Environmental enrichment is crucial for the welfare and behaviour of animals, yet zebrafish (Danio rerio) and medaka (Oryzias latipes) are often housed in barren, uniform conditions. When enrichment is provided, its implementation is often inconsistent and lacks a species-specific approach. This study investigated preferences for physical and sensorial enrichment in a group of six fish for both species, using a four-choice plus maze. Physical enrichment choice was provided as structural elements or images, while sensorial enrichment involved arms illuminated with red, green, blue, or white light. Fish occupancy was video recorded at multiple timepoints (09:00, 10:00 and 16:00) over 48 h-8 timepoints in total. Zebrafish preferred overall physically enriched arms, particularly those with gravel and plants. Notably, there were changes in preference for physical enrichment throughout the experiment, suggesting that diurnal differences are an important factor in choice. Zebrafish exhibited time-dependent preferences for red light early and late in the day. Medaka showed strongest preference for the bottom-and-wall image arm, followed by the gravel-and-plant arm over the barren arm, and preferred white and blue light, with preferences shifting between these two lights over time. These results reveal species-specific enrichment preferences and highlight important temporal dynamics of choice preference behaviour, emphasizing the need for adequate acclimation and careful selection of multiple observation timepoints to ensure accurate assessment of behavioural preferences.

Fishes are by far the most species-rich group of vertebrates, with 36,105 species currently recognized, approximately the same number of species as that of all non-fish vertebrates combined. Recent decades have witnessed dramatic population declines for many fish species, together with a loss in overall fish biodiversity. Globally, fish biodiversity is being threatened by a multitude of anthropogenic impacts including overfishing, habitat loss, pollution, aquaculture, loss of river connectivity, climate change and the impact of alien species. Nowhere is the world's biodiversity crisis more acute than in freshwater ecosystems. While rivers, lakes and wetlands cover less than 1% of the planet's total surface, they are home to over half the world´s fish species. One-third of freshwater fishes are now threatened with extinction, and 80 species have become extinct in recent years. This review covers the main drivers of declining fish biodiversity and details remedial strategies aimed at conserving both marine and freshwater fish biodiversity. The preservation of genetic resources through the cryobanking of reproductive cells and tissues, collectively known as germplasm, will increasingly become a valuable tool in the conservation of fish biodiversity. It is expected that the ability to establish cryobanks for the full range of fish germplasm, including sperm, oocytes, embryos and germ cells will be of key importance to the conservation of threatened fish species, as well as to fishery management and aquaculture. With the help of a range of emerging reproductive technologies, frozen germplasm will play a key role in future in situ and ex situ conservation initiatives. The rapid advance in next-generation sequencing technologies, together with the proliferation of resources such as fully sequenced fish genomes, is expected to result in a rapid expansion in the application of conservation genomics to the field of fish conservation and be instrumental in formulating future management strategies directed at conserving fish biodiversity, through both in situ and ex situ conservation initiatives.

Mortality and morbidity can be high when farmed Atlantic salmon (Salmo salar) are exposed to sudden environmental changes such as transfer from freshwater rearing tanks to marine sea cages. Implementing environmental enrichment (EE) during the rearing of juvenile Atlantic salmon may increase behavioural flexibility thereby improving the ability of the fish to adapt to unexpected stress-inducing environmental change. In this study, juvenile salmon were randomly assigned to one of three different EE treatments (stones, periodic release of air bubbles, or exercise via increased water flow) or a barren control treatment during rearing in tanks in a freshwater flow-through aquaculture system with automated pellet feeding. After 40 days, randomly selected fish from each rearing tank were exposed to a novel environment challenge (NEC) by placing them in pairs in aquaria compartments, all having stones and continuous bubble aeration, for 35 days (n = 4 pairs/treatment). The fish were hand-fed with food pellets during the NEC, with the assessment of daily feeding behaviour used as a proxy for their rate of adaptation to the NEC. In addition, the fish were fed shrimp pieces as a novel food challenge (NFC) during the final 3 days of the NEC. We found that the control treatment was not different in any behavioural measures from any of the enrichment groups. All groups showed a similar habituation pattern based on food motivation in the first nine days in the novel environment. Thereafter, fish from the stones and bubbles groups performed more effective feeding (actively searching for food and consuming more than 50 % of the daily food ration) compared to fish from the exercise group. Fish from the exercise group accepted the novel food at a higher rate compared to fish from the bubbles treatment. No treatment differences were observed in agonistic behaviour or locomotory activity. Our results suggest that changes in environmental enrichment impact adaptation capabilities and coping with novelty in different ways.

