Similar to 𩽙 (U+29F59). ⿰魚鱀 is not encoded, this is an orphan character.

The text of the corresponding Wikipedia entry https://zh.wikipedia.org/wiki/白鱀豚 looks like this:


If the text is accurate, both ⿰鱼𬶨 or ⿰鱼暨 seem acceptable.

It seems very probable that ⿰鱼𬶨 is a mistake for ⿰鱼暨 (simplified form of U+29F59 𩽙) in the two modern evidences given (someone just added a fish radical to the left of 𬶨 without noticing that the fish radical form of the character has 暨 on the right). If ⿰鱼𬶨 is correct we would expect ⿰魚鱀 to exist as well, but it does not.

The two evidences provided are both general texts which just mention Baiji dolphin in passing, so they cannot be considered to be authoritative sources. Please try to find additional evidence for either ⿰鱼𬶨 or ⿰鱼暨 from a zoological source that specifically discusses the Baiji dolphin. If there is no additional evidence then the character should be withdrawn. If additional evidence shows ⿰鱼暨 then suggest to change IDS and glyph to ⿰鱼暨.

地名大词典 is the only source for this form of 鳍, obviously this book misused a raw character, withdraw it.