EXPERT TOPIC: Salmonids

SALMONIDS

38 | May | June 2016 - International Aquafeed

Welcome to Expert Topic. Each issue will take an in-depth look at a particular species and how its feed is managed.

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Iran

The Salmonidae family, collectively known as Salmonids, comprises of salmon, trout, chars, freshwater whitefishes, and graylings, but it is the trout and Atlantic salmon, of the genus Salmo, which gives the family their name. A slender teleost fish, they can range in size between just 13 cm to a whopping 2 m in length. With a single

row of sharp teeth, Salmonids are predators, choosing to feed on smaller fish, aquatic insects and small crustaceans. Despite spawning in fresh water Salmonids are mostly anadromous, spending their lives at sea, choosing only to return to rivers to reproduce.

Our Salmonid focus ‘Rainbow Trout’ is native to the Pacific drainages of North America, ranging from Alaska to Mexico, although many countries report rainbow trout farming production, primarily areas in Europe, North America, Chile, Japan and Australia. Since 1874 it has been introduced to waters on all continents except Antarctica, for recreational angling and aquaculture purposes. Production greatly expanding in the 1950s as pelleted feeds were developed. Trout fisheries are maintained,

or culture practised, in the upland catchments of many tropical and sub-tropical countries of Asia, East Africa and South America. As a result, several local domesticated strains have developed (e.g. Shasta and Kamloops), while others have been arisen through mass selection and cross-breeding for improved cultural qualities.

The rainbow trout is a hardy fish that is easy to spawn, fast growing, tolerant to a wide range of environments and handling, and the large fry can be easily weaned on to an artificial diet (usually feeding on zooplankton). They are capable of occupying many different habitats, ranging from an anadromous life history, to permanently inhabiting lakes.

The anadromous strain is known for its rapid growth, achieving 7-10 kg within 3 years, whereas the freshwater strain can only attain 4.5 kg in the same time span. The species can withstand vast ranges of temperature variation (0-27 °C), but spawning and growth occurs in a narrower range (9-14 °C). The optimum water temperature for rainbow trout culture is below 21 °C. As a result, temperature and food availability influence growth and maturation, causing age at maturity to vary; though it is usually 3-4 years.

Source FAO

1 Salmonidae

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Austrianorth America

International Aquafeed - May | June 2016 | 39

SALMONIDSTHE USE OF PREBIOTICS IN SALMONID DIETSNatural alternatives for improving production

by Fernando Roberti, Biorigin

Fish production has been gaining increasing importance in the protein market and in the animal nutrition sector. Estimates indicate that it will reach 160 million tonnes by 2030, with an increasing participation of aquaculture1. In this context, for supporting the intensification of the production, which on one hand requires increasingly

revenues, but on the other predisposes to disease outbreaks, the use of functional compounds becomes essential.

To achieve an economically viable aquaculture, it is fundamental to maximise nutrient digestibility and retention, feed conversion rate, dietary nutrient balance, and minimising fish mortality in antibiotic-free conditions through the development of health-promoting diets. In this context, the inclusion of prebiotics into fish diets plays a very important role for these results to be reached.

PrebioticsPrebiotics are non-digestible compounds able to modulate gut

microbiota and to selectively stimulate the growth of beneficial bacteria2. Among prebiotics, mannanoligosacharides (MOS) from the yeast Saccharomyces cerevisiae have been widely researched and applied in animal nutrition.

The main described effects of MOS are related to pathogen colonisation blocking, alongside growth and feed conversion improvement. The use of MOS as a pathogen colonisation blocker evolves from the concept that some sugars as mannose could be used as inhibitors of pathogen adhesion to intestinal cells3.

Therefore, the objective of including MOS in aquaculture feeds is to reduce intestinal attachment of pathogenic bacteria

by using a component that resists the passage along the gut during digestion and mimics the specific carbohydrates groups of intestinal cells4.

Moreover, MOS are also a fermentation substract for beneficial bacteria which are able to produce organic acids. This fermentation, in addition to promoting the growth of these beneficial bacteria populations, leads to an acidification of the intestinal environment due to the acid production. Importantly, some of these acids are used as the major energy sources by some gut cells5, helping to maintain the intestinal integrity.

Together, all these benefits generate a healthy environment, which will favor nutrients digestibility and absorption. To reach these benefits, however, it is important to select a good MOS product that presents the following characteristics: high mannan content, high mannan exposure – obtained from a suitable production process – which ensures a good pathogens adhesion, and a high total carbohydrates content for fermentation purposes.

