IntoductionWith the expansion of the aquaculture industry, the need arises for scalable, reliable, and robust methods to assess fish behaviour in sea cages to guide operational management, which includes feeding optimisation and welfare assessments. Fish cage monitoring utilising either acoustic transmitters or underwater cameras is well-studied. However, the relationship between those two different measurement types seems to have not been explored, nor have they been evaluated together in one experimental site.MethodsIn our 1-month study, we compared the activity of 14 sentinel fish and the artificial intelligence (AI)-inferred speed of individuals from the European seabass (Dicentrarchus labrax) sea cage population in three feeding trials. Comparisons include a maximum activity comparison using persistent peaks, fish behavioural pattern establishment and retention, and periodical behavioural patterns.ResultsOur results demonstrate that under certain circumstances, both technologies are interchangeable from the perspective of persistent peaks and periodicity, but complementary when it comes to behaviour analysis such as food anticipatory behaviour (FAB).DiscussionWe anticipate that our findings will stimulate advances where multiple sensor types are in use to achieve a more holistic understanding of fish behaviour in the aquaculture sector using underwater technologies.

IntroductionFarmed fish like European seabass (Dicentrarchus labrax) anticipate meals if these are provided at one or multiple fixed times during the day. The increase in locomotor activity is typically known as food anticipatory activity (FAA) and can be observed several hours prior to feeding. Measuring FAA is often done by demand feeders or external sensors such as cameras or light curtains. However, purely locomotor-activity-based FAA may provide an incomplete view of feeding and prefeeding behaviour.MethodsHere, we show that FAA can be measured through passive acoustic telemetry utilising three different approaches and suggest that adding more means to food anticipation detection is beneficial. We compared the diving behaviour, acceleration activity, and temperature of 22 tagged individuals over the period of 12 days and observed FAA through locomotor activity, depth position, and density-based unsupervised clustering (i.e., DBSCAN).ResultsOur results demonstrate that the position- and density-based methods also provide expressions of anticipatory behaviour that can be interchangeable with locomotor-driven FAA or precede it.DiscussionWe, therefore, support a unified framework for food anticipation: FAA should only describe locomotor-driven FAA. Food anticipatory positioning (FAP) should be a term for position-based (P-FAP) and density-based (D-FAP) methods for food anticipation. Lastly, FAP, together with the newly defined FAA, should become part of an umbrella term that is already in use: food anticipatory behaviour (FAB). Our work provides data-driven approaches to each FAB category and compares them with each other. Furthermore, accurate FAB windows through FAA and FAP can help increase fish welfare in the aquaculture industry, and the more approaches available, the more flexible and more robust the usage of FAB for a holistic view can be achieved.

Environmental challenges related to open sea cage production of Atlantic salmon have sparked interest in developing commercial-scale semi-closed sea systems for post-smolt Atlantic salmon (100–1000 g). Determining the mass-specific water flow required by post-smolts will largely influence the design and dimensioning of such systems. In this experiment, post-smolts were exposed to four levels of specific water flow: 0.2, 0.3, 0.4, and 0.5 L kg fish−1 min−1. All treatments involved flow-through seawater with full oxygenation, a salinity of 34‰, and a mean temperature of 9.3 °C. The stocking density was kept stable at 75 kg m−3. Water pH decreased with reduced flow, while partial pressure of carbon dioxide (pCO2) and total ammonia nitrogen (TAN) in the water increased. The increase in water CO2 was reflected in the blood with increased pCO2, HCO3−, and decreased Cl− in the lowest water flow treatment (0.2 L kg fish−1 min−1), indicating a typical regulatory response to increased water CO2 over the eight-week experimental period. No negative effects on osmoregulation, external macroscopic welfare, or performance indicators were observed, suggesting that within the time period of this experiment, post-smolts can compensate for reductions in water flow down to 0.2 L kg fish−1 min−1. However, to avoid activating and exhausting potentially energy-costly physiological regulatory mechanisms, it is suggested to keep specific water flow above 0.3 L kg fish−1 min−1 in large-scale operations with semi-closed sea systems at intermediate temperatures.

