Fish Welfare Research Articles

Comparative assessment of plasma cortisol and fecal corticoid metabolites (FCM) of Atlantic salmon (Salmo salar L.) subjected to acute- and long-term stress

Farmed Atlantic salmon (Salmo salar L.) are subject to a variety of stressors throughout production. Elevations of cortisol level in the blood are one of the major endocrine primary stress responses in vertebrates and are widely used as stress indicator. However, blood sampling is invasive and stressful procedures. Cortisol instantly released into the blood at sampling can easily interfere with the initial stress response of interest. Fecal corticoid metabolites (FCM) have been suggested for a less invasive assessment of stress in fish. In the present study, we evaluated stress responses by using plasma cortisol and FCM as stress indicators in Atlantic salmon. Atlantic salmon (with average weight 42 g) were exposed to 3 different stressors: 1) parr-smolt transformation; 2) infestation with salmon lice (Lepeophtheirus salmonis) for two-weeks; 3) infection with infectious salmon anaemia virus (ISAV) for four weeks. The results demonstrated that FCM levels correlated well with the plasma cortisol levels at single-point sampling during long-term stress (p < 0.05). Significant increases of both plasma cortisol and FCM were found two weeks after initiated 24 h light. Each tank was sampled twice, with around 40-min interval. The chasing during the first sampling was used as an acute stressor. The effects of sampling procedures on plasma cortisol and FCM levels are compared. Blood and feces were analyzed by an Enzyme-Linked Immunosorbent Assay (ELISA). Plasma cortisol and FCM levels increased within 40 min after fish perceived an acute stressor when the average weight of fish was below 100 g. Attenuated cortisol stress responses to acute stress were found after fish experiencing long-term stress. Collectively, FCM reflects changes in plasma cortisol and can be an alternative to analysis of plasma cortisol in Atlantic salmon, complementing evidence of using FCM as stress indicators for monitoring and improving fish welfare. The results highlight that standard sampling procedures and good experimental designs should be established to ensure robust and repeatable results in the study of stress based on the FCM measurement. Future study will investigate representative timepoints for FCM sampling, with particular attention to the types of stress and sizes of the fish.

Effects of temperature and photoperiod on growth, physiological, and behavioral performance in steelhead trout (Oncorhynchus mykiss) under indoor aquaculture condition

Light and temperature are necessary conditions for migratory fish. The assessment of fish physiology and behavior is important for identifying fish welfare, but also for the assessment of the optimal setting of recirculating aquaculture systems (RASs). This study aimed to explore the interactive effect of photoperiod and temperature on steelhead trout culture. Four treatments were set up with specific settings were as follows: a LP-LT group treated with 16L:8D and 12°C, a LP-HT group treated with 16L:8D and 16°C, a SP-LT group treated with 12L:12D and 12°C, and a SP-HT group treated with 12L:12D and 16°C. Growth performance, behavioral and physiological parameters were measured. Two indexes, locomotor activity and social interaction were used for behavioral analysis, and the results were applied to interpret the behavioral responses to the photoperiod and temperature stimulation in juveniles. The growth performances were significantly lower in treatments LP-LT and SP-LT. The treatment LP-HT had significantly higher growth performance than the other treatments, but no significant differences were noted in survival rate and coefficient of variation. The results of fish behavior indicated that the movement of juveniles should be primarily monitored at high temperatures or long photoperiods, and the state parameters should be primarily monitored at low temperatures or short photoperiods. The results of the physiological parameters showed that the recovery time from stress varied among different treatments. After 60 days of the experiment, superoxide dismutase and alanine aminotransferase dropped back to their initial level. The results of Na+-K+-ATPase showed that although the combined effect of photoperiod and temperature could advance the time of smoltification, it may result in poorer salt tolerance. Our findings underscore the importance of the interaction of photoperiod and temperature on steelhead trout culture. The outcome could provide guidance for the development of effective aquaculture systems.

