Marine lipids: Differentiating farmed and wild fish and mussels

Overview

GC-FID and 2D GC/MS methods were used to differentiate wild and farmed fish and molluscs from their lipid contents, a process which should help to identify farmed fish that are being passed off as wild fish.

Farmed fish fatty acids

The Organization for Economic and Cooperation Development predicted that fish yields from aquaculture would grow by 35% in the decade leading up to 2022, while traditional fishing would rise by just 5%. That growth demonstrates the importance of farmed fish to the global population, with aquafarmed seafood expected to constitute a far greater proportion of the seafood eaten worldwide.

In general, consumers seem to accept that seafood has to be farmed to keep up with demand and help to preserve stocks in the wild. However, some people perceive it to be inferior to wild fish and there is a temptation for unscrupulous dealers to pass off farmed fish for wild fish. Not only does this food fraud present economic opportunities for the fish sellers, it can also affect the diet of the consumer because the fatty acid compositions of the two types are different.

A number of studies have reported that the fatty acids in aquafarmed fish vary from those in wild fish, and those of freshwater fish differ from those living in seawater. Now, a new study by scientists in Italy has examined the link between the diet of farmed and wild fish and their fatty acid contents. Rosaria Costa and colleagues from the University of Messina and the University Campus Bio-Medico of Rome focused on two species of fish that are commonly cultivated in Italy, as well as mussels and clams.

Fish and molluscs

Sea bass and gilthead sea bream were taken from an aquafarm in Italy and caught by traditional methods in the Strait of Messina. Mussels were collected from a farm in a minor salt lake and clams were harvested from the salt lake Ganzirri.

The lipids were extracted from the flesh and subjected to transesterification so that they would pass easily through a gas chromatography column before analysis by GC with field ionisation detection and 2D GC/MS. Using two linked columns having different properties in the GC/MS system introduces a second degree of separation which helps to separate closely eluting compounds and provide purer mass spectra.

Farmed fish contained a greater overall weight of fatty acids. Looking at particular groups of acids, omega-3 fatty acids were more abundant in wild fish, with cultivated fish tending to contain greater amounts of omega-6 acids and oleic acid. The relative contents of docosahexaenoic, palmitic, stearic and arachidonic acids in cultivated and wild fish were 4 vs. 21, 13 vs. 22, 4 vs. 13 and 0.2 vs. 11%, respectively.

The differences were attributed to diet. Fish oil and rapeseed oil which are present in the feed for aquafarmed fish contain high contents of oleic, linoleic and linolenic acid, so it is natural that they were transferred to the fish. In addition, erucic acid, a minor component of rapeseed oil, was also found in the farmed fish.

One other factor that the researchers considered was the farming conditions but sea bass raised in pools on land or in cages in the open sea showed little variation in their fatty acid contents, presumably because they were given the same diet.

The GCxGC distribution patterns were displayed in 2D graphs. The first axis represented carbon number and the proximity of double bonds to the end of the acyl chain (the omega number) while the second dimension represented the degree of unsaturation. The distribution helps to identify unknown fatty acids from their position in the plot. The plots also illustrate the ease with which farmed and wild fish can be differentiated by visual examination.

The fatty acid contents of the shellfish were markedly different to those of the fish, having lower amounts of monounsaturated acids and higher amounts of polyunsaturated fatty acids. The dominant acid in mussels was eicosapentaenoic acid whereas that in clams was docosahexaenoic acid.

Mercury accumulation

The research team also compared the amount of mercury in wild and farmed fish. Shellfish have been reported to accumulate heavy metals from the sea as they grow but the mussels and clams had ultralow levels of mercury at 0.005 mg/kg.

In contrast, the levels in wild fish in the open sea were about 10-fold higher than for aquafarmed fish, averaging 0.556 and 0.063 mg/kg for gilthead, respectively. These high contents are at the maximum tolerated for mercury in fish according to EC regulations.

As the researchers commented: "This finding emphasizes the role of bioaccumulation of toxins from pollution in the Mediterranean Sea up the food chain and into the flesh of wild-caught fish versus aquafarmed fish, which were raised in the same waters but were fed a controlled diet."

The GC-FID and GCxGC/MS results demonstrate an easy procedure for the differentiation of farmed and wild fish to help prevent food fraud. They method could also be used to support fish farmers as they try to optimise the diet for the best fatty acid composition in the fish.

However, the findings will present a dilemma for some consumers who prefer "natural" food. Do they continue to eat fish caught in the wild and expose themselves to high mercury levels, or do they convert to farmed fish which are safer?