Food Science Universe (FSU)

Factors Affecting the Quality of Fish


Fish is eaten in almost every part of the world. The nutritional value of fish is very good especially the presence of polyunsaturated fatty acids makes it best for human consumption. It biological value of protein is considered best in all muscle foods. Other than nutrition some cultures eat fish as a staple food. So, it is very important to provide fish and fish-related products of high quality. It is only possible if the quality of fish maintains throughout the production chain means from farms or any capture area until consumer consumption. If products are made on a high level of machinery but their initial quality is not good, then they will be not processed according to the desire of the processor and consumer. Quality is taken as a broad term for food products. Its impact is very high to make successful of any product. Fish is a perishable food, and its quality maintenance is very complex. This report is written about some major factors that can alter the quality value of fish regarding nutrition, economy, shelf life, and sensory characteristics. Effect of feeding on different nutrients and their effect on further processing included in the report. It is also included in this report that slaughtering requires special care to get rid of spoilage microbes and ease further processing. Other factors like environment, primary processing steps, and packaging are all described briefly in this report. It is also needed to know that quality factors are not only the responsibility of one person. It should be maintained by every stakeholder of the process.


Quality is a broad term, influenced by many situations and concepts. It covers freshness, appearance, safety, degree of spoilage, presence of chemicals, and parasites of food and food products. Its definition varies according to consumer requirements. Generally, it is defined as the acceptability of food or foodstuff is called food quality. According to International Standardization Organization (ISO), food quality is food without defects, adulteration, and fraud. They also added that it also covers the need of customers and their expectations about the product. Quality is a major influencing feature to buy products. According to the European Commission website, 96% of buying influence is dependent on product quality (European Commission). Fish is a popular food and use as a staple food in many areas of the world. It is the only source in many areas to get daily life protein levels. Many countries rely on fish for the omega-3 polyunsaturated fatty acid requirement that can be very helpful against many human diseases and also a rich source of minerals and vitamins (Zsuzsanna et al., 2011). So, that is why quality fish products are very important for everyone. Its high level of processing and marketing is only achieved by using good quality fish and maintaining the quality during different processing procedures. The customer only attracts those companies who provide the best quality products consistently.

According to FAO, it is estimated that 25% of fish catch loss occurs due to poor postharvest quality management. It is a huge amount and can be fatal for any type of industry. This loss is not all but can be minimized by just putting some quality standards during the processing chain. It is important for this to identify major factors that can change or destroy the quality of fish. This report is comprised of those factors that can influence the quality of fish. Fish quality is different for different consumers. Some need high-fat content; some need lean fish, and some require good color with full nutrients. So, it is very important to provide fish according to desire quality attributes.

In many cases, fresh fish is considered as best for the end product, but in the case of smoked fish slightly older fish are considered good in quality because the filleting and smoking process becomes easier and quicker (Huss, 1995). Processing also alters the quality attributes. Different procedures have different effects on the product. It is necessary to maintain the quality of the entire chain of processing from farm to fork. Fish nutrition, a big quality attribute can be affected by the feeding of fish. Feed will help to develop different prospects like lean fat ratio, minerals contents, and feed also affect the sensory properties like color. handling and environment are also affecting the quality of fish and fish can become hazardous due to the presence of microbes in it which can come from the environment and the utensils in which fish are handled while transportation. Slaughtering is another step where quality can be affected. Sometimes consumer only uses sensory evaluation to buy fish. Consumers assume freshness mostly by naked eye evaluation. Just like if farmed salmon looks good in color, smell, and texture they considered it a good quality product. Same as this if salmon looks pale and soft it is considered to be fatter fish. The texture is also important regarding the gapping problems, rigor mortis, and other postharvest changes. It is also considered a quality attribute. in this report, different important factors were discussed which affect the quality of fish and fish products.


Feeding is the most important factor that determines the fish quality regarding different aspects like, nutrition value, color, etc. It is very important to provide feed according to the species and required results. The nutrient value in fish affects the composition of fish. Protein a major part of fish feed can influence the composition of fish. If the feed is high in protein it will increase the protein level of the muscles of fish and it increased the weight of the product (Shyong et al., 1998). In another study which is done on Lepomis cyanellus it is noted that a high level of protein can increase the body protein in fish (Webster et al., 1995). Dietary protein can increase growth up to a certain level after this protein intake goes to store as fat after the deamination process (Shaerer, 2001). The amino acid of the whole body did not change in composition due to the source of protein it remains unaffected (Schwarz and Kirchgessner, 1988). So, if the feed contains more protein its growth is high but the fat content of viscera and carcass also becomes high due to deamination or extra protein (Shaerer, 2001).