Importance of intestinal health for fishBeyond the benefits to performance, maintaining good

intestinal health is particularly important since many infectious diseases initiate from the colonisation of the gut mucosa by pathogens, and the efficiency of the intestinal barrier against this process depends on the intestinal integrity and on the balance of comensal bacteria6. In addition, the more intact the intestinal barrier, the more pathogens will be avoided to translocate over stressing conditions, reducing the risks of the development of systemic frames.

Trial with rainbow troutsA study* performed in a semi-intensive farm located in

Mazandaran, Iran, evaluated the effects of the dosages 0, 0.1, 0.25 and 0.4 percent of MOS (ActiveMOS, Biorigin, Brazil)

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included in a commercial feed for rainbow trouts. After 7 days of acclimation to the control diet, 300 fish juveniles were randomly distributed into 12 tanks, with 25 fish in each tank and 3 replicates (tank) per treatment (dosage of MOS), where they were kept and evaluated during 60 days. Performance and survival were evaluated once every 2 weeks and intestinal bacteria evaluation was conducted at the end of the nutritional trial.

According to Figures 1 and 2, all groups supplemented with ActiveMOS had greater final body weight and lower feed conversion rate. Among the different dosages, the supply of 1kg of ActiveMOS/ton of feed led to the most interesting results for final body weight and feed conversion rate. Concerning gut microbiota, there was a trend of increase on lactic acid bacteria populations in the dosage of 0.1 percent. It is important to point out that dosages should be adjusted in function of phase, feed intake, among other factors.

These results make clear the effects of MOS on fish performance, through the greater final body weight and improved feed conversion rate. The modulation of gut microbiota, with increased populations of beneficial bacteria that improve intestinal health, are the major explanation for the obtantion of these results.

ConclusionThe positive effects of mannanoligosacharides have already

been proven in fish species. It is highly recommended to consider the inclusion of these prebiotics as functional and natural solutions for the design of health-promoting diets, as well as of diets for early stages of fish production.

*Denji et al. Effect of dietary prebiotic mannan oligossacharide

(MOS) on growth performance, intestinal microflora, body composition, haematological and serum biochemical parameters of rainbow trout (Oncorhynchus mykiss) juvenile. Journal of Fisheries and Aquatic Science, v. 10, p. 255-265, 2015.

References available upon request

Figure 1: Final body weight of rainbow trout fed different levels of MOS

Figure 2: Feed conversion rate (FCR) of rainbow trout fed different levels of MOS


SALMONIDSWAYS TO IMPROVE SALMONID DISEASE RESISTANCE

by Benedict Standen & Rui Gonçalves, Biomin

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With an increasing pressure to provide affordable protein to an ever-growing population, aquaculture practices are expanding and intensifying. High stocking densities can lead to animals becoming

stressed and immunocompromised, while also favoring the proliferation of pathogens.

Unfortunately, disease outbreaks are inevitable and this represents a major constraint for the sustainable development of the industry. Nowhere is this more obvious than in high value species, such as salmonids, where disease outbreaks can lead to significant economic losses.

Salmon culture is often considered the holy grail of aquaculture. Consequently, as an industry it has worked hard to reduce its dependency on antibiotics, relying primarily on vaccination for disease control. Although this approach has been effective, it also has its limitations. This is especially true in cold water fish because antibody production is temperature dependent: thus it can

Figure 1: Atlantic salmon with external appearance of a large ‘furuncle’ under the skin (a). Insert shows open furuncle and (b) opened peritoneal cavity of an Atlantic salmon with furunculosis showing extensive hemorrhaging in the peritoneal fat and wall (yellow arrows) and within muscle (red arrow). SOURCE: www.agriculture.gov.au

a b


take a long time for fish to develop resistance through adaptive mechanisms. The labour intensive and costly vaccination process can also result in high levels of stress for fish.

The use of novel feed additives –including enhanced acidifiers, probiotics and yeast-based immunostimulants— to provide immediate prophylactic protection may comprise a less stressful and more convenient alternative.

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Figure 2: Survivaloffishfedeitheracontroldiet,ordietsupplemented with Biotronic® Top3 after A. salmonicida challenge. Bars represent averages between three infection routes, IP injection, immersion and cohabitation.

Figure 3: Rainbow trout with petechial lesions around the operculum (a) and the mouth and tongue(b)afterartificialinfection with Y. ruckeri. Image (c) shows the internal organs after infection. Of particular interest are the petechial lesions on the pyloric caeca (arrow) and the bloodfilledintestine(arrowhead).

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b

c


AcidifiersAcidifiers represent an exciting approach to pathogen control.

Acidifiers have two modes of action; firstly in their dissociated form they create a hostile environment for pathogens by lowering the pH. Secondly, they can have a direct antimicrobial effect by entering pathogenic cells. Once in the cytoplasm the acids will dissociate, reducing cytoplasmic pH, disrupting protein and enzyme synthesis and ultimately killing the pathogen from the inside.

