Pete begins by quoting Tom Fort’s description in The Book of Eels of the eel as a “creature of mystery” and then suggests that perhaps one reason that they are relatively undocumented is that they were historically “so abundant and ubiquitous in their occurrence”, that “few people had ever really thought to study the species’ ecology in much detail.”
In recent decades, the global European eel population has undergone a catastrophic decline; numbers are now estimated at just 5% of what they were in the 1970s. This is particularly concerning, says Pete, owing to the abundance of predator and prey species that are linked within an eel’s food web.
Although several contributory factors, including global climate change, changes in oceanic current patterns, overexploitation, habitat loss and degradation, pollution, parasitic infections, manmade obstacles in rivers and entrainment and impingement at water abstractions, can be linked to the decrease in eel populations, “the simple fact remains that no one really knows the cause of the decline.”
Efforts are currently being made to sustain and increase eel numbers through measures as a regulation brought in by the European Commission in 2007 that requires member states to establish national eel management plans that include measures to help increase the numbers of adult eels leaving inland waters to cross the Atlantic to spawn in the Sargasso Sea. In the UK, this falls under the Eels Regulations 2009. Here, eel passes are being installed to aid upstream migration and there are some restocking efforts.
Eel-tile eel pass on a weir
However, says Pete, these plans “often do not consider the habitat upstream or downstream of where these interventions are planned.” Habitat creation or enhancements could facilitate the recovery of European eel stocks and should be considered, he argues. It is not true, he continues, that eels can live anywhere and need nothing but water to survive. Although they are found in almost any aquatic habitat, including rivers, seas, ponds, lakes and even polluted canals, ditches, drains and sewers, eels do require certain things of their chosen dwelling places. Factors including water velocity and depth, aquatic undergrowth and the presence of burrowing and refuge areas all play a part in determining whether a water body provides a suitable habitat for the eel.
Pete releasing an eel during surveys
“When river managers consider habitat improvements, they should also consider the requirements of eels. This would benefit multiple aquatic species and species that predate on eels such as bitterns and otters,” Pete concludes.
The article can be read in full on the Inside Ecology website.