The fine art of gill protection PROTEC GILL Protec Gill - designed to support gill health and recovery during disease, environmental and treatment challenges.
GILLS AN IMPORTANT MULTI-FUNCTIONING ORGAN Fish gills are the most important site of contact between the fish and the water. They are not only responsible for the exchange of the respiratory gases (oxygen and carbon dioxide) they also play a part in maintaining salt balance and excreting waste. Skretting s gill handbook has been developed so you can learn more about this life sustaining organ, and how maintaining good gill health will enable fish to perform to their full potential. GILL STRUCTURE Gills lie in what s known as the branchial cavity and are protected by a hard bony flap called an operculum. If you lift the operculum, you will see four gill arches. On each gill arch, there are lines of frills called filaments. On the top and bottom of the filaments, there is a series of secondary folds called lamella. Inside the lamella, you find tiny blood vessels. The lamella walls consist of one cell layer, and the entire lamella is about 10 microns thick; a little more than half the thickness of a strand of human hair. This branching structure gives the gills its large surface which is about 50 times larger than the area of skin covering the body. Gill surface area declines with increasing fish weight which is why larger fish often are the first to succumb to oxygen depletion. Lamella Filament blood vessels Illustration showing how blood vessels coming from the heart (in blue) and going to the body (in red) runs through the gill arch. The water flows between the gill arches, through the gill filaments, passing each gill lamella.
GILL FUNCTION GAS EXCHANGE When the fish opens its mouth, water flows in, past the gills, and out of the gill operculum in one direction, while the blood in the gill flows in the opposite direction through the blood vessels in the lamella. This process is called the counter-current principle. The combination of the large surface area, the counter-current principle and the short distance between blood and water makes the gills very efficient. Gills can extract up to 80% of the oxygen from the water passing through them. CONTROLLING THE BODY S WATER CONTENT Chloride secretory cells located in the gill lamella play an important role in the transportation of salts to and from the blood into the surrounding water; this process is known as osmoregulation. In freshwater, water will continuously diffuse into the fish so it has to actively uptake salts from the water to get enough ions in the body for it to function properly. In seawater this process is reversed and fish will have a lower concentration of salts than that of the surrounding Lamella seawater, so it loses water and gains salts. Fish need to get rid of this excess salt to avoid dehydration. ELIMINATION OF WASTE The process of getting energy from feed produces waste and a large amount of this is in the form of nitrogen. Nitrogen combines with other molecules in the body to produce toxic compounds such as ammonia. About 80 to 90% of a fishes nitrogenous waste is excreted as ammonia by the gills, with the remainder excreted by the kidney as urea. Again, the short distance between the blood and the water the fish swims in is useful, as The counter-current principle: A small segment of one of the gill ammonia can easily be transported filaments where the lamellae are located. Water and blood flow from the blood stream to the water via in opposite directions past the lamellae making oxygen uptake very efficient. the thin lamella wall.
WHAT HAPPENS WHEN GILLS ARE COMPROMISED? There are many potential threats to gill integrity and function such as disease, poor water quality and the side-effects of treatments. The very design that makes gills so efficient means that they are also vulnerable to damage. The most common response of the gill to any disorder are inflammatory responses in which the distinguishing feature is either. 1) Hyperplasia - the enlargement of an organ caused by an increase in the reproduction rate of its cell 2) Hypertrophy the enlargement of an organ or tissue from the increase in size of its cells. Both hyperplasia and hypertrophy will lead to the lamellae becoming stuck or clubbed together. Chronic irritation is also likely to cause a thickening of the mucous layer. These inflammatory symptoms can significantly reduce the surface area available for gas and salt exchange as well as excretion. The effects on farmed fish populations can be reduced appetite, a lower tolerance to stress and in severe cases mortality. Today, gill disease is one of the most common causes of production loss in aquaculture. AGD AMOEBA CHALLENGES GILL HEALTH An increasingly common challenge to gill health is from the parasitic amoeba Neoparamoeba perurans which causes amoebic gill disease (AGD). Amoebas are single celled organisms. In the human intestine we have several naturally-occurring amoebas, while other amoebas that end up in food or drink can cause illness, for instance dysentery. N. perurans only infects fish in sea water and was first observed in Tasmania in 1985. By 2015 nine of the twelve Atlantic salmon producing countries worldwide had reported confirmed cases of the disease. Even though amoebas are small, they are large compared to the gill filaments, with a length of up to 50μm. N. perurans can float well in water and actively move over the surface of gills. They reproduce quickly and easily by cell division. When the amoeba attaches to the gills, its presence causes a severe increase in the number cells (hyperplasia) within the gill tissue which in turn leads to the fusion of the gill lamellar. These are the white spots visible when you open the gill operculum and study the gill filaments. If you look at the damage under a microscope, you can see that the lamellae are clumped together. Therefore, the gills lose their surface area, and the gill capacity is poorer. When the gills are severely attacked, the fish can suffocate. In Northern hemisphere salmon production regions, AGD is most common in the autumn
when the temperature starts to drop from the summer highs. The severity of the disease reduces when the temperature falls to its lowest winter levels. It has been proven that N. perurans can survive for at least 14 days in open sea water, and further spread one kilometer from the source. It also exists on other species living in the sea, such as cleaner fish, shellfish and hydroids. The risk of disease is likely to be higher in farmed salmon, as the density of fish and nutrients is higher. A common scoring system has now been adopted almost uniformly across the industry to monitor the gills to see if the fish has been infected by P. perurans. The damage is scored in individual fish on a scale of 1 to 5. More information about gill scoring can be found on Skretting s AGD poster and scorecard. A score of 5 indicates the most serious damage, where most of the gill tissue is altered and there is a higher risk of mortality. It is therefore common to treat the fish before the average gill score of the unit has escalated. AGD is treated with hydrogen peroxide or freshwater baths. This requires time and resources, and the treatment can be stressful for the fish. Skretting research has shown that the combination of several of our tried and tested functional ingredients can modulate key immune system parameters which are known to support gill structure and function. Amoebas may form runners used for movement and to catch food.