Fat is the next main component nutritionally can be affected by the feed. It is believed that fat is a big and important parameter for farmers, manufacturers, and customers. That is why before catching starvation applied to reduce fat levels in fish as farmers can easily increase the weight with the addition of fat in the diet (Morris, 2001). The fat content of fish is directly proportional to the fat content they have in the diet as like any other animal group (west and York, 1998) but the storage location of fat in the body is different for different species. It is also reported in a study of Atlantic salmon that if dietary fat is high then it affects the visceral fat more than the fat in fillets (Hillstad et al., 1998). The study of Atlantic halibut shows that the fat content of the fish increased with the size of the fish and dietary fat level (Nortvedt and Tuene, 1998). The fatty acid profile in fish also gets changes with the fatty acid present in the feed. Salmon shows the same fatty acid profile as present in its feed and this factor also affects the organoleptic properties of fish flesh (Bergstrom 1989). The moisture content of the fat is also very related to fat content. Moisture content reduces with an increase in fat deposition in the fish bodies. The relationship between fat and moisture is very reliable to determine the fat level of farmed fish (Shaerer, 2001). Carbohydrates are not a big concern in fish feed as fish have very less amount of carbohydrate content. Its effect on the carcass is very minor, some time excess carbohydrates can increase the level of visceral fat (Bergot, 1989). A high level of fiber can increase the somatic index by increasing the size of the fish stomach and hence yield will be reduced(Hilton, 1983). If carbohydrates are absent in the diet their requirement can be fulfilled by gluconeogenesis with the help of amino acids. But carbohydrates are still useful as a cheap energy source and starch is also used for the gelatinization process in feed (Morris, 2001). Mineral content is also influenced by feed as if the feed is without essential minerals it will affect the nutrition level of fish (Schwarz, 1995). It is believed after a study by Skonberg et al.(1997) that if the diet is without phosphorus then it can increase the fat content of the whole body. The absence of vitamins also disrupts energy partition and deposition of fat can be increased. Minerals like selenium and chromium also have a good impact on flesh quality (Morris, 2001).

Feeding also can influence the color of flesh. Carotenoid pigments are needed in farmed salmon to look as same as wild salmon flesh. For this purpose, astaxanthin and/or canthaxanthin are mixed in fish feed which will provide the same orange-red color to fish fillets. Wild salmon gets these from natural sources like seaweed (Baker, 2001). So, the addition of minimum astaxanthin in fish feed is very important. Roche SalmoFan is an international standard color assessment tool and Nickell and Springate (2001) provided some stats about minimum levels of pigments require in feed to achieve the Roche Salmofan scale. Table.1 describes those levels.

Tab.1: Minimum astaxanthin level requirement to reach target color (Nickell and Springate, 2001).

These are all feeding effects on fish and will also affect the products. Selection of true feed with good feeding time intervals will help to achieve good quality products.


The surroundings of fish also affect their quality. Although copper is good for fish as a micronutrient if the pipe system is made up of copper it can increase its amount in fish and it will become toxic. The same happened for iron and zinc. Other elements that can affect the fish are the presence of mercury, lead, and cadmium in water sources. These elements are not essential for fish, but they can mix in water through wastewater, and other pollution factors (Zsuzsanna et al., 2011). Polluted water can also be a carrier of different pathogens like salmonella and listeria (Laidler, 2001). So, it is very important to make sure that the fish environment is well designed and made up in a protective way to eliminate the chance of these chemical and biological hazards. It is also required to implement the strict rules about pathogens and chemical hazards set by different organizations.


Handling fish prior to slaughtering is a very important factor that can influence the quality of fish. fish starvation, prior to slaughtering is needed to plan according to guidelines. Mostly nowadays 2-3 days of the starvation period observed depend on the water temperature and to ensure how to log fish will take to empty its gut. Two reasons behind the emptying of the gut, the first are it will lower down the level of food spoilage organisms and the second one is that by this process oxygen levels remain high in fish (Wall, 2001). Starvation makes changes in the physical structure of fish. fish become thin and its head size increased (Lie and Huse, 1992). The skin color also becomes darker due to starvation (Roberts and Bullock, 1989).

