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China is the largest aquaculture producer in the world. Antibiotics were extensively used to ensure the development of the intensive aquaculture; however, the use of antibiotics causes safety- and environment-associated problems. As an alternative strategy to antibiotics, aquatic probiotics have attracted attention. The microbial organisms used as probiotics or tested as potential probiotics in Chinese aquaculture belong to various taxonomic divisions, including Actinobacteria, Bacteroidetes, Firmicutes, Proteobacteria and yeast. Moreover, the mixture of probiotic strains and synbiotics are also widely used. Studies on the mode of action of aquatic probiotics have extended our understanding of the probiotic effects, and novel mechanisms have been discovered, such as interference of quorum sensing. However, use of probiotics in Chinese aquaculture is still at an initial stage, and there are potential risks for some probiotic applications in aquaculture. Further regulation and management are required to normalize the production and usage of aquatic probiotics. In this review, we discuss species, effects, and mode of actions of probiotics in Chinese aquaculture since 2008. Challenges and future directions for research are also discussed., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Calanus finmarchicus and Calanus glacialis are keystone zooplankton species in North Atlantic and Arctic marine ecosystems because they form a link in the trophic transfer of nutritious lipids from phytoplankton to predators on higher trophic levels. These calanoid copepods spend several months of the year in deep waters in a dormant state called diapause, after which they emerge in surface waters to feed and reproduce during the spring phytoplankton bloom. Disruption of diapause timing could have dramatic consequences for marine ecosystems. In the present study, Calanus C5 copepodites were collected in a Norwegian fjord during diapause and were subsequently experimentally exposed to the water-soluble fraction of a naphthenic North Sea crude oil during diapause termination. The copepods were sampled repeatedly while progressing toward adulthood and were analyzed for utilization of lipid stores and for differential expression of genes involved in lipid metabolism. Our results indicate that water-soluble fraction exposure led to a temporary pause in lipid catabolism, suggested by (i) slower utilization of lipid stores in water-soluble fraction-exposed C5 copepodites and (ii) more genes in the [beta]-oxidation pathway being downregulated in water-soluble fraction-exposed C5 copepodites than in the control C5 copepodites. Because lipid content and/or composition may be an important trigger for termination of diapause, our results imply that the timing of diapause termination and subsequent migration to the surface may be delayed if copepods are exposed to oil pollution during diapause or diapause termination. This delay could have detrimental effects on ecosystem dynamics., Introduction The trophic transfer of highly nutritious lipids from phytoplankton to predators through small herbivores is essential for maintaining oceanic ecosystem dynamics (Lee et al., 2006). The calanoid copepods Calanus [...]

New de novo sources of omega 3 (n-3) long chain polyunsaturated fatty acids (LC-PUFA) are required as alternatives to fish oil in aquafeeds in order to maintain adequate levels of the beneficial fatty acids, eicosapentaenoic and docosahexaenoic (EPA and DHA, respectively). The present study investigated the use of an EPA+DHA oil derived from transgenic Camelina sativa in Atlantic salmon (Salmo salar) feeds containing low levels of fishmeal (35%) and fish oil (10%), reflecting current commercial formulations, to determine the impacts on tissue fatty acid profile, intestinal transcriptome, and health of farmed salmon. Post-smolt Atlantic salmon were fed for 12-weeks with one of three experimental diets containing either a blend of fish oil/rapeseed oil (FO), wild-type camelina oil (WCO) or transgenic camelina oil (DCO) as added lipid source. The DCO diet did not affect any of the fish performance or health parameters studied. Analyses of the mid and hindgut transcriptomes showed only mild effects on metabolism. Flesh of fish fed the DCO diet accumulated almost double the amount of n-3 LC-PUFA than fish fed the FO or WCO diets, indicating that these oils from transgenic oilseeds offer the opportunity to increase the n-3 LC-PUFA in farmed fish to levels comparable to those found a decade ago. [ABSTRACT FROM AUTHOR]