Stunting, characterized by impaired growth following seawater transfer, poses a significant challenge in the aquaculture of Atlantic salmon (Salmo salar). Stunts exhibit markedly reduced growth compared to normally developing conspecific individuals. This condition is associated with increased mortality and heightened susceptibility to infectious agents, compromising both fish welfare and profitability through decreased production efficiency. This study investigates the underlying mechanisms of stunting by analyzing key physiological markers of smoltification, including gene expression of gill-specific ion transporters and genes involved in endocrine regulation, as well as hormone profiles in salmon raised in a full-scale aquaculture facility. A longitudinal approach was employed, sampling from pre-smolt through early post-smolt phases to characterize the physiological adaptations critical for seawater tolerance. Gill gene expression analysis showed distinct patterns in the Na+/K+-ATPase (Nka) subunits. Elevated nka alpha 1a and lower nka alpha 3 expression in the stunted group suggested incomplete osmoregulatory adaptation. The expression patterns of pituitary tsh beta b, which are linked to photoperiodic regulation, demonstrated a potential desynchrony within the population, suggesting individual variation in photoperiod responsiveness before seawater transfer. Stunts had significantly lower plasma levels of thyroxine (T4), triiodothyronine (T3), and growth hormone (Gh), but cortisol levels were similar to normal salmon. These results highlight crucial endocrine and osmoregulatory mechanisms that differ from those in normally developing salmon and may be linked to the underlying cause of stunting, particularly when connected to compressed photoperiod smoltification regimes during intensive salmon farming production.

Fishes are by far the most species-rich group of vertebrates, with 36,105 species currently recognised, approximately the same number of species as that of all non-fish vertebrates combined. Recent decades have witnessed dramatic population declines for many fish species, together with a loss in overall fish biodiversity. Globally, fish biodiversity is being threatened by a multitude of anthropogenic impacts including overfishing, habitat loss, pollution, aquaculture, loss of river connectivity, climate change and the impact of alien species. Nowhere is the world’s biodiversity crisis more acute than in freshwater ecosystems. While rivers, lakes and wetlands cover less than 1% of the planet’s total surface, they are home to over half the world´s fish species. One third of freshwater fishes are now threatened with extinction, and 80 species are already extinct. This review covers the main drivers of declining fish biodiversity, and details remedial strategies aimed at conserving both marine and freshwater fish biodiversity. The preservation of genetic resources through the cryobanking of reproductive cells and tissues, collectively known as germplasm, will be a valuable tool in the conservation of fish biodiversity. With the help of a range of emerging reproductive technologies, frozen germplasm will play a key role in future in situ and ex situ conservation initiatives. The ability to establish cryo-banks for the full range of fish germplasm, including sperm, oocytes, embryos and germ cells represents a powerful tool for use in the conservation of threatened fish species. The rapid advance in next-generation sequencing technologies, together with the proliferation of resources such as fully sequenced fish genomes is expected to result in a rapid expansion in the application of conservation genomics to the conservation and management of fish populations, and be instrumental in formulating mitigations directed at conserving fish biodiversity, through both in situ and ex situ conservation initiatives.

