Definition of blackfin cisco
: a deep-bodied whitefish (Coregonus nigripinnis synonym Leucichthys nigripinnis) with bluish-black fins that formerly inhabited the Great Lakes but is now considered highly endangered or extinct
- The blackfin cisco disappeared from its once-expansive range in the Great Lakes by 1969, but at least it has managed to escape extinction: a small population, classified by the government of Canada as threatened, survives in unknown numbers in Lake Nipigon, Ontario.
- —Scott Weidensaul, The Ghost with Trembling Wings, 2002
