| Summary: | Abstract Background Information on European silver eel Anguilla anguilla anatomy was collected to gain information on limitations on size and placement of electronic tags. Findings To reduce the eel’s ability to bite its own sutures, it may be an advantage to make surgical incisions close to the head, but this increases the risk of cutting the liver. Recommended placement of an incision was slightly further from the head than one-fourth of the fish’s body length ( L T ) to avoid damaging the liver. Long, flexible tags comprising various components can be adjusted to the narrow body cavity and undulating movements of eels. There was space for surgically implanting a 100 mm long tag (11 mm in diameter) in the body cavity of eels with L T ≥380 mm. During gastric tagging, tag length is limited by stomach length. Silver eels with L T 380 to 998 mm had stomach lengths of 47 to 185 mm, indicating that there was space for short gastric tags in the smallest eels, but that there was space for relatively long tags in larger eels. The distance from the snout to the start of the stomach constituted 15 to 23% of L T , indicating how far the transmitter should be inserted during tagging. Conclusion This information aids the development of tags and tagging methods that consider the unique morphological and behavioral features of eels.
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