Effect of fasting on self-feeding activity in juvenile sea bass (Dicentrarchus labrax)

In various experiments under self-feeding conditions, sea bass groups could be divided into three categories regarding feeder actuation: high, low and zero-triggering fish. In all cases few high-triggering fish were responsible for a high percentage of the feed delivery. A question was raised about the role played by feeding motivation in such high-triggering status acquisition. It was approached by applying a 3-week fasting period in order to induce similar negative specific growth rate (SGR) in two groups of fish of similar mean weight but with either a low or a high coefficient of variation for weight (CV_w) (T_low: CV_w ∼ 11%, 3 tanks of 60 fish each; T_high: CV_w ∼ 20%, 3 tanks of 60 fish each). These groups were created to test the consistency of behavioural responses in two different contexts (i.e. two population size-distributions). During the follow-up period of 40 days, the group level feed-demand behaviour was not strongly modified by the fasting period and there were no differences between T_low and T_high groups. Complete growth compensation was the same in all tanks as observed at the end of the experiment. At the individual level, high-triggering fish were exactly the same individuals before and after the fasting period. Up to four high-triggering fish could be observed according to the tank and when several fish were performing high-triggering activity, their rankings were sometimes reversed after the fasting period. High-triggering fish increased their activity levels after the fasting period showing behavioural plasticity. High-triggering status could neither be explained by an initial lower SGR nor a sex effect, nor by any of the measured physiological blood parameters. Thus, individual's triggering activity levels could be related to personality and/or metabolic traits but further research is required to confirm this assumption.     

Releasing of the European sea bass Dicentrarchus labrax (Linnaeus) in the Adriatic Sea: Large-volume versus intensively cultured juveniles

The main goals of this project were to evaluate if artificial reefs are suitable sites for releasing hatchery-reared sea bass and if intensively and extensive large-volume cultured sea bass are suitable to be released into the wild for stock enhancement purposes. Large-volume cultured bass were reared in lower densities compared to intensively cultured ones and, when fry were about 90 days old, were transferred into external ponds connected to the channels of the surrounding marsh, where that they could integrate pellet food sources with live prey. Intensively cultured bass were fed for 55 days with Artemia salina (Linnaeus, 1758) and then with pellets. Underwater visual census (UVC) and fishing sampling were carried out to verify the presence of the tagged specimens at the artificial reef. A low mortality rate after tagging was observed and good tag retention was recorded. Individuals dead after tagging were statistically smaller than survivors. A total of 45 tagged bass (42 large-volume and 3 intensively cultured) were returned by fishers and 16 specimens were observed during UVC (15 large-volume and 1 intensively cultured). The majority of recaptured bass concentrated in the surroundings of river mouths and harbours suggesting that, after release, sea bass migrated towards shallower and brackish waters. Subsequently, as they grew, they came back towards deeper waters and tended to aggregate around artificial structures. Analysis of stomach contents of returned individuals confirmed their dependence on hard-substrate food items. The study provides evidence to support the suitability of large-volume juveniles for restocking purposes, due to their ability to prey on wild food and their endurance to the stress caused by release operations.     

Effects of feeding and hypoxia on cardiac performance and gastrointestinal blood flow during critical speed swimming in the sea bass Dicentrarchus labrax

Previous studies have shown that if European sea bass are exercised after feeding, they can achieve a significantly higher maximum metabolic rate (MMR) than when fasted. They can meet combined metabolic demands of digestion (specific dynamic action, SDA) and maximal aerobic exercise, with no decline in swimming performance. If, however, exposed to mild hypoxia (50% saturation), bass no longer achieve higher MMR after feeding but they swim as well fed as fasted, due to an apparent ability to defer the SDA response. This study explored patterns of cardiac output (Q_A) and blood flow to the gastrointestinal tract (Q_GI) associated with the higher MMR after feeding, and with the ability to prioritise swimming in hypoxia. Sea bass (mean mass ~ 325 g, forklength ~ 27 cm) were instrumented with flow probes to measure Q_A and Q_GI during an incremental critical swimming speed (U_crit) protocol in a tunnel respirometer, to compare each animal either fasted or 6 h after a meal of fish fillet equal to 3% body mass. Feeding raised oxygen uptake (M_O2) prior to exercise, an SDA response associated with increased Q_A (+ 30%) and Q_GI (+ 100%) compared to fasted values. As expected, when exercised the fed bass maintained the SDA load throughout the protocol and achieved 14% higher MMR than when fasted, and the same U_crit (~ 100 cm s^-1). Both fed and fasted bass showed pronounced increases in Q_A and decreases in Q_GI during exercise and the higher MMR of fed bass was not associated with higher maximum Q_A relative to when fasted, or to any differences in Q_GI at maximum Q_A. In hypoxia prior to exercise, metabolic and cardiac responses to feeding were similar compared to normoxia. Hypoxia caused an almost 60% reduction to MMR and 30% reduction to U_crit, but neither of these traits differed between fed or fasted bass. Despite hypoxic limitations to MMR and U_crit, maximum Q_A and patterns of Q_GI during exercise in fasted and fed bass were similar to normoxia. Estimating GI oxygen supply from Q_GI indicated that the ability of bass to prioritise aerobic exercise over SDA when metabolically limited by hypoxia was linked to an ability to defer elements of the SDA response occurring outside the GI tract.     

