中华鲟 (Chinese sturgeon, Acipenser sinensis) is a 'living fossil' protected species in the Yangtze, often called 长江鱼王 ('king of Yangtze fish'). Sturgeon are also the source of caviar (鱼子酱).
鱼 (fish) sits on the left as the indexing radical, immediately flagging 鲟 as a species name. Sturgeon are ancient cartilaginous river fish, and 鲟 sits alongside 鲤 (carp), 鲨 (shark), 鳕 (cod) - all built on the same fish-plus-phonetic template.
寻 (xún) supplies the sound, unchanged. No semantic contribution - fish-name characters typically borrow any well-pronounced piece. Compare 浔 (riverbank, also xún) and 荨 (nettle) in the same phonetic family.
