The prey we have most frequently seen the otters eating are eels, which they appear to take with ease. I’ll never forget seeing a mother otter standing in the water watching her 2 cubs, she seems completely untroubled with a small eel squirming desperately in her mouth.
Without doubt the most exciting prey we discovered the otter hunting is the lamprey. I knew very little of the lamprey until we saw the otter catch and eat one. They are an ancient species that existed before the time of the dinosaurs.
There is more information about lampreys on these pages;