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Development and validation of methodology for the confirmation of the origin of wild and farmed salmon and other fish
Project Code: Q01031
Lees, D ; Thomas, M
Legislation recently introduced into the European Community (EC/2065/2001) requires that all fishery and aquaculture products on sale to consumers specify a minimum amount of
information about the fish being used. This includes details of the catch area and method of production, i.e. whether the fish is wild or farmed.
There has been a considerable increase in aquaculture over the last twenty years. Salmon is the most widely farmed fish species, but thanks to improvements in aquaculture techniques a number of other species such as cod, halibut, sea bass, sea bream are also being farmed.
The main aim of this study was to investigate possible methods which could differentiate between wild and farmed fish and to provide some information on the geographical catch or farmed area. The main focus of the project was wild and farmed salmon which is by far the most important farmed species available.
From previous studies it had been shown that staple isotope content found in salmon reflects both the environment in which it is grown also the composition of the diet consumed. Theprinciple objective of the project described in this report was to expand on these early studies by applying the stable isotope approach to a larger authentic data set and to compare the results with those obtained using classical compositional analyses such as gas
This work was carried out as part of a wider project funded by the European Commission:
COFAWS, Contract N° G6RD-CT-2001-00512. A consortium was brought together to study different analytical techniques and, in particular, to provide well-characterised samples of
authentic wild and farmed salmon. The analytical techniques included, in addition to isotopic and compositional analysis, the investigation of high-resolution 13C and 1H NMR
spectroscopies as profiling methods to differentiate between wild and farmed salmon and distinguish geographical origin.
Stable isotope ratio analysis is based on the quantitative determination of hydrogen, carbon, nitrogen and oxygen isotopes at natural abundance levels. Isotopic content of a compound can vary according to its source and thereby provide information on the origin of the food product. Greater resolution can be achieved by multi-dimensional studies in which the information is combined from (i) measuring a number of different isotope ratios (13C/12C, 18O/16O, 15N/14N) using both isotope ratio mass spectrometry (IRMS) and (ii) measuring the non-statistical distribution of deuterium within the product using quantitative nuclear magnetic resonance spectroscopy (2H SNIF-NMR). These techniques were applied to various components of the salmon: lipids and derived products such as fatty acids, glycerol, phospholipids; salmon flesh; water. The project sample set contained over 300 fish. Wild and farmed salmon from different geographical origins (Scotland, Ireland, Norway, Iceland, Faroe Islands, Canada, Alaska, Tasmania) and different seasons were studied, in addition to more limited sample sets of cod, sea-bass and halibut.
Despite promising results obtained in previous studies, 2H-SNIF-NMR was unable on its own to provide clear discrimination of the samples in this larger sample set, both in terms of geographical origin and method of production. The experimental procedure is timeconsuming and requires lengthy spectrometer time, factors which probably outweigh the
benefits of the additional discrimination provided.
A combination of GC fatty acid composition and the δ18O and δ15N IRMS measurements of the fish oil and choline respectively appear to provide conclusive differentiation between wild and farmed salmon. Both these methods closely reflect the fish diet, which is the principle factor of discrimination as farmed fish feed contains higher proportions of vegetable oil. The isotopic data can also provide added indication on the use of C3- or C4- derived vegetable oils. These methods could therefore be used to monitor the type of oil used in fish feed and could be included in an overall traceability system for the fish producers.
The results obtained regarding geographical origin are not as clear-cut. In most cases Alaskan wild salmon forms a clearly differentiated group. Although this is to be expected since the fish is a different species, the methods can still be used to confirm a labelling claim of "wild, Alaskan" salmon. Since the analytical results closely correlate with the feed used, the different proportions of vegetable oil used for Canadian farmed salmon enables this origin to be distinguished from North Atlantic farmed fish. Wild Atlantic and wild Pacific can also be differentiated.
The extension of this methodology to other fish species gave promising results albeit on smaller sample sets. These other fish - cod, sea-bass, halibut - are much leaner species
making it more difficult to extract oil from the flesh for the analyses. An experimental protocol was worked up to get over this difficulty.
The analytical database, suitable for distinguishing between farmed and wild salmon and containing the GC and isotopic results, has been incorporated into a user-friendly statistical tool to assist interpretation of the data. Further work should include a full inter-collaborative study of the IRMS method for validation purposes and the analyses of more authentic samples of fish other than salmon in order to build up the data base.
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