EXPERT TOPIC - SALMON

November | December 2012

EXPERT TOPIC - SALMON

The International magazine for the aquaculture feed industry

International Aquafeed is published five times a year by Perendale Publishers Ltd of the United Kingdom.All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies, the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of information published. ©Copyright 2012 Perendale Publishers Ltd. All rights reserved. No part of this publication may be reproduced in any form or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1464-0058

http://www.aquafeed.co.uk

30 | InternatIonal AquAFeed | november-December 2012

EXPERT T●PIC

november-December 2012 | InternatIonal AquAFeed | 31

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

SALMONEXPERT TOPIC

IAF12.06.indd 30 07/11/2012 17:41

30 | InternatIonal AquAFeed | november-December 2012 november-December 2012 | InternatIonal AquAFeed | 31

EXPERT T●PIC

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Atlantic Canadaby Pamela Parker, Executive Director, Atlantic Canada Fish Farmers Association, Canada

Atlantic Canada is the birthplace of Canada’s salmon farming industry. Canada’s first commercial harvest of farmed Atlantic salmon in took place

in Lord’s Cove, Deer Island in 1979.

Today, aquaculture is a $2.1 billion industry in Canada, employing over 15,000 workers.

Atlantic Canada produces approximately 55,000 metric tonnes of salmon annually, 30 per cent of Canada’s farmed salmon production. The sector is one of the region’s biggest economic drivers generating over $435 million in revenue and employing over 3,500 people. In many rural coastal communities, salmon farming is the major employer and further growth potential exists. Both production and employment are poised to grow significantly in the near future with the launch of Nova Scotia’s aquaculture development strategy and with continued focus on develop-ment in Newfoundland. Salmon is already the largest agri-food export in New Brunswick.

Although the vast majority of finfish farmers grow salmon, many companies are growing other finfish species such as cod, trout, arctic char, sturgeon and halibut as well as mussels and

seaweeds from integrated multi-trophic aquacul-ture farms. Of the fish farmed in Atlantic Canada, approximately 60 percent is exported to the United States.

Canada has vast and dynamic ecosystems and while some farm management practices vary depending on the environment, no Canadian salmon producer uses hormones, dyes or chemi-cals in their feed and our farmed salmon is not genetically modified. Less than three percent of salmon feed contains an antibiotic.

Because salmon farming is science-based, our environmental and fish health management practices are continually changing and improving as new research or technology emerges.

Canada leads the development of fishmeal and fish oil replacement in salmon feed. In the 1990s, wild fish based ingredients in feeds were as high as 80 per cent. Today, it’s as low as 20 per cent.

Atlantic Canadian feed producers work with top researchers to develop their own feed using local ingredients whenever possible. The fish waste from our processing facilities is now being used to produce other animal feeds (pets, poultry) so that we are a net protein producer.

All the salmon farming companies operating in eastern Canada are pri-vately owned and operated by Atlantic Canadians. Our salmon farmers are pas-sionate and hardworking people who are committed to building a locally based, globally competitive and environmentally sustainable industry that will continue to bring prosperity to our coastal com-munities.

The Atlantic Canada Fish Farmers Association (ACFFA) is an industry-fund-

ed association that has been working on behalf of the salmon farming industry in the mari-time region since 1987. The ACFFA represents over 95 percent of salmon production in New Brunswick and Nova Scotia in addition to a wide range of businesses and organization in the supply and service, technological and research sectors.

The ACFFA takes a leadership role in the development and implementation of strategies that are focused on fish health and welfare, environmental stewardship, innovation and social responsibility within our communities.

More InforMatIon:Website: www.atlanticfishfarmers.com

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IAF12.06.indd 31 07/11/2012 17:41

New Zealand by Adam Hicks, Aquaculture New Zealand, New Zealand

Since its beginnings in the 1970s, New Zealand’s salmon farming industry has evolved from a group of

innovative pioneers, to a profes-sional, specialised and quality food production sector focused on envi-ronmental sustainability, food safety and value added marketing.