Japanese medaka (Oryzias latipes) has been used as a model organism in different research fields, including reproductive physiology. Sperm motility is the most important marker for male fertility in fish and, thus, reproduction success. However, because of small volume of ejaculate and short motility duration, it is still challenging to manage the sperm collection and analysis in small model fish. In the present study, we aimed to investigate sperm motility and to optimize sperm collection, short-term sperm storage, and cryopreservation in Japanese medaka (Oryzias latipes). Using two different approaches for sperm collection: testes dissection and abdominal massage, different housing conditions and activating the sperm with different activation solutions, we investigated immediate sperm motility. In the second part of this study, we used different osmolalities of immobilization solution, Hank's Balanced Salt Solution (HBSS) for sperm storage at 0, 2 and 3 h after sperm collection. Finally, the sperm were cryopreserved using methanol as cryoprotectant and HBSS as extender at two different osmolalities, and post-thaw sperm motility was investigated. The highest post-activating sperm motility was achieved in the groups activated by the extender at 300 mOsm/kg. The quality of sperm remained unaffected by co-housing with females or with males only. Furthermore, Hanks’ Balanced Salt Solution (HBSS) with an osmolality of 600 mOsm/kg demonstrated its efficacy as a suitable extender for sperm storage, preserving motility and progressivity for 3 h. The highest post-thaw motility was around 35%. There were no significant differences between post-thaw motility in different groups. We also found that post-thaw incubation on ice can maintain the motility of the sperm for up to one hour after thawing.

Hydrogen sulphide (H2S) poses a major threat in marine land-based recirculating aquaculture systems (RAS) leading to acute mortality in sensitive fish species such as Atlantic salmon (Salmo salar). To date, little is known about the effects of H2S on the physiology and behavior of the species. The present study analyzed Atlantic salmon swimming behavior in response to H2S in a controlled trial. The setup consisted of two Recirculating Aquaculture Systems (RAS) in parallel. The control RAS comprised of one fish tank (800 L, 10 kg fish/m3 (approximate to 70 fish)), while the exposure RAS included two fish tanks (800 L; 10 and 30 kg fish/ m3 (approximate to 70 and 200 fish)). Swimming behavior was monitored via a submerged custom-built stereo camera system and an overhead surveillance camera. Fish (smolt, approximate to 114 g) were exposed once a day for 10 consecutive days to increasing H2S concentrations, from approximate to 1- up to approximate to 68 mu g/L (2 mu M). Continuous measurements of dissolved H2S, O2 and CO2 were taken using a real-time monitoring system. Three swimming parameters were extrapolated from video recordings using machine learning algorithms: i) speed, ii) pattern (representing whether the fish swim in a straight or zigzagging direction) and iii) dispersion (indicative of schooling behavior). The results showed that fish reacted rapidly to H2S, with a stress response characterized by faster swimming speed, erratic pattern, and loss of schooling behavior. The response was concentration-dependent, increasing linearly up to 30-40 mu g/L, above which a clear threshold was observed. Notably, concentrations around 40-50 mu g/L, induced significantly greater behavioral changes compared to lower concentrations, and further increases in H2S did not lead to additional changes in behavior. Swimming parameters quickly returned to basal levels, comparable to the one's prior exposure, once H2S was no longer present in the water. This study provides new insights on the sensitivity of Atlantic salmon to acute H2S exposure and highlights the potential behind the use of machine vision as an early warning tool for poor water quality in RAS.

