Although findings showed that their mercury levels were within safety limits for human consumption (if consumed in moderation, particularly by children and pregnant women), swordfish were in poor physical state, indicating unfavourable conditions in the north-west Mediterranean.
A large, migratory predator, swordfish is one of the most valuable species in the EU market and is intensively fished: in 2020, 224 tons were landed in Catalan ports. The species is also extensively studied; however, research on their physiological condition is sparse, and this is important to inform sustainable fishing practices. As well as being under pressure from fishing, Mediterranean swordfish are affected by genetic isolation and mercury contamination. Since they are at the top of the food web, they are susceptible to build-up of contaminants, through biomagnification (concentration in tissues of organisms at higher levels in a food chain) and bioaccumulation across their lifespan. This poses a food safety issue to people who consume swordfish, but also has serious long-term effects on the fish themselves.
To gain information on the condition of Mediterranean swordfish, their exposure to mercury and aspects of the food web that influence their mercury intake, the researchers used a multi-method approach. They sampled 26 adult swordfish caught in depths of 20 metres in the Catalan Sea in June–August 2018. The fish were measured and weighed, and stomachs and other organs were extracted and frozen before being sent for laboratory analysis.
First analysing the contents of the swordfishes’ stomachs, the researchers found that cephalopods (in particular, squid such as Todarodessagittatus) comprised more than 90% of ingested food; the remainder was teleost fish (including pilchards, Sardina pilchardus) and a small amount of crustacean.
Then the researchers rated the condition of the swordfish according to length and eviscerated mass, and liver lipid reserves. Energy reserves – chiefly stored fat – are an indicator of fish health; in swordfish, most lipid content is in the liver. The researchers found that liver mass relative to body weight decreased in relation to increasing fish size – an indication of poor overall condition and potentially unfavourable food sources. They also noted that the relative weight of livers to total body weight was lower in these fish than in swordfish sampled in the North Atlantic in the 1930s, also suggesting less access to food in this Mediterranean study. Liver lipids, meanwhile, ranged from 4–22 % of wet weight, again decreasing with fish size. This could be explained by an increased need for energy in muscles, in larger fish, but further research is needed to clarify the cause.
The researchers then inferred mercury intake and biomagnification rates from samples of swordfish prey fish analysed for mercury content and stable isotopes, which offer information about the food web. All prey fish were contaminated with mercury. Average levels in cephalopods were significantly lower than in teleosts, but cephalopod ingestion accounted for 87% of the mercury intake for Catalan swordfish in the study.
Overall, the results showed a clear indication that mercury biomagnification was taking place in the swordfish food web. Biomagnification factors varied among prey species; while examples of significant biomagnification occurred with most cephalopod species, however, a bioreduction was observed for the spotted barracuda (Arctozenos risso).
Mean mercury levels in the swordfish were below EU maximum regulatory levels1, however, any change in diet towards benthic prey (those living on the seabed) could lead to a rise in biomagnification of mercury in swordfish (since benthic organisms contain generally higher mercury concentrations than those living in the open sea (pelagic organisms)).
Changing environmental conditions in the Mediterranean may favour certain species over others, at various levels in the food chain, the researchers note. The size and condition of small pelagic fish in the north-west Mediterranean is probably related to changes in quality or quantity of zooplankton, for example, and squid – which generally feed on those fish – will then be affected. Any change detrimental to the pelagic community, including squid, could lead swordfish to consume more benthic prey, which contain more mercury. A separate study2 has noted that climate change will likely increase human exposure to mercury from marine fish, as rising sea temperatures alter the food web, and species such as tuna consume more benthic prey.
The researchers also note that levels found in swordfish at this time may still be enough to act as endocrine disruptors in the fish, causing dysfunctions in their reproductive systems and altering their ability to adjust to outside agents, which can weaken populations. In light of these findings, future management of Mediterranean swordfish populations should integrate monitoring of physiological, feeding and ecotoxicological data, they say.
- European Commission (2006). Commission Regulation (EC) n°1881/2006 of 19 December of 2006 setting maximum levels of certain contaminants in foodstuffs 37 (4). Available from: https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2006:364:0005:0024:EN:PDF
- Schartup, A.T., Thackray, C.P., Qureshi, A., Dassuncao, C., Gillespie, K., Hanke, A. and Sunderland, E.M. (2019) Climate change and overfishing increase neurotoxicant in marine predators. Nature, 572 (7771): 648–650.
Biton-Porsmoguer, S., Bănaru, D., Harmelin-Vivien, M., Béarez, P., Bouchoucha, M., Marco-Miralles, F., Marquès, M. and Lloret, J. (2022). A study of trophic structure, physiological condition and mercury biomagnification in swordfish (Xiphias gladius): Evidence of unfavourable conditions for the swordfish population in the Western Mediterranean. Marine Pollution Bulletin, 176: 113411.
To cite this article/service:
“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.
Notes on content:
The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.
- Publication date
- 11 January 2023
- Directorate-General for Environment