In the present study the impact on autochthonous (adherent) bacteria in proximal intestine ( PI) and distal intestine ( DI) of Atlantic cod ( Gadus morhua L.) was evaluated following feeding of a control diet and a diet supplemented with 5% chitin. The autochthonous gut bacteria were investigated using denaturing gradient gel electrophoresis ( DGGE). Analysis of the microbiota associated with PI and DI of Atlantic cod indicate that dietary chitin modulate the intestinal bacterial community. For example, band 25 ( Escherichia coli-like), band 14 ( Anaerorhabdus furcosa-like) and band 29 (uncultured bacterium-like) in PI were depressed by dietary chitin ( P < 0.05). The number of bands (23.7 ± 5.4) in DI of fish fed chitin was marginally higher than the control fish (16.7 ± 2.1) ( P = 0.065), and the relative abundances of band 6 (swine faecal bacterium-like) were marginally stimulated by dietary chitin ( P = 0.095). Furthermore, the present study reports several novel sequences not previously reported in the gastrointestinal tract of Atlantic cod. Whether the dietary effect of chitin on gut bacterial community has any positive effect of fish health merits further investigations. [ABSTRACT FROM AUTHOR]

Intensive fish production worldwide has increased the risk of infectious diseases. However, before any infection can be established, pathogens must penetrate the primary barrier. In fish, the three major routes of infection are the skin, gills and gastrointestinal (GI) tract. The GI tract is essentially a muscular tube lined by a mucous membrane of columnar epithelial cells that exhibit a regional variation in structure and function. In the last two decades, our understanding of the endocytosis and translocation of bacteria across this mucosa, and the sorts of cell damage caused by pathogenic bacteria, has increased. Electron microscopy has made a valuable contribution to this knowledge. In the fish-farming industry, severe economic losses are caused by furunculosis (agent, Aeromonas salmonicida spp. salmonicida) and vibriosis [agent, Vibrio ( Listonella) anguillarum]. This article provides an overview of the GI tract of fish from an electron microscopical perspective focusing on cellular damage (specific attack on tight junctions and desmosomes) caused by pathogenic bacteria, and interactions between the ‘good’ intestinal bacteria [e.g. lactic acid bacteria (LAB)] and pathogens. Using different in vitro methods, several studies have demonstrated that co-incubation of Atlantic salmon ( Salmo salar L.) foregut (proximal intestine) with LAB and pathogens can have beneficial effects, the cell damage caused by the pathogens being prevented, to some extent, by the LAB. However, there is uncertainty over whether or not similar effects are observed in other species such as Atlantic cod ( Gadus morhua L.). When discussing cellular damage in the GI tract of fish caused by pathogenic bacteria, several important questions arise including: (1) Do different pathogenic bacteria use different mechanisms to infect the gut? (2) Does the gradual development of the GI tract from larva to adult affect infection? (3) Are there different infection patterns between different fish species? The present article addresses these and other questions. [ABSTRACT FROM AUTHOR]

Abstract: Replicate groups of Atlantic cod were fed 5 diets in which fish meal protein was replaced with 25% (25 PP), 50% (50 PP), 75% (75 PP) and 100% (100 PP) of a plant protein mixture containing 50% wheat gluten, 36% soy protein concentrate and 14% bioprocessed soybean meal. After 28 weeks, fish were sacrificed and analysed for growth, feed conversion, nutrient digestibility (see [Hansen, A.-C., Rosenlund, G., Karlsen, Ø., Koppe, W., Hemre, G.-I., Accepted for publication. Total replacement of fish meal with plant proteins in diets for Atlantic cod (Gadus morhua L.) I: Effects on growth and protein retention. Aquaculture]) and health status. Except for small reductions in the mean cell volume and haemoglobin content of red blood corpuscles, health indicators in blood and plasma were mostly unaffected by diet. Although the apparent digestibility coefficient (ADC) of nutrients was only marginally affected by diet [Hansen, A.-C., Rosenlund, G., Karlsen, Ø., Koppe, W., Hemre, G.-I., Accepted for publication. Total replacement of fish meal with plant proteins in diets for Atlantic cod (Gadus morhua L.) I: Effects on growth and protein retention. Aquaculture], faeces moisture content increased non-linearly peaking in fish fed the 100 PP diet where dry matter was only 11% compared to 18% in the control fish. This indicates a progression towards a diarrhoea-like condition in 100 PP fish. Feeding plant proteins did not affect gut relative weight but did affect intestinal morphology. In particular, there was a general increase in cellularity of the lamina propria and a modest goblet cell hypertrophy and hyperplasia which peaked in fish fed the 100 PP diet. More severe enteritis-like conditions were rare and found in only two fish fed the 100 PP diet. The dietary effect on goblet cells may be caused by phytate or fibres rather than the primary enteritis-inducing compounds found in untreated soybean meal. Hindgut segments seemed to be more affected than midgut thus suggesting an up-concentration of the causative agents of intestinal alteration in hindgut regions. Soyasaponins were not degraded during gut transit although the various conjugates were hydrolysed increasing the level of un-conjugated soyasaponins in the faeces. The intestinal level of heat shock protein 70 (HSP70) was unaffected by diet except in one replicate pen of fish fed the 100 PP diet where expression increased fourfold compared to the other replicate and other diets. The reason for this remains obscure. In conclusion, the plant ingredients used in the present study appear not to affect cod health to any major degree when used to replace up to 75% of fish meal protein. An increase to 100% may induce enteritis-like conditions in the guts of some fish and also activate HSP70 stress-gene mRNA expression in the intestine. [Copyright &y& Elsevier]