Acute central effects of corticotropin-releasing factor (CRF) on energy balance: Effects of age and gender

Previously demonstrated age-related changes in the catabolic melanocortin system that may contribute to middle-aged obesity and aging anorexia, raise the question of the potential involvement of corticotropin-releasing factor (CRF) in these phenomena, as this catabolic hypothalamic mediator acts downstream to melanocortins. Catabolic effects of CRF were shown to be mediated by both CRF1 (hypermetabolism) and CRF2 (anorexia) receptors. To test the potential role of CRF in age-related obesity and aging anorexia, we investigated acute central effects of the peptide on energy balance in male and female rats during the course of aging.Effects of an intracerebroventricular CRF injection on food intake (FI), oxygen-consumption (VO₂), core- and tail skin temperatures (Tc and Ts) were studied in male and female Wistar rats of five different age-groups (from 3- to 24-month). Anorexigenic responsiveness was tested during 180-min re-feeding (FeedScale) following 24-h fasting. Thermoregulatory analysis was performed by indirect calorimetry (Oxymax) complemented by thermocouples recording Tc and Ts (indicating heat loss).CRF suppressed FI in 3-month male and female animals. In males, CRF-induced anorexia declined with aging, whereas in females it was maintained in all groups. The peptide increased VO₂ and Tc in all male age-groups, while the weaker hypermetabolic response characterizing 3-month females declined rapidly with aging.Thus, age-related alterations in acute central anorexigenic and hypermetabolic effects of CRF show different non-parallel patterns in males and females. Our findings underline the importance of gender differences. They also call the attention to the differential age-related changes in the CRF1 and CRF2 receptor systems.     

Nutritional condition, starvation status and growth of early juvenile Japanese sea bass (Lateolabrax japonicus) related to prey distribution and feeding in the nursery ground

The nutritional condition and protein growth rates of Japanese temperate bass larvae and juveniles were studied in relation to prey distribution and feeding habits in the nursery grounds in Chikugo estuary, Ariake Sea, Japan. Samples were collected from a wide spatial area covering the nursery grounds of the fish in March and April 2003. Food habits of the fish were analyzed by examining the gut contents. Fish condition was evaluated by using RNA/DNA ratio and other nucleic acid-based indices and protein growth rates. The nucleic acid contents in individually frozen larvae and juveniles were quantified by fluorometric method. Two distinguished feeding patterns, determined by the distribution of prey copepods, were identified along the nursery ground. The first pattern showed the dependency of the fish on the calanoid copepod Sinocalanus sinensis, which was the single dominant prey in low saline upper river areas and the second pattern involved a multi-species dietary habit mainly dominated by Acartia omorii, Oithona davisae and Paracalanus parvus. Values of RNA, DNA, total protein, growth rates and for all the nucleic acid-based indices were higher in upstream areas than in the downstream areas. The proportion of the starving fish was higher in the downstream areas than in the upstream areas. Condition of juvenile sea bass was not equal throughout the nursery grounds; fish in the upper river were in better condition than those in the lower estuary. We speculated that utilization of S. sinensis, which appears a suitable prey item and provide a better foraging environment in the upstream nursery ground, is one of the key factors for early survival and growth of Japanese temperate bass larvae and juveniles in the Chikugo estuary.     

In vivo regulation of GLUT2 mRNA in sea bass (Dicentrarchus labrax) in response to acute and chronic hypoxia

The expression and regulation of sodium-independent glucose transporter (GLUT)-2, in relation to hypoxia has not yet been explored in fish or other vertebrates. In this study, the complete open-reading frame for sea bass GLUT2 was isolated and deposited in the GenBank. The predicted 12 transmembrane domains of the protein (508 amino acids) are presented. A phylogenetic tree was constructed on GLUT2 sequences of sea bass and those of other teleost, amphibian, avian, and mammalian species. We also analyzed acute and chronic hypoxia-induced changes in the expression of hepatic GLUT2 mRNA, using one-tube, two-temperature, real-time RT-PCR with which gene expression can be absolutely quantified by the standard curve method. The number of GLUT2 mRNA copies was significantly increased in response to both acute (1.9 mg/L, dissolved oxygen for 4 h) and chronic (4.3 mg/L, DO for 15 days) hypoxia conditions. The hypoxia-related changes in GLUT2 mRNA copy number support the view that GLUT2 is involved in the adaptation response to hypoxia in sea bass, a marine hypoxia-sensitive species. We realize that the GLUT2 mRNA levels in our study do not measure the physiological effects produced by the protein. Thus, we can only speculate that, under hypoxic conditions, GLUT2 probably functions to allow the glucose produced from liver glycogen to leave the hepatocytes.     

Stress-induced effects on feeding behavior and growth performance of the sea bass (<Emphasis Type="Italic">Dicentrarchus labrax</Emphasis>): a self-feeding approach

Repetitive aquaculture-related protocols may act as cyclic stressors that induce chronic stress in cultured fish. The sea bass is particularly sensitive to stressful conditions and the mere presence of humans will disturb feeding behavior. In this paper, we study whether chronic stress induced by repetition of acute stress protocols affects long-term feeding behavior and growth performance in sea bass and whether exogenous cortisol may induce stress-like changes in these parameters. We demonstrate that both chronic stress and dietary cortisol decrease food intake and have a negative effect on feed conversion efficiency, severely impairing sea bass performance. Both experimental approaches induced changes in the daily feeding activity by lengthening the active feeding periods. Fish subjected to a cyclic stressor modify their daily feeding pattern in an attempt to avoid interference with the time of the stressor. The delay in feeding when fish are acutely and repeatedly stressed could be of substantial adaptive importance.     

Saline preferendum for the European sea bass, Dicentrarchus labrax, larvae and juveniles: effect of salinity on early development and sex determination

The sea bass, Dicentrarchus labrax, is an eurhyaline marine fish. Juveniles of this species are known to frequent estuaries and lagoons where salinity is lower than in the open sea. Sex determination occurs during this phase of fish life and has been shown labile and sensitive to environmental factors. In this work, the effect of rearing salinity on sex determination and early development of the sea bass was investigated. An excess of males (87%) was found in all groups and the salinity level [(natural sea water salinity, mean: 37‰) vs. (15‰)], when maintained constant, had no effect on the sex-ratio. The transfer from low to high salinity at 93 days post-fertilization (p.f.) increased the percentage of males (93%) suggesting that sexual differentiation in this species may be influenced by such an osmotic stress. Growth was improved by a 15‰ salinity at the beginning of larval rearing (14 days p.f.) and at the end of pre-growing (234-458 days p.f.), periods during which low temperatures were applied. Survival during larval rearing and nursery were also improved in the groups reared at low salinity and so was swimbladder inflation. These results show that sea bass juveniles have a low saline preferendum, a finding that corresponds to the conditions they actually encounter in the wild during their juvenile ecophase; sex determination is not directly modulated by the salinity level but seems to be subjected to complex environmental regulations.     

Effects of ionizing radiation and modified atmosphere packaging on the shelf life of aqua-cultured sea bass (Dicentrarchus labrax)

The major problems concerning marketing and distribution of seafoods are their high perishability due mainly to contamination by spoilage and pathogenic microorganisms. The reduction, destruction or inhibition of microbial contaminants using single or combined treatments might lead to efficient preservation methods. In this research we examined the feasibility of modified atmosphere packaging and irradiation treatments on the extension of the shelf-life of fresh aqua-cultured sea bass Dicentrarchus labrax species. The quality and the freshness of sea bass packaged in modified atmosphere or γ-irradiated were evaluated by assessing their microbiological, sensory and chemical properties. The gas mixture composed by 60:35:5 CO₂/N₂/O₂ resulted as the best inhibitory treatment to extend the shelf life of sea bass. A major MDA containment and microbial decontamination in irradiated samples at 3 kGy were also found in respect to the control samples.     

Effects of starvation on valine and alanine transport across the intestinal mucosal border in sea bass,Dicentrarchus labrax

Summary kinetics of intestinal transport of l-alanine and l-valine (substrates of the A-system and the L-system, respectively, in mammals) across the brush-border membrane in sea bass, Dicentrarchus labrax, were studied on intact mucosa using a short-term uptake technique. When fish were starved for 4–8 weeks, total influx (mucosa-to-cell) of valine fell owing to disappearance or modification of the diffusion component. The maximum influx rate of saturable component increased but its affinity (reflected by the Michaelis constant) decreased. Alanine transport by Na⁺-dependent and diffusion pathways was unchanged after starvation Fasting also induced an almost 20% decrease in the length of intestinal microvilli.Abbreviations K _d diffusional constant - K _m Michaelis constant - V _max maximum influx rate     

Effects of postmortem storage temperature on sea bass (Dicentrarchus labrax) muscle protein degradation: analysis by 2-D DIGE and MS

Storage conditions are known to be important for postmortem deterioration of fish muscle, and temperature is one of the factors with the strongest impact on this process. In order to shed light on the influence of temperature on the status of sea bass (Dicentrarchus labrax) muscle proteins during postmortem storage, a 2-D DIGE and mass spectrometry study was performed on fish kept at either 1 or 18°C for 5 days. As expected, the greatest alterations in sea bass filet protein composition were observed upon postmortem storage at 18°C, with distinct changes appearing in the 2-D protein profile after 5 days of storage at this temperature. In particular, degradation of the myofibrillar protein myosin heavy chain and of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase, among the most abundant muscle proteins, could be clearly observed upon storage at higher temperatures. Although to a lesser extent, however, several proteins were observed to vary in abundance also upon storage for 5 days at 1°C. In particular, one of the most interesting observations was the rapid and significant decrease in the abundance of nucleoside diphosphate kinase B and phosphoglycerate mutase 2, which was observed also at low storage temperatures and appeared to be temperature-independent. The results of this study offer new knowledge on changes occurring in sea bass muscle proteins during postmortem storage at different temperatures and provide indications on protein degradation trends that might be useful for monitoring freshness of fish and quality of storage conditions.     

Autotomy reduces feeding, energy storage and growth of the sea star Stichaster striatus

We evaluated the effect of autotomy on feeding, energy storage and growth of juvenile Stichaster striatus kept in the laboratory for five months with a limited supply of the mussel Semimytilus algosus. Autotomy strongly decreased feeding, energy storage and growth. Intact juveniles showed a ∼ 3 fold higher feeding rate than autotomized individuals throughout the experiment. Intact juveniles also had a higher (∼ 5 fold) energy content per pyloric caeca in each arm. This was mainly due to higher lipid content, the main proximate constituent of pyloric caeca. Intact juveniles showed a greater growth rate and reached a greater size than autotomized individuals, more evident for underwater mass than radius length. The reduced capacity to feed reduced energy intake in autotomized individuals. However, low energy reserves along with low growth in autotomized sea stars, support the hypothesis that juveniles of this species allocate energy to regeneration to the detriment of growth. This was also supported by the ∼ 25% of arm length regeneration after 5 mo. Remaining small could increase risk of lethal predation, however, S. striatus may reduce predation risk by using crevices and kelp holdfasts as refuges from predators. Given the strong impact of autotomy on feeding, regeneration of arms to recover full capacity to forage and grow seems a better strategy for juvenile S. striatus, than merely growing.     

Mercury chloride effects on the function and cellular integrity of sea bass (Dicentrarchus labrax) head kidney macrophages

The objective of this work was to study mercury chloride effects on the function and integrity of sea bass (Dicentrarchus labrax) head kidney macrophages (S-HKM), and to evaluate the response of HgCl2-exposed cells to macrophage activating factor(s) (MAF) produced by sea bass head kidney leukocytes. There was considerable variability in the effects of HgCl2 on the production of reactive oxygen species (ROS) by S-HKM. When incubated with HgCl2, cells from five out of nine fish tested showed a decrease in ROS production as compared to cells incubated with medium alone. In those cultures, MAF addition prevented the mercury chloride-induced decrease in ROS production. In other S-HKM cultures isolated from different fish, mercury chloride abrogated the up-regulating effect of MAF on the respiratory burst. MAF activation of the phagocytic activity of S-HKM was also impaired by HgCl2 addition. Mercury chloride induced apoptosis in S-HKM cultures and MAF addition prevented this effect.     