We are the world’s largest pro-ducer of the premium Chinook (King) Salmon, with our 2011 harvest of 14,000 tonnes accounting for roughly 84 percent of total global production.

Last year the New Zealand salmon industry generated $128 million in revenue and provided employment for hundreds of Kiwis.

Roughly half of all salmon farmed in New Zealand, is consumed in New Zealand. It is readily available at local supermarkets and restaurants – much of it served in family kitchens and backyard barbecues. The remainder is exported to over 30 countries includ-ing Japan, US, Australia, Hong Kong and Canada.

The premium species of salmon, King Salmon is prized for its char-acteristic rich flavour, delicate soft texture and high Omega-3 content. King Salmon is much harder to grow than Atlantic salmon, but yields a much higher quality product.

Our farmed King Salmon are grown in the pristine, colder waters off the South Island with the majority in sea pens in Marlborough, Canterbury and Southland regions. The farms are

located in areas selected for their isola-tion, water quality and flow. After being placed within a seawater farm, a salmon generally takes 19 - 31 months to grow to an optimum market size of around 3.5 – 4 kg. There are also a number of small fresh water farms operating in the McKenzie Country hydroelectric- canals.

New Zealand producers (New Zealand King Salmon, Sanford, Akaroa Salmon, Mt Cook Alpine Salmon, Benmore Salmon and High Country Salmon) are focused on nurturing the salmon throughout their natural growth cycle to ensure fish welfare and guaran-tee high quality and safe salmon for the consumer.

International feed production com-panies Skretting, Ridley, Biomar and Reliance supply the majority of New Zealand’s salmon feed. The food is specially blended for King Salmon with fishmeal and fish oil, with some produc-ers also incorporating plant proteins and oils and by-products from the poultry and meat industries, from animals raised for human consumption.

The New Zealand salmon farming industry now produces more fish pro-tein than it consumes – with some pro-ducers achieving conversion rates better than 1:1.19. Information supplied by feed producers show the wild fish pro-tein used in feed production is sourced primarily from the well-managed and sustainable Peruvian anchovy fishery (www.fishsource.org).

Core to the industry, is an uncom-promising commitment to the respon-sible management of our resources. Our Environmental Codes of Practise are independently recognised as world leading, and our farming operations are highly regulated and closely monitored to meet the strict environmental condi-tions of the New Zealand Resource Management Act.

Salmon farming is an industry that New Zealand can be proud of and at the same time be excited about for our future.

More InforMatIon:Website: www.salmon.org.nz or www.aquaculture.org.nzThe history of New Zealand salmon farming history has been captured in Swimming Upstream, and is available by emailing [email protected]

2


EXPERT T●PIC


IAF12.06.indd 32 07/11/2012 17:41

British Columbia, Canadaby Mary Ellen Walling, Executive Director, BC Salmon Farmers Association, Canada

Salmon farming is the largest agricul-tural business in British Columbia. It produces around 80,000 metric tonnes annually with a value of US

$450 million. The industry employs 6,000

people, of which 2,000 are employed directly by farming companies. The domestic demand for BC salmon is strong but the fish also exported to the USA and

some specialty markets in Japan, Asia and India.

The BC Salmon Farmers Association works in various ways

to look after the needs of its members. For example, regulatory responsibility for the industry has recently been transferred from the provincial to the federal govern-ment. However, there is no specific aqua-culture legislation in Canadian law. This means farmers have to work within existing, older acts which are not always relevant to the industry. The association is working with the Canadian Aquaculture Industry Alliance to advocate for national regulation for aquaculture.

Bringing the industry together to effec-tively manage fish health is also a prior-ity for the association. Following the 2002-03

IHN outbreak, the association developed a viral management plan designed to respond to future incidents of disease more effectively. This plan was implemented in May 2012, when IHN was detected at a farm in north of Tofino. There were culls at three farms and weekly farm tours were postponed but the spread of the disease was halted.

There is a strong environmental move-ment in BC. The association is committed to providing good information and engaging with questions from the public. It has also worked with the WWF on its Salmon Aquaculture Dialogue.

More InforMatIon

Website: www.salmonfarmers.org

3by Mary Ellen Walling, Executive Director,

almon farming is the largest agricul-tural business in British Columbia. It produces around 80,000 metric tonnes annually with a value of US

$450 million. The industry employs 6,000

the industry has recently been transferred from the provincial to the federal govern-ment. However, there is no specific aqua-culture legislation in Canadian law. This means farmers have to work within existing, older acts which are not always relevant to the industry. The association is working with the Canadian Aquaculture Industry Alliance to advocate for national regulation for aquaculture.

Bringing the industry together to effec-tively manage fish health is also a prior-ity for the association. Following the 2002-03

Dialogue.

More InforMatIon

Website: www.salmonfarmers.org


EXPERT T●PIC

A sustainable approach to aquacultureThe BioSustain programme targets the need for a sustainable approach in food production, by evaluating and documenting the sustainability profile of different feed types.

For further information please visit www.biosustain.no

www.biomar.com

W O R L D C L A S S F I S H F E E DBIOSUSTAIN

IAF12.06.indd 33 07/11/2012 17:41

http://www.biosustain.no

Immunonutritionin fish farming:

A natural and sustainable solutionby D. Pacitti, S. A. M. Martin, C.J. Secombes , Scottish Fish Immunology Research Centre, Institute of Biological and Environmental Sciences, University of Aberdeen, United Kingdom

The rise of aquaculture has been one of the most profound develop-ments in global food production over the past 100 years, with pro-

duction approximately doubling each decade. Aquaculture now delivers 39 percent of aquatic food products with the FAO record-ing 310 species under culture in 2010.

Among these, salmonid fish (primarily rainbow trout and Atlantic salmon) are the most intensively farmed fish in more than 30 countries representing 90 percent of global marine aquaculture production. Salmonid production, particularly Atlantic salmon, increased from 299,000 tonnes in 1990 to 1.9 million tonnes in 2010, at an average annual rate exceeding 10 percent. Salmon is one of the food categories that is growing at a significantly higher rate than the world’s human population (FAO, 2012).

However, the salmon farming industry is vulnerable to the adverse impacts of disease. For example, in 2007 an outbreak of infec-tious salmon anaemia (ISA) in Chile caused more than $2 billion in losses and reduced by half the Chilean production of Atlantic salmon (Godoy et al, 2008). The common causative agents of infectious diseases in aquaculture include a range of bacteria, viruses, parasites and oomycetes.

Whilst vaccines exist for some of these dis-eases, it is clear that additional measures are

needed to give a suite of approaches to disease control to farm managers. This article will focus on one such approach, involving the optimisation of the mineral component of the diet. Knowledge of the impact of mineral nutrition on immunological

function and health status of fish, together with our greater understand-

ing of the salmonid genome and a new suite of molecular tools, may offer a new

perspective enabling better prophylactic con-trol of stress and disease.

Fish immunologyThe immune system protects an organism

against disease and participates in the main-tenance of stable conditions during develop-ment and growth, inflammatory reactions and tissue injury. As in the human immune system, the fish immune system is divided into innate and adaptive components.

The innate system is an ‘ancient’ system that is based on a non-specific recognition of a pathogen, that gives an instant reaction but has a short duration. The innate immune system is of prime importance in the immune defence of fish and is commonly divided into three compartments: the epithelial/mucosal barrier, secreted soluble mediators (e.g. complement system, interferons, anti-microbial peptides) and the cellular components (e.g. phagocytic cells such as macrophages and granu-locytes).

The epithelial and mucosal barrier of the skin, gills and alimentary tract is an extremely important bar-rier in fish, being constantly immersed in

media con-taining poten-

tially harmful agents. The

humoral and cel-lular defences repre-

sent the first response of the organism once sub-

ject to pathogen attack. However, a second encounter with the same pathogen will not result in an enhanced response.

