The deepwater cisco was one of the largest ciscoes in the Great Lakes. Its average length was 30 cm and it was about 1.0 kilogram in weight. Occurring only in Lakes Huron and Michigan, and inhabiting waters between 50 and 150 metres deep, it was difficult to distinguish from other ciscoes and was possibly the same species as the shortjaw cisco. The deepwater cisco was distinguished by usually having fewer than 33 gill rakers, relatively long pectoral fins, and unpigmented jaws. It was a silvery colