Fish farming

Fish farming is the practice of raising and killing fishes and other animals (such as crustacea and amphibians), mainly for food. Fish farming has been growing significantly for many decades. Between 1970 and 2006 the industry grew at a rate of 6.9% per annum,1 and in recent years close to half of the marine animal products eaten by humans has been farmed fish.2 These fishes are also used to feed other animals: more than 2.5 millon tons of fishes are used to produce cat food every year.3

It has been estimated that between 37 and 120 billion fishes are killed every year,4 which does not include the other sentient animals who are also killed in aquatic farms, either by being raised there for human consumption or by being fed to other animals. Crustacea raised on fish farms are commonly fed molluscs which have been passed through grounding mills that destroy their shells, as well as fishing subproducts including rests of fish.

Many species of fish are bred in farms; however, some are bred in greater quantities than others. The most prevalent are carps, tilapias, sturgeons, salmons and catfishes.5 As for crustacea, most species of them cannot be raised in farms due to their small size and to diseases they suffer in fish farms. Those that are bred in farms are pacific white shrimp and giant tiger prawn (Penaeus monodon).

Those who defend fish farming say that it will solve the problem of the scarcity of fishes and other aquatic animals due to fishing. This defense does not take into account the fishes’ capacity to suffer or their interest in staying alive. It considers the benefit to humans of the exploitation of aquatic animals. The aim of fish farming is to achieve the maximum production of fishes and other animals for consumption at the lowest cost. This leads to a disregard of the interests of the exploited aquatic animals, resulting in uncomfortable or miserable lives and early, often painful deaths.

It is not possible to keep animals in fish farms without causing them harm. Fishes are routinely moved out of water to be measured, their tanks are cleaned with harmful chemicals, and their lives are made generally unpleasant by manual handling and habitat disturbance. Furthermore, by definition, raising animals in farms for consumption means they are eventually killed.

Fishes on fish farms can be grown in natural ecosystems (lakes, rivers, or oceans) or in fish farm tanks. There are three main types of fish farm, defined by the ways the animals are raised: extensive, semi-intensive and intensive.

In extensive systems the animals get food from their environment and are not fed by humans; humans control only the environment where the animals are kept. Populations are controlled by manipulating environmental variables such as nutrients, light and the condition of the water. The fishes are kept in a manner that prevents escape and allows for easy capture. The capture of these fishes is sometimes referred to by terms such as “collecting” and “harvesting,” words whose use is euphemistic and inappropriate since they are generally used only for non-sentient plant life.

In semi-intensive systems fishes are in a semi-controlled environment. Part of their food comes from farmers while the rest comes from the environment. Other variables in their environment are also managed, such as the circulation of water. This allows fishes to be raised in higher densities than is possible in extensive fish farms, which as we will see below causes discomfort, illness, and injury.

Finally, in intensive farms the living conditions of fishes, feeding and reproduction are completely under human control. Fish density is very high in intensive farms.

In addition to current fish farms, research is currently underway to convert larger natural bodies of water, which are closed or almost closed off from other water systems, into massive extensive or semi-intensive farms.

How crustaceans are bred

Crustacea egg stocks are increased by various captive breeding techniques One method, in which females are captured, involves administering thermal shock, which induces them to lay their eggs.

These animals can lay several hundreds of thousands of eggs, which can hatch in as little as one day. Another method of captive reproductive technique involves capturing (“collecting”) larvae. Larvae are kept in deposits in hatcheries where the circulation of water is controlled. After 2-3 weeks, they become postlarvae and are carried to bigger deposits with open circulation of water, called nurseries, where they spend between one and one-and-a-half months. When the weight of the postlarvae reaches between 1 and 2 grams, they enter the pre-fattening stage of their life in captivity, and are transported to “fattening ponds” for consumption. Although the processes of breeding and fattening the animals is often carried out in a single facility, there are specialized companies that use multiple facilities for captive reproduction (known as “nursery farming”). Fattening ponds may be located in an intertidal area, with mesh barriers that allow water to circulate.

Crustacea can also be bred in tanks of water with floodgates that allow for new water to come into the tanks from a sea, lake, or river. They are later transferred to a fattening pond. Many postlarval shrimps die during this process. The survivors in the fattening ponds are captured from the ponds several months later by nets or by draining the ponds.

How fishes are bred

As with crustacea, there are several stages in raising fishes. First, fries (young fishes) are commonly bred in captivity, although they can also be captured. Adult fishes of reproductive age may also be captured, but they are often (and increasingly) bred and raised in captivity too. Some fishes, such as eels, are always captured in the wild because it is not possible to breed them in captivity.

In order for fishes to reproduce they must be in low-stress environments. The breeding animals are kept in tanks with much lower densities of animals than the ones in which they are kept for growth (fattening). The room in their enclosure is minimal and can be as little as one m3 of water per fish. Fishes used for breeding are sometimes allowed to reproduce at their own rhythm, but they are often induced to lay their eggs.

Inducing egg laying can be done with a variety of hormones, such as injections of gonadothropins or human chorionic gonadotropin (which can be obtained from women’s urine.

Sometimes laid eggs are easily collected, because fertilized eggs float while unfertilized eggs sink. In other cases, the collection of eggs is carried out by a technique euphemistically called “abdominal massage.” Simply, the abdominal area of the fish is pressed until the eggs are forced out of the body, a method which is extremely stressful and harmful to their health. In some cases, an artificial catheter is used with this process. The catheter is introduced through the urogenital opening into the body cavity of the female in order to open the ovarian funnels. Then, the abdominal pressing is used to push the eggs into the catheter from which they fall into a receptacle.6

After the eggs are collected, they are kept in hatcheries for several days until the larvae come out of the eggs. The larvae are then carried out to larvae deposits, which are commonly small cylindrical tanks with a constantly renewed supply of water. The primary reason it is done is simply because huge numbers of larvae would die if it were not done. The more larvae that survive, the greater the profit.

Once these animals develop from larvae to fries and their weight is about one or two grams, they are either moved to bigger, pre-fattening, tanks, sold to other aquaculture businesses, or released in the wild to be fished out later. The pre-fattening process has the purpose of acclimating fish to the kind of food they will be given during the fattening process and to the crowded conditions they will be forced to endure. In some cases, the fishes must also adjust to a switch from fresh to salted water.

During all stages of fish development, the normal growing process is affected by crowding, which alters their normal development in ways that can be harmful.7

When their size allows the fishes to be moved without the risk of many deaths during the move, they are transported to fattening tanks.8 In fattening tanks, animals often compete for food, so food must be provided to them regularly and in small amounts so the stronger fishes don’t eat it all and leave the weaker to starve.

How animals suffer in fish factories

Fishes in fish farms are harmed in numerous ways. As with land animals, even if their lives were fine, they would still be harmed by premature death, which deprives them of potential positive experiences in the future. But they are also harmed because they have a poor quality of life. Some of the causes for this are:

Handling and transportation

Transporting animals to fish factories inflicts great psychological stress on them from which it takes a long time to recover.9 Physical agitation triggers the symptoms of stress,10 and makes the animals more susceptible to disease.11 It has been established, for instance, that stressed fishes suffer more from white spot disease.12

Lack of space and crowded conditions

In fish factories, fishes are commonly crowded in tiny spaces. This happens systematically in the case of trouts and salmons,13 seabass,14 seabram,15 or gilt-head bream.16 Not having space to move around and having so many other animals around cause stress.17

The relationship between fish concentration and harm suffered is not necessarily linear. In the case of salmons, for instance, the negative effects can be seen only once a certain density has been reached, and then the increase in negative effects may be greater than the proportional change as new individuals are added.18 In addition to the stress of crowding, other factors such as decrease in water quality add to their stress and discomfort.19 Crowded conditions also affect the availability of oxygen. Fishes depend on oxygen dissolved in the water, and when the oxygen level falls below certain levels, they can suffer great stress and health issues. In extreme cases, they can die from asphyxiation.