In contrast, the adaptive arm is character-

ised by specific activity, which is not a heritable trait but reflects the immune experience of each individual. The response of the adaptive immune system is relatively slow initially but is long lasting and has a memory component, giving faster and larger responses on a second encounter. The main effector cells are a differ-ent white blood cell type called lymphocytes. During infection, the fast but generally short-lived innate immune response precedes the longer lasting more specific adaptive immune response. In fish this lag period can be as much as 10-12 weeks, which has to be kept in mind when considering prophylactic immunological control of fish disease (Magnadottir, 2010).

ImmunonutritionTraditionally the use of antimicrobials

and vaccination has been used to fight disease in fish farms. Today, farmed Atlantic salmon are routinely vaccinated against a

number of bacte-

rial and viral diseases before seawater transfer.

However, fish vaccinology is still a young and maturing science, and vac-

cines for many pathogens have not yet been developed.

It is a well-accepted concept that appro-priate feed and feeding regimes support optimum health. However the sustainability of fishmeal and fish oil stocks has brought about changes in aquafeed formulations that are demanding a greater understanding of the role that alternative ingredients, feed addi-tives, macro- and micro-nutrients and their balance plays as they can directly or indirectly influence fish health and immune function (Figure 1).

4


EXPERT T●PIC


Immunonutritionin fish farming:

A natural and sustainable solutionby D. Pacitti, S. A. M. Martin, C.J. Secombes , Scottish Fish Immunology Research Centre, Institute of Biological and Environmental Sciences, University of Aberdeen, United Kingdom

he rise of aquaculture has been one of the most profound develop-ments in global food production over the past 100 years, with pro-

duction approximately doubling each decade. Aquaculture now delivers 39 percent of aquatic food products with the FAO record-ing 310 species under culture in 2010.

Among these, salmonid fish (primarily rainbow trout and Atlantic salmon) are the most intensively farmed fish in more than 30 countries representing 90 percent of global marine aquaculture production. Salmonid production, particularly Atlantic salmon, increased from 299,000 tonnes in 1990 to 1.9 million tonnes in 2010, at an average annual rate exceeding 10 percent. Salmon is one of the food categories that is growing at a significantly higher rate than the world’s human population (FAO, 2012).

However, the salmon farming industry is vulnerable to the adverse impacts of disease. For example, in 2007 an outbreak of infec-tious salmon anaemia (ISA) in Chile caused more than $2 billion in losses and reduced by half the Chilean production of Atlantic salmon

, 2008). The common causative agents of infectious diseases in aquaculture include a range of bacteria, viruses, parasites

Whilst vaccines exist for some of these dis-

approach, involving the optimisation of the mineral component of the diet. Knowledge of the impact of mineral nutrition on immunological

function and health status of fish, together with our greater understand-

ing of the salmonid genome and a new suite of molecular tools, may offer a new

perspective enabling better prophylactic con-trol of stress and disease.

Fish immunologyThe immune system protects an organism

against disease and participates in the main-tenance of stable conditions during develop-ment and growth, inflammatory reactions and tissue injury. As in the human immune system, the fish immune system is divided into innate and adaptive components.

The innate system is an ‘ancient’ system that is based on a non-specific recognition of a pathogen, that gives an instant reaction but has a short duration. The innate immune system is of prime importance in the immune defence of fish and is commonly divided into three compartments: the epithelial/mucosal barrier, secreted soluble mediators (e.g. complement system, interferons, anti-microbial peptides) and the cellular components (e.g. phagocytic cells such as macrophages and granu-locytes).

