Abstract
Salmon aquaculture is in great need of good quality balanced protein and lipid sources, particularly marine omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA), to sustain a further development of the industry. One possibility is to harvest mesopelagic marine layers. Therefore, the current project analysed mesopelagic hauls from three cruises (November 2015 to October 2016) collected from the inner fjord systems around Bergen and in open-waters off Tromsø and Ålesund, Norway. Jellyfish, krill, shrimps and small amounts of the mesopelagic fish, Maurolicus muelleri and Benthosema glaciale, dominated the mixed mesopelagic hauls. Lipid content ranged between 35-40% of dry matter with two samples from autumn being 21 and 13%, with the latter haul being almost exclusively krill. In contrast, M. muelleri and B. glaciale had lipid contents of around 54 and 47% respectively. Overall, lipid was a relatively good source of marine n-3 LC-PUFA, EPA and DHA, being in the range of 15–20% of fatty acids which increased in lean samples. However, many of the trawl hauls contained wax esters (7 out of 9 hauls), equivalent to 40% or more of the lipid, with B. glaciale containing almost 90% wax esters of lipid. This presents a challenge if used in salmon diets, as their utilisation is limited. Protein contents ranged between 45-50%, increasing in lean samples. The essential amino acid content was well above the requirements for Atlantic salmon (Salmo salar) with B. glaciale generally containing higher levels compared to M. muelleri. Leucine, lysine and valine levels were particularly high. Hauls from open-waters contained mixtures of amphipods resulting in cadmium levels exceeding the maximum allowable level in feedstuffs. Arsenic levels were high or borderline. Reducing crustacean mix in hauls appear to be the only option to reduce these levels, whereas mesopelagic fish contained low levels of all heavy metals. In summary, the mesopelagic layer contains protein and lipid sources that could supply raw materials to the salmon aquaculture industry. However, high levels of wax esters, cadmium and arsenic needs to be addressed.
Highlights
The continual growth of the global population coupled with its de mand for seafood has placed further pressures on the wild capture fisheries that are already at or beyond their capacity (FAO, 2018)
The decrease in fish oil use, with a concomitant rise in plant oil inclusion, has been of particular concern owing to the associated decline in the health-beneficial omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA), eicosapentae noic acid (EPA; 20:5n-3) and docosahexanoic acid (DHA; 22:6n-3) in the farmed fish product (Sprague et al, 2016)
The level of EPA and DHA in feeds have been reduced from 20% of the dietary lipid in the late 1990’s to less than 5% in current feeds (Einen et al, 1998; Ytrestøyl et al, 2015), subsequently compromising the value of farmed salmon as a prime source of these fatty acids for human consumers (De Roos et al, 2017; Sprague et al, 2016)
Summary
The continual growth of the global population coupled with its de mand for seafood has placed further pressures on the wild capture fisheries that are already at or beyond their capacity (FAO, 2018). Many of the farmed species, carnivorous marine species such as Atlantic salmon (Salmo salar), seabass (Dicentrarchus labrax) and seabream (Sparus aurata), have traditionally relied upon the inclusion of raw ingredients sourced from wild fisheries in order to satisfy their nutritional requirements. Efforts to replace these finite ingredients with alternatives, typically of terrestrial origin, have resulted in the Norwegian salmon feed industry shifting from 90% marine ingredient inclusion in feeds in 1990 to less than 30% inclusion in 2013 (Ytrestøyl et al, 2015). Few other sources can supply raw materials in the quantities needed for a growing salmon industry without seriously affecting the production cost
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