The effect that lack of space has on certain fishes is much worse for those animals that form social hierarchies20 and can lead to aggressive behavior21 including cannibalism.22

Light disturbance

Artificial lightning, sometimes created by underwater lamps, can be used to speed up fish growth.23 This is done in particular in hatcheries by reducing the sleeping time of the fries and providing them with more time to feed.In species such as salmonids, this changes the animals’ maturing time, so they are bigger when they are killed. But the bright lamps can disturb them and even affect their feeding habits as they try to avoid the lamps.24

In the case for common salmon, light alteration and high temperatures have been both identified as the main factors causing vertebral deformities.25

Hunger

Hunger and malnourishment can occur at several stages of the growth of animals in fish factories, for reasons such as the competition for food between the animals. Apart from hunger, there are other ways lack of food harms animals. For instance, the deprivation of food also means an increase in dorsal fin erosion in trouts fin,26 which can cause difficulties in swimming and reduce the chance of survival. It has also been observed that Atlantic salmons swim more slowly and make less of an effort to feed themselves when they are not properly fed.27

Harm to the health of farmed sea animals

The situation presented above causes these animals stress, which leads to further harm, since that compromises their health.28 But there are other reasons why their health is wrecked. Animals often suffer wounds due to overcrowding, which can easily lead to infections. The close contact between fish bodies and their cages as well as the bodies of other fishes leads to abrasions, which can also easily become infected.

Chemical variations in the water, which can easily occur due to the overcrowded environment, can cause the animals to be particularly sensitive to illnesses they might not otherwise contract. Sometimes these diseased fishes are killed.

For more information on this see our page on the diseases of fish and crustacea.

To prevent against infection and mass death, animals in fish farms are given antibiotics, many of which have negative side effects, including suppressed immunity.29 Some of the antibiotics increase stress.30 It is also important to note that both the diseases and the antibiotics affect not only the animals kept in fish farms, but also others living in the wild in surrounding areas.31

Death at fish farms

Due to all the reasons we have seen above, death rates before slaughter are very high in fish farms.32 But of course they all die early deaths whether by illness or at the hands of humans. Fishes and other sentient aquatic animals are killed in different painful ways, in most cases while they are totally conscious. Their suffering starts before their deaths, since they commonly are in pain and distress while they are being transported to the place where they are killed.33 In addition, they are often starved before their deaths. It takes time for food to be digested and assimilated into the body to create more flesh, and any food given to animals shortly before their deaths will not be converted into new flesh. It is often considered a waste to feed the animals any food that won’t become new flesh, so they are not fed and go hungry before they are slaughtered.34

Other animals killed to feed animals in fish farms

It is also important to note that other animals (mainly crustacea and fishes) are used to feed those that are raised in aquatic farms. Therefore, these animals are also victims of the human consumption of fishes and other aquatic animals. In addition to feeding animals in fish farms with the bodies of other fishes, more than half of the fish fat production from fishes captured or grown in farms is used to feed salmons.

Further readings

Acerete, L.; Balasch, J. C.; Espinosa, E.; Josa, A. & Tort, L. (2004) “Physiological responses in Eurasian perch (Perca fluviatilis L.) subjected to stress by transport and handling”, Aquaculture, 237, pp. 167-178.

Alanara, A.; Winberg, S.; Brannas, E.; Kiessling, A.; Hoglund, E. & Elofsson, U. (1998) “Feeding behaviour, brain serotonergic activity levels, and energy reserves of Arctic char (Salvelinus alpinus) within a dominance hierarchy”, Canadian Journal of Zoology, 76, pp. 212-220.

Andrew, J. E.; Holm, J.; Kadri, S. & Huntingford, F. A. (2004) “The effect of competition on the feeding efficiency and feed handling behaviour in gilthead sea bream (Sparus aurata L.) held in tanks”, Aquaculture, 232, pp. 317-331.