The epithelial and mucosal barrier of the skin, gills and alimentary tract is an extremely important bar-rier in fish, being constantly immersed in

media con-taining poten-

tially harmful agents. The

humoral and cel-lular defences repre-

sent the first response of the organism once sub-

ject to pathogen attack. However, a second encounter with the same pathogen will not result in an enhanced response.

immune system is relatively slow initially but is long lasting and has a memory component, giving faster and larger responses on a second encounter. The main effector cells are a differ-ent white blood cell type called lymphocytes. During infection, the fast but generally short-lived innate immune response precedes the longer lasting more specific adaptive immune response. In fish this lag period can be as much as 10-12 weeks, which has to be kept in mind when considering prophylactic immunological control of fish disease (Magnadottir, 2010).

ImmunonutritionTraditionally the use of antimicrobials

and vaccination has been used to fight disease in fish farms. Today, farmed Atlantic salmon are routinely vaccinated against a

number of bacte-

rial and viral diseases before seawater transfer.

However, fish vaccinology is still a young and maturing science, and vac-

cines for many pathogens have not yet been developed.

It is a well-accepted concept that appro-priate feed and feeding regimes support optimum health. However the sustainability of fishmeal and fish oil stocks has brought about changes in aquafeed formulations that are demanding a greater understanding of the role that alternative ingredients, feed addi-tives, macro- and micro-nutrients and their balance plays as they can directly or indirectly influence fish health and immune function

44

IAF12.06.indd 34 07/11/2012 17:41

In terms of macronutrients, the protein (and amino acids), carbohydrate and lipid/fatty acid components can all impact on health status. Dietary proteins provide essen-tial and non-essential amino acids, which have a central role in defence mechanisms, as they are required for the synthesis of an array of proteins involved in immune functions. The use of alternative plant proteins has still to be optimised for growth and immune function. Lipids provide energy and meet the

essen-tial fatty acid

requirements of the animal. It is known that

several polyunsaturated or monounsaturated fatty acids are

involved in different immune functions, exerting their influence through changes in membrane fluidity, eicosanoid synthesis, for-mation of lipid peroxides, regulation of gene expression, apoptosis, alteration of antigen presentation, or modulation of intestinal microbiota. All of these processes and path-ways have significant roles in inflammation and disease resistance.

The micronutrients also represent a funda-mental component of fish diets. Micronutrients comprise of vitamins (e.g. A, C and E), carotenoids (e.g. β-carotene, α-carotene and γ-carotene) and minerals (e.g. calcium, mag-nesium, iron, copper, zinc and selenium). Since many micronutrients are involved in several biological pathways, an inadequate intake can lead to adverse effects on fish health due to deficiency.

VitaminsVitamins are organic compounds required

in small amounts in the diet, because they play major roles in growth, physiology, and

metabolism. However in certain circumstanc-es, when the fish is exposed to certain kinds of stress, the required amount may be two to three times higher.

Vitamin A has essential roles in vision, growth, bone development, reproduction and normal maintenance of epithelial tissue. Some important functions of vitamin A include regula-tion of cellular differentiation and proliferation,

resist-ance to

infection as well as embry-

onic development and growth.

Vitamin C (or ascorbic acid) is a co-factor for several enzymatic

reactions, including col-lagen synthesis and the produc-tion of stress hormones by interrenal and chromaffin cells. Vitamin C itself is also a reduc-tive compound that acts as an antioxidant dur-ing oxidative stress.

Vitamin E compounds are the major chain-breaking antioxi-dant; they have an important role in maintain-ing the home-ostasis of labile metabolites (such as vitamins and unsaturated fatty acids) and in protecting the cell membranes from oxidative damage.

Vitamin sup-plemented diets

can be given during aquaculture operations that are stressful and potentially immunosup-pressive. They are essential for a variety of biological and physiological functions including increased disease resistance and wound heal-ing. A study conducted in rainbow trout fed diets supplemented with vitamin C, showed that this molecule increased complement activity and lymphocyte proliferation. Other studies have revealed that ascorbic acid sup-plementation is able to alleviate the adverse effects due to hypoxic conditions and tem-perature fluctuations (Oliva-Teles, 2012).