Environmental light is perceived and anticipated by organisms to synchronize their biological cycles. Therefore, artificial light at night (ALAN) disrupts both diurnal and seasonal biological rhythms. Reproduction is a complex physiological process involving integration of environmental signals by the brain, and release of endocrine signals by the pituitary that regulate gametogenesis and spawning. In addition, males from many species form a dominance hierarchy that, through a combination of aggressive and protective behavior, influences their reproductive success. In this study, we investigated the effect of ALAN and continuous daylight on the behavior and fitness of male fish within a dominance hierarchy using a model fish, the Japanese medaka. In normal light/ dark cycles, male medaka establish a hierarchy with the dominant males being more aggressive and remaining closer to the female thus limiting the access of subordinate males to females during spawning. However, determination of the paternity of the progeny revealed that even though subordinate males spend less time with the females, they are, in normal light conditions, equally successful at producing progeny due to an efficient sneaking behavior. Continuous daylight completely inhibited the establishment of male hierarchy, whereas ALAN did not affect it. Nonetheless, when exposed to ALAN, subordinate males fertilize far fewer eggs. Furthermore, we found that when exposed to ALAN, subordinate males produced lower quality sperm than dominant males. Surprisingly, we found no differences in circulating sex steroid levels, pituitary gonadotropin levels, or gonadosomatic index between dominant and subordinate males, neither in control nor ALAN condition. This study is the first to report an effect of ALAN on sperm quality leading to a modification of male fertilization success in any vertebrate. While this work was performed in a model fish species, our results suggest that in urban areas ALAN may impact the genetic diversity of species displaying dominance behavior.

ABSTRACTEnvironmental light is perceived and anticipated by organisms to synchronize their biological cycles. Therefore, exposure to artificial light at night could disrupt diurnal and seasonal rhythmicity. Reproduction is a complex physiological process involving integration of environmental signals by the brain, and release of endocrine signals by the pituitary that regulate gametogenesis and spawning. In addition, males from many species form a dominance hierarchy that, through a combination of aggressive and protective behavior, influences their reproductive success. In this study, we investigated the effect of different light regimes, including light pollution at night and continuous light, on the fitness of male fish within a dominance hierarchy using a model fish, the Japanese medaka. In normal light/dark rhythm conditions, we observed that dominant males are more aggressive, remain closer to the female, and spend ten-fold more time spawning than subordinates. By using males with different genotypes, we determined the paternity of the progeny and found that even though subordinate males spend less time with the females, they are equally successful at fertilizing eggs in normal light conditions due to an efficient sneaking behavior. However, when exposed to light at night, dominant males fertilize more eggs. We indeed found that when exposed to nocturnal light pollution, dominant males produce higher quality sperm than subordinate males. Surprisingly, we did not find differences in circulating sex steroid levels, pituitary gonadotropin levels, or gonadosomatic index between dominant and subordinate males, neither in control nor night light condition. Continuous light was found to completely inhibit establishment of male hierarchy. This study is the first to report an effect of light pollution on sperm quality with an impact on male fertilization success in any vertebrate. It has broad implications for fish ecology in urban areas with potential impacts on the genetic diversity of these fish populations.GRAPHICAL ABSTRACT:Highlights:In a triadic relationship, subordinate medaka show sneaking behavior during female spawning, allowing them to produce about 50% of the offspring.Continuous light inhibits establishment of male hierarchy.Light pollution at night shifts the balance of reproductive efficiency between dominant and subordinate males, benefiting dominant males.Exposure to light pollution at night improves sperm quality in dominant fish relative to subordinates, but does not clearly affect reproductive hormone levels.

Glucocorticoid (GC) stress hormones are well-known for their impact on phenotypic traits ranging from immune function to behaviour and cognition. For that reason, consistent aspects of an individual's physiological stress response (i.e. GC responsiveness) can predict major elements of life-history trajectory. Zebrafish (Danio rerio) emerge as a promising model to study such consistent trait correlations, including the development of individual stress coping styles, i.e. consistent associations between physiological and behavioral traits. However, consistency in GC responsiveness of this popular animal model remains to be confirmed. Such a study has so far been hampered by the small-bodied nature and insufficient blood volume of this species to provide repeated measurements of circulating GCs. Here, we adopted a technique that allows for repeated, non-invasive sampling of individual zebrafish by quantifying GCs from holding water. Our findings indicate consistency of the magnitude of post-stress GC production over several consecutive stress events in zebrafish. Moreover, water-borne GCs reflect individual variation in GC responsiveness with the strongest consistency seen in males.