Evaluating the repetitive mucus extraction effects on mucus biomarkers, mucous cells, and the skin-barrier status in a marine fish model

Among all the mucosal barriers, the skin and its surrounding mucus are possibly the main defensive tool against changes in the environment that can be harmful for fish. Due to the extraction of this mucus being less invasive, the study of its production and functions has attracted great interest in recent years. However, there are still many gaps concerning the sampling process as well as the possible alterations in skin integrity and mucus composition. In the current study, the effects of skin mucus extraction were determined by comparing the effects of a single extraction (single extraction group, SEG) with those of three successive extractions separated by 3 days (repetitive extractions group, REG). Intact skin histology without mucus extraction (ØEG) and both plasma and skin mucus biomarkers and antibacterial capacities were also assessed. Regarding the skin histology and skin barrier properties, both the SEG and REG did not show differences in the intact skin. Interestingly, repetitive mucus extractions seemed to activate skin mucus turnover, significantly increasing the number of small-sized mucous cells (cell area&amp;lt; 100 µm2) and reducing the number of large-sized mucous cells (cell area &amp;gt; 150 µm2). Repetitive extractions significantly decreased the amounts of soluble protein and increased cortisol secretion. These metabolites remained unaltered in the plasma, indicating different responses in the plasma and mucus. Despite changes in the mucus biomarkers, antibacterial capacity against pathogenic bacteria (Pseudomonas anguilliseptica and Vibrio anguillarum) was maintained in both the plasma and mucus irrespective of the number of mucus extractions. Overall, the mucus sampling protocol had little effect on skin integrity and mucus antibacterial properties, only modifying the amounts of soluble protein exuded and stimulating mucous cell replacement. This protocol is a feasible and minimally invasive way of studying and monitoring fish health and welfare and can be used as an alternative or a complement to plasma analysis. This methodology can be transferred to farm culture conditions and be very useful for studying threatened species in order to preserve fish welfare.

Environmental Enrichment for Rainbow Trout Fingerlings: A Case Study Using Shelters in an Organic Trout Farm.

Physical enrichment can improve the welfare of captive fish. Previous research has shown that fish often show preference for enriched environments, which can also result in improvements in growth performance. However, effects of enrichment are not always positive and the design and extent of the enrichment needs to be carefully considered. In this regard, information in real aquaculture scenarios is limited. The aim of this study was to serve as a proof of concept to test the feasibility of using simple PVC immersed shelters as a tool for better welfare in an organic rainbow trout farm. Our shelters induced little extra work in farm routines and had no negative effects on fish performance, health or mortality. The behavioral assessment pointed to a preference for sheltered areas in undisturbed conditions. However, no benefits were observed in terms of stress responses during standardized stress tests, and fish showed no obvious shelter-seeking behavior after disturbance. The results in terms of shelter-seeking behavior were probably limited by the short duration of the experiment, which was due to the farm's routines and needs. It is recommended that strategies for enrichment in real scenarios should be tested covering a relevant part of the life cycle of the fish in captivity, to fully account for their potential to improve welfare in aquaculture.

Temporal Variations in Hematological, Immunological and Serum Biochemical Parameters of Rainbow Trout (Oncorhynchus mykiss) Acclimated to High-Saline Water in the Northern Aegean Sea

Abstract The present study evaluated the health status of rainbow trout (30.24±5.34 g) acclimated to high-saline water (28‰). Among adaptation procedures, gradually-acclimated (for 48 h) fish showed best performance, which were then introduced to the offshore cage-farm for further monitoring of fish health in marine environment over a long-term period of 100 days, until harvest weight of 319.9±48.51 g. Fish health and welfare was evaluated by means of biochemical parameters (viscerasomatic index, hepatosomatic index, mesenteric fat index, and spleen somatic index), hematological parameters (serum glucose, triglyceride, cholesterol, total protein, albumin, globulin), and immune parameters (lysozyme, myeloperoxidase, respiratory burst and potential killing values). Gradual acclimation to high-saline water did not show any adverse effects on health parameters. No significant differences were found in biometric measures (P&gt;0.05). However, a time-dependent increase was recorded in hematology markers (P&lt;0.05) after seawater transfer, and the health status improved over long term from March 7 to June 14, 2018. All serum biochemical markers, except the total protein showed significant alterations (P&lt;0.05) in long term but without detrimental influence by day 100. Therefore, it seems possible to expand trout farms from brackish water sites to higher saline environments up to 28‰ salinity, with no detrimental impacts on fish health, that in turn may significantly contribute to the extension of potential aquaculture sites to wider areas.

Microplastic exposure and consumption increases susceptibility to gyrodactylosis and host mortality for a freshwater fish.

