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The word aquaculture is used in order to describe different kind of applications that involve the culture of marine organisms such as fish, oyster or seaweed in seawater, freshwater and brackish environments. Given the aforementioned generic use of aquaculture, a more restrictive approach might be useful with regard to the type of the organism, whose culture is described (principles, p.3). As far as fish culture is concerned, according to FAO, the world's dependence on fish has never been higher than today, with global aquaculture counting for half of the fish consumed (http://www.fao.org/3/a-i3807e.pdf). The above argument is driven by the fact that global population is increasing, which consequently means that humans' demand in food production is also rising (FAO, meeting the food...). It is notable that between 2010 and 2012, the global fish farming production rose from 32,4 to 66,4 million tones (http://www.fao.org/3/a-i3720e.pdf, p. 20). What is more, fishes are also an invaluable source of proteins, comparably to those derived from poultry, beef and pork, counting also to their extensive farming (Principles, p.4).
Among all the species within the fish aquaculture industry, Atlantic salmon (Salmon salar L.) has gained a high popularity on an international level within the last few decades and is considered as a species of increased economic value (http://www.fao.org/3/a-i3720e.pdf, p. 39, 66). Its farming demonstrates the advances of intensive aquaculture in terms of technological improvement, obtained through scientific research and investment (salmonid, p.1). It was first introduced in Norway and Scotland but it was only a matter of time until its farming would expand in different countries around the world (principles, p. 352). While the reins with the regard to Atlantic salmon aquaculture seems to be held particularly by Norway, Chile has also recently grown as one of the leading farm industries of Atlantic salmon, although, as it is mentioned by FAO, their production suffered in the period between 2009 and 2011 owing to the onset of disease (salmonid, p. 13, http://www.fao.org/3/a-i3720e.pdf, p. 39).
In general, salmon aquaculture is carried out either as a closed culture or through ranching programs. With regard to the first case, both hatching and the production of the smolts (young fish) usually takes place in controlled freshwater environments, while the main growth of the fish is carried out in some kind of cage systems in the sea. The key point of the aforementioned form of culture is that fishes are captured during their whole life cycle (marine harvest, p. 29, principles p. 365, salmonid, p.1, 123). Ranching projects, on the contrary, take advantage of the fact that salmon species are anadromous, which means that they born in fresh water, leave the freshwater as smolts in order to complete their growth in the open sea and finally instinctively return to the places, in which they have born in order to reproduce (marine harvest, p. 5, salmonid p.1). The young are kept confined under controlled conditions until they are ready to be released and when they are back to their birth place, after two years approximately, they are recaptured (marine harvest, p. 29, salmonid p.1-2).
This transition phase of the Atlantic salmon from the freshwater to the marine environment is a highly challenging procedure, correlated with high mortality rates in the marine environment. These high mortality rates are primarily driven by factors, associated with the production of smolts, which are considered as the most vulnerable stage in the life cycle of Atlantic salmon (Kroglund et al. 2007; Thorstad et al. 2013). The factors involved are anthropogenic, including acid deposition and pollutants, as well as naturally occurring acidification episodes resulting from spring snowmelt, rainfalls or deposition of sea salts in the freshwater environment, where the production of smolts takes place (Serrano 2005; Thorstad et al. 2013). It is well established that chronic acidification, caused by anthropogenic activity, have severely affected the populations of Atlantic salmon in many rivers in Norway and elsewhere (Kroglund et al. 2007; Kroglund et al. 2008). In particular, acidification refers to a reduction in pH (increased concentration of H+), combined with a mobilization of aluminium derived from the soil, resulting in increased concentrations in the water, which are considered toxic to the fish. Nonetheless, an extremely acidic environment could be proven lethal to the fish, by itself (Kroglund et al. 2008).
2. Acidification and increased concentrations of aluminium: addressing the effects and possibilities for recovery
As it is mentioned above, an increase in the concentration of H+ and its subsequent precipitation results in the mobilization of aluminium from the soil. Aluminium in the water occurs in a wide variety of forms with a corresponding variety of toxicity. However, the inorganic, monomeric form of Al (Ali) is the one, which is considered as the most toxic to the fish and the one, whose concentration increases as a response to a lower pH (Serrano 2005; Thorstad et al. 2013). While high concentrations of hydrogen ions seems to interfere with the membrane's permeability, Ali is binding to the gills, thus accumulating and resulting in ion deregulation and respiratory disorders (Serrano 2005; Thorstad et al. 2013) (Gensemer & Playle 1999). On the contrary, an environment whose concentration in various organic molecules is high could result in reduced inorganic Al in the water, since Ali and organic matter would bind so that less of the most toxic Al would be available to be bound to the gills (Kroglund et al. 2007) (Gensemer & Playle 1999).
A research conducted by Thorstad et al. (2013) aimed at evaluating the survival of the migrating population of Atlantic salmon smolts exposed to different concentrations of Al at a moderately acidified freshwater, between 5.88 and 5.98, suggests that both the concentration of Al and the time that the exposure lasts are of extreme significance, affecting the survival in the marine environment. At this point, it is worth mentioning that in hatcheries and smolt production facilities, the pH of the freshwater should range between 6 and 9 (salmonids p.51). In the aforementioned study, the smolts which have been exposed to high concentrations of Al for 48 hours showed approximately the same mortality rates during the migration process with those that have been exposed to low concentrations of Al, but the smolts whose exposure to high Al concentrations lasted for 90 hours displayed an 100% mortality almost right away after they have been released. However, according to a research conducted by Kroglund et al. (2007), the length of the exposure is not recognised as a significant parameter with respect to the marine survival, since both short-Al and high-Al treated groups had more or less the same responses.
With the respect to the effect that high concentrations of aluminium has to the fish, it is well established that aluminium, which is concentrated to the gills, promote an inhibitory effect of the enzyme Na+, K+ - ATPase. This enzyme is an important element of the smoltification process, which is merely a physiological pre-adaptation to the marine environment, and its activity is growing during this process. Such an inhibitory effect could be proven detrimental, since smolts would not be able to tolerate seawater, and consequently survive (Kroglund et al. 2007; Monette and McCormick 2008; Thorstad et al. 2013).
And while a severe acidification episode could considerably decrease the production of smolts, moderate acidification conditions such as those described above could trigger a series of physiological responses such as a glucose increase, indicating that the fish is consuming energy as an effort to keep stable their internal conditions at the expense of their growth. It is possible that growth could be reduced to half during long periods of exposure (Kroglund et al. 2007). By suppressing their growth, smolts also reduce the possibilities to survive during migration as well as in the more challenging marine environment, where the predation levels are considerably high (Thorstad et al. 2013).
IMPORTANT TO DETERMINE THE MINIMUM WATER QUALITY IF WE WANT TO UTILIZE SALMON EFFECTIVELY
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