Carotenoids Carotenoids (tetraterpenoid organic pig-

ments) are naturally occurring in plants and some other photosynthetic organisms (some

types of bacteria and fungi). They protect cells against oxidative injury and ensure optimal cellular functions, including apoptosis, cell sig-nalling and gene regulation. The immunopro-tective functions of the carotenoids depend very much on the equilibrium between the intra- and extracellular milieu and on the type and concentration of the carotenoid.

Despite the role of carotenoids have in the nutrition of several fish and crustacean species, only few studies have considered them in rela-tion to the health of the organism. In rainbow trout, activities of lysozyme, complement,


EXPERT T●PIC

health status. Dietary proteins provide essen-tial and non-essential amino acids, which have a central role in defence mechanisms, as they are required for the synthesis of an array of proteins involved in immune functions. The use of alternative plant proteins has still to be optimised for growth and immune function. Lipids provide energy and meet the

essen-tial fatty acid

requirements of the animal. It is known that

several polyunsaturated or monounsaturated fatty acids are

involved in different immune functions, exerting their influence through changes in membrane fluidity, eicosanoid synthesis, for-mation of lipid peroxides, regulation of gene expression, apoptosis, alteration of antigen presentation, or modulation of intestinal microbiota. All of these processes and path-ways have significant roles in inflammation and disease resistance.

The micronutrients also represent a funda-mental component of fish diets. Micronutrients comprise of vitamins (e.g. A, C and E), carotenoids (e.g. β-carotene, α-carotene and γ-carotene) and minerals (e.g. calcium, mag-nesium, iron, copper, zinc and selenium). Since many micronutrients are involved in several biological pathways, an inadequate intake can lead to adverse effects on fish health due to deficiency.

VitaminsVitamins are organic compounds required

in small amounts in the diet, because they

three times higher. Vitamin A has essential roles in vision,

growth, bone development, reproduction and normal maintenance of epithelial tissue. Some important functions of vitamin A include regula-tion of cellular differentiation and proliferation,

resist-ance to

infection as well as embry-

onic development and growth.

Vitamin C (or ascorbic acid) is a co-factor for several enzymatic

reactions, including col-lagen synthesis and the produc-tion of stress hormones by interrenal and chromaffin cells. Vitamin C itself is also a reduc-tive compound that acts as an antioxidant dur-ing oxidative stress.

Vitamin E compounds are the major chain-breaking antioxi-dant; they have an important role in maintain-ing the home-ostasis of labile metabolites (such as vitamins and unsaturated fatty acids) and in protecting the cell membranes from oxidative damage.

Vitamin sup-

biological and physiological functions including increased disease resistance and wound heal-ing. A study conducted in rainbow trout fed diets supplemented with vitamin C, showed that this molecule increased complement activity and lymphocyte proliferation. Other studies have revealed that ascorbic acid sup-plementation is able to alleviate the adverse effects due to hypoxic conditions and tem-perature fluctuations (Oliva-Teles, 2012).

Carotenoids Carotenoids (tetraterpenoid organic pig-

ments) are naturally occurring in plants and some other photosynthetic organisms (some

types of bacteria and fungi). They protect cells against oxidative injury and ensure optimal cellular functions, including apoptosis, cell sig-nalling and gene regulation. The immunopro-tective functions of the carotenoids depend very much on the equilibrium between the intra- and extracellular milieu and on the type and concentration of the carotenoid.

Despite the role of carotenoids have in the nutrition of several fish and crustacean species, only few studies have considered them in rela-tion to the health of the organism. In rainbow trout, activities of lysozyme, complement,

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phagocytes and non-specific cytotoxicity can be elevated upon β-carotene and astaxanthin supplementation. These effects can be further enhanced when using diets enriched for vita-mins A, C and E. In a subsequent investigation, the same researchers validated the benefits of carotenoids derived from marine algae, which improved humoral as well as cellular responses (Kiron, 2012).

Minerals Minerals are another important component

in the fish diet. In many cases their importance is under-estimated and as a consequence their amount in fish diets can be below the required level. Moreover, several studies have shown that certain minerals, when provided to fish at doses marginally above essential levels can effectively boost immune responses and increase stress resistance.

However, it is important not to exceed the tolerated level with mineral augmenta-tion, because toxic effects may occur (Figure 2). In higher vertebrates minerals are known to impact general organism homeostasis and

immunity. Among the wide range of minerals essential for organism welfare, zinc and selenium have received particular attention. They are a required component for more than 300 differ-ent enzymes, which makes them fundamental for the proper functioning of many metabolic processes in the organism, including the immune response (Ferenčík and Ebringer, 2003).

Zinc is essential due to its vital structural and/or catalytic importance in several proteins that play important roles in fish growth, reproduc-tion, development, vision and immune function. Consequently for fish, of the essential metals, zinc is second in quantitative importance only to iron. Dietary zinc minimum requirements range between 15–60 mg kg–1 dry mass of diet (it varies slightly amongst different fish species), with the maximum level that is permitted in fish diets by the European Union being 250 mg kg-1.

Previous studies have shown a toxic effect in rainbow trout fed zinc at concentrations ranging between 500-1000 mg kg–1. It may exert its toxicity by interfering with intracellular calcium homeostasis, and affecting hepatic copper and haemoglobin levels.

In contrast, zinc supports a healthy immune system and is needed for wound healing. Indeed, zinc deficiency has been shown to compromise antibody produc-tion, leading to reduced titres post-immunisation. Adequate zinc status is essential for proliferation, matu-ration and differentiation of cells of the adaptive immune response. Studies conducted on dietary zinc supplementation have shown an increased level of circulating lym-phocytes in the blood and chemo-taxis of macrophages, leading to an overall improved disease resistance.

Selenium (Se) is another important trace element for fish because it is a constituent of more than 30 seleno-proteins with fundamental structural and enzymatic roles in the cell. Se is primarily involved in antioxidant defences, reproduction, synthesis of thyroid hormones and the immune response. The Se requirement is esti-mated to be 0.15-0.38 mg kg–1 (it also slightly varies amongst different fish species), with the maximum level in fish diets permitted by the European Union being 0.5 mg kg-1.

Selenium toxicity occurs in rain-bow trout when the dietary intake exceeds 13 mg kg-1. Se-deficient diets can profoundly affect the antioxi-dant defences, metabolism and the immune response in fish. In Se defi-ciency, cell/tissue integrity can more easily be compromised by oxidative stress and inflammatory disorders can occur.

Different studies, conducted

both in mammalian and fish models, have shown that Se augmentation is able to alleviate inflammatory reactions, boost the phagocytic and killing capacity of the cell mediated immune response, and increase the expression of cellular components responsible for efficient antiviral-defences.

Typically the dose range between levels giving deficiency and those giving toxicity for different minerals is quite narrow, and does not leave a big margin for their supplementa-tion. Apart from concentration level, another important aspect is the bioavailability of these micronutrients in the diet.

Factors influencing bioavailability include the level and form of the nutrient, particle size and digestibility of the diet, nutrient interactions which may be either synergistic or antagonistic, stress and pathological conditions of the fish, waterborne mineral concentration and the species under con-sideration. Of these factors, those related to the chemical state are particularly important. If the mineral is present in the diet in insoluble and indigestible form, uptake can be affected.

Moreover, the element can form insoluble and non-absorbable substances in the gastrointestinal tract of the animal that may either prevent or reduce its uptake, transport and metabolism. Commonly, minerals can be provided to the fish either as inor-ganic salts or as chelated or organic forms.

In recent years, there has been considerable interest in the use of organic trace minerals rather than salts, on the grounds that they are more bio-available or more similar, than inorganic sources, to forms that occur in the organism. If the metal chelate or complex is stable in the digestive tract, the metal would be protected from forming complexes with other dietary components that can inhibit absorption, allowing greater assimilation.

