase 9 and claudin 10 had increased transcription at a density of 100 kg/m 3 . Together, these results suggest structural alterations in the skin epithelium at densities ≥ 100 kg/m 3 . In the specific water flow experiment, four different water flow levels were established (0.2, 0.3, 0.4 and 0.5 l/kg/min) while the fish density was kept constant at 75 kg/m 3 . After eight weeks, transcription of mucin-like 2 and -5ac, inducible nitric oxide synthase, lysozyme and cathepsin B and -L increased in skin samples from fish reared in tanks with a specific water flow of ≤ 0.3 l/kg/min. Increased transcription of these genes implies activation of stress and immune responses in skin at low specific water flow. Results from this study suggests that skin is a sensitive organ for environmental changes, and suggests several molecular indicators that may be valuable in predicting the effects of varying rearing conditions on skin health. Further validation through long-term studies, combined with other health parameters is required for practical recommendations regarding critical fish density and water flow for optimal fish health and performance in semi-closed production systems. Statement of relevance This paper is of high importance for the industry as it investigates the effects of fish density and specific water flow on skin properties in semi-closed-containment-systems in Atlantic salmon post-smolt production. Increased fish densities and reduced specific water flow can increase production profitability. Thus, biological limits related to these two factors are important to identify to avoid reduced animal welfare. [ABSTRACT FROM AUTHOR]

Evolution has since the very beginning resulted in organisms which can sort fitness-related information from noise, evaluate it and respond to it. In animals, the architecture for proximate control of behaviour and physiology has been gradually evolving since before the Cambrian explosion of animal phyla. It integrates many different survival circuits, for example for danger, feeding and reproduction, and operates through reflexes, instincts, homeostatic drives and precursors to human emotions. Although teleost brains differ substantially from the much better understood brains of terrestrial vertebrates, their anatomy, physiology and neurochemistry all point towards a common and malleable architectural template with strong and flexible effects on fish behaviour and elements of personality. We describe the main components of this architecture and its role in fish behaviour from the perspectives of adaptation, evolutionary history and gene pools. Much research is needed, as several of the basic assumptions for architectural control of behaviour and physiology in teleosts are not thoroughly investigated. We think the architecture for behavioural control can be used to change ecosystem models from a bottom-up perspective to also include behaviourally mediated trophic cascades and trait-mediated indirect effects. We also discuss the utility of modelling based on proximate architectural control for fish welfare studies. [ABSTRACT FROM AUTHOR]

In global aquaculture, fish welfare is attached increasing importance to. Fish behavior holds important information of its living welfare. Thus, in order to detect special behaviors concerned with fish welfare in a recirculating aquaculture system (RAS), a modified kinetic energy model of the whole shoal instead tracking individuals inside a shoal was proposed in this study. The dispersion, velocity and turning angle of the shoal were regarded as the key factors in this model. First, with the reflective regions of the water surface taken into account, two alternative ways were presented to extract the necessary spatial behavioral characteristics of the shoal. Then, optical flow, entropy and statistics were adopted to measure the dispersion status (dispersion of individuals in space) and the changing magnitude of the motion (behavioral characteristics) of the shoal. Finally, the modified kinetic energy model was obtained by combining the dispersion measurement with the changing magnitude of the behavioral characteristics. Referring to the human observation and the evacuation of the gastro-intestine contents-based long experiment of Nile tilapia ( Oreochromis niloticus ) in RAS, the proposed model shows good performance in detection of the emergent gathering and scattering behaviors with 97.20 ± 1.23% success rate at least and 0.61 ± 0.08% missing report rate at most. [ABSTRACT FROM AUTHOR]

Dispatch of finfish using the ikijime method (also known as ikejime or brain spiking) is the fastest and most humane way to kill fish. However, fish brains are small and vary in location between species groups. Without guidance, recreational fishers can find it difficult to pith the brain of a live fish accurately, potentially resulting in unnecessary stress. A lack of information on how to perform ikijime was considered a barrier to the widespread uptake of this method. This project filled the information gap by developing resources that provide accurate information on the ikijime procedure and pinpoint the location of the brain of fish commonly captured by recreational fishers. To assist with communicating this information to recreational fishers and the broader community, a new website was developed, together with the Ikijime Tool series of phone apps, which provide access to interactive photograph/radiograph overlays revealing the brain location of 100 species of fish from 38 families. These resources raise awareness of fish welfare issues and improve the ability of recreational fishers to dispatch fish humanely using best practice methods. [ABSTRACT FROM AUTHOR]

In the present experiment, the characteristics of minimally handled, directly slaughtered farmed Atlantic cod were compared to cod subjected to 2 h preslaughter handling stress. The results strongly indicate that increasing the storage temperature from 0 to 5°C had a larger negative impact on the quality characteristics than the handling stress applied. Further, there may be interaction effects between handling, storage temperature, and storage time. [ABSTRACT FROM PUBLISHER]

Methylmercury (MeHg) is an environmental contaminant of increasing relevance as a seafood safety hazard that affects the health and welfare of fish. Non-invasive, on-line methodologies to monitor and evaluate the behavior of a fish system in aquaculture may make the identification of altered systems feasible--for example, due to the presence of agents that compromise their welfare and wholesomeness--and find a place in the implementation of Hazard Analysis and Critical Control Points and Fish Welfare Assurance Systems. The Shannon entropy (SE) of a European seabass (Dicentrarchus labrax) system has been shown to differentiate MeHg-treated from non-treated fish, the former displaying a lower SE value than the latter. However, little is known about the initial evolution of the system after removal of the toxicant. To help to cover this gap, the present work aims at providing information about the evolution of the SE of a European seabass system during a recuperation period of 11 days following a two-week treatment with 4 μg· MeHg/L. The results indicate that the SE of the system did not show a recovery trend during the examined period, displaying erratic responses with daily fluctuations and lacking a tendency to reach the initial SE values. [ABSTRACT FROM AUTHOR]

The EU Directive on animal experimentation suggests that all protected animals should have enrichment to improve welfare yet relatively little research has been conducted on the impact of enrichment in fish. Studies employing enrichment in zebrafish have been contradictory and all fish species should be provided with species-specific enrichments relevant to their ecology. Salmonids are important experimental models in studies within aquaculture, toxicology and natural ecosystems. This study therefore sought to establish whether an enriched environment in an experimental aquarium may promote improved welfare in rainbow trout ( Oncorhynchus mykiss ) by enhancing their recovery from invasive procedures. Trout were held individually in either barren (no tank ornamentation) or enriched (gravel, plants and an area of cover) conditions. Recovery rates after a noxious stimulus and a standard stressor were investigated by monitoring behaviour, opercular beat rate and plasma cortisol concentrations. Fish were randomly assigned to one of four treatment groups: Control (undisturbed), Sham (handled but not manipulated), Stress (air emersion) and Pain (subcutaneous injection of acetic acid). The results suggest that for rainbow trout environmental enrichment appears to promote recovery and ameliorate adverse effects following a stressor. However, recovery rate did not differ between environments in the pain treatment groups. Thus environmental enrichment may not be an important factor when the fish is responding to a painful stimulus. These results have important implications for the husbandry and welfare of captive rainbow trout and possibly other salmonids and suggest that enriched environments may be preferable to barren environments in experimental studies. [ABSTRACT FROM AUTHOR]