THE FUNCTIONAL FEED DEVELOPMENT PROCESS At Skretting our rigorous process of functional feed development starts by screening substances in the laboratory to establish the potential of the nutrient to either affect pathogens directly or to modulate the immune system to achieve a positive effect. As these innovative substances will be added to feed formulations the ingredients with potential are then assessed for aspects such as stability, yields and bioavailability in finished feed products. DEVELOPING PROTEC GILL The role that functional feed ingredients play in supporting farmed fish health and in turn to sustainable and economic production has been a major focus for Skretting Aquaculture Research Centre over the last 25 years. In 1992, Skretting launched its first functional feed called Response, whose ingredients were designed to strengthen the fish s immune defense systems. Since then, our researchers have discovered more functional ingredients that have been added to the feed that work synergistically to improve fish health. With these upgrades, the name of the feed was changed to Protec. Today, the Protec solution is used for a number of species worldwide, and is effective in preparing the fish for upcoming stress and challenges in farm conditions, such as handling, vaccination and diseases. With an extensive array of laboratory research, and repeated infection trials, a new feed that better supports gill health and recovery during environmental, treatment and disease challenges such as AGD has been developed. The feed is called Protec Gill and, as the name implies, it contains the full Protec package, with added components that ensure better gill health. Protec Gill has produced higher survival rates in three controlled AGD infection trials, and analysis of these results has shown that the fish fed Protec Gill had significantly lower rates of mortality.
Protec ingredients inhibit the growth of amoeba when exposed to mucus from fish fed different functional ingredients: 97 96 95 Growth Percentage 94 93 92 91 90 89 88 87 Neg. Control Protec Protec Gill Pos. Control Results show that when P. perurans amoeba are exposed to the mucus of Protec gill fed salmon the growth of amoeba is inhibited.
Protec Gill Increases Survival in fish challenged with P. Perurans 100 90 Percentage Survival 80 70 60 50 40 30 20 10 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 Days after infection Control Protec Gill Challenge trials using Protec Gill against AGD infections were replicated three times to confirm results. The fish that received Protec Gill had the lowest mortality rates every time, and showed 27.1% 20.3% and 42.4% higher survival rates.
Protec Gill supports osmoregulation in fish challenged with P. Perurans 174 173 Blood Sodium (mmol/l) 172 171 170 169 168 167 166 165 164 Control Protec Gill In seawater fish must replace the water which they constantly lose through osmosis. However, drinking seawater brings with it a large quantity of salt into the blood and this has to be removed by chloride secretory cells in the gills, which actively transport salts from the blood back into the surrounding water. Measuring blood sodium levels gives an indication of the integrity of gill structure and the efficiency of gill function. Unhealthy or damaged gills will be less efficient in removing salts from the blood. Research shows that Protec Gill fed fish are better able to maintain osmotic regulation during an AGD challenge; the principle cause of death in AGD infected fish.
Protec Gill increases the expression of antibodies Relative Expression 16 14 12 10 8 6 4 2 Control Protec 0 IgM IgT An antibody, also known as an immunoglobulin, is a large, Y-shaped protein produced mainly by plasma cells that is used by the immune system to identify and neutralise pathogens such as bacteria and viruses. Immunoglobulin M, or IgM for short, is a basic antibody that is produced by B cells. It is the first antibody to appear in response to initial exposure to an antigen. IgT is the first monomeric immunoglobulin to be shown in teleost fish. Recently discovered, IgT is produced following exposure to toxins, pathogens or other foreign substance and is predominantly found in the fish mucosal surfaces, suggesting a possible role of IgT in mucosal immunity.
Protec Gill increases anti-bacterial potential 1000 900 800 ** Iysozymes (U/ml) 700 600 500 400 300 200 * 100 0 Control Protec Protec Gill Lysozymes are enzymes that increase the rate of destruction of the cell walls of certain bacteria. Lysozyme is one of a number of immune related components occurring in fish mucus and indicates the importance that mucus plays in providing the first line of defence against a broad spectrum of pathogens. Research shows that the mucus produced by the skin and gills in Protec Gill fed fish have higher levels of lysozyme and anti-bacterial potential.
SKRETTING S RECOMMENDATIONS FOR MAINTAINING GOOD GILL HEALTH Proven functional feed solutions like Protec should be included as part of a structured approach to performance and health management and alongside other strategies such as vaccination, disease monitoring, and water quality management. When specific known gill stressors do develop, a change to Protec Gill will focus the benefits of functional nutrients to where they will have the most impact. RECOMMENDATIONS FOR PROTEC GILL USE Typical recommendations for Protec Gill use in Scottish seawater salmonid production: Before and During Gill Health Challenges Feeding Protec Gill 2 to 4 weeks ahead of known gill health risk periods and/or at the commencement of gill disease outbreaks until its resolution, will provide additional mitigation against the effects of the challenge. Before and After Bath Treatments Feeding Protec Gill for 2 weeks before and 2 weeks after Hydrogen Peroxide (H 2 O 2 ) treatments will support the regeneration of gill structure and function. Skretting is the global leader in the production of innovative and sustainable nutritional solutions for aquaculture. We deliver high quality feeds and services to farmers around the world, so that they can produce healthy and delicious food in a sustainable manner. Skretting produces feed in 18 countries and has an annual production of 2 million tonnes of high quality feed to a wide range of fish and shrimp species in all life stages. www.skretting.co.uk