The transfer success of farmed post-smolt Atlantic salmon (Salmo salar) to sea-cages rely on neural adaptions to promote stress resilience. As low temperatures impact physiology, this suggests that off-season transfer to cold waters may be challenging. To address this, post-smolts reared at 13 degrees C seawater were abruptly transferred to 10 degrees C, 7 degrees C, and 4 degrees C, then acclimated to these respective temperatures for 58-days followed by an acute challenge test (ACT) using confinement stress. Plasma and brain samples were collected after i) the abrupt temperature transfer at 1-h and 1-day, ii) 58-days of acclimation, and iii) 1-h post ACT. In tandem to measuring plasma cortisol levels, the expression of key genes involved in telencephalic regulation (crf, crfbp, mr, gr1, gr2 and hsd11b2) and neural plasticity (neurod, bdnf, pcna, and cfos) were analyzed. Post-smolts exposed to the 7 degrees C and 4 degrees C displayed the largest alteration in telencephalic functions, differentially regulating mr and gr1, to elevate the mr/g1 ratio for downregulating Gr1, proposing an elevated stress loads. After acclimation, these coincided with blunted stress responses capacities to ACTs for both cortisol and telencephalic neural activity (cfos), suggesting a continuation of challenges and reduced the capacity to mount a stress response. Concomitantly, these telencephalic alterations in CRs coincided with a differential modulation in neural plasticity, measured as elevated bdnf and neurod during the abrupt transfer period (acute) and after acclimation (prolonged), respectively, revealing neural responses are still robustly maintained to retain a degree of stress resilience. However, exposure of post-smolts to 4 degrees C clearly induced the most adverse and suppressive effects in telencephalic functions, cued by a suppression in pcna and stress response capacities, downregulation in the CRF system, and largest elevation in the mr/g1 ratio. Conversely, acclimating post-smolts to 7 degrees C elevated 11hsdb2 proposing a greater inhibition of cortisol action that may point to still adequate maintenance of CR and neural processes. Taken together, these findings show that cold temperatures alter key neural processes required for maintaining proper stress management, providing an alternative explanation for reductions in fish stress reactivity commonly observed with declining temperature. Therefore, exposing post-smolts at 13 degrees C to temperature reductions of 6 degrees C or greater should be avoided in aquaculture.

Smoltification in salmonids occurs during spring in response to increasing photoperiod to prepare for marine life. Smoltification is associated with increased hypo-osmoregulatory ability and enhanced growth potential, mediated by growth hormone and insulin-like growth factor (IGF)-1. Rainbow trout is uniquely insensitive to the induction of smoltification-associated changes by photoperiod, such as the activation of gill Na+,K+-ATPase (NKA). We measured the circulating IGF-1 and IGF-binding protein (IGFBP)-2b levels in yearling rainbow trout exposed to natural and manipulated photoperiods during spring and correlated these with gill NKA activity and body size. Although the effect of photoperiod manipulation on body size and circulating IGF-1 and IGFBP-2b was negligible, they were positively correlated with gill NKA activity in fish under simulated natural photoperiod. We next pit-tagged yearling rainbow trout and fed them a restricted ration or to satiation under a natural photoperiod. In April, gill NKA activity was higher in the satiation group than in the restricted group and positively correlated with body size and growth rate. In addition, circulating IGFBP-2b was positively correlated with gill NKA, size and growth, whereas circulating IGF-1 was correlated only with size and growth. The relationship between circulating IGF-1 and growth intensified from May to June, suggesting that the IGF-1-growth relationship was disrupted in April when gill NKA was activated. Two additional IGFBPs were related to growth parameters but not to gill NKA activity. The present study suggests that circulating IGFBP-2b and IGF-1 mediate the size-dependent activation of gill NKA in yearling rainbow trout during spring.