Male Atlantic salmon (Salmo salar) display different sexual strategies, maturing either as parr during the freshwater phase (as sneaky spawners), or as post smolts following one or several years at sea. First sexual maturation (puberty) occurs at different times depending on environmental and genetic factors. To improve our knowledge on the timing (age and season) of first sexual maturation in Atlantic salmon male parr, we investigated pubertal activation in second generation farmed salmon from the Norwegian river Figgjo, reared under natural conditions of photoperiod and water temperature. Histological analysis, in combination with morphometric measurements, plasma androgen levels and pituitary gonadotropin gene expression analysis revealed that, as previously reported, some male parr initiated early sexual maturation in spring at one year of age. Interestingly, some male parr were observed to initiate sexual maturation already in autumn, six months after hatching (under-yearlings), much earlier than reported in previous studies. One-year old maturing males showed a low induction in gonadotropin levels, while under-yearling maturing males displayed a significant increase in fshb transcripts as compared to immature/early-maturing fish. Plasma testosterone, detectable also in immature/ early-maturing males, increased constantly during testes development, while 11-ketotestosterone, undetectable in immature/early-maturing and early maturing males, increased during more advanced stages of maturation. A mild feminization of the testes (ovotestes) was detected in a subset of samples. This study brings new knowledge on the little investigated field of sexually maturing under-yearlings in Atlantic salmon. This is also the first study comparing the physiology of under-yearling vs one-year old maturing male parr, thus bringing new insights to the remarkable plasticity of Atlantic salmon puberty.

It is well-established that sustained exercise training can enhance brain plasticity and boost cognitive performance in mammals, but this phenomenon has not received much attention in fish. The aim of this study was to determine whether sustained swimming exercise can enhance brain plasticity in juvenile Atlantic salmon. Brain plasticity was assessed by both mapping the whole telencephalon transcriptome and conducting telencephalic region-specific microdissections on target genes. We found that 1772 transcripts were differentially expressed between the exercise and control groups. Gene ontology (GO) analysis identified 195 and 272 GO categories with a significant overrepresentation of up- or downregulated transcripts, respectively. A multitude of these GO categories was associated with neuronal excitability, neuronal signalling, cell proliferation and neurite outgrowth (i.e. cognition-related neuronal markers). Additionally, we found an increase inproliferating cell nuclear antigen(pcna) after both three and eight weeks of exercise in the equivalent to the hippocampus in fish. Furthermore, the expression of the neural plasticity markerssynaptotagmin(syt) andbrain-derived neurotrophic factor(bdnf) were also increased due to exercise in the equivalent to the lateral septum in fish. In conclusion, this is the first time that swimming exercise has been directly linked to increased telencephalic neurogenesis and neural plasticity in a teleost, and our results pave the way for future studies on exercise-induced neuroplasticity in fish.

The teleost fishes are the largest and most diverse vertebrate group, accounting for nearly half of all known vertebrate species. Teleost fish exhibit greater species diversity than any other group of vertebrates and this is reflected in the unique variety of different reproductive strategies displayed by fish. Fish have always been an important resource for humans worldwide, especially as food. While wild capture fisheries have historically been the main source of fish, the farming of fish (aquaculture) is increasingly becoming the more dominant source of food fish, and is predicted to account for 60% of total global fish production by 2030.Fishes are increasingly threatened by a wide range of anthropogenic impacts, including loss of habitat, pollution, invasive species and over-exploitation. In addition, climate change, especially the consequences of global warming, can impact fish at all levels of biological organization from the individual to the population level, influencing both physiological and ecological processes in a variety of direct and indirect ways. As such, there is an urgent need to protect and conserve the huge genetic diversity offered by this diverse vertebrate group, not just as a source of genes for contemporary breeding and for protection against the consequences of climate change and disease, but also as part of our national heritage. While the cryopreservation of reproductive cells is a means of achieving these objectives, currently only fish sperm can be successfully frozen. Due to their large size, large yolk compartment, low membrane permeability and high chilling sensitivity, successful and reproducible protocols for the cryopreservation of fish oocytes and embryos still remains elusive. However, significant advances have been made in the cryopreservation of primordial germ cells as an alternative means of conserving both paternal and maternal genomes. Although more research needs to be carried out on how these cells can be optimally applied to emerging reproductive technologies, including transplantation techniques and surrogate broodstock technologies, the successful cryopreservation of fish germ cells, and the establishment of genetic resource banks, offers the possibility of both conserving and restoring threatened species. Further, current and future conservation efforts need to consider the impact of climate change in both in situ conservation and reintroduction efforts.In conclusion, it is anticipated that the successful cryopreservation of fish germplasm will result in a range of economic, ecological and societal benefits. In partnership with emerging assisted reproductive technologies, the successful cryopreservation of fish germplasm will lead to more efficient reproduction in aquaculture, assist selective breeding programmes, and be of crucial importance to future species conservation actions.