Ashley, P. J. (2007) “Fish welfare: Current issues in aquaculture”, Applied Animal Behaviour Science, 104, pp. 199-235.

Bell, A.; Bron, J.; Turnbull, J. F.; Adams, C. E. & Huntingford F. A. (2002) “Factors influencing the welfare of farmed Atlantic salmon (Salmo salar) in commercial marine cages”, Research in Veterinary Science, 72 (Suppl. A), pp. 7-8.

Borgatti, R. & Buck, E. H. (2004) Open ocean aquaculture: CRS report for Congress, Congressional Research Service, Washington: Library of Congress.

Chandroo, K. P.; Yue, S. & Moccia, R. D. (2004) “An evaluation of current perspectives on consciousness and pain in fishes”, Fish and Fisheries, 5, pp. 281-295.

Chiua, A.; Lib, L.; Guob, S.; Baib, J.; Fedora, C. & Naylora, R. L. (2013) “Feed and fishmeal use in the production of carp and tilapia in China”, Aquaculture, 414-415, pp. 127-134.

Cutts, C. J.; Metcalfe, N. B. & Taylor, A. C. (2002) “Fish may fight rather than feed in a novel environment: Metabolic rate and feeding motivation in juvenile Atlantic salmon”, Journal of Fish Biology, 61, pp. 1540-1548.

Devlin, R. H.; d’Andrade, M.; Uh, M. & Biagi, C. A. (2004) “Population effects of growth hormone transgenic coho salmon depend on food availability and genotype by environment interactions”, Proceedings of the National Academy of Sciences of the United States of America, 101, pp. 9303-9308.

Ellis, T.; North, B.; Scott, A. P.; Bromage, N. R. & Porter, M. (2001) “What is stocking density”, Trout News, 32, pp. 35-37.

Erikson, U.; Gansel, L.; Frank, K.; Svendsen, E. & Digre, H. (2016) “Crowding of Atlantic salmon in net-pen before slaughter”, Aquaculture, 465, pp. 395-400.

Esteve, C. & Alcaide, E. (2009) “Influence of diseases on the wild eel stock: The case of Albufera Lake”, Aquaculture, 289, pp. 143-149.

Farm Animal Welfare Council (FAWC) (1996) “Report on the welfare of farmed fish”, fawc.org.uk [accessed on 14 May 2013].

Hart, P. J. B. (1993) “Teleost foraging: Facts and theories”, in Pitcher, T. J. (ed.) Behaviour of teleost fishes, 2nd ed., London: Chapman & Hall, pp. 253-284.

Håstein, T. (2004) “Animal welfare issues relating to aquaculture”, in World Organisation for Animal Health (OIE) Global Conference on Animal Welfare: An OIE Initiative. Proceedings, Luxembourg: Office for Official Publications of the European Communities, pp. 219-231 [accessed on 17 May 2013].

Huntingford, F. A.; Adams, C.; Braithwaite, V. A.; Kadri, S.; Pottinger, T. G.; Sandøe, P. & Turnbull, J. F. (2006) “Current issues in fish welfare”, Journal of Fish Biology, 68, pp. 332-372.

Johnson, S. C.; Treasurer, J. W.; Bravo, S.; Nagasawa, K. & Kabata, Z. (2004) “A review of the impact of parasitic copepods on marine aquaculture”, Zoological Studies, 43, pp. 229-243.

New, M. B. (2002) Farming freshwater prawns: A manual for the culture of the giant river prawn (Macrobrachium rosenbergii), Rome: Food and Agriculture Organization of the United Nations [accessed on 23 February 2013].

Robb, D. H. F. & Kestin, S. C. (2002) “Methods used to kill fish: Field observations and literature reviewed”, Animal Welfare, 11, pp. 269-282.