A longer period of starvation can also affect the quality of fish. during starvation fish use deposited fat as fuel for metabolic rates (Love, 1980). It is also needed to set the stocking density with limits. Wrong density selection will cause a lot of problems in fish quality. Fish can get stress and this factor will lower down the quality of fish. it can also cause abrasion on the flanks, tails, and snout. If seals are there near fish cages, they can also attack fish and can cause injury or killing of fish, it mostly happens in sea cages (Wall, 2001). So, stocking density requirements are also important to keep in control. Then the next thing to consider is that if the slaughtering place is away from the cages then it is necessary to keep things according to guidelines. Toeing is used for cages mostly, but it is important to toe the cage in not very fast speed mode and not against the tides. It can cause damages to fish as fish can be crowded at the back of the cage. It can also increase the stress levels in fish. The well-boat transfer is another method to transport fish to a required slaughtering area. This method is now becoming more acceptable in fish industries. Transportation methods should be chosen by keeping in view the effect of the use of pumps, diseases factor, and water quality where to transfer. It is also important to provide rest to fish prior to slaughter. The flesh of fish that rested before slaughtering always shows good quality (Wall, 2001).

Before slaughtering, fish is delivered to the slaughtering/killing area by crowding with the help of a pump, braille, or airlifting system. These systems can be harmful to fish if not maintained according to guidelines. Lack of oxygen can kill the fish in this procedure. Too much crowding can decrease oxygen too early. Organic matter on dirt nets also lowers down the oxygen level.  This oxygen reduction cause excitability in fish and this excitability can cause abrasion, skin ulceration, eye, and snout bruising. Regularly monitoring the oxygen level in this step will help to maintain the quality of fish. According to Wall (2001) it is experienced that one of the biggest factors in poor flesh quality is crowding damage. So, it is necessary to use modern ways of fish transport systems to avoid any damage to fish quality. Also, keep an eye on and reduce vigorous fish muscle activities, because it can become a quality reducer effect. Monitoring of suitable stress indicators and oxygen levels should be checked continuously during transportation (Erikson, 2001).


Slaughtering is a point where the animal turned into meat and this step affects many quality characteristics for example color, a chief sensory quality factor, starts changing right after slaughtering. Mostly, when slaughtering is discussed regarding quality it means ethical and fleshes quality effects.

Different types of killing methods are used in fish industries. The oldest method of fish-killing is death in air killing. In this method, fish are exposed to air in absence of water, in this way its gills stopped working, and fish are killed. Fish try to escape in this process and use all the oxygen very fast hence its brain death is done quickly. High temperature also decreases the death time in this method (kestin et al., 1991). The ice slurry method is also used to kill fish. The time remains high in this method, but fish activity is less in this and reduces the stress rate. Exsanguination, this method is mostly used for large size fish. The sharp knife used to cut the gills and bleeding will help in the death of fish. it can take 5 minutes minimum before death.

Direct evisceration is also used as a killing method. This method is applied mostly for wild-caught. Viscera removal helps in bleeding and fish will die from anoxia. Anesthesia is added in water which has fish and it makes fish unconscious. Fish need 10-30 minutes for anesthetization. But this method is not being used in many countries due to the use of chemicals (Robb, 2001). Carbon dioxide addition in water is also used and it works as acid. It will lower down the pH of fish blood. It took 5 minutes to take brain death in salmon during the carbon dioxide method (Kestin, 1995). Electric shocks, percussive stunning (hard shot on the head), and spiked with a sharp tool in the head are also used for killing methods.

These all methods are divided into fast and slow methods. The slow methods are allowing fish to move more, and the fast methods kill the fish as soon as applied. The first effect of slaughtering will show on the muscles. Fish have red and white muscles and use them for movements. Muscles contain proteins and if the movement of fish does not regularize, lactic acid can be produced. During normal movement fish used red muscles but when their activity increases it starts to use white muscles and anaerobic respiration starts in fish which produces lactic acid. This will low down the pH of the flesh and flesh quality decrease will be shown clearly. Drop-in pH not only affects the muscles but also affects the color of flesh and lowering pH can cause loss of the desired color in fillets (Robb,2001).