Biotronic® Top3 (BIOMIN GmbH) is a commercial acid-based product that contains a phytochemical component which prevents bacterial pathogens from dividing, and also has a quorum quenching effect by disrupting bacterial communication. In order to improve the mode of action of the previous components, Biotronic® Top3 also includes a unique permeabilising complex which weakens the cell wall of Gram-negative pathogens by breaking down the lipopolysaccharide layer.

The efficacy of Biotronic® Top3 in providing protection against Aeromonas salmonicida in rainbow trout was assessed. Although A. salmonicida infection is not unique to salmonids, it is the etiological agent of furunculosis, a serious septicemic disease which causes large losses within the salmonid industry (Figure 1). Rainbow trout were split into two triplicate treatments; a control treatment fed a commercial diet and a test treatment fed a commercial diet supplemented with Biotronic® Top3 at a final dose of 0.8 g/kg. After 175 days, fish were exposed to A. salmonicida via three routes of infection; challenge by intraperitoneal (IP) injection, immersion and cohabitation. After 35 days the survival rates were assessed.

Unsurprisingly, IP injection caused the highest mortalities when compared with other infection routes and the protective effect of Biotronic® Top3 was more pronounced in this group. Rainbow trout in the control group showed 75 percent mortality, significantly higher than the group receiving the Biotronic® Top3 supplemented diet where mortality was just 30 percent.

In the fish challenged by immersion, mortality remained unchanged (approximately 30%) by dietary regime. In the cohabitation infection route group, mortality was 10 percent in control fish, while no mortalities were observed in fish treated with Biotronic® Top3 (Figure 2). Fish surviving the challenge were examined for the presence of A. salmonicida using microbiological and molecular methods.

Many of these fish tested positive for the pathogen, suggesting that they might be asymptomatic carriers. The proportion of carrier fish was lower in the Biotronic® Top3 group (25%) when compared with those that received a control diet without the supplement (75%). These results suggest that Biotronic® Top3 may help to provide protection against A. salmonicida and also to reduce the spread of disease by removing the pathogen from previously infected fish.

ProbioticsProbiotics can also improve disease resistance in salmonids.

It is well known that probiotics, particularly lactic acid bacteria (LAB), can inhibit pathogen growth directly via the production of bacteriocins. In vitro trials demonstrate that the LAB strains in AquaStar® Growout (BIOMIN GmbH) can provide a broad spectrum of antagonism against some of the most important aquaculture pathogens including Aeromonas, Edwardsiella, Streptococcus, Vibrio and Yersinia.

To investigate this effect in vivo, rainbow trout were split into three treatments, control vs AquaStar® Growout (at 2 g/kg and 5 g/kg), fed for eight weeks and then subsequently challenged with Yersinia ruckeri (via oral intubation). Y. ruckeri is a Gram-negative pathogen which causes enteric red mouth disease in salmonids (Figure 3). After two weeks the survival was significantly higher (43% and 38%) in both AquaStar® treatments (low and high dose, respectively) when compared with the control treatment (12%) (Figure 4).

Immune stimulationWhile acidifiers, and to a certain extent probiotics, may improve

disease resistance by providing direct pathogen antagonism, other feed additives may provide protection by stimulating the hosts own immune system. For example, Levabon® Aquagrow E (BIOMIN GmbH) is an autolysed yeast product specifically developed to bring immuno-modulatory benefits to aquatic animals.

To test the effect on disease resistance, rainbow trout were split into two treatments, control (basal diet) and a diet supplemented with Levabon® Aquagrow at 4 g/kg. After eight weeks of feeding the experimental diets, fish were challenged with Y. ruckeri, via immersion, at a dose of 1.6 x 104 CFU/ml. Dead and moribund fish were monitored and after two weeks the survival rate was calculated. In the unsupplemented control treatment, survival was 68 percent, significantly lower than the Levabon® Aquagrow supplemented diets, where survival was 86 percent (Figure 5).

ConclusionFor some individuals, the issue of disease control in aquaculture

may appear as an insurmountable challenge. However, this should be viewed as an opportunity to embrace and promote the use of sustainable feed additives, such as enhanced acidifiers, probiotics and immunostimulants. While the exact mechanisms which underpin their effectiveness may still elude us, it is clear that their use in aquafeeds can provide protection against a range of pathogens, ultimately resulting in improved survival, improved production and consequently higher profitability.

Figure 4:Survivaloffishfedeitheracontroldiet,ordietsupplemented with AquaStar® Growout (low and high dose) after Y. ruckeri challenge.

Figure 5: Survivaloffishfedeitheracontroldiet,ordietsupplemented with Levabon® Aquagrow after Y. ruckeri challenge.