Abstract: Groups of rainbow trout (Oncorhynchus mykiss Walbaum) in feeding (guts filled with digesta) or food-deprived (3 days of diet deprivation) states were subjected to 15 min of acute stress. Blood samples and intestinal tissue were collected and prepared for physiological, chemical and ultrastructural analysis immediately before stress, and at 4 and 48 h post-stress. Haematocrit, plasma cortisol and lactate levels increased following stress, and the response appeared to be more pronounced in food-deprived than in fed fish. Plasma glucose appeared to increase more in fed fish. Stress did not seem to cause massive tissue damage as measured by cellular leakage of transaminase enzymes into the blood. Furthermore, the plasma oxidative stress marker malondialdehyde did not increase markedly following stress. The content of malondialdehyde did not change following stress neither in midgut nor hindgut sections, and nor did membrane lipid class and fatty acid compositions. Ultrastructural studies showed that stress caused significant widening of the tight junctions between enterocytes in the midgut, with little effect seen in the hindgut. Fed fish appeared to experience more damage than food-deprived fish as judged by the ultrastructural analysis. But these changes were to a large extent transient and cellular organization in the midgut had returned to normal values within 2 days. Intestinal paracellular permeability of the midgut increased 4 h post-stress and was further increased 48 h post-stress of food-deprived fish, whereas no effect was seen in fed trout. In hindgut of food-deprived fish, the permeability appeared to increase sometime after 4 h and was still elevated 48 h after stress. No change in permeability occurred in fed fish. The adherent microbial population level and composition in hindgut was significantly reduced following stress, while the level increased in faeces. This suggests that substantial amounts of mucus are peeled off following stress. It is concluded that, under the present experimental conditions, acute stress causes cellular alteration in rainbow trout gastrointestinal tract. Ultrastructural damages are mainly observed in midgut, but most changes appear to be transient returning to normal levels within 48 h. Changes in adherent intestine microbial population level and compositions together with increased intestinal paracellular permeability following acute stress suggest a possible increased susceptibility to bacterial infections after stress and that active feeding may have a protective role. [Copyright &y& Elsevier]

