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Aquaculture has grown into a major industry in Norway in the past three decades, contributing to economic growth and employment especially along the western and northern coastline (The Norwegian Ministry of Fisheries and Coastal Affairs 2010). Atlantic salmon is the dominating species within this industry making up 93% of the Norwegian fish farming production in 2011, with more than one million tons produced (Statistics Norway 2012). Moreover, Norway has become the world's largest producer and exporter of Atlantic salmon with a total export weight of 978 048 metric tons representing a value of 29 197 million NOK in 2011 (Norwegian Directorate of Fisheries 2012). Farmed salmon is therefore an important export product from Norway and aquaculture industry contributes substantially to the country's economy (Hindar et al. 2006; Liu et al. 2011).
The Norwegian salmon industry has grown very fast. It started as a small local family business in the early 1970s and it rapidly developed into a modern, intensive, integrated and globalised industry controlled by only a few multinational companies (Liu et al., 2010; Pettersen & Alsos, 2007). This great growth is mainly due to new technologies and innovations that allow more control over the production process, higher productivity and lower production costs (Asche, 2008). A number of environmental concerns have emerged due to this phenomenal growth. Farm escapes into the wild represent one of them as the rapid expansion of salmon farming has resulted in increased numbers of escaped farmed salmon from the marine net pens and hatcheries (or smolt farms?) (Lund et al. 1991; Thorstad et al. 2008). These escaped fish may have the potential to survive and invade natural salmon rivers (Glover et al. 2012).
In Norway farm salmon represent on average 11-35% of the wild spawning populations, getting up to 80% of some small populations in rivers located close to fish farms (Hindar et al. 2006). The risks that these intrusions pose to native salmon populations is a hot debated topic, especially in the countries where salmon farming and wild salmon coexist (Ford & Myers 2008). Adverse environmental impacts including ecological and genetic effects (Fig. 1) caused by escaped farmed salmon on wild salmon populations are scientifically documented (Thorstad et al. 2008). Fleming et al. (2000) reported on the significant potential for resource (such as space and prey) competition between farm and hybrid juveniles and their wild counterparts due to the overlap in their habitat use and diet. In addition, escaped juveniles grow faster and are generally more aggressive, which can cause stress and lead to the displacement of native fish, even increasing their mortality (Fleming et al. 2000; McGinnity et al. 1997; 2003). The same authors also demonstrated that escaped farm salmon are able to successfully interbreed with wild salmon, although their breeding performance is lower. Farm Atlantic salmon has been subject to selective breeding and domestication throughout its production and therefore differs genetically from wild populations (Gjøen & Bentsen 1997; Roberge et al. 2007) and displays reduced genetic variation (Skaala et al. 2004). Owing to this fact and as mentioned in Liu et al. (2011), the interbreeding between wild and farmed salmon causes changes in genotypes and loss of genetic variation in wild salmon populations as well as a reduction in the fitness and productivity of wild salmon. Besides the risks associated with the competition and genetic interactions between farmed and wild salmon, other negative effects include the potential transfer of pathogens and diseases through infected escaped fish (Naylor et al. 2005). For instance, furunculosis disease is believed to have been transmitted to wild stocks from a large number of infected farm salmon that escaped from Norwegian fish farms in 1988-1989 (Naylor et al. 2005).
Fig. 1.1. Summary of potential risks imposed by farm-escaped salmon on populations. Addapted from (Meager & Skjaeraasen 2006-2009)
Apart from the environmental impacts already described, the economic consequences of escapees to fish farmers should also be considered. In Norway, the direct economic cost through loss of stock is relatively small since reported escapes account for less than 0.2% of the salmon that is held in the net pens annually (Jensen et al. 2010). However, the major cost of escapes is indirect since escape events are often reported by the press and thereby generate criticism and a bad reputation to the industry (Jensen et al. 2010).
As a conclusion of the above and if salmon farming and healthy wild salmon populations are to coexist in the future, measures to reduce the number of farm escapees must be implemented (Glover 2010). The Norwegian government is well aware of the problem and has therefore established a national strategy plan against escapes that compiles all the information and actions required to prevent and reduced them. In fact, the number of farmed escaped salmon already seems to have decreased after the Norwegian technical standard NS 9415 was introduced in 2004 for the use of certified equipment in all fish farms (Jensen et al. 2010). Norwegian authorities also mandate immediate reporting and recapture efforts after escape events and there are penalties for the breach of these escape-related regulations (Naylor et al. 2005). Despite these legal obligations there is evidence of unreported escape events (Skilbrei & Jørgensen 2010). These unreported escapes may be unintentional (fish farmers not aware of it) or intentional (fish farmers with-holding information after escape incidents) (Glover et al. 2008). Therefore there is increasing opinion about the need to develop a method for labeling farmed fish in order to identify the origin of escapees and potentially, use it as a tool to detect aquaculture sites in need of better husbandry practices (Adey et al. 2009) and to prosecute fish farmers breaching the regulations (Glover 2010).