Microplastics have been found in all surveyed ecosystems and in the diet of multiple species. Detrimental health impacts of microplastic consumption include reduced growth and fecundity, metabolic stress and immune alterations for both invertebrates and vertebrates. Limited information exists, however, on how disease resistance may be affected by microplastic exposure and consumption. Here, the impact of microplastic (0.01 and 0.05 mg l-1 of polypropylene) on fish host susceptibility to disease and mortality was assessed using the guppy Poecilia reticulata-gyrodactylid Gyrodactylus turnbulli system. Fish exposed to and/or consuming microplastic at both concentrations demonstrated significantly higher pathogen burdens over time compared with fish fed a plastic-free diet. Furthermore, microplastic (at both tested concentrations) was associated with increased mortality events for fish within all treatments, regardless of host infection status. This study adds to the growing body of evidence showing that microplastic pollution can be detrimental to fish welfare by reducing disease resistance.

THE USE OF ENVIRONMENTALLY-SAFE POLUMS IN AQUACULTURE

Introduction. In the conditions of anthropogenic pollution of water, environment-safe drugs and implementing of immunomodulatory drugs are becoming increasingly common. Schemes of their use in fisheries are being developed. The article presents and analyzes the potential use of biologically active additives (BAA), namely probiotics, prebiotics and yeasts, both domestically and internationally produced, in terms of their effect on the fish body.&#x0D; Materials and methods of research. Search for literature data on the use of biologically active substances in fish farming, namely probiotics, prebiotics and yeast.&#x0D; Research results. Considering the negative impact of prophylactic and therapeutic use of antibiotics in aquaculture, the use of dietary immunostimulants has been proposed as an alternative to antimicrobial drugs. In this sense, functional dietary supplements, including pre-, probiotics and yeasts, are receiving increasing attention as an environmental strategy to improve fish health.&#x0D; Probiotics are the objects of comprehensive scientific research and an important product on the world market. The use of probiotics as biocontrol agents in aquaculture is increasing. The benefits of such additives include increased nutritional value, inhibition of pathogens and enhanced immune response by increasing white blood cells and phagocytosis. They improve the quality of the growing environment, protect fish from biological hazards, and modulate physiological processes that ultimately contribute to the health and welfare of fish in aquaculture. Probiotics also enhance growth performance and feed utilization in aquatic animals by increasing the activity of digestive enzymes.&#x0D; The beneficial effects of prebiotics are due to by-products resulting from the fermentation of intestinal commensal bacteria. Among the many health benefits attributed to prebiotics is the modulation of the immune system. They directly enhance the innate immune response, including activation of phagocytosis, neutrophils, alternative complement system, and increased lysozyme activity.&#x0D; Another environmentally friendly product that has been proposed as a dietary supplement is yeasts. Research on yeast products in fish diets has focused on their role in nutritional and functional supplements that contribute to the immune responses and gut health of fish.&#x0D; Conclusions. Various studies of pro- and prebiotics in fish have shown the following results: effects on growth, gut microbiota, resistance to pathogenic bacteria and parameters of innate immunity such as alternative complement activity (ACH50), lysozyme activity, natural hemagglutination activity, respiratory burst, superoxide dismutase activity and phagocytic activity.All the above studies demonstrate that the addition of nutritional supplements to feed, such as immunostimulants, is an alternative method for the prevention and control of various diseases in aquaculture.

Development of integrated multitrophic aquaculture–based cage rearing system in an underutilized fishing port and its application in marine stock enhancement

Human activity and global climate change have severely affected marine ecosystems and fishery resources. Habitat conservation and stock enhancement are considered effective methods. Moreover, with the gradual disappearance of fishery resources, fishing ports have become underutilized spaces. Currently, 73 of the 221 fishing ports in Taiwan are underutilized. Therefore, we, for the first time, developed an integrated multitrophic aquaculture (IMTA)-based cage rearing system suitable for stock enhancement and applied it in an optimal underutilized fishing port after the site evaluation and selection of 17 potential fishing ports fishing. We further tested that hypothesis that hatchery-produced organisms can be reared and monitored appropriately in this cage rearing system with good survival and growth as well as less environmental impact and handling stress. Through the collocation of various release organisms of different trophic levels, the cage rearing system can reduce environmental impacts as evidenced by the steady water quality (stable pH and undetectable levels of ammonia nitrogen, nitrates, and nitrites). As for the fish welfare, this semiartificial rearing system could also reduce the discomforts of hatchery-produced organisms after transportation and facilitate their adaptations to the released environments as evidenced by positive growth and high survival rates (94%–98%). The cultured and naturally grown shellfish and algae on the cage nets could provide habitats for hatchery-produced and wild organisms that facilitate habitat conservation and stock enhancement. Taken together, we have demonstrated that it is feasible to implement this novel IMTA-based cage rearing system in an underutilized fishing port required for marine stock enhancement.