Rose, J. D. (2002) “The neurobehavioural nature of fishes and the question of awareness and pain”, Reviews in Fisheries Science, 10, pp. 1-38.

Skjervold, P. O.: Fjaera, P. B.; Ostby, P. B. & Einen, O. (2001) “Live-chilling and crowding stress before slaughter of Atlantic salmon (Salmo salar)”, Aquaculture, 192, pp. 267-282.

Sneddon, L. U.; Braithwaite, V. A. & Gentle, M. J. (2003) “Do fishes have nociceptors? Evidence for the evolution of a vertebrate sensory system”, Proceedings of the Royal Society London B, 270, pp. 1115-1121 [accessed on 20 June 2014].

Soderberg, R. W.; Meade, J. W. & Redell, L. A. (1993) “Growth, survival, and food conversion of Atlantic salmon reared at four different densities with common water quality”, The Progressive Fish-Culturist, 55, pp. 29-31.

Southgate, P. & Wall, T. (2001) “Welfare of farmed fish at slaughter”, In Practice, 23, pp. 277-284.

Tacon, A. G. J. & Metian, M. (2009) “Fishing for aquaculture: Non-food use of small pelagic forage fish – A global perspective”, Reviews in Fisheries Science, 17, pp. 305-317.

Wedermeyer, G. A. (1997) “Effects of rearing conditions on the health and physiological quality of fish in intensive culture”, in Barton, B. A.; Wedemeyer, G. A.; Pankhurst, N. W.; Kraak, G. Van der; Sumpter, J. P.; McDonald, G.; Milligan, L.; Schreck, C. B.; Potinger, T. D.; Pickering, A. D.; Balm, P. H. M.; Fletcher, T. C. & Morgan, J. D. Fish stress and health in aquaculture, New York: Cambridge University Press, pp. 35-71.

Wendelaar-Bonga, S. E. W. (1997) “The stress response in fish”, Physiological Reviews, 77, pp. 591-625.

Notes

1 Bostock, J.; McAndrew, B.; Richards, R.; Jauncey, K.; Telfer, T.; Lorenzen, K.; Little, D.; Ross, L.; Handisyde, N.; Gatward, I. & Corner, R. (2010) “Aquaculture: Global status and trends”, Philosophical Transactions of The Royal Society B: Biological Sciences, 365, pp. 2897-2912.

2 A study on the importance of fish farming stated in recent years: “[a]quaculture contributed 43 per cent of aquatic animal food for human consumption in 2007 (e.g. fish, crustaceans and molluscs, but excluding mammals, reptiles and aquatic plants) and is expected to grow further to meet the future demand.” Ibid.

3 De Silva, S. S. & Turchini, G. M. (2008) “Towards understanding the impacts of the pet food industry on world fish and seafood supplies”, Journal of Agricultural and Environmental Ethics, 21, pp. 459-467.

5 Food and Agriculture Organization of the United Nations (2011) Fisheries and aquaculture statistics: Aquaculture production, [Rome]: Food and Agriculture Organization of the United Nations [accessed on 11 January 2013].

6 Szczepkowski, M. & Kolma, R. (2011) “A simple method for collecting sturgeon eggs using a catheter”, Archives of Polish Fisheries, 19, pp. 123-128.

7 Moreau, D. T. R. & Fleming, I. A. (2011) “Enhanced growth reduces precocial male maturation in Atlantic salmon”, Functional Ecology, 26, pp. 399-405.

8 An acceptable size for transport varies according to the species and weight. For instance, eels they are moved when they weigh about 5 grams, while in the case of basses or turnops it may weigh up to 40 grams by the time transport is considered. In the case of salmonids, their weight can vary significantly depending on the time of the year at which transport is done, from around 15-20 grams in the spring to up to 100 grams in the fall. For some species, such as trouts, if they are carried out to fattening tanks in the winter, they can be already up to weigh up to 200 grams.