Detecting the degree of establishment of a non-indigenous species in coastal ecosystems: sea bass <Emphasis Type="Italic">Dicentrarchus labrax</Emphasis> escapes from sea cages in Canary Islands (Northeastern Central Atlantic)

This article gives an insight into the degree of establishment and potential impact of escaped sea bass in shallow coastal habitats of Tenerife. We studied spatial distribution, abundance, size frequency and use of habitat by visual census. Stomach contents and gonadal maturity of escaped sea bass were analysed. Results show that abundance of escaped sea bass is related to distance from to the nearest fish farm and to the habitat complexity. The most frequent size found in the wild corresponds to that of first marketing size. Stomach contents indicate that escaped sea bass were able to exploit natural resources, especially fish. The presence of a female with mature gonads indicates that conditions for successful maturation exist. However, further studies are needed to ensure whether or not escaped sea bass are able to establish self-reproducing populations. Thus, we can conclude that sea bass seem to be already established in shallow coastal habitats of Tenerife, but the population may depend mainly on escapees. As a result of being an opportunistic piscivore, sea bass could be predating on and competing with other native species that are economically important. These results are useful to develop a risk assessment based on quantitative data to predict negative effects. Moreover, management policies such as communication of massive escapes or mitigation measures are needed to prevent future impact on the marine environment. Handling editor: P. Viaroli     

Assessment of growth characteristics,the survival rate and body composition of Asian sea bass Lates calcarifer (Bloch, 1790) under different feeding rates in closed aquaculture system

One of the essential factors to be addressed in the development of aquaculture is the feeding regime. This study was investigated to assess the effects of feeding rate on growth performance, feed utilization, chemical body composition survival rate, cannibalism and morphological indices of Asian Seabass, Lates calcarifer. Intended for the trial, one hundred forty sea bass individuals with an average weight 5.47 ± 0.11 g were randomly distributed in 4 concrete tanks (914 cm × 183 cm 122 cm) each; length × width × depth) and volume 18,399 L, for total 68 days. The fishes were fed with a pelleted diet containing 46% crude protein for different feeding groups designated as (T1, T2, T3 and T4). The feed was supplied with a rate of (T1) 3%, (T2) 4%, (T3) 6%, and (T4) 9% of fish biomass per day and feeding frequency were maintained three times per day to all the groups. At the end of the trial water physicochemical parameters was in acceptable range for Asian sea bass growth. The average daily weight gain (g), weight gain (g) and specific growth rate (%) was significantly higher (p < 0.05) in T3 and T4 as compared to T1 and T2 group. The poorest feed conversion ratio was recorded in T1 group with 3% biomass per day. The cannibalism rate was significantly (p < 0.05) higher in T1 (3%) compared to T3 and T4 treatment. The morphological indices, condition factors (CF), viscerosomatic index and hepatosomatic index (HSI) was significantly higher in T4 group as compared to other treatments. The protein, moisture and ash contents of the whole biomass of the Asian sea bass were not significantly influenced by feeding rate. The fat levels in the fish bodies increased significantly (p < 0.05) with increasing feeding ratio. The phenomenal regression indicates that 6.5% feeding rate per day is optimum for best growth performance, survival and minimum cannibalism rate for Asian sea bass in captivity. The outcome of the finding will help in promotion for not only the coastal aquaculture in Pakistan abut also elsewhere.     

Effects of feeding regime on growth rate in the Mediterranean Sea anemone Actinia equina (Linnaeus)

Polyps of Actinia equina are the most common sea anemones in the rocky intertidal zone of the Mediterranean coast of Israel, where they occur in one of the southernmost populations of this species in the northern hemisphere. We examined effects of feeding rate on polyp growth at ambient sea temperature for this population. Under laboratory conditions, polyps were left unfed, or were fed with brine shrimp (Artemia) once every 2 weeks, once a week, or twice a week. Of the four experimental treatments, only feeding twice a week resulted in polyp growth; under all other regimes, the sea anemones lost body mass. We conclude that a high rate of feeding is required at sea temperatures in the eastern Mediterranean, where these sea anemones may have high metabolic rates relative to more northern populations.     

