Assessing the Influence of Salmon Farming through Total Lipids, Fatty Acids, and Trace Elements in the Liver and Muscle of Wild Saithe Pollachius virens

Saithe Pollachius virens are attracted to uneaten salmon feed underneath cages at open-cage salmon farms in Norway. The aggregated Saithe have modified their feeding habits as they have switched from wild prey to uneaten food pellets, which could lead to physiological and biochemical changes in the Saithe. Variations in profiles of total lipids, fatty acids, and trace elements in Saithe liver and muscle were measured to evaluate the influence of fish feed from salmon farms on wild Saithe populations. Farm-aggregated Saithe had higher fat content in liver tissues than did individuals captured more than 25 km away from farms, but no clear differences were found in muscle tissues. High proportions of fatty acids of terrestrial origin, such as oleic, linoleic, and linolenic acids, in liver and muscle tissues of farm-aggregated Saithe reflected the presence of wild Saithe at farms. Accordingly, low proportions of arachidonic, eicosapentaenoic, and docosahexaenoic acids in Saithe tissues mirrored the feeding activity at farms. Variations in specific trace element signatures among fish groups also revealed the farming influence on wild Saithe. High levels of Fe, As, Se, Zn, and B in liver, but also As, B, Li, Hg, and Sr in muscle of Saithe captured away from farms indicated the absence of feeding at farms. Many fish species are attracted to natural or artificial floating objects, often referred to as “fish aggregation devices” (FADs) (Dempster and Taquet 2004). Marine fish farms may serve as FADs by providing uneaten fish feed and structural habitat and by attracting small prey species (Sanchez-Jerez et al. 2011). Since fishing is not allowed near farms, these effects of fish farming may create conflicts with local fisheries because wild fish stocks become less available for Subject editor: Anthony Overton, East Carolina University, Greenville, North Carolina P. Arechavala-Lopez, B.-S. Sæther, F. Marhuenda-Egea, P. Sanchez-Jerez, and I. Uglem This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The moral rights of the named author(s) have been asserted. *Corresponding author: [email protected] Received March 26, 2014; accepted December 10, 2014 59 Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science 7:59–67, 2015 Published with license by American Fisheries Society ISSN: 1942-5120 online DOI: 10.1080/19425120.2014.1001540 exploitation. Moreover, a diet shift from wild prey to a diet consisting partly of artificial fish feed also may affect fish quality and fish fillet organoleptic characteristics (Carss 1990; Skog et al. 2003; Ottera et al. 2009; Dempster et al. 2011). Saithe Pollachius virens is an important commercial fish species in Norway, and they typically occur in pelagic schools along the coast during part of their migration and range extensively through a wide number of fjords (Bjordal and Skar 1992; Bjordal and Johnstone 1993). In addition, Saithe is one of the most abundant wild fish species found around Norwegian salmon farms (Dempster et al. 2009). Wild Saithe reside near fish farm facilities for several months (Bjordal and Skar 1992; Bjordal and Johnstone 1993; Uglem et al. 2009; Dempster et al. 2009, 2010), a sufficient period to cause physiological changes and modification of metabolic profiles due to a diet switch from wild prey to uneaten feed pellets (Skog et al. 2003; Dempster et al. 2009; Ottera et al. 2009; Bustnes et al. 2010; Fernandez-Jover et al. 2011a, 2011b). Significant differences in body condition, relative liver size, lipid content, and fatty acid (FA) composition in both muscle and liver of Saithe have been reported in previous studies for farm-aggregated and unaggregated fish. Saithe fillets from a fjord without salmon farms tasted better than those collected in a fjord where farms were present (Skog et al. 2003). Besides providing nutrient inputs, fish farms may be supplying additional sources of other trace elements, since fish diets are enriched with various essential elements, including copper (Cu), iron (Fe), zinc (Zn), manganese (Mn), cobalt (Co), and chrome (Cr) among others (CIESM 2007). In addition, Cu is still used as an antifouling treatment (copper-based algaecides) for the net-pens and related equipment (e.g., Solberg et al. 2002; Brook and Mahnken 2003; Braithwaite and McEvoy 2005; Braithwaite et al. 2007). Bustnes et al. (2011) found that mercury (Hg) concentrations in the livers of farmaggregated Saithe were higher than in unaggregated fish, but not at critically elevated levels of public health concern, suggesting that the distribution of Hg and other elements in Saithe and Atlantic Cod Gadus morhua in Norwegian coastal waters may be influenced by a combination of habitat use, diet, geochemical conditions, and water chemistry rather than farming activity alone. The overall knowledge about the presence and origin of essential and nonessential trace elements in Saithe populations is still sparse. To address conflicts between fish farming and fisheries, a quantitative tool to determine retrospectively whether Saithe have been eating feed pellets would be useful. In some cases the fish might have been feeding actively on pellets over prolonged periods even though pellets are not found in stomach samples. The evacuation time for food consumed in gadoids usually varies (Andersen 2001), and stomach analyses would thus only reveal recent feeding on artificial fish feed. Enlarged livers can be used as an indication of active feeding by fish on salmon pellets (Dempster et al. 2009, 2011); however, it is also possible that Saithe will develop enlarged livers due to feeding on natural prey with high fat content, like Atlantic Herring Clupea harengus or Capelin Mallotus villosus. Thus, the FA and trace element (TE) profiles of commercial fish feed, which differ from those in natural food, might be used to compare their biochemical variation with those in tissues to characterize the dietary prehistory of Saithe. For instance, FA profiles in liver, muscle, and eggs vary between farm-aggregated and unaggregated gadoids (Skog et al. 2003; FernandezJover et al. 2011b; Uglem et al. 2012). In particular, the FAs from vegetable oils vary between the two groups, since vegetable fats are used as a substitution for marine fat in artificial fish feed (e.g., Bell et al. 2001, 2003). In the current study we examined whether TEs in addition to FAs and lipid content could be used to distinguish between farm-aggregated and unaggregated Saithe. The specific objectives of the study were to (1) compare the composition of total lipids, FAs, and TEs between Saithe captured both at salmon farms and in areas having no farming activity, and (2) determine the reliability of using these compounds as indicators to detect the influence of salmon farming on wild Saithe assemblages.