The successful transfer of farmed post-smolt Atlantic salmon (Salmo salar) depends on proper stress responses and cognitive functions during the early seawater (SW) phase. However, with increasing summer oceanic temperatures, these processes may become a challenge, implicating allostasis and welfare. Therefore, we examined the effect of post-smolt transfer from 10°C SW to elevated temperatures (13°C, 16°C, and 18°C) on plasma cortisol and telencephalic genes modulating cognition ( neurod, bdnf, pcna , and c-fos ) and stress-axis regulation ( crf, crfbp, mr, gr1, gr2 , and hsd11b2 ). Fish were sampled at i) 1 day following transfer, ii) 45 days of acclimation, and iii) 45 days and 1 h after an acute challenge test (ACT) using confinement stress. Fish transferred to 13°C retained stress responses, elevating levels of cortisol, crf, mr, gr2, c-fos , and bdnf and maintaining levels of neurod and pcna . Contrastingly, although cortisol increased at 16°C, telencephalic genes reverted to an inhibition of stress responses, increasing crfbp and gr1 complemented with dampened bdnf , neurod , and c-fos responses. However, transferring post-smolts to 18°C showed the most adverse effects, having absent stress responses (cortisol and c-fos ), elevated crfbp , and a suppression of hsd11b2 and neurod . The hsd11b2 downregulation implies low cortisol inhibition in line with absent modulations in corticosteroid receptors and stress responses. These results suggest that the transfer to 16°C and 18°C inhibits the normal reactive response of post-smolts. Following acclimation (45 days), cortisol levels were basal for all groups; however, post-smolts at 16°C and 18°C maintained a telencephalic inhibition of key regulatory genes ( crf , mr , gr2 , and hsd11b2 ), alongside a lower mr/gr1 ratio, an indicator of chronic allostatic load. Moreover, neural plasticity ( neurod and pcna ) was suppressed at 16°C and 18°C, suggesting impacts of elevated allostatic loads with potentially inferior cognitive capacities. Despite maintaining similar plasma cortisol responses to ACTs, post-smolts at 16°C and 18°C elevated neural activation ( c-fos ) to stress, implying greater challenges, with the 18°C group also elevating the level of bdnf . In summary, the telencephalon shows that post-smolts transferred to 16°C and 18°C continue to struggle with the thermal allostatic loads even after acclimation, which is not revealed by plasma cortisol levels, grounding the importance of telencephalic measures in identifying environmental thresholds and hidden challenges.

Photoreceptive inputs to the teleost brain are perceived as image of the visual world and as photo-modulation of neuroendocrine and neuronal signals. The retina and pineal organ are major receptive organs with projections to various parts of the brain, but in the past decades deep brain photoreceptors have emerged as candidates for photoreceptive inputs, either independent or in combination with projections from light sensory organs. This study aimed to test the effects of narrow bandwidth light using light-emitting diodes technology on brain neural activity through putative opsin stimulation in Atlantic salmon. The expression of c-fos , a known marker of neural activity, was compared in situ between dark-adapted salmon parr and following light stimulation with different wavelengths. c-fos expression increased with duration of light stimulation and the strongest signal was obtained in fish exposed to light for 120 minutes. Distinct and specific brain regions were activated following dark to light stimulation, such as the habenula, suprachiasmatic nucleus, thalamus, and hypothalamus. The c-fos expression was overlapping with photoreceptors expressing melanopsin and/or vertebrate ancient opsin, suggesting a potential direct activation by light. Interestingly in the habenula, a distinct ring of vertebrate ancient opsin and melanopsin expressing cells is overlapping with c-fos expression after neural activation. Salmon exposed to different spectra had neural activation in similar brain regions. The most apparent difference was melanopsin expression in the lateral cells of the lateral tuberal nuclus in the hypothalamus, which appeared to be specifically activated by red light. Light-stimulated neuronal activity in the deep brain was limited to subpopulations of neurons, mainly in regions with neuronal modulation activity, retinal and pineal innervations and known presence of nonvisual photoreceptors. The overlapping expression patterns of c-fos and nonvisual opsins support direct light stimulation of deep brain photoreceptors and the importance of these systems in light induced brain activity.