Moreover, the ingestion of metals in the inor-ganic form might facilitate the formation of reactive ions which can promote oxidative stress in the gastro-intestinal tract. The use of organic chelated minerals is regarded as a more natural method of trace element supplementation and may give a larger safe range for supplementation (Watanabe et al., 1997).

In the case of zinc and selenium, two prod-ucts called Bio-Plex® and Sel-Plex® have been produced by Alltech, to provide respectively zinc and selenium augmentation into the animal diet. Both contain a relatively higher amount of these two metals complexed into organic compounds derived from yeast. Numerous studies have already been conducted in different models (mice, poultry, pigs and fish) showing the benefits of mineral-yeast enriched diets on animal welfare. The mineral-enriched diets can provide a relatively inexpensive, sustainable and consumer friendly approach to improve fish production, with a negligible impact on the environment.

Moreover, a better tolerance of higher con-centrations of these two metals as yeast-derived ingredients in animal feed has been found. This combined with an increased activity of cellular


EXPERT T●PIC


Figure 1: The concept of immunonutrition in health maintenance (modified from Kiron, 2012)

Figure 2: Schematic representation of the relationship between element intake, tissue element concentration and health indices. The curve represents an essential trace element which may produce adverse health effects in conditions of deficiency or excessive exposure. Intake A & B represent intakes which produce minimal statistical significant changes from normal value of one or more health indices due to deficiency or toxicity respectively (Modified from Spivey et al, 1982)

IAF12.06.indd 36 07/11/2012 17:41

components involved in stress resistance and immune responses in animals fed such diets, leads to the conclusion that farmed animal feeds enriched with organic metal compounds are safe at higher Se/ Zn doses. However, more investigations are needed to better elucidate to what extent these compounds can improve the fish immune response and resistance to stressors.

ConclusionsIt is important to ensure that diet composition

meets the fish required level of essential nutrients. This has been done to a large extent with growth in mind but it is also a possible strategy that could effectively increase fish health status. Micronutrient augmentation in particular may represent a sus-tainable and environmental/consumer friendly approach to improve fish responses to many kinds of stress (farm operations and disease outbreaks). The concentration and the form in which these micronutrients are delivered to fish must be taken into account and be optimised. New ingredients and additives are emerging on the market, and give an opportunity to produce new formulations to ensure a higher assimilation of these components and reduce the potential for adverse affects of micronutrient augmentation.

References

Available on request

Amoebic gill disease (AGD) first emerged as a problem in the 1980s in Tasmania; it is now a disease of inter-

national significance. AGD has now been identified on the west coast of USA, Chile, New Zealand, Japan, South Africa, Ireland, Scotland, France, Spain and Norway.

Current methodologies of controlling this disease involve bathing the fish in either freshwater for an extended period of time; or in hydrogen peroxide for a short period of time.

Despite the fact that AGD has been around for several decades, there are still significant gaps in our knowledge about this disease. The causative agent was only identified relatively recently. In

order for us to develop more effective control strategies for AGD, we need to improve our knowledge of the organism itself and the epidemiology of the disease. As examples – where does the amoeba live when a site is fallowed? Does it have a reservoir in wild populations of fish? Can it live independent of a host, for how long? What depths does it prefer? Is it phototactic? What environmental conditions favour amoeba proliferation? Does AGD have a link with biofouling or harmful algae? Under normal culture situ-ations, Chinook salmon are immune to AGD, why? Will ingredient substitution in the feed have any influence on AGD?

Despite the disease being around for almost 30 years, we have still a long way to go before we have total understanding of the disease we are trying to defeat.

5Managing AGD (Amoebic Gill Disease) in Atlantic salmon: Still a long way to go

by SmartAqua, Australia


EXPERT T●PIC

IAF12.06.indd 37 07/11/2012 17:41

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VOLUME 15 I S SUE 6 2 012

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EXPERT TOPIC– Salmon

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