adipose fin
US /ˈæd.ɪ.poʊs fɪn/
UK /ˈæd.ɪ.poʊs fɪn/
1.
a small, fleshy, rayless fin on the back of some fish, such as salmon and catfish, located between the dorsal fin and the tail
:
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The salmon's adipose fin is a key identifier for its species.
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Catfish are easily recognized by their prominent adipose fin.