Dictionary of Ichthyology  

slippery as an eel = an idiom, sliding from one's grasp as easily as an eel (which has an elongate, muscular body, no firm protrusions to grasp, small scales and a slimy skin, making it difficult to hold)

Krystal Lapierre and Anguilla rostrata at Champlain Bridge, Ottawa, River, 28 June 2005. Photograph by Brian W. Coad.

Canadian Museum of Nature