9 Bandeen, J. & Leatherland, J. F. (1997) “Transportation and handling stress of white suckers raised in cages”, Aquaculture International, 5, pp. 385-396. Iversen, M.; Finstad, B. & Nilssen, K. J. (1998) “Recovery from loading and transport stress in Atlantic salmon (Salmo salar L.) smolts”, Aquaculture, 168, pp. 387-394. Rouger, Y.; Aubin, J.; Breton, B.; Fauconneau, B.; Fostier, A.; Le Bail, P.; Loir, M.; Prunet, P. & Maisse, G. (1998) “Response of rainbow trout (Oncorhynchus mykiss) to transport stress”, Bulletin Francais de la Peche et de la Pisciculture, 350-351, pp. 511-519. Barton, B. A. (2000a) “Salmonid fishes differ in their cortisol and glucose responses to handling and transport stress”, North American Journal of Aquaculture, 62, pp. 12-18. Sandodden, R.; Findstad, B. & Iversen, M. (2001) “Transport stress in Atlantic salmon (Salmo salar L.): Anaesthesia and recovery”, Aquaculture Research, 32, pp. 87-90. Chandroo, K. P.; Cooke, S. J.; McKinley, R. S. & Moccia, R. D. (2005) “Use of electromyogram telemetry to assess the behavioural and energetic responses of rainbow trout, Oncorhynchus mykiss (Walbaum) to transportation stress”, Aquaculture Research, 36, pp. 1226-1238.

10 Pickering, A. D. (1998) “Stress responses in farmed fish”, in Black, K. D. & Pickering, A. D. (eds.) Biology of farmed fish, Sheffield: Sheffield Academic Press, pp. 222-255.

11 Strangeland, K.; Hoie, S. & Taksdal, T. (1996) “Experimental induction of infectious pancreatic necrosis in Atlantic salmon (Salmo salar L.) post-smolts”, Journal of Fish Diseases, 19, pp. 323-327.

13 Ewing, R. D. & Ewing, S. K. (1995) “Review of the effects of rearing density on the survival to adulthood for Pacific salmon”, Progressive Fish-Culturist, 57, pp. 1-25.

14 Vazzana, M.; Cammarata, M.; Cooper, E. L. & Parrinello, N. (2002) “Confinement stress in seabass (Dicentrarchus labrax) depresses peritoneal leukocyte cytotoxicity”, Aquaculture, 210, pp. 231-243.

15 Rotllant, J. & Tort, L. (1997) “Cortisol and glucose responses after acute stress by net handling in the sparid red porgy previously subjected to crowding stress”, Journal of Fish Biology, 51, pp. 21-28.

16 Montero, D.; Izquierdo, M. S.; Tort, L.; Robaina, L. & Vergara, J. M. (1999) “High stocking density produces crowding stress altering some physiological and biochemical parameters in gilthead seabream, Sparus auratus, juveniles”, Fish Physiology and Biochemistry, 20, pp. 53-60.

17 Gornati, R.; Papis, E.; Rimoldi, S.; Terova, G.; Saroglia, M. & Bernardini, G. (2004) “Rearing density influences the expression of stress-related genes in sea bass (Dicentrarchus labrax L.)”, Gene, 341, pp. 111-118. Iguchi, K.; Ogawa, K.; Nagae, M. & Ito, F. (2003) “The influence of rearing density on stress response and disease susceptibility of ayu (Plecoglossus altivelis)”, Aquaculture, 220, pp. 515-523. Iversen, M.; Finstad, B. & Nilssen, K. J. (1998) “Recovery from loading and transport stress in Atlantic salmon (Salmo salar L.) smolts”, op. cit. Ellis, T.; North, B.; Scott, A. P.; Bromage, N. R.; Porter, M. & Gadd, D. (2002) “The relationships between stocking density and welfare in farmed rainbow trout”, Journal of Fish Biology, 61, pp. 493-531. Barton, B. A.; Ribas, L.; Acerete, L. & Tort, L. (2005) “Effects of chronic confinement on physiological responses of juvenile gilthead sea bream, Sparus aurata L., to acute handling”, Aquaculture Research, 36, pp. 172-179. Barton, B. A.; Schreck, C. B. & Barton, L. D. (1987) “Effects of chronic cortisol administration and daily acute stress on growth, physiological conditions, and stress responses in juvenile rainbow trout”, Diseases of Aquatic Organisms, 2, pp. 173-185. Arends, R. J.; Mancera, J. M.; Munoz, J. L.; Bonga, S. E. W. & Flik, G. (1999) “The stress response of the gilthead sea bream (Sparus aurata L.) to air exposure and confinement”, Journal of Endocrinology, 163, pp. 149-157.