Vitality as a measure of animal welfare during purse seine pumping related crowding of Atlantic mackerel (Scomber scrombrus)

The impacts of wild capture fishing on animal welfare are poorly understood. During purse seine fishing for Atlantic mackerel (Scomber scrombrus), catches are crowded to high densities to facilitate pumping onboard. This study aimed to monitor fish welfare during crowding events in the Norwegian purse seine fishery, and to identify relevant drivers. We first correlated a suite of neuro-endocrine, physiological and physical stress responses (integrated into a single measure of welfare using multivariate analysis) to the behavioural vitality of individual mackerel in controlled crowding trials in aquaculture cages. Vitality was found to be a useful measure of welfare. We then assessed individual fish vitality onboard a commercial purse seiner. Catch welfare, measured using vitality, was observed to be negatively impacted during pumping related crowding. Larger catches and longer crowding exposure times resulted in greater negative impacts. Vitality was not significantly impacted by crowding density or dissolved oxygen concentrations inside the net, although methodological limitations limited accurate measurement of these parameters. Blood lactate levels correlated negatively with vitality, suggesting that high-intensity anaerobic locomotory activity was associated with the reduction in welfare. Based on these findings, catch welfare could be improved by targeting smaller schools to minimise crowding exposure times.

Considering Two Aspects of Fish Welfare on African Catfish (Clarias gariepinus) Fillet throughout Postmortem Condition: Efficiency and Mechanisms.

Knowledge about fish welfare and its impact on fish fillet quality is still insufficient. Therefore, the influence of two aspects of fish welfare (slaughtering method: bled and unbled fish; fish stock densities: 90, 120, and 150 kg·m-3) on African catfish fillet quality during postmortem conditions was investigated. The aim of study was to determine (i) the efficiency of bleeding on oxidation progress and (ii) the influence of stock density on fillet quality. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed a higher protein loss in the unbled than in the bled groups, especially in the heavy myosin chain (MHC) band. However, density did not show any influence on protein profile. Western blot analysis showed fewer oxidized carbonyls in the bled than in the unbled groups; higher oxidation development, microbial growth, and lower hardness were observed in unbled fillets. Additionally, hardness was higher at 90 and 120 kg·m-3 densities in bled fillet compared to 150 kg·m-3. The first three days of storage showed a higher oxidation rate in unbled fillets than in bled fillets, confirming the contribution of hemoglobin to oxidation development with different mechanisms of protein oxidation. The obtained results revealed the same fillet quality in all aspects at either 90 or 120 (kg·m-3) stock densities, which would suggest 120 kg·m-3 for the fishery industry. However, higher stocking density in this study would not be appropriate for fish welfare.

Use of Bacillus Strains Producing Lichenase and Laminarinase Enzymes as Fish Feed Additives

To obtain high efficiency from animals, it is necessary to maximize the ability to benefit from feed, as well as to protect animal health. One of the important applications in this direction is feed additives. Feed additives are plant and animal products and microorganisms that increase fish production and welfare, improve digestion and digestive system microflora and contribute to the preservation of nutrients and feed. Although plant and animal tissues are used in enzyme production, the use of microorganisms is in the first place. After the use of molecular genetic techniques, the genes responsible for enzyme production were cloned in microorganisms, making it possible to produce enzymes more purely and cheaply on an industrial scale.

The effectiveness of bite-shortened hooks for reducing handling time and injury of small-bodied freshwater fish captured by recreational anglers