The gonadotropins follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) play essential roles in vertebrate reproduction. This article presents data on molecular weight validation of recombinant medaka (Oryzias latipes) (md) gonadotropins Fshβ (mdFshβ), Lhβ (mdLhβ), Fshβα (mdFshβα), and Lhβα (mdLhβα) generated by Pichia pastoris, as well as data on a validation of produced antibodies against Fshβ and Lhβ by Western blot analysis. Furthermore, the article includes data on Fsh and Lh protein levels in male medaka pituitaries using recombinant mdFshβα and mdLhβα within enzyme-linked immunosorbent assays (ELISAs), in which protein amounts were analyzed related to body weight and age of the fish. This dataset is associated with the research article entitled "Medaka Follicle-stimulating hormone (Fsh) and Luteinizing hormone (Lh): Developmental profiles of pituitary protein and gene expression" (Burow et al., in press).

Conspicuous carotenoid ornamentation is considered a signal of individual "quality" and one of the most intensely studied traits found to co-vary with parasitism. Since it has been suggested that only "high quality" individuals have enough resources to express excessive sexual ornaments and resist parasites, current theory struggles to explain cases where the brightest individuals carry the most parasites. Surprisingly little emphasis has been put on the contrasting routes to fitness utilized by different parasite species inhabiting the same host. Using Arctic charr (Salvelinus alpinus) as model species, we hypothesized that skin redness and allocation of carotenoids between skin and muscle (redness ratio) will be positively and negatively associated with parasites using the fish as an intermediate and final host, respectively. Both pigment parameters were indeed positively associated with abundances of parasites awaiting trophic transmission (Diplostomum sp. and Diphyllobothrium spp.) and negatively associated with the abundance of adult Eubothrium salvelini tapeworms. These empirical data demonstrate that contrasting associations between carotenoid coloration and parasite intensities relates to the specific premises of different parasite species and life cycle stages.

It is well-established that sustained exercise training can enhance brain plasticity and boost cognitive performance in mammals, but this phenomenon has not received much attention in fish. The aim of this study was to determine whether sustained swimming exercise can enhance brain plasticity in juvenile Atlantic salmon. Brain plasticity was assessed by both mapping the whole telencephalon transcriptome and conducting telencephalic region-specific microdissections on target genes. We found that 1772 transcripts were differentially expressed between the exercise and control groups. Gene ontology (GO) analysis identified 195 and 272 GO categories with a significant overrepresentation of up- or downregulated transcripts, respectively. A multitude of these GO categories was associated with neuronal excitability, neuronal signalling, cell proliferation and neurite outgrowth (i.e. cognition-related neuronal markers). Additionally, we found an increase in proliferating cell nuclear antigen (pcna) after both three and eight weeks of exercise in the equivalent to the hippocampus in fish. Furthermore, the expression of the neural plasticity markers synaptotagmin (syt) and brain-derived neurotrophic factor (bdnf) were also increased due to exercise in the equivalent to the lateral septum in fish. In conclusion, this is the first time that swimming exercise has been directly linked to increased telencephalic neurogenesis and neural plasticity in a teleost, and our results pave the way for future studies on exercise-induced neuroplasticity in fish.