In view of the need for a reliable method for identifying escapees, the Norwegian Directorate of Fisheries established a committee to evaluate a series of marking techniques (Glover 2010; Naylor et al. 2005), including physical tags, bar-code and genetic marks, among others. Moreover, a genetic method developed by Glover et al. (2008) has already been successfully implemented in a number of court cases to identify the farm of origin of recaptured escaped salmon. However, this method faces some challenges and Glover (2010) suggested that non-genetic supplementary techniques would be required in the future in order to increase precision and assist genetic assignment tests. For instance, fish scale microchemistry (Adey et al. 2009) and scale fatty acid profile (Grahl-Nielsen & Glover 2010) could be potential tools since they have been shown to differ amongst reared Atlantic salmon groups.
According to the eventual necessity for alternative tagging techniques, the idea behind this thesis research was to develop a simple and inexpensive method that allows us not only to distinguish between farmed and wild Atlantic salmon, but also to track the escaped salmon back to the farm of origin. The results of a pilot study performed by our group (data not published) suggested that the incorporation of rare earth elements (REEs) in fish scales following supplementation to the feed would be worthwhile to investigate as a potential tagging method.
Chemical marking offers the possibility to mark large groups of fish and individual handling is not required, which reduces labour-intensity and improves animal welfare. On the other hand, the REEs have the potential to be successful chemical markers since of them are non-radioactive and therefore easy to handle ,they are incorporated in the bony tissues, and have been shown to have a long retention time, and they are relatively inexpensive. Based on this, an experiment was designed in order to evaluate some of the REE as possible elemental tracers.
Tagging methods have a long history of use as tools in the study of animal populations to provide information related to stock identification, population size, migration patterns, growth and survival rates or the contribution of farmed fish to fisheries programs (Thorsteinsson 2002). Many techniques have been used to mark fish. Some of these include external marks such as morphological characteristics, physical attached tags, mutilations (e.g. fin clipping, cold-branding or tattooing) or externally applied dyes/pigments, which have been conventionally used
Previous works with REEs
Rare earth elements (REEs):
The term "rare earth elements" refers to a set of 17 chemical elements in the group III of the periodic table (Figure), specifically to the 15 lanthanide elements plus yttrium and scandium (Humphries 2012). The last two elements are commonly classed as REEs due to similar physiochemical properties and co-occurrence in nature (Tse 2011). The lanthanides include a series of elements with atomic number ranging from 57 to 71; in increasing order of atomic number: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium (Kagan et al. 1988)
Fig. 1 Seventeen REEs (indicated in yellow)
Figure: Rare earth elements (highlighted in yellow)and their position in the periodic table (Hou 2006)
The REEs are generally classified into two groups according to their atomic number: the light rare earths which include the elements with atomic numbers 57 through 63 (lanthanum to europium) and the heavy rare earths, including those with atomic numbers ranging from 64 to 71(gadolinium through to lutetium) as well as yttrium (Schüler et al. 2011). The term "rare earths" is misleading since they are neither rare nor earths. In fact, these metallic elements (with the exception of promethium, the only radioactive REE) are moderately abundant in the Earth's crust, with some being even more abundant than copper, lead, gold, and platinum (Humphries 2012). There is a peculiarity when it comes to the terrestrial contents of the REEs and that is a decreasing content of the elements with increasing atomic weight as well as a higher frequency of those elements with even atomic number (Kabata-Pendias & Pendias 2001). This fact explains that the heavier and odd-numbered REEs are more precious and difficult to obtain and therefore tend to be more expensive. The classification and abundances of the REEs are provided in table. Despite their relative abundance the REEs are not often found in concentrated form as rare earth minerals, which make them economically challenging to exploit (EPA 2012). Additionally, these metals share many similar properties and therefore tend to occur together in mineral deposits and are difficult to isolate (Castor & Hedrick 2006). A great number of minerals are known to contain REEs but, for industrial production they are principally mined from bastnasite and monazite ores, which are enriched in LREEs and account for approximately 95% of the currently used REEs (Redling 2006).
Table: Classification and abundances of the rare earth elements.
1: Classification of the REEs according to the atomic number (Schüler et al. 2011).
2: Crustal abundance in ppm (EPA 2012).
The REEs have a wide variety of applications in several different fields such as catalysts, lighting, metallurgy and many others. Furthermore, their use in modern technology has dramatically increased over the past few years, being incorporated in growing markets such as battery alloys, ceramics and permanent magnets, among others (Goonan 2011). The diverse applications of the REEs are illustrated in figure.
chart - The Many Uses of Rare Earth Elements
Figure: Major end uses and applications of the REEs (Robinson 2011).
Information on chemical properties was gathered and selected based on its relevance to this thesis. Most of the
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