Modified bite-shortened hooks show promise for enabling easy handling and rapid release of fish captured by recreational anglers, with the potential to reduce injury and stress of released fish. This study investigated whether bite-shortened modified hooks were effective at improving fish welfare relative to more traditional hook configurations. We evaluated how hook type (jigs vs. Aberdeen), hook length (regular, shortened by 4 mm or shortened by 6 mm), and the presence or absence of a barb influenced the landing rates, handling time, hooking depth, ease of hook removal, and injury of Bluegill (Lepomis macrochirus). There was significantly more fish landed using jig style (87.7%) than Aberdeen hooks (80.5%). The landing rates varied significantly among hook configurations with standard barbed hooks having the highest landing success (96%) and very bite-shortened hooks having the lowest (67%) landing success. Once landed, self-release (i.e., unhooked without use of hands) was most common for fish caught on very bite-shortened (shortened by 6 mm), followed by short bite-shortened (shortened by 4 mm), and then regular barbless hooks (hook not modified), while regular barbed hooks had the lowest self-release rate. Very short bite-shortened and short bite-shortened barbless hook configurations resulted in shallower hook depths and were less likely to cause injury compared to barbed and barbless regular length hooks. This research suggests that bite shortened barbless hooks allow for easy and rapid self-release while retaining reasonably high rates of landing success, suggesting that bite-shortened hooks could be a useful management tool for recreational fisheries.

Gilthead Seabream Liver Integrative Proteomics and Metabolomics Analysis Reveals Regulation by Different Prosurvival Pathways in the Metabolic Adaptation to Stress

The study of the molecular mechanisms of stress appraisal on farmed fish is paramount to ensuring a sustainable aquaculture. Stress exposure can either culminate in the organism's adaptation or aggravate into a metabolic shutdown, characterized by irreversible cellular damage and deleterious effects on fish performance, welfare, and survival. Multiomics can improve our understanding of the complex stressed phenotype in fish and the molecular mediators that regulate the underlying processes of the molecular stress response. We profiled the stress proteome and metabolome of Sparus aurata responding to different challenges common to aquaculture production, characterizing the disturbed pathways in the fish liver, i.e., the central organ in mounting the stress response. Label-free shotgun proteomics and untargeted metabolomics analyses identified 1738 proteins and 120 metabolites, separately. Mass spectrometry data have been made fully accessible via ProteomeXchange, with the identifier PXD036392, and via MetaboLights, with the identifier MTBLS5940. Integrative multivariate statistical analysis, performed with data integration analysis for biomarker discovery using latent components (DIABLO), depicted the 10 most-relevant features. Functional analysis of these selected features revealed an intricate network of regulatory components, modulating different signaling pathways related to cellular stress, e.g., the mTORC1 pathway, the unfolded protein response, endocytosis, and autophagy to different extents according to the stress nature. These results shed light on the dynamics and extent of this species' metabolic reprogramming under chronic stress, supporting future studies on stress markers' discovery and fish welfare research.

Increasing Dip Net Mesh Size Results in More Fin Splits in Post-Smolt Atlantic Salmon (Salmo salar)

ABSTRACT Dip nets are widely used to retrieve fish from the water but can cause injuries to the fish. The objective of this study was to document the potential effects of dip net mesh size with respect to external injuries, more specifically fin splits and scale losses, in Atlantic salmon (Salmo salar). The study included 273 post-smolt Atlantic salmon from two fish batches referred to as the Tank group ( w ˉ = 178 ± 36 g, n = 198) and the Cage group ( w ˉ = 1319 ± 334 g, n = 75). Four identical dip nets with mesh sizes of 5, 10, 15, and 20 mm, respectively, were used to net fish out of their enclosures before the external injuries were assessed by visual inspection. The results showed that the number and severity of fin splits increased with dip net mesh size, but no effect of mesh size on the percentage of scale losses was found. Dip-netting of five fish at a time instead of one with the 20 mm dip net, had no statistically significant effect on the examined injuries. It is concluded that the use of dip nets with smaller mesh sizes can be positive for fish welfare by reducing fin splitting.

Dietary Arthrospira platensis in Rainbow Trout (Oncorhynchus mykiss): A Means to Reduce Threats Caused by CdCl2 Exposure?

The rainbow trout (Oncorhynchus mykiss) is one of the most commercially sought-after freshwater fish species and one of the most farmed in the world. On the other hand, aquaculture breeding frequently results in outbreaks of infectious diseases and pests, and compromises the production and welfare of fish. Arthrospira platensis (known as "Spirulina") has been used as a supplement in diets to enhance fish welfare in recent years because of its beneficial properties. This study aimed to assess the possible protective effects of Arthrospira platensis on rainbow trout specimens exposed to three different doses of the toxicant CdCl2. The experiment was carried out using five experimental treatments of 40 individuals each: control group; group II (0.2 mg CdCl2 per kg of commercial fish feed); group III (0.2 mg Kg-1 of CdCl2 plus 2.5 g per kg of A. platensis); group IV (0.2 mg Kg-1 of CdCl2 plus 5 g per kg of A. platensis); group V (0.2 mg Kg-1 of CdCl2 plus 10 g per kg of A. platensis). During the experiment, dietary supplementation of A. platensis normalized all serum and blood parameters altered by the presence of CdCl2. A. platensis also had a protective effect on markers of oxidative stress.