The paucity of information on understanding the regulatory mechanisms that are involved in the control of piscine Fsh and Lh synthesis, secretion, and function, prompted the present work. Part of the problem is related to the molecular heterogeneity and the unavailability of Fsh and Lh assays for quantifying gonadotropins, in particular assays regarding the measurement of Fsh, and such assays are available today for only a few teleost species. The present study reports the development and validation of competitive ELISAs for quantitative determination of medaka Fsh and Lh by first producing medaka recombinant (md) gonadotropins mdFshβ, mdLhβ, mdFshβα, and mdLhβα by Pichia pastoris, generating specific antibodies against their respective β subunits, and their use within the development of ELISAs. The advantages of this protocol include: •The reproducibility of the ELISA demonstrated was relatively high, as shown by reasonably low intra- (Fsh 2.7%, Lh 3%) and interassay CVs (Fsh 5.3%, Lh 5.7%).•The high degree of parallelism between serial dilutions of the recombinant and native pituitary-derived Fsh and Lh, may be a sign of similar structures and immunologically similarity.•Two new competitive ELISAs for the quantification of medaka Fsh and Lh were established for the first time.

Fish reared for stocking programs are severely stimulus-deprived compared to their wild conspecifics raised under natural conditions. This leads to reduced behavioural plasticity and low post-release survival of stocked fish. Environmental enrichment can have positive effects on important life-skills, such as predator avoidance and foraging behaviour, but the neural mechanisms underpinning these behavioural changes are still largely unknown. In this study, juvenile Atlantic salmon (Salmo salar) were reared in an enriched hatchery environment for seven weeks, after which neurobiological characteristics and post-release survival were compared to fish reared under normal hatchery conditions. Using in situ hybridisation and qPCR, we quantified the expression of brain-derived neurotrophic factor (bdnf) and the neural activity marker cfos in telencephalic subregions associated with relational memory, emotional learning, and stress reactivity. Aside from lower expression of bdnf in the Dlv (a region associated with relational memory) of enriched salmon, we observed no other significant effects of enrichment in the studied regions. Exposure to an enriched environment increased post-release survival during a five-month residence in a natural river by 51%. Thus, we demonstrate that environmental enrichment can improve stocking success of Atlantic salmon parr and that EE is associated with changes in bdnf expression in the fish's hippocampus-equivalent structure.

The hormone melatonin connects environmental cues, such as photoperiod and temperature, with a number of physiological and behavioural processes, including seasonal reproduction, through binding to their cognate receptors. This study reports the structural, functional and physiological characterization of five high-affinity melatonin receptors (Mtnr1aaα, Mtnr1aaβ, Mtnr1ab, Mtnr1al, Mtnr1b) in Atlantic salmon. Phylogenetic analysis clustered salmon melatonin receptors into three monophyletic groups, Mtnr1A, Mtnr1Al and Mtnr1B, but no functional representative of the Mtnr1C group. Contrary to previous studies in vertebrates, pharmacological characterization of four receptors in COS-7, CHO and SH-SY5Y cell lines (Mtnr1Aaα, Mtnr1Aaβ, Mtnr1Ab, Mtnr1B) showed induction of intracellular cAMP levels following 2-iodomelatonin or melatonin exposure. No consistent response was measured after N-acetyl-serotonin or serotonin exposure. Melatonin receptor genes were expressed at all levels of the hypothalamo-pituitary-gonad axis, with three genes (mtnr1aaβ, mtnr1ab and mtnr1b) detected in the pituitary. Pituitary receptors displayed daily fluctuations in mRNA levels during spring, prior to the onset of gonadal maturation, but not in autumn, strongly implying a direct involvement of melatonin in seasonal processes regulated by the pituitary. To the best of our knowledge, this is the first report of cAMP induction mediated via melatonin receptors in a teleost species.