The effect of temperature on feeding and growth characteristics of the sea urchin Lytechinus variegatus fed a formulated feed

We investigated the effect of culture temperature on feed intake, absorption, organismal growth, and tissue production of Lytechinus variegatus by culturing individuals at three different temperatures representing the normal range of temperature exposure in wild populations in the northern Gulf of Mexico. Large L. variegatus (ca. 42 + 0.6 mm diameter, 36 + 1.3 g wet weight, n = 97) were collected at St. Joseph Bay, Florida, in October 2001. Eight sea urchins were held individually in 1-L containers within an 80-L aquarium with recirculated synthetic seawater at 32-ppt salinity. Three aquaria with the containers were each placed in three incubators at temperatures of 16, 22, or 28 °C for 8 weeks. Sea urchins held at 22 °C had the highest rate of feed intake. Feed intake in individuals held at 16 °C decreased significantly during the first 2 weeks of exposure and then increased to values not significantly different from those held at 28 °C by week 6. The dry matter absorption efficiency of individuals held at 28 °C was significantly higher than those held at 16 °C or 22 °C. The percentage of organic matter in the feces did not vary significantly with temperature. Individuals increased significantly in diameter, wet weight, and gonad weight at all temperatures. The wet weights of individuals held at 22 °C were significantly higher than those held at 28 °C or 16 °C, which did not differ significantly. The gut weight varied inversely with temperature. The wet weight of gonads of individuals held at 22 °C was significantly higher than those held at 28 °C, but neither differed significantly from those held at 16 °C. Production efficiencies, both organismal and gonadal, were inversely proportional to temperature, indicating that the overall metabolic cost of production increased with increasing temperatures. Organism production efficiencies were lower and gonad production efficiencies were higher than those found in small sea urchins, emphasizing that patterns of nutrient allocation vary between small and large sea urchins. Physiological processes associated with feed intake, absorption, and nutrient allocations vary with temperature, but allow the sea urchins to maintain growth and gonad production at a variety of temperatures. These data suggest that temperatures near the upper limits do not promote efficient use of resources, an important consideration for future commercial culture. Since gonad (roe) production is the ultimate goal of many aquaculture operations, gonad production efficiencies will provide a valuable tool for evaluating the efficacy of various feeds and feeding conditions on gonad production.     

Red porgy (Pagrus pagrus) larval feeding performance and behavior at the onset of exogenous feeding

The feeding performance and behavior at the onset of exogenous feeding, 3 to 4 days after hatching (DAH), were studied in red porgy Pagrus pagrus larvae. Similar feeding efficiency and intensity were achieved for two feeding treatments (live or freeze-dried rotifers) suggesting that prey movement is not decisive for their detection and capture and demonstrating that at first feeding red porgy larvae can ingest inert food. Larvae feeding performance was not affected by a diet shift between treatments. Based on maximum rotifers consumption and gut evacuation time at 18 °C, the daily ration was estimated as 14.035 μg, considering 14 h of feeding and a 25% egg:female rotifer ratio. Larval swimming activity measured by video recording showed a close association with gut fullness and similar swimming patterns for 3 and 4 DAH larvae. However, 20.3% larger mouth gape and 54.6% higher swimming speed of the older larvae should provide a better feeding performance and more energy needed for growth.     

Effects of water salinity on melatonin levels in plasma and peripheral tissues and on melatonin binding sites in European sea bass (Dicentrarchus labrax)

Sea bass is an euryhaline fish that lives in a wide range of salinities and migrates seasonally from lagoons to the open sea. However, to date, the influence of water salinity on sea bass melatonin levels has not been reported. Here, we evaluated the differences in plasma and tissue melatonin contents and melatonin binding sites in sea bass under four different salinity levels: seawater (36‰), isotonic water (15‰), brackish water (4‰) and freshwater (0‰). The melatonin content was evaluated in plasma, whole brain, gills, intestine and kidney, while melatonin binding sites were analyzed in different brain regions and in the neural retina. Plasma melatonin levels at mid-dark varied, the lowest value occurring in seawater (102 pg/mL), and the highest in freshwater (151 pg/mL). In gills and intestine, however, the highest melatonin values were found in the seawater group (209 and 627 pg/g tissue, respectively). Melatonin binding sites in the brain also varied with salinity, with the highest density observed at the lower salinities in the optic tectum, cerebellum and hypothalamus (30.3, 13.0, and 8.0 fmol/mg protein, respectively). Melatonin binding sites in the retina showed a similar pattern, with the highest values being observed in freshwater. Taken together, these results reveal that salinity influences melatonin production and modifies the density of binding sites, which suggests that this hormone could play a role in timing seasonal events in sea bass, including those linked to fish migration between waters of different salinities for reproduction and spawning.