Seasonal effects of macrophyte growth on rainbow trout habitat availability and selection in a low‐gradient,groundwater‐dominated river

Rainbow trout habitat use is often described in high‐gradient, runoff‐driven, heterotrophic streams where geomorphic features and overhanging riparian vegetation provide channel complexity and cover. However, many rainbow trout populations thrive in rivers with contrasting aquatic habitat. We describe rainbow trout habitat use in a low‐gradient, groundwater‐dominated tailwater river where river flow management and macrophyte growth and senescence largely govern available trout habitat. In the summers of 2013 and 2014, available aquatic habitat (depth, velocity, macrophyte cover, substrate size) was quantified, while individual trout location was determined by radio telemetry and linked to environmental variables. Detailed habitat surveys indicate that macrophyte cover increases throughout the summer and is a strong determinant of in‐stream habitat characteristics. Paired logistic regression shows that adult rainbow trout prefer greater depths. Water depth increases with macrophyte abundance at both reach and local scales as plants restrict flow, and available trout habitat is linked to this seasonal pattern. When macrophyte abundance is high, adult trout show secondary preference for localised areas of lower macrophyte cover but otherwise show no selectivity for macrophyte cover, velocity or substrate size. Results suggest that submerged aquatic plants increased the quantity and quality of rainbow trout habitat as a source of channel complexity and cover. Macrophytes may play a similar role in other low‐gradient streams and should not be overlooked by fisheries managers considering habitat suitability.     

Effects of low‐level dietary microalgae supplementation on the distal intestinal microbiome of farmed rainbow trout Oncorhynchus mykiss (Walbaum)

In this study, high throughput 16S rRNA sequencing was used to investigate the effect of a novel whole‐cell dietary microalgae meal (Schizochytrium limacinum), on the distal intestinal microbiome of farmed rainbow trout, Oncorhynchus mykiss. Heterotrophic microalgae are rich in omega 3 polyunsaturated fatty acids, can be produced sustainably and have been shown to have beneficial effects on host health. After a 15‐week trial period, microbial community profiles were compared between the distal intestinal contents of fish fed either a control diet or a treatment diet that partially replaced fish oil with microalgae meal, at a substitution level of 5%. The results of this research showed that the microbial communities of both fish populations were composed of similar microbial taxa, however, the treatment group fed the microalgae supplement possessed a greater level of microbial diversity than those in the control group. A limited number of bacterial taxa were discriminatory between diets and were significantly elevated in the treatment group, notably operational taxonomic units (OTUs) assigned to the genera Streptococcus, Leuconostoc, Lactobacillus, Lactococcus and Weissella. However, the overall structure of the intestinal microbiome between control and treatment groups was not found to be significantly different. The treatment group displayed a heavier mean weight and condition factor at the end of the trial period. The results of this study suggest that the tested microalgae meal can be used as a replacement for a proportion of fish oil in aquafeeds, with minor changes to the intestinal microbiome of farmed rainbow trout, and positive effects on growth.     

Assessing the bioavailability of L‐methionine and a methionine hydroxy analogue (MHA‐Ca) compared to DL‐methionine in rainbow trout (Oncorhynchus mykiss)

Different synthetic forms of supplemental methionine (Met) are produced commercially by various manufacturers. Limited information exists on the bioavailability of these different forms in commercially relevant fish species. The objective of this study was to compare the relative bioavailability of L‐methionine and a methionine hydroxy analogue calcium salt (MHA‐Ca) to DL‐methionine in a commercially relevant species, rainbow trout, using a linear slope‐ratio assay. In addition, the ability of the rainbow trout to effectively convert the D‐isomer of methionine into the biologically active L‐isomer was studied by comparing the relative bioavailability of L‐methionine to DL‐methionine. Nine (9) diets were created from a methionine‐deficient basal diet which was supplemented with graded equimolar levels of methionine (0.1%, 0.2%, 0.3% of diet by weight) from either DL‐methionine, L‐methionine and methionine hydroxy analogue calcium salt (MHA‐Ca). Diets were fed in triplicate (IBW = 24.0 ± 0.6 g) for 12 weeks using a pair‐feeding protocol. The basal diet was formulated to be deficient in both methionine (0.5%) and cysteine (0.3%), whereas meeting all other known nutritional requirements of rainbow trout. MHA‐Ca was found to be less available (P < 0.05) than DL‐methionine with relative bioavailability values of 69%, 60% and 73% based upon weight gain, growth rate (TGC) and retained nitrogen values respectively. No significant differences (P > 0.05) in bioavailability were found between DL‐methionine and L‐methionine. These findings lead to the conclusion that differences in bioavailability do exist between sources of synthetic methionine, additionally findings suggest that rainbow trout are able to effectively convert the D‐isomer of methionine into the L‐isomer without significantly affecting fish performance.     

Non‐invasive recording of brain function in rainbow trout: Evaluations of the effects of MS‐222 anaesthesia induction

Effective methods of producing instantaneous and irreversible unconsciousness at the time of slaughter are crucial for ensuring animal welfare in commercial aquaculture. However, the traditional method of visually evaluating unconsciousness has been shown to be insufficient and may lead to misjudgements of stunning efficiency. In this study, we developed a non‐invasive technique that measures brain activity in fish as an alternative to traditional invasive, brain implants and used it to determine when a change in consciousness occurs in trout during anaesthesia induction. Nine rainbow trout (Oncorhynchus mykiss) were equipped with a custom designed silicone cup fitted with electrodes and submerged in 10°C water with dissolved MS‐222. During anaesthesia, the state of consciousness was assessed by recordings of electroencephalogram (EEG). The EEG recordings were analysed both by visually evoked responses from light stimulation (VERs) and from changes in signal amplitude, median frequency and relative signal power. According to the loss of VERs and decrease in signal amplitude, trout transitioned to surgical depth of anaesthesia within 5 min. Our results show that consciousness, or loss of, can be determined using a non‐invasive system to record EEG in fish.     

Assessment of the Antibacterial Effect of Chitosan Coated with Heracleum persicum Oil on Rainbow Trout Quality

Today, due to the rapid spoilage of fish, the use of natural preservatives is a priority over those of synthetic varieties. Also, the natural antimicrobial effects of essential oils can help to increase shelf life. There are very few studies concerning the use of essential oils in this regard. In the present study, the antibacterial effect of chitosan coated with Heracleum persicum oil was investigated on the quality of rainbow trout. The control and the coated fish samples were analyzed periodically by generalized estimating equation (GEE) for total viable count (TVC), psychrophilic bacteria, total volatile base nitrogen (TVB-N), polyunsaturated fatty acids (PUFA), texture, taste, odor, and overall acceptance parameters. The results showed that by increasing the amount of chitosan, TVC, psychrophilic bacteria, and TVB-N decreased while PUFA, texture, taste, odor, and overall acceptance parameters increased.     

一种安全有效提高虹鳟苗种成活率的技术

为安全有效地提高虹鳟苗种成活率,应用1种生物制剂——抗菌肽对虹鳟苗种进行浸泡处理。通过比较苗种成活率,检测SOD、ALP、CAT、IL、IFN、TNF等6种免疫相关酶表达量,采用扫描电镜检测苗种体表细菌情况,确定抗菌肽浸泡对虹鳟成活率的影响。试验结果显示,自破膜期开始,用40%抗菌肽浸泡10次,试验组6种免疫相关酶表达量明显高于对照组,试验组苗种的成活率高达95%以上,对照组苗种的成活率则低于30%。电镜观察结果显示,试验组仔鱼体表细菌数量显著减少。试验结果表明,用抗菌肽浸泡法能显著提高虹鳟苗种成活率。     

Projected impacts of climate change on stream salmonids with implications for resilience‐based management

The sustainability of freshwater fisheries is increasingly affected by climate warming, habitat alteration, invasive species and other drivers of global change. The State of Michigan, USA, contains ecologically, socioeconomically valuable coldwater stream salmonid fisheries that are highly susceptible to these ecological alterations. Thus, there is a need for future management approaches that promote resilient stream ecosystems that absorb change amidst disturbances. Fisheries professionals in Michigan are responding to this need by designing a comprehensive management plan for stream brook charr (Salvelinus fontinalis), brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) populations. To assist in developing such a plan, we used stream‐specific regression models to forecast thermal habitat suitability in streams throughout Michigan from 2006 to 2056 under different predicted climate change scenarios. As baseflow index (i.e., relative groundwater input) increased, stream thermal sensitivity (i.e., relative susceptibility to temperature change) decreased. Thus, the magnitude of temperature warming and frequency of thermal habitat degradation were lowest in streams with the highest baseflow indices. Thermal habitats were most suitable in rainbow trout streams as this species has a wider temperature range for growth (12.0–22.5 °C) compared to brook charr (11.0–20.5 °C) and brown trout (12.0–20.0 °C). Our study promotes resilience‐based salmonid management by providing a methodology for stream temperature and thermal habitat suitability prediction. Fisheries professionals can use this approach to protect coldwater habitats and drivers of stream cooling and ultimately conserve resilient salmonid populations amidst global change.     

Dietary profile of n‐3 series LC‐PUFAs in rainbow trout under regular stripping condition: Semen production and quality,hepato‐somatic index,haemato‐immunologic values,oxidative stress and fatty acid composition of liver,muscle and semen

In this study, the semen production and quality, hepato‐somatic index, haemato‐immunologic values, oxidative stress and the fatty acid contents in liver, muscle and semen of rainbow trout fed diets supplemented n‐3 series long‐chain polyunsaturated fatty acids (LC‐PUFAs) under regular stripping condition were investigated. For this aim, three diets (Control, D1 and D2) were prepared. These diets were contained n‐3 LC‐PUFAs (as a percentage of dietary total fatty acid) at 3.14%, 7.84% and 13.63% respectively. Experimental fish were fed with the control and test diets. The highest hepato‐somatic index, spermatologic (semen pH and volume, sperm motility and density), haematologic (haematocrit value, haemoglobin, erythrocyte count, corpuscular volume, haemoglobin and its concentration in corpuscular), immunologic (nitroblue tetrazolium activity, leucocyte count, phagocytic index, protein and immunoglobulin [IgM] in total plasma) and antioxidants (reduced glutathione, catalase, glutathione peroxidase and superoxide dismutase) values were found in fish fed the D2, D1 and control diets respectively (p < 0.01). Increase in the dietary n‐3 LC‐PUFAs was not significantly (p > 0.01) increased the oxidative stress (malondialdehyde) in fish. The results indicated that the n‐3 LC‐PUFAs at 13.63% level of total fatty acid in the diet could increase the semen production and quality, hepato‐somatic index, haematologic and immunologic values, and the n‐3 LC‐PUFA contents in liver, muscle and semen of rainbow trout broodstock under regular stripping condition.     

An evaluation of DVAqua®, a fully‐fermented yeast culture,during long‐term hatchery rearing of McConaughy strain rainbow trout

The addition of a proprietary, fully‐fermented yeast Saccahromyces cerevisiae culture supplement (DVAqua^®, Diamond V Mills, Cedar Rapids, IA, USA) was evaluated during long‐term feeding of McConaughy strain rainbow trout Oncorhynchus mykiss. Beginning at initial feeding and continuing for 408 days of hatchery rearing, the trout received either a commercially‐manufactured feed or the same feed containing 0.125 g kg^?1 DVAqua. This study was conducted at a production level as part of normal (real‐world) hatchery operations, with the fish periodically inventoried and moved into different rearing units. Although no rearing‐tank replication occurred during the first 54 days of feeding, multiple tanks and raceways were used thereafter. Fish in rearing units receiving DVAqua supplementation exhibited less mortality, particularly during the earlier rearing stages. During the final 177 days of rearing in six raceways, DVAqua‐fed McConaughy strain trout were significantly larger and had a significantly improved feed conversion ratio. The overall feed conversion ratio for the entire duration of the study was 1.17 in the fish receiving DVAqua supplementation compared to 1.21 in the control group. Despite the limitations of this study, the use of DVAqua is recommended for McConaughy strain rainbow trout and other less‐domesticated, more difficult‐to‐rear salmonids.     

Optimizing zinc supplementation levels of rainbow trout (Oncorhynchus mykiss) fed practical type fishmeal‐ and plant‐based diets

Rainbow trout (23.1 ± 0.4 g) were fed either a fishmeal‐ or plant‐based diet supplemented with various levels of zinc (0, 15, 30, 60 or 120 mg kg^?1) for 12 weeks. Trout fed the fishmeal diet had significantly higher weight gain than with the plant‐based diet. Zinc supplementation in the fishmeal diet had no effect on growth performance, suggesting that additional dietary supplementation of zinc is not required. However, in trout fed the plant‐based diet, growth increased significantly up to 30 mg kg^?1 zinc after which growth was not affected. Trout fed the plant‐based diet containing no zinc exhibited severe growth retardation, and in fish fed the 0 and 15 mg kg^?1 zinc diets, cataracts were present. Use of broken‐line quadratic modelling suggests that dietary supplementation of zinc needed to prevent deficiency and promote adequate growth in rainbow trout fed the plant‐based diet in this study was 30.1 mg kg^?1 (80 mg kg^?1 total dietary zinc). This is higher than the NRC (2011, Nutrient Requirements of Fish and Shrimp) dietary recommended level of 15 mg kg^?1 for rainbow trout. Following the NRC recommendation could lead to zinc deficiency in rainbow trout fed a plant‐based diet.     

Supplementing enzymes to extruded,soybean‐based diet improves breakdown of non‐starch polysaccharides in rainbow trout (Oncorhynchus mykiss)

Plant‐based feed ingredients typically contain remnants of dietary fibres [DF; non‐starch polysaccharides (NSP) and lignin] that have various antinutritive effects in carnivorous fish. Exogenous enzymes have been shown to improve the apparent digestibility coefficients (ADC) of plant‐based diets presumably by assisting in the breakdown of NSP. This study examined the effects on NSP degradation when supplementing β‐glucanase, xylanase, protease or a mix of the three enzymes to an extruded, juvenile rainbow trout (Oncorhynchus mykiss) diet containing 344 g kg^?1 de‐hulled, solvent‐extracted soybean meal (SBM). The NSP content in the non‐supplemented control diet and in faecal samples from the dietary treatment groups was analysed to determine the recovery/apparent digestibility of cellulose and total non‐cellulosic polysaccharide (T‐NCP) sugar monomers. The enzymes had significant, positive effects at the pH range and temperature prevailing in the gastrointestinal tract: β‐glucanase improved the ADC of mannose, galactose and uronic acids; xylanase and protease improved the ADC of xylose; and protease furthermore improved the ADC of mannose and uronic acids. There were no effects when supplementing all three enzymes together. In conclusion, exogenous enzymes may potentially be applied to fish feed containing SBM, assisting in the breakdown of NSP and alleviating some of the antinutritive effects.     

The olive oil by‐product in ‘rainbow trout Onchorynchus mykyss (Walbaum)’ farming: productive results and quality of the product

The aim of this work is to investigate olive oil by‐product [vegetation water (VW)] inclusion in rainbow trout diet and its effect on the productive traits and the quality of the product. Two levels of VW inclusion were used and one control group was included. Fish diets were isonitrogeonous (crude protein 40%) and isoenergetic (18 MJ kg^?1 DM). Two thousand and four hundred rainbow trout were used. An in vivo digestibility experiment was performed in order to determine diets’ digestibility. All the fish diets and fillets were analysed to determine the proximate and fatty acid composition. On final fish fillet, lipid oxidation was determined at 0, 24, 48, 72, 96 and 192 h of storage using the 2‐thiobarbituric acid method (thiobarbituric acid reactive substances). Aroma analyses on the final cooked and raw fillet were performed using an electronic nose. The VW inclusion partially reduced protein digestibility. The fish growth varied between 1.08% and 1.1% day. The supplemental level of VW led to a better antioxidant status of fish fillet, in particular, in the fillet sample after 72 h of fillet conservation. Principal component analysis in raw and cooked fish fillet indicates that the VW inclusion in the fish diet led to aroma modification on fish fillet.     

Reducing bacterial density in the semen of rainbow trout (Oncorhynchus mykiss) by applying gradient centrifugation and swim‐up washing methods

The objective of this study was to demonstrate the effectiveness of disinfection procedures to reduce bacterial load of rainbow trout, Oncorhynchus mykiss (Walbaum 1792), semen. Fresh semen was obtained from 3–4‐year‐old male species by abdominal sampling of sperm into pre‐cooled test tubes. After sperm cryopreservation and thawing, experiments were accomplished at 4–9°C. ‘Swim‐up’ and gradient centrifugation were used as a sperm washing method with commercial kits. Phosphate buffered saline was also used as washing solution. Bacterial growth tests were employed before and after washing the semen samples. Samples were inoculated on tryptic soy agar (TSA), modified Anacker and Ordal agar (MAOA) as well as brain heart infusion (BHI) agar. After using ‘swim‐up’ method for washing the semen, many bacterial colonies were observed. However, after semen washing with gradient centrifugation, lower bacterial growth was observed on TSA, MAOA and BHI. Some motile (40%) spermatozoa were obtained doing gradient washing procedure. Although sperm motility was not satisfactory, apparently the gradient centrifugation method reduced bacterial contamination as known from the mammalians.     

Supplementation of plant‐based diets for rainbow trout (Oncorhynchus mykiss) with macro‐minerals and inositol

Replacement of fish meal with plant products in aquafeeds results in the elimination of dietary compounds which may be important for optimal growth and physiology. A study was conducted to determine if supplementation with macro‐minerals and/or inositol would improve performance of rainbow trout (Oncorhynchus mykiss) fed a plant‐based diet. Four iso‐caloric and iso‐nitrogenous diets (40 g kg^?1 protein and 15 g kg^?1 lipid) were formulated and consisted of a fish meal‐based control diet (control) and three plant‐based experimental diets. Plant‐based diets were supplemented with either macro‐minerals and inositol (+MM+I), no macro‐minerals with inositol (?MM+I), and no macro‐minerals and no inositol (?MM?I). Sodium chloride, potassium chloride and magnesium oxide were the sources used in the macro‐mineral premix. There was no effect of diet on survival, but there was an effect of diet on weight gain, FCR, feed intake, HSI and nutrient retentions. Significant liver pathology was observed in trout fed plant‐based diets without MM supplementation. Supplementation of MM and inositol significantly improved weight gain of trout fed a plant‐based diet. Supplementation of MM and/or inositol also improved PRE and ERE. This study demonstrates the importance of supplementing these nutrients to trout fed fish meal free diets.