Effect of water quality ununiformity on production of marine medaka

The large amount of organic impurities deposited at the bottom of an intensive aquaculture pond often leads to the deterioration of the surrounding environment, creating additional environmental stresses on the farmed fish and affecting its growth. This study investigated the effect of water quality ununiformity on growth, feeding behavior, and stress tolerance of marine medaka (Oryzias melastigma) through differences of residence time in different water quality areas in the same aquaculture system. Through the pre-test, the flow field distribution in the aquarium for cultured experiment was clarified, and the water body in the aquarium was divided into the central area (Ce) and the corner area (Co) according to the water quality difference in different flow fields. Then, seven residence time gradients were set in Co, which were 24 h, 14.4 h, 7.2 h, 4.8 h, 2.4 h, and 0 h, respectively. That was, they accounted for 100% (P100), 60% (P60), 30% (P30), 20% (P20), 10% (P10), and 0% (P0) of a whole day, respectively. The results showed that, all water quality indicators such as dissolved oxygen (DO), pH, nitrite (NO2−-N) and non-ionic ammonia (NH3−N) in Ce and Co were consistent with the requirements of the standard (GB11607–89) during the 28-day cultured experiment, but the water quality in Co was gradually worse than that in Ce, with the prolonging cultured period. In spite of increasing the body length and body weight of medaka in all treatments, their growth rates had significant differences among several treatments (P0 = P10 = P20 > P30 > P60 = P100). And the fish's growth, feeding efficiency, and stress resistance decreased with the increase of residence time in Co (P < 0.05). Through nonlinear regression analysis, it was predicted that the accumulation of residence time of marine medaka stayed in the coner area was >6.667 h, its feeding behavior would be affected, and the corresponding appetite and food intake, immunity to disease and growth rate would all decreased. Attaining uniformity of water quality in aquaculture system may help improve fish welfare and increase the sustainability of aquaculture.

Modified kinetic energy feature-based graph convolutional network for fish appetite grading using time-limited data in aquaculture

Feed has the greatest impact on the carbon footprint of the aquaculture, and also determines the water quality in aquaculture to a great extent. Making appropriate feeding control strategies is one of the most effective ways to promote cleaner production as well as fish welfare in aquaculture. Reliable and accurate fish appetite grading especially based on time-limited data is a prerequisite for achieving high-precision and reasonable feeding control in practical production. To date, however, few efforts have been done on this challenge. For these, regarding Micropterus salmoides as the experimental fish, a novel and practical method, based on a modified kinetic energy feature-based graph convolutional network (GCN), was developed in this study. First, graphs were constructed based on the extracted modified kinetic energy features and their temporal correlation. Then, with the help of a series of the convolution and global pooling operations, a GCN model was customized based on the constructed graphs. Following this, the customized GCN model was enriched by the self-attention pooling mechanism and customized network structure. Results show that the proposed GCN-based approach outperforms other typical state-of-the-art methods in fish appetite grading, and the grading accuracy obtained here could be 98.60% using only the first 4.2 seconds as well as the first 8.3 seconds of input data, which is not much different from that (98.89%) using full-length (25 second-long) input data. What’s more, compared to the recurrent neural network (RNN)-based method which performance is closest to our method, the space complexity of the proposed approach here can better satisfy the requirements of real aquaculture, in which the quantity of the trainable parameters here is only 6.4% ~ 31.8% of the RNN-based method. In summary, the proposed modified kinetic energy feature-based GCN approach is favorable for the appetite grading of fish like Micropterus salmoides with time-limited data, which is a promising approach in dealing with feeding control tasks and alleviating the water environmental burden in aquaculture.