Abstract: Despite gains in commercial salmonid aquaculture production by using underyearling smolts (0+), poor performance in seawater has been noted. This study assesses how artificial photoperiods in seawater may improve the performance. Seawater-transferred 0+ Atlantic salmon were exposed to natural light (NL) or continuous light (LL) from October to midsummer, or to LL from October to January followed by natural light (LL–NL), or NL from October to January and then LL to midsummer. All fish experienced natural light after midsummer. The photoperiods changed the seasonal growth pattern and harvest weights became significantly different (LL: 3445±46 g; NL–LL: 3207±45 g; LL–NL: 3092±56 g; NL: 2854±42 g). Growth of NL fish decreased from autumn through winter and increased in spring. LL fish grew better than NL fish until spring and was 21% heavier at slaughter. Switching from LL to NL in January reduced growth rate from March onwards to a level slightly lower than the seasonal growth pattern of the NL fish. The onset of light in January (NL–LL) resulted in a temporary decrease in growth for 7 weeks followed by a major weight increase with growth rate being significantly higher than LL fish from late April until October. NL–LL fish never managed to catch up with the LL fish, but were significantly larger than the NL fish. Changes in condition factor with light treatment demonstrated clearly the photoperiodic influence on the seasonal growth pattern. Two vaccines were used in all photoperiods. The fish given the vaccine that gave least adherences (lesion score 1.8 vs. 2.3) became 20% heavier than those given the other vaccine, and no interaction between photoperiod and vaccine was observed. Muscle lipid and astaxanthin contents in June were highest in LL fish (lipid: 9.1%; astaxanthin: 4.4 mg kg−1) and lowest in NL fish (lipid: 4.1%; astaxanthin: 2.6 mg kg−1). These differences largely disappeared by November, although fish exposed to light following seawater transfer (LL and LL–NL) had a tendency to contain higher levels of astaxanthin than those on natural light (NL and NL–LL). Both lipid and astaxanthin contents in fillet correlated positively with fish weight. The incidence of sexual maturation was 4% in LL and LL–NL, 1.5% in NL and 0% in NL–LL. In the seawater-phase of underyearling autumn-transferred smolts, we recommend using the NL–LL photoperiod regime. This light treatment gave a slightly smaller fish than LL at harvest, but there were no losses due to sexual maturation. [Copyright &y& Elsevier]

Increased cortisol production and oxygen consumption are commonly used as indicators for stress in most fish species. For comparative studies measuring acute stress responsiveness, it is often assumed that the cortisol is sampled at peak response. However, the effect of acclimated temperature on cortisol response is largely unknown, and so is also the dynamics of oxygen uptake following stress. In the present study we acclimated Atlantic salmon post-smolt to four different temperature within the species relevant temperature spectrum (4, 8, 12 and 17 °C), and applied an acute stressor of 60 s of confinement. Cortisol, as measured in water, was sampled before and up to 360 min following the stressor, while oxygen consumption rates was estimated per minute. Basal cortisol production increased significantly with temperature, which must be considered when calculating the cortisol response per se. The cortisol response followed a Gaussian pattern, where the amplitude (max value) increased with temperature while the curve width (time before peak and recovery) decreased. Also, the total amount of cortisol released in response to the stressor increased with temperature. The cortisol peak at 4, 8, 12 and 17 °C was respectively measured after 123.75 ± 7.2, 67.5 ± 9.7, 56.25 ± 3.75 and 33.74 ± 3.75 min. The stress induced peak in oxygen uptake increased with temperature in an exponential manner, and both peak value and the total oxygen uptake above baseline levels was three times higher at 17 than at 12 °C, while the total oxygen uptake was similar between 4, 8, 12 °C. This strong difference in oxygen uptake between 12 and 17 °C was not reflected by the cortisol levels, and suggests that salmon are extra vulnerable for creating hypoxic conditions when stressed at high temperature. The current work suggests when to sample cortisol for attaining peak values at a given acclimated temperatures in salmon post-smolt. This should allow for better comparison of cortisol values between studies. [ABSTRACT FROM AUTHOR]

The possibility to prepare for and respond to challenges in a propermanner is essential to cope with a changing environment, and learning allows fish to up or downregulate the stress response based on experience. The regulation of the response to predicted needs should be easier inmore predictable environments.Weexposed salmon parr to chasing of either 15 s (weak stressor) or 5min (strong stressor) twice daily for a 7-day learning period, with chasing either announced by a 30 s light signal (conditioned) or not announced (unconditioned). The behavioural response to the light signalwas different between the conditioned and unconditioned groups, demonstrating that conditioned groups associated the signal with chasing. We could, however, not demonstrate any effect on the stress response of anticipation. The fish habituated to repeated stress exposures with a similar decrease in oxygen hyperconsumption in all groups. Due to habituation, possible effects of predictable announcement of a stressor on the physiological stress response may not have been expressed in this study. Plasma cortisol concentrations 1 h after light signal and chasing the day after the training period was moderate in all groups although higher after 5 min chasing (13 ng ml-1) than 15 s chasing (7 ng ml-1). There was no physiological stress response after exposure to the light signal only after the learning period. We argue that the benefit of predictability of stressors is limited when the fish have no way to avoid the stressor. [ABSTRACT FROM AUTHOR]