Sensory effects on fish flesh due to slaughtering methods investigated by Sigholt et al. (1997) in which it is identified that stress during slaughtering can make texture hard and lower the odor. This character also makes the fish hard to process. So, the selection of slaughtering method according to further processing and product type is very important to maintain quality in the production chain.

Primary Processing

Hygiene control is very important to start the processing fish. slaughtered fish have blood which is the best food for microbes. Farmed fish reared in a good environment have less bacterial load but if it has an abrasion on the skin these bacteria can invade in muscles and will grow there rapidly. The chance of parasites in farmed fish is less than wild fish. Autolysis also can start, and fish will lose special odor. So, loss of odor or even spoilage can reduce by controlling temperature if further production is delayed. The temperature control mostly depends on the species, size, and conditions of the farm. Like if fish is caught from high-temperature water it will take more time to get chilled (FAO 1973).

Evisceration is almost done in every fish immediately after slaughtering before marketing. This process is very important to reduce enzymatic activity. Hygiene control is applied during this process as it is believed that most of the microbes remain on the outer side of fish, and if hygiene is not controlled these microbes can penetrate into the flesh. Actually, flesh from healthy fish is sterile right at the time of slaughtering because of the immune system present in fish (Rørå et al., 2001). This process also helps in the removal of parasites from fish because most of the parasites are present in the intestines of the fish, even in some countries it is compulsory to remove evisceration to get rid of nematodes (Chory, 1988). If the fish feed actively and remain un-eviscerated, it will spoil much earlier even if kept in a cool area(Meyer et al., 1986). Another advantage of evisceration is that it will reduce the chance of belly bursting which is caused by the autolysis of un-eviscerated fish. belly bursting produces a very bad flavor in flesh (Shewan, 1961). The chilling process also takes less time in eviscerated fish (Price et al., 1991). This process can be mechanical or handheld. But most of the industries now doing it in a mechanical way. Mostly evisceration is done by cutting the belly portion in this way that the muscle part is not affected by cutting. The cut is done in the fat-rich portion which helps to avoid muscle damage and the bacteria present on the outer side of fish skin also does not penetrate in muscles easily. Some machine removes the viscera without cutting the belly part. Rinsing and cleaning of belly portion by mouth side. These machines remove the viscera by sucking it through moth. This method looks good that no cutting requires but it is difficult to handle the quality of this procedure. It is very important to protect the membrane on the muscles which protects them from bacterial invasion, but this type of evisceration or gutting can make it difficult to prevent it (Rørå et al., 2001).


Filleting is the most important step in reading marketing and flesh yield quality. Filleting also affects the economy, transportation, and consumer satisfaction. Most of the flesh loss occurs in this procedure if not handled correctly. The filleting process can be done by using machines and also done by hands. The skilled worker can do filleting with good efficiency but still, it is risky and hard work to do. The loss of flesh can increase in the hand fillet method. That is why most of the companies started this process with machines. Sometimes it is required to produce flesh with no bones in the muscles, these fillets are called v-shaped fillets, and this will also cause extra loss of flesh. Machines also can produce specific shapes and if any other shape is tried to produce the yield of flesh will decrease. It is noted that if filleting is done after the resolution of rigor mortis it can be done more easily and correctly mostly after 2-4 days (Rørå et al., 2001). But according to Shaw et al.(1984) fillets produced after 7 days of post-mortem showed more shelf life increase as compared to 1 and 4 days. If filleting is done pre-rigor then pin-bones removal will be difficult due to because of tightness of flesh. Also, the characteristics of pre-rigor fillets like texture, color, and cooking properties will be different (Sørensen et al., 1997). These different characteristics will lose the image of the product in the market. This effect also disturbs the product to take up salt and salt distribution in flesh during the salting process if required (Fennema 1990).

Another problem associated with fillets is fillet gaping. These fillets are hard to process and also not acceptable in the market. It can be happened due to slow cooling, not good handling of fish, season, slaughtering of unrested fish. by applying good primary processing we can decrease fillet gaping (Jerrett et al., 1996). It is also needed to use sharp knives and blades to overcome the fillet gaping problems. The primary processing step is very important, it converts the fish into processable flesh and also increases the shelf life of the product. It also allows marketing the of fresh product which will attract the consumer. During farming, it is possible to alter some characteristics of the fish, but primary processing helps to get the desired characteristics according to marketing and further processing (Rørå et al., 2001).