Long-term body tactile stimulation reduces aggression and improves productive performance in Nile tilapia groups

One concern of the Anthropocene is the effects of human activities on animal welfare, revealing the urgency to mitigate impacts of rearing environments. Body tactile stimulation (TS), like massage therapy, has emerged as an enrichment method to counteract stress and anxiety in vertebrates. In the current study, we evaluated the effects of long-term TS on four-member groups of male Nile tilapia, a worldwide reared species whose socially aggressive behavior is an essential source of stress. We placed a rectangular PVC frame fitted with vertical plastic sticks sided with silicone bristles in the center of aquarium to enable the fish to receive body TS when passing through the bristles. A similar apparatus without bristles was used as the control. Fish subjected to TS for 21 days showed a gradual lowering of overt fights over time, but with no reduction in cortisol or androgen levels. Nevertheless, TS improved the specific growth rate, maintained balanced length/weight gain, and increased feed efficiency, probably owing to the lowered energy expenditure during fights. Thus, we show for the first time that long-term TS provided by a simple device can be used as a tool to improve the welfare and productive performance of territorial fish.

Study of the Effect of Dietary Agavin Supplementation in Blood Parameters and Antioxidant Enzymes of Juvenile Nile Tilapia (Oreochromis niloticus) under Stress Conditions

High-density aquaculture produces oxidative stress that affects fish welfare and leads to poor growth. Agavin is a fructan that regulates the antioxidant response and inhibits stress-related alterations in blood metabolites (cortisol and blood lipids), preventing oxidative damage in fish. This study evaluated the effect of dietary agavin on the hepatic antioxidant response and blood metabolites in Nile tilapia under high rearing density. A negative control, a positive control, and experimental diets supplemented with 20 and 40 g/kg agavin were formulated. Nile tilapia (1.04 g ± 0.01 g initial body weight) were fed for 80 days to evaluate the antioxidant response and blood metabolites. After 90 days, fish were exposed to high density (63 kg/m3) for twenty days, except for the negative control (low-density). Under high-density stress, cortisol, cholesterol, and triglyceride levels decreased in fish fed diets with agavin compared with the positive control (high density). CAT and SOD activity significantly increased in fish fed agavin diets; malondialdehyde levels decreased under high density compared with both controls. These results suggest that dietary agavin supplementation promotes the antioxidant response and prevents stress-related alterations in blood parameters in Nile tilapia under high rearing density through its antioxidant properties and, probably, has a prebiotic effect.

Physical and social enrichment influences the adaptability-related behaviors of black rockfish Sebastes schlegelii: An effect mediated by social behaviors, HPI axis and neurogenesis

Environmental enrichment is a promising method to increase fish ecological fitness in hatchery release and to improve fish welfare in the aquaculture industry. However, little is known about the effects of physical and social enrichment on fish behavioral development and the relevant regulatory mechanisms. To address this knowledge gap, a typical aquaculture fish, black rockfish Sebastes schlegelii, was exposed to contrasting environments (i.e., physically enriched environment vs. barren environment, and socially low-density environment vs. high-density environment) for six weeks. Subsequently, the adaptability-related behaviors and social interactions during the captive period were determined, and stress-related physiological parameters and neurogenesis-related gene markers were measured. In general, the fish from an enriched environment expressed significantly higher sheltering propensity and anti-predator ability while less risk-taking frequency than the fish from a barren environment. And meanwhile, the fish from a low-density environment presented significantly less risk-taking behavior and more anti-predator responses than high-density fish. A structurally barren environment significantly increased fish aggressive frequency and locomotor activity compared with an enriched environment, whereas the social environment did not affect fish social interactions. For physiological status, the hypothalamus-pituitary-interrenal (HPI) axis and corticosteroid receptor system activities of barren fish were significantly higher than those of enriched fish, and meanwhile, the high-density fish also presented significantly higher physiological stress than low-density fish. For neurogenesis processes, an enriched environment did not affect fish brain cell proliferation but increased telencephalon neuronal differentiation ratio and immature neuron number. In contrast, a low-density environment significantly stimulated cell proliferating activity in telencephalon but did not affect brain neuronal differentiation activity. These novel results verified the conspicuous effects of physical and social environments on the adaptability-related behaviors in aquaculture fish, and finally, we proposed a novel regulatory pathway for fish behavioral plasticity, i.e., environmental stimuli - social interactions - HPI axis - corticosteroid receptor system - neurogenesis - adaptability-related behaviors. These results may lay the foundations for restoring fishery resources and for improving fish welfare, add novel knowledge to fish biology and environmental enrichment framework, and help deeply understand the mechanisms of environmental effects on marine animal behaviors.