Fishy measurements: AAS provides baseline heavy metal load

Canned heavy metal

Samples of tinned sardines, originating from six countries have been analyzed for total arsenic, cadmium, lead and mercury content using AAS. The analysis provides a useful baseline for a foodstuff - small pelagic fish - that could become increasingly important in a possible sustainable future.

Some observers have suggested that pelagic fish, those species among which are counted sardines, that live in the open water of the sea away from shorelines, reefs and the seabed, could be a major staple of future food supply given the diminishing stocks of non-pelagic species, including cod, tuna, haddock, flounder, and salmon, through overfishing. It is important, therefore, to understand the current profile of toxic metals present in such species, especially given that pollutant such as arsenic are a major food-borne risk for cancer and cardiovascular disease and may be present at concentrations above acceptable safety levels.

Total fishy metal

John Shiber of the Kentucky Community & Technical College System in Prestonsburg, Kentucky, USA has sampled tinned sardines from 17 sources in six countries, including Canada, Morocco, Norway, Poland, and Thailand, and on sale in the US. He used furnace atomic absorption spectrophotometry to analyse composites of three to four fish for total arsenic (As), cadmium (Cd) and lead (Pb) content and for mercury (Hg) used thermal decomposition amalgamation and AAS. The results were as follows in micrograms per gram of wet sample: arsenic 0.49-1.87, cadmium less than 0.01-0.07, lead less than 0.06-0.27, and mercury less than 0.09. Shiber points out that the concentrations were similar to those reported by others but given that there are no internationally agreed upon guidelines for arsenic or cadmium in fish products it is difficult to comment on whether or not the figures obtained would be considered acceptable. Shiber does point out that incidence of cancers and cardiovascular diseases associated with arsenic ingestion are "extraordinarily high" in his region and suggests that the relationship between human health and arsenic ingestion must be scrutinised more closely, especially given the increasing use of pelagic fish in our diets.

"The most recent Food and Agriculture Organization (FAO) Report on the State of World Fisheries and Aquaculture points out that over half of the world's monitored fish stocks have been fully exploited, while another quarter is overexploited, depleted or slowly recovering," says Shiber, "Increased absence of large predatory species causes fishermen to 'fish down the marine food web' and seek smaller fish like sardines and sardine-types marketed as sardines ('herring', 'sprats', 'pilchards', etc.) for commercial purposes." He adds that pelagic species are not only currently cheaper, but they are also higher in omega-3 fatty acids and so considered useful nutritionally speaking. Importantly, their small size and shorter lifespan apparently precludes their ability to concentrate the elevated mercury levels that larger predatory fish do as is apparent from Shiber's data, although other toxic elements, in particular arsenic, do seem to be present at potentially worrying levels.

Safety at sea

More attention must now be paid to the accumulation of hazardous materials, such as heavy metals, in these smaller fish species. "This pilot study is valuable to that end and might serve as a reference point for similar future studies," concludes Shiber. "Determining the countries of sample origin and the likely species analyzed allows for correlating results with the condition of the water in the region where the fish are caught and is, thus, useful to such work." Shiber suggests that other packing ingredients, brine, oil, tomato products and packaging materials must also be included in the assessment of heavy metal content of fish products in follow-on research in this field.

"The legislative and epidemiological ramifications cannot be predicted, as not enough work has [yet] been done on heavy metals in smaller fish species or in food generally," Shiber told SpectroscopyNOW.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.