18 Turnbull, J. F.; Bell, A.; Adams, C. E.; Bron, J. & Huntingford, F. A. (2005) “Stocking density and welfare of cage farmed Atlantic salmon: Application of a multivariate analysis”, Aquaculture, 243, pp. 121-132.

19 Scott, A. P.; Pinillos, M. & Ellis, T. (2001) “Why measure steroids in fish plasma when you can measure them in water?”, in Goos, H. J. Th.; Rastogi, R. K.; Vaudry, H. & Pierantoni, R. (eds.) Perspectives in comparative endrocrinology: Unity and diversity, Bologna: Monduzzi, pp. 1291-1295. Ellis, T.; North, B.; Scott, A. P.; Bromage, N. R.; Porter, M. & Gadd, D. (2002) “The relationships between density and welfare in farmed rainbow trout”, op. cit.

20 Ejike, C. & Schreck, C. B. (1980) “Stress and social hierarchy rank in coho salmon”, Transactions of the American Fisheries Society, 109, pp. 423-426.

21 Greaves, K. & Tuene, S. (2001) “The form and context of aggressive behaviour in farmed Atlantic halibut (Hippoglossus hippoglossus L.)”, Aquaculture, 193, pp. 139-147.

22 Katavić, I. & Jug-dujaković, J. (1989) “Cannibalism as a factor affecting the survival”, Aquaculture, 77, pp. 135-143. Folkvord, A. & Otteråb, H. (1993) “Effects of initial size distribution, day length, and feeding frequency on growth, survival, and cannibalism in juvenile Atlantic cod (Gadus morhua L.)”, Aquaculture, 114, pp. 243-260. Baras, E. & Jobling, M. (2002) “Dynamics of intracohort cannibalism in cultured fish”, Aquaculture Research, 33, pp. 461-479.

23 Puvanendran, V. & Brown, J. A. (2002) “Foraging, growth and survival of Atlantic cod larvae reared in different light intensities and photoperiods”, Aquaculture, 214, pp. 131-151.

24 Empirical reserach has shown that many fishes avoid bright lights. Atlantic salmons, for instance, avoid bright light in the water surface, except when they need to stand it in order to feed themselves. See Fernö, A.; Huse, I.; Juell, J. E. & Bjordal, A. (1995) “Vertical distribution of Atlantic salmon (Salmo salar L.) in net pens: Trade-off between surface light avoidance and food attraction”, Aquaculture, 132, pp. 285-296; Juell, J. E.; Oppedal, F.; Boxaspen, K. & Taranger, G. L. (2003) “Submerged light increases swimming depth and reduces fish density of Atlantic salmon Salmo salar L. in production cages”, Aquaculture Research, 34, pp. 469-477.

25 Fjelldal, P. G.; & Hansen, T.; Breck, O.; Ørnsrud, R.; Lock, E.-J.; Waagbø, R.; Wargelius, A. & Eckhard Witten, P. (2012) “Vertebral deformities in farmed Atlantic salmon (Salmo salar L.) – etiology and pathology”, Journal of Applied Ichthyology, 28, pp. 433-440.

