Some facts and thoughts from eel work in Belgium.

As the eel is a long-lived, carnivorous, benthic and lipid-rich species, it is particularly prone to the accumulation of noxious chemical compounds, especially lipophilic contaminants like polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) and brominated flame retardants (BFRs). At the Research Institute for Nature and Forest (INBO), a monitoring network was set up to measure contaminants in the eel over Flanders during a 15-year research programme, starting from 1994. More than 3000 eels from 376 locations were analysed for PCBs, OCPs, heavy metals and some other compounds.
The data showed clearly that eels in their yellow stage are excellent chemical bioindicators; contaminant profiles in the studied eels are fingerprints of the contamination pressure on the site where they grow up.
Results generated status reports and distribution maps of eel pollution for some 30 substances. Most substances are present all over Flanders, but there is considerable variation between river basins, dependent on land use. Contaminant analysis in eel is able to pinpoint specific pollution sources, like some volatile organic compounds in very specific locations, very high BFR levels in eels from areas with intensive textile industry, or high lindane levels in some rivers under agricultural pressure. We could demonstrate that banned chemicals like DDT are still in use in some places. Within the study period, trend analysis indicated significant reductions in PCBs and many OCPs. Also for some heavy metals (lead, arsenic, nickel and chromium), concentrations decreased in the eel, but this was not the case for cadmium and mercury.
Self-caught eels are much esteemed by fishermen, but considering the eel’s high contaminant body burden, consumption constitutes a potential risk for human health. On many sites levels of specific compounds exceed legal maximum levels.
Several contaminants were assessed for their potential impact on the eel population. Contaminants may impact on various levels of biological organisation from molecular, individual to population and community, hence the nature of the effects varies to a very wide extent. Despite a very high internal load of endocrine disrupters, we did not find any effects on vitellogenin levels in immature yellow eel. However, a significant negative correlation between heavy metal pollution load and condition was observed, suggesting an impact of pollution on the health of sub-adult eels. In strongly polluted eels a reduced genetic variability was observed. New advances in gene expression profiling, using either microarrays or RNA-seq, offers the opportunity to investigate the effects of pollutants at the genome-wide level and also showed significant impacts.
It was further demonstrated that fat stores and condition decreased significantly during the last 15 years in eels in Flanders and The Netherlands, jeopardizing a normal migration and successful reproduction of this long-distance migrator. Belpaire et al (2009) hypothesized that pollution is a major driver for this decrease in fat reserves.
Part of the work presented here has been summarized in two reviews. Geeraerts and Belpaire (2010) reviewed the potential effects of contamination on the eel and Belpaire et al. (2016) presented a simplified concept describing how reprotoxic chemicals may influence the status of the stock of the European eel.
Belpaire C.G.J., G. Goemans, C. Geeraerts, P. Quataert, K. Parmentier, P. Hagel and J. De Boer. 2009. Decreasing eel stocks: Survival of the fattest? Ecol. Freshw. Fish. 18(2):197-214.
Belpaire, C., Pujolar, J.M., Geeraerts, C., and Maes, G., 2016. Contaminants in Eels and their Role in the Collapse of the Eel Stocks. in press. In Biology and Ecology of Anguillid Eels T. Arai (Ed.)
Geeraerts, C. and C. Belpaire. 2010. The effects of contaminants in European eel: a review. Ecotoxicology (2010) 19: 239–266