Producing a larger post-smolt in recirculating aquaculture systems (RAS) could shorten the production time in sea cages and potentially reduce mortality. Knowledge of the biological requirements of post-smolts in closed-containment systems is however lacking. In the present study, the effects of salinity and water velocity on growth, survival, health, and welfare of Atlantic salmon reared in RAS were examined. Salmon smolts were stocked in three separate RAS with salinities of 12, 22, and 32 parts per thousand and subjected to high (1.0 body lengths per s(-1)) or low (0.3 body lengths second(-1)) water velocity. Growth performance, survival, welfare, and physiological stress responses were monitored until the fish reached a bodyweight of around 450 g. Growth rate was higher at lower salinity and higher water velocity generally had a positive effect on growth in all salinities. Feed conversion ratio was lower at 12 parts per thousand compared to the 22 and 32 parts per thousand when the fish were between 250 and 450 g. Higher mortality, elevated plasma cortisol levels, higher incidence of cataract, and a higher expression of stress-induced genes in the skin (iNOS, Muc5ac-like) indicated a negative effect of higher salinity on fish welfare. Male maturation was low (<1%), and not affected by salinity or water velocity.

Photoperiod is thought to be the main zeitgeber that induces smoltification in salmonids. However, its effects on the smoltification of rainbow trout (Oncorhynchus mykiss) are not fully understood and no published data documents the effects of the photoperiod regime currently used commercially, continuous light (LL). The present study compared the effect of four different photoperiod regimes (i.e. advanced phase photoperiod (APP), delayed phase photoperiod (DPP), LL and simulated natural photoperiod (SNP)) on the smoltification and growth of juvenile rainbow trout during their freshwater phase of winter-spring and the following summer post smolt phase. Smoltification was evaluated by monitoring gill Na+,K+–ATPase (NKA) activity and transcription of NKA α-subunit isoforms 1a and 1b, and Na+,K+,2Cl‐ cotransporter 1a. Growth was measured as specific growth rate of both length and weight, and through molecular growth proxies such as the levels of circulating insulin-like growth factor 1 (IGF-I) in plasma and transcription of igf-I, igf binding protein 1b (igfbp1b), growth hormone receptor 1 (ghr1) and cathepsin L (ctsl) in the liver. Results indicate that APP induces a longer smolt window and higher levels of plasma IGF-I in both freshwater and seawater (two months post transfer), while DPP led to a shorter smolt window, lower plasma IGF-I levels in freshwater and seawater, an earlier decrease in liver igf-I and ctsl transcription in freshwater (as seen by modelling over time) and lower specific growth rate in freshwater. The transcription analysis of osmoregulatory genes complemented NKA activity and allowed for the detection of a transient response to light and of differences between the osmoregulatory capacity of parr and desmolted fish. Furthermore, an upregulation of the liver transcription of igf-I, ghr1 and ctsl was found in all treatments during the smolt window, which corresponded to the periods with highest growth. Finally, both plasma IGF-I and liver igf-I in seawater were found to be significantly correlated to fish growth in seawater. However, our data did not show that plasma IGF-I prior to seawater transfer could be used as a reliable predictor of growth in seawater. Overall, and especially when compared with other salmonid species, photoperiod seems to be a weaker inducer of smoltification in rainbow trout, according to the parameters that were tested, suggesting that other environmental cues might be more important drivers of this process.

Environmental temperature has a considerate impact over near all aspects of physiology in both wild and cultured salmonids. In addition to climate change, insight regarding the effects of temperature over physiology is crucial for the aquaculture sector, as temperature differences during the seaward transfer of fish from (semi‐) closed containment systems to open sea pens may be great and ultimately dependent on the prevailing season. For example, transferring of fish will likely occur towards higher temperatures during the summer months and lower temperatures during winter months. Further, open‐pen rearing of post‐smolt salmonids occur throughout the year at varying seasonal temperatures. Due to these temperature differences, it is therefore important to know how temperature influences tissue characteristics and modulates physiological functioning and adaptation, which processes maybe challenged during stress, thereby contributing to the stress response and overall robustness of the fish. Here we present how differences in temperature, ranging from 4°C to 18°C, affects Atlantic salmon (Salmo salar L.) post‐smolt primary barrier composition and functioning of osmoregulation in seawater. Our results suggest that primary barrier characteristics change with shifting temperatures, resulting in new physiological set points to match osmoregulatory demand in those conditions. Further, we elucidate how prior acclimation history affects the capacity of these barrier systems to respond when confronted by an additional acute stress. In this study, we investigated key changes in gill and skin phospholipid (PL) and fatty acid (FA) compositions utilizing electrospray ionization‐tandem mass spectrometry (ESI‐MS/MS) and gas chromatography (GC‐FID), in concert with qPCR expression profile changes of vital genes involved in osmoregulation, thereby aiming to understand how these correlate to changing patterns in primary barrier performance at different temperatures.Support or Funding InformationCenter for Research Based Innovation, “CtrlAqua” Project Number 237856/030This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The circadian and seasonal actions of melatonin are mediated by high affinity G-protein coupled receptors (melatonin receptors, MTRs), classified into phylogenetically distinct subtypes based on sequence divergence and pharmacological characteristics. Three vertebrate MTR subtypes are currently described: MT1 (MTNR1A), MT2 (MTNR1B), and Mel1c (MTNR1C / GPR50), which exhibit distinct affinities, tissue distributions and signaling properties. We present phylogenetic and comparative genomic analyses supporting a revised classification of the vertebrate MTR family. We demonstrate four ancestral vertebrate MTRs, including a novel molecule hereafter named Mel1d. We reconstructed the evolution of each vertebrate MTR, detailing genetic losses in addition to gains resulting from whole genome duplication events in teleost fishes. We show that Mel1d was lost separately in mammals and birds and has been previously mistaken for an MT1 paralogue. The genetic and functional diversity of vertebrate MTRs is more complex than appreciated, with implications for our understanding of melatonin actions in different taxa. The significance of our findings, including the existence of Mel1d, are discussed in an evolutionary and functional context accommodating a robust phylogenetic assignment of MTR gene family structure.

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.

Atlantic salmon has demonstrated the capacity to physiologically acclimate when exposed to increased pCO2 levels in aquaculture flow‐through systems. Less is known about tolerance to increased pCO2 in Recirculation Aquaculture System (RAS). In the present study, Atlantic salmon post‐smolt in a RAS were kept at 12 ppt (12°C) and exposed to six different pCO2 levels ranging from <5 mg/L (PCO2<1.83 mm Hg), 12 mg/L (PCO2=4.40 mm Hg), 19 mg/L (PCO2=6.94 mm Hg), 26 mg/L (PCO2=9.50 mm Hg), 33 mg/L (PCO2=12.06 mm Hg) and 40 mg/L (PCO2=14.61 mm Hg). After 12 week treatments fish were transferred to full strength seawater with the lowest tested pCO2 level. We found that post‐smolts demonstrated a classic transient physiological compensatory response to cope with elevated pCO2, despite a clear and persistent decrease in growth rate when reared above 19 mg/L CO2. After twelve weeks of variable pCO2 exposures post‐smolt were challenged with rapid changes from high (40 mg/L) to low (5 mg/L) pCO2 levels and vice‐versa. Here we also will present physiological compensatory responses 1 and 24 hours and up to 6 days following rapid changes in pCO2 exposure.Support or Funding InformationCenter for Research Based Innovation, “CtrlAqua” Project Number 237856/030This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

•nkaα1a, nkaα1b and nkcc1a provide relevant information that the NKA activity does not reflect.•While nkaα1a and nkcc1a transcription are mainly regulated by temperature, nkaα1b is regulated mainly by photoperiod.•High water temperature could potentially compromise the adaptive and innate immune response of rainbow trout.•Increased water temperature (8 °C) during winter does not provide clear advantages for smoltification or size at harvesting.

Individuals in a fish population differ in key life-history traits such as growth rate and body size. This raises the question of whether such traits cluster along a fast-slow growth continuum according to a pace-of-life syndrome (POLS). Fish species like salmonids may develop a bimodal size distribution, providing an opportunity to study the relationships between individual growth and behavioural responsiveness. Here we test whether proactive characteristics (bold behaviour coupled with low post-stress cortisol production) are related to fast growth and developmental rate in Atlantic salmon, Salmo salar. Boldness was tested in a highly controlled two-tank hypoxia test were oxygen levels were gradually decreased in one of the tanks. All fish became inactive close to the bottom at 70% oxygen saturation. At 40% oxygen saturation level a fraction of the fish actively sought to avoid hypoxia. A proactive stress coping style was verified by lower cortisol response to a standardized stressor. Two distinct clusters of bimodal growth trajectories were identified, with fast growth and early smoltification in 80% of the total population. There was a higher frequency of proactive than reactive individuals in this fast-developing fraction of fish. The smolts were associated with higher post-stress plasma cortisol than parr, and the proactive smolts leaving hypoxia had significant lower post-stress cortisol than the stayers. The study demonstrated a link between a proactive coping and fast growth and developmental ratio and suggests that selection for domestic production traits promotes this trait cluster.

The use of closed containment (CCS) or semi-closed containment systems (S-CCS) for Atlantic salmon Salmo salar aquaculture is under evaluation in Norway. One such system is the Preline S-CCS, a floating raceway system that pumps water from 35 m depth creating a constant current through the system. Exposing fish to moderate water currents is considered aerobic exercise and it is often perceived as positive for fish welfare, growth, food utilization, muscle development and cardiac health. The present study compared fish reared in the Preline S-CCS and in a reference open pen. Samples were taken in fresh water before being transferred to the seawater systems and after 1, 2 and 4 months in seawater and analysed for growth, mortality, muscle development and plasma insulin-like growth factor I (IGF-I) levels. Moreover, gene transcription were determined in the skeletal muscle [igf-I, insulin-like growth factor 1 receptor a (igf1ra) and insulin-like growth factor 1 binding protein 1a (igf1bp1a)] and cardiac transcription factors [myocyte-specific enhancer factor 2C (mef2c), gata4 and vascular endothelial growth factor (vegf)]. While the results suggest that post-smolts in Preline S-CCS were smaller than reference fish, fish from Preline S-CCS have less accumulated mortality at the end of the experiment and showed 2.44 times more small muscle fibres than the reference group fish after 4 months in seawater. These results confirmed what was previously observed in the second generation of Preline. Similar levels of big muscle fibres between Preline S-CCS and reference suggest a similar hypertrophy of muscle fibres even with lower IGF-I expression in the Preline S-CCS. Cardiac gene transcription suggests cardiac hypertrophy was observed after 4 months in seawater in the Preline S-CCS group. Altogether, Preline S-CCS is a promising technology able to produce more robust S. salar with a faster growth and lower mortality in the subsequent standard open cage system growth period.

The 10th International Workshop on Salmon Smoltification was held in Norway from 14–August 18, 2017. Organized by L. Ebbesson, T. Nilsen, S. Stefansson and S. Handeland, the meeting was held at the University of Bergen for the first 2 days, and then at the delightful seaside resort Solstrand Hotel in Osøyro, Norway. The meeting also included an "upstream migration" to the Norwegian Wild Salmon Centre in Laerdal. The meeting was attended by 34 participants from 10 countries that included Australia, Canada, Chile, Denmark, France, Japan, Norway, Sweden, the UK and the USA. The first International Workshop on Salmon Smoltification was held in La Jolla California and was focused on problems associated with the developing aquaculture industry, especially challenges associated with moving animals into seawater net pens. Since then the meeting has been held around the world, including Stirling in Scotland, UK, Trondheim in Norway, St. Andrews in New Brunswick, Canada, Muonio in Finland, Westport in Ireland, Tono in Japan, Corvallis in Oregon and Reykjavík and Hólar in Iceland. Over time the focus of the meeting has shifted to include hatchery fish that are part of restoration efforts, and most recently an increasing examination of fish in the wild. The emphasis on wild fish and improving hatchery conditions for mitigation of wild fish populations is especially important in light of the continuing declines in salmon populations in many areas and the looming threats of dams, climate change and other human activities that threaten the sustainability of salmon populations. Salmon smolts make a gigantic niche shift as they migrate from freshwater rivers to the ocean, and undergo preparatory adaptations that allow them to survive and thrive in this new environment. Part of the continuing information gap for smolts is understanding the environmental factors that most contribute to fitness and survival at this critical stage. Photoperiod sets overall seasonal timing for smolting, but many of the details of photoperiod action are still unknown. There appears to be a critical photoperiod for smolt development (Strand et al., 2018), which may differ for different (e.g., more northern) populations. The extent and evolutionary flexibility of photoperiod control of smolting has important implications for the response of salmon populations to climate change, particularly given the large number of populations of salmon from the Atlantic and Pacific Oceans that are of conservation concern and supported by supplementation from hatchery-reared fish. Consequently, understanding the role early rearing environment has on development and successful transformation from parr to smolts remains an important area of investigation. Hatchery produced smolts have been found to have lower survival compared with wild smolts, but the reasons for this remain unclear. Limitations in habitat complexity in hatcheries may result in poor learning capacity. Self et al. (2018a) found that artificial rearing structure provided in a hatchery setting had only minor effects on movement and predator avoidance behaviour of juvenile steelhead Oncorhynchus mykiss (Walbaum 1792), indicating that not all hatchery interventions will lead to improved survival. In an examination of migratory patterns of Atlantic salmon Salmo salar L. 1758 in a Norwegian River influenced by a large lake, Barlaup et al. (2018) found much later migration in hatchery reared fish compared with wild fish. This could have substantial effects on subsequent seawater survival and understanding of the reasons for these altered migratory patterns would benefit all hatchery release programmes. Effects even earlier in development, through maternal effects on size, may affect early growth and thus the demography of smolt development (Self et al., 2018b). Changes to migratory corridors have the potential to affect downstream movement patterns in both hatchery and wild-reared smolts. Dams can have a dramatic effect on smolt survival, but the extent of effects is highly dependent on the height, location, turbine type and other factors. Moore et al. (2018) found that passage of S. salar smolts through an Archimedes screw hydropower turbine at a low head dam in southern England, UK, did not greatly affect migratory behaviour and survival through the rest of the fish's riverine and estuarine migration. Intercepting and tagging wild-reared smolts during migration is necessary to assess survival during the marine phase, but the effect of such procedures is largely unknown. The effect of tagging wild S. salar smolts caught in a small stream in southern England varied; capture of smolts following mild winters or at night decreased their probability of return as adults (Riley et al., 2018). This information will be important in guiding future tagging programmes to avoid artefacts and negative consequences of tagging programmes. There is increasing evidence of population-specific migratory and physiological traits that affect fitness and survival of smolts. Masu salmon Oncorhynchus masou (Brevoort 1856) are largely anadromous, though landlocked, riverine populations are the rule in their more southerly distribution in the south of Japan. Inatani et al. (2018) found that anadromous O. masou from the north of Japan had increased gill Na+–K+-ATPase activity and salinity tolerance in spring, whereas fish from landlocked populations in the south did not change in spring but showed modest increases in smolt appearance and salinity tolerance in autumn. It is less clear if populations in the same river catchment have evolved differential traits related to smolt development. Elsner & Shrimpton (2018) compared populations of coho salmon Oncorhynchus kistuch (Walbaum 1792) from high and low in the Fraser River catchment by rearing them under common conditions before and during smolt development. They did not find substantial differences in the timing or duration of gill Na+–K+-ATPase activity (a common indicator of smolt development), transcription levels of its isoforms or growth hormone and prolactin receptors, suggesting that the onset and duration of the smolt window may not be influenced by location in the same river system. Consistent with these findings, Bassett et al. (2018) found that gill Na+–K+-ATPase activity increased as fish moved downstream and entered the estuary, but did not find population-related differences. Further work is needed in other river systems examining other smolt-related traits to determine if within-river population differences can be found. Successful smolting is clearly dependent on factors that the fishes experience in the freshwater environment such as pathogens and stress. The intestine is an important barrier to pathogens, but stress leads to intestinal barrier dysfunction. Sundh et al. (2018) found that acute stress also reduced gastrointestinal blood flow, although mechanical occlusion of blood flow to the gut did not induce barrier dysfunction. Developing therapeutic programmes that are effective and do not have a deleterious influence on the aquatic environment is often difficult. Dietary supplementation with oregano essential oil reduced infection rate of two common ectoparasites of chum salmon Oncorhynchus keta (Walbaum 1792) in Japan, improved growth in freshwater and resulted in higher survival rates following transfer to seawater (Mizuno et al., 2018). Ultimately the best measure of successful completion of the parr–smolt transformation is marine survival and performance following seawater entry. Given likely effects of climate change on the ocean, knowledge of how biotic and abiotic variables will influence seawater tolerance in smolts is important. Brown et al. (2018) found that large S. salar (10 fold greater than wild smolts) reared under a non-stimulatory photoperiod in which smolt development was minimized were able to acclimate to seawater, indicating that large size can help overcome the challenges of seawater exposure. Consequently, increases in temperature and potential productivity in the freshwater environment may have some benefits for smolt survival. There is also an interaction among environmental variables during seawater exposure of smolts. Salmo salar smolts showed greater ionoregulatory perturbations with exposure to higher salinity at warmer temperatures; effects that were not seen with just transfer to warmer water at constant high or low salinity (Vargas-Chacoff et al., 2018). Along with the reduced capacity for ionoregulation in seawater at high temperatures, decreases in gill mRNA for the Na+–K+-ATPase α1a and α1b isoforms suggested a reduced capacity to elevate or maintain high levels of ion transporters at high temperatures. Ocean acidification due to increases in atmospheric CO2 potentially introduces a compounding stressor for smolts entering seawater. Exposure to high CO2 did not alter salinity tolerance in S. salar and growth rate in seawater was greater than under control conditions (McCormick & Regish, 2018). Significantly lower plasma chloride levels for smolts transferred into seawater with high levels of CO2 may be associated with elevated plasma bicarbonate, a common response of fish to ocean acidification. The effects of ocean acidification, therefore, may not be all negative for salmonids to complete their anadromous life cycle. Although, much of the workshop focused on wild fish or husbandry strategies to produce competent smolts that will mimic wild-reared fish, production of aquaculture fish remains an important line of investigation. Salmon production protocols are rapidly changing towards industrialized production facilities and protocols with larger sized smolts up to 400–500 g. The rationale being that bigger fish are more robust and thus have increased performance and survival in seawater. To reach these large smolt sizes, the aquaculture industry is testing and adopting photoperiod manipulation with ever shorter winter signals, as well as use of intermediate salinities. The increased use of artificial environmental cues has raised concerns that the industry is losing or altering the timing and extent of critical smoltification traits (such as osmoregulatory ability, brain development and immune responses) that will affect post-smolt performance. For such changes to be sustainable more research and improved technological solutions will be required. Evaluation of semi-closed containment systems compared with open net pens for S. salar aquaculture revealed lower accumulated mortality, but the higher flow and exchange rates resulted in more small muscle fibres and cardiac hypertrophy than reference fish (Balseiro et al., 2018). Since the first smolt workshop in 1981 (and nine stimulating meetings later), our understanding of environmental effects on smolts has increased considerably, particularly with the use of new molecular tools to characterized physiological changes. In combination with these new methods, behavioural changes during the parr–smolt transformation have also been investigated revealing the complexity of responses exhibited among fish within a population. Population-specific effects have also been shown, but the genetic basis of differences in smolting are still largely unknown. Given the importance of seasonal changes in environment of stimulating the parr–smolt transformation, understanding the effects of climate changes such as temperature and ocean acidification on the life cycles is crucial to the long-term sustainability of anadromous salmonids that have so much economic and cultural importance. We expect that future lines of investigation will reveal important fitness traits for smolts under wild, hatchery-release and aquaculture settings. The interaction between behaviour and physiology will differ among rearing environments, but how domestication in the short and long-term affect these fitness traits is unclear. Understanding environmental interactions with smolts, therefore, will be critical to maintaining salmon populations in a world increasingly affected by human activity.

In nonmammalian vertebrates, photoreception takes place in the deep brain already early in development, but knowledge is lacking about the functions of these nonvisual photoreceptive systems. Prior to hatching, Atlantic halibut has a transient bilateral cluster of photoreceptive cells in the hindbrain. The cluster is imbedded in a neuronal network projecting to the narrow belt of hatching glands in the yolk sac. In halibut, hatching is inhibited in light and activated by transfer to darkness and c-fos analysis during hatching shows that the hindbrain cluster and hatching glands have neural activation. Unexpectedly, the hindbrain cluster expresses dual photopigments, vertebrate ancient opsin and melanopsin. Evolutionarily, these opsins are believed to belong to different classes of photopigments found in rhabdomeric and ciliary photoreceptors. The concept that an organism develops transient light sensitivity to target critical aspects of life history transitions as hatching provides a fascinating landscape to investigate the timing of other biological events.