Atlantic salmon (Salmo salar L.) experience periodic drops in dissolved oxygen (O2) inside aquaculture sea cages. In order to evaluate whether fish function or welfare is compromised during such drops in O2 (termed hypoxia), it is necessary to establish the limit for acceptable O2 reductions; the hypoxia tolerance threshold. In the present study, effects of temperature (6, 12 and 18°C) and hypoxia acclimation (33days of hypoxic periods down to 50% O2 occurring every 6h at 16°C) on the routine oxygen consumption rate (ṀO2) and the limiting oxygen saturation (LOS, defined as the hypoxia tolerance threshold) were investigated in undisturbed, fed fish kept in groups with the aim of resembling commercial aquaculture conditions. ṀO2 was measured using open respirometry where a progressive decline in O2 was caused by fish O2 consumption during a period of low water turnover. LOS was defined as the O2 below which fish were no longer able to uphold routine ṀO2. Both ṀO2 and LOS were found to increase exponentially with temperature (Q10 =2.7 for ṀO2 and 1.8 for LOS), but no effect of hypoxia acclimation was found. The mean (±SE) LOS at 6, 12, 16 and 18°C was 30±1, 39±1, 47±1 and 55±2% of air saturation respectively. The variation in LOS within and across temperatures was to a large extent explained by variation in ṀO2 (R2 =0.93), suggesting that LOS increases with any factor that raises the metabolic rate of Atlantic salmon. This study provides aquaculturists and legislators with a lower limit for acceptable drops in O2 at temperatures relevant for the on-growing phase in Atlantic salmon aquaculture, and discusses the variation in LOS that can be expected at a given temperature. [ABSTRACT FROM AUTHOR]

Abstract: Using reward conditioning to change the cognitive interpretation of stressors has been suggested as a method to reduce stress in farmed fish. We studied the modification of behavioural and physiological responses of Atlantic cod to a stressor. Eight groups of cod were exposed to a moving dip net six times daily over 7 days. Half were rewarded with food 10s after dip net exposure (trace reward conditioning), while the remaining groups were fed 30min after dip net exposure (unrewarded). The swimming speed and oxygen consumption responses after dip net exposure in the unrewarded groups declined at a rate of 13% and 12%trial−1, respectively. The swimming speed response was lost completely, while 37% of the initial oxygen hyper consumption response remained after habituation. The rewarded groups learnt to associate dip net exposure with food delivery. When the dip net stopped moving, they approached it before gathering in the feeding area. Due to this anticipatory behaviour, swimming speed and oxygen consumption remained elevated after dip net exposure in the rewarded groups. Plasma cortisol and glucose levels remained low after dip net exposure on the last day in both groups. The experiment demonstrates that a stimulus that initially evokes a fright response can become a positive signal of a forthcoming reward. [Copyright &y& Elsevier]

Abstract: The digestive tract of fish is essentially a muscular tube lined by a mucous membrane of columnar epithelial cells that exhibit regional variation in structure and function. In the last two decades, our understanding of translocation of bacteria in the fish gut has increased, and electron microscopy has contributed to this knowledge. Translocation is the movement of viable and non-viable bacteria and bacterial products (such as endotoxin) from the intestinal lumen through the epithelial mucosa to infect otherwise sterile tissues. This paper provides an overview of the gastrointestinal tract of larvae and fry from an electron microscopical perspective focussing on factors (mucus, interactions between probiotics and pathogens, and stress) affecting translocation of bacteria (transcellular and paracellular), cellular damage (specific attack on tight junctions and desmosomes caused by pathogenic bacteria) and pathogenesis. Furthermore, the paper will present information on the occurrence and translocation of pathogenic bacteria in the gastrointestinal tract of larvae, fry and adult fish. Currently, it is generally accepted that the major routes of infection in fish are through the skin, gills and gastrointestinal tract. As the gastrointestinal tract is a potential port of entry for pathogens, the use of electron microscopy in studies on microbiota in fish digestive tract is a valuable method for increasing our understanding of the mechanisms involved in entry of the pathogens. Possible mechanisms involved in translocation of bacteria in fish gastrointestinal tract, autochthonous (indigenous) and pathogenic, are discussed. [Copyright &y& Elsevier]