After filleting, if fish is stored before further processing or marketing it is required to ensure that its oxidation factor should be in control. Lipid oxidation is an autocatalytic procedure and can start in fish by enzymes, light, by any initiator. First hydroperoxides are produced in oxidation which is a tasteless and odorless compound. Then it will convert into ketones and aldehydes if the process is not stopped and favorable conditions easily available. These compounds have little smell but this smell is responsible for rancid odor further (Milo and Grosch, 1993). Sometimes this process goes further, and ketones and aldehydes react with amino acids and affect the color and texture of the products. This process happened in the presence of lipids, oxygen, pro-oxidants like metals (iron, copper, etc.). These elements increase the rate of oxidation. It is necessary to freeze the product as soon as possible because it will suppress the oxidation process. It should not exceed two days. If needed to store for long period, then the skin of fish should not remove from high-fat areas. If fish is minced, then it should be kept in large blocks so that its surface area will be reduced. Washing after filleting if done, then it should be done as quickly as possible and also use antioxidants in the water. Place the product away from the light because more light initiates the oxidation chance high. Mild heat treatment for the inactivation of enzymes is another option to reduce the effect of oxidation (Undeland, 2001).

Other Processing Operations

Salting is the process mostly done in fish processing for preservation purposes. It is the oldest method of preservation. Dry salting and brine solution can be used for this process and it will affect the nutrients and sensory quality. Moisture, as a major favorable component for spoilage-causing microbes, can be reduced by applying brine or dry salt. It is reported by Hafez et al.,(2019) that moisture content of anchovy drop-down from 75.5% to 54.16 when salt applied. It is also needed to know that if the salt concentration increased the water reduction also increased. Salting can reduce the level of water-soluble vitamins as water is removed from fish. Salted fish have a frisk texture and are easy to break. Taste also gets changed but mostly smoking is applied after salting which makes the taste acceptable.

Smoking is another processing step that can be done on fish products. Smoking develops a specific aroma in fish, and it is a very popular method for many fish products. It is very necessary to control the temperature during smoking because it can produce different carcinogenic compounds in fish which can be classified as chemical hazards. Smoke can be applied in two ways cold smoke and hot smoke. Cold smoking shows a better result to reduce the production of carcinogenic compounds and also provide better smoking taste. The selection of wood is also very important for smoking because it also alters the aroma characteristics.


Fish is a perishable product. Packaging provides it a suitable preservative so that its shelf life increased almost more than 100 times. These days mostly modified atmospheric packaging (MAP) technique is used for fish and fish products (Day, 2001). Different gasses or mixtures of gasses are used in this technique. Oxygen, carbon dioxide, nitrogen, or vacuum are used for packaging. MAP is depending on the type of packaging material being used and the type of product. The targeted spoilage organism is also fallen in the list of factors that will be affected by the gasses in the package. If the combination of gasses or vacuum is not according to requirement it can cause problems with the product. Like if oxygen will add to any package it will increase oxidation of the product and can not control the spoilage of the product. It is also important to provide temperature according to the need of the product even after packaging in MAP. Leaking check test should be performed regularly during packaging to ensure that package is sealed properly (Noseda et al., 2013).


Quality is very important to maintain throughout the supply chain. It is the responsibility of every stakeholder to perform its duty to ensure good quality products. Fish is a perishable food, and its quality is also very fragile. It can affect every step in the fish line from farm to fork. Every farmer or industry that rare or processes the fish needs to work on guidelines set by the different organizations. Farmer should choose the feed according to the fish species, next market, and temperature of the environment. Same advice for industry or any processor that without maintaining quality continuously no one can capture the market.


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  • (date 26/11/2020).

Submit To: Odd-Ivar Lekang (Associate Professor, Norwegian University of Life Sciences NMBU, Norway)

Submit By: Muhammad Umar (Norwegian University of Life Sciences NMBU, Norway)

Student No: 110155

Course: AQF200- Aquatic Food Primary Production: Fishery and Aquaculture

Study Program: AQ Food (Safety and Quality)



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