27 Andrew, J. E.; Noble, C.; Kadri, S.; Jewell, H. & Huntingford, F. A. (2002) “The effects of demand feeding on swimming speed and feeding responses in Atlantic salmon Salmo salar L., gilthead sea bream Sparus aurata L. and European sea bass Dicentrarchus labrax L. in sea cages”, Aquaculture Research, 33, pp. 501-507.

28 Barton, B. A. (2000b) “Stress in fishes: A diversity of responses”, American Zoologist, 40, pp. 937-1937. Conte, F. S. (2004) “Stress and the welfare of cultured fish”, Applied Animal Behaviour Science, 86, pp. 205-223. Contreras-Sanchez, W. M.; Schreck, C. B.; Fitzpatrick, M. S. & Pereira, C. B. (1998) “Effects of stress on the reproductive performance of rainbow trout (Oncorhynchus mykiss)”, Biology of Reproduction, 58, pp. 439-447.

29 Rijkers, G. T.; Teunissen, A. G.; Van Oosterom, R. & Van Muiswinkel, W. B. (1980) “The immune system of cyprinid fish. The immunosuppressive effect of the antibiotic oxytetracycline in carp (Cyprinus carpio L.)”, Aquaculture, 19, pp. 177-189.

30 Yildiz, H. Y. & Pulatsu, S. (1999) “Evaluation of the secondary stress response in healthy Nile tilapia (Oreochromis niloticus L.) after treatment with a mixture of formalin, malachite green and methylene blue”, Aquaculture Research, 30, pp. 379-383. Griffin, B. R.; Davis, K. B. & Schlenk, D. (1999) “Effect of simulated copper sulphate on stress indicators in channel catfish”, Journal of Aquatic Animal Health, 11, pp. 231-236. Griffin, B. R.; Davis, K. B.; Darwish, A. & Straus, D. L. (2002) “Effect of exposure to potassium permanganate on stress indicators in channel catfish”, Journal of the World Aquaculture Society, 33, pp. 1-9. Thorburn, M. A.; Teare, G. F.; Martin, S. W. & Moccia, R. D. (2001) “Group-level factors associated with chemotherapeutic treatment regiments in land-based troutfarms in Ontario, Canada”, Preventative Veterinary Medicine, 50, pp. 451-466. Sørum, U. & Damsgard, B. (2003) “Effects of anaesthetisation and vaccination on feed intake and growth of Atlantic salmon (Salmo salar L.)”, Aquaculture, 232, pp. 333-341.

31 Krkošek, M.; Lewis, M. A.; Morton, A.; Frazer, L. N. & Volpe, J. P. (2006) “Epizootics of wild fish induced by farm fish”, Proceedings of the National Academy of Sciences, 103, pp. 15506-15510. Johansen, L. H.; Jensen, I.; Mikkelsen, H.; Bjørn, P. A.; Jansen, P. A. & Bergh, O. (2011) “Disease interaction and pathogens exchange between wild and farmed fish populations with special reference to Norway”, Aquaculture, 315, pp. 167-186.

32 Another factor for this to happen is that fishes, as well as other animals kept in fish farms, are r-strategists in which gene traits cannot be easily recognized and selected by humans as those of more characteristically K-strategists are. This makes more difficult to select those who can resist certain conditions, and makes higher the probability that they die due to them.

33 Erikson, U.; Sigholt, T. & Seland, A. (1997) “Handling stress and water quality during live transportation and slaughter of Atlantic salmon (Salmo salar)”, Aquaculture, 149, pp. 243-252. Iversen, M.; Finstad, B.; McKinley, R. S.; Eliassen, R. A.; Carlsen, K. T. & Evjen, T. (2005) “Stress responses in Atlantic salmon (Salmo salar L.) smolts during commercial well boat transports, and effects on survival after transfer to sea”, Aquaculture, 243, pp. 373-382. Alanara, A. & Brannas, E. (1996) “Dominance in demand-feeding behaviour in Arctic charr and rainbow trout: The effect of stocking density”, Journal of Fish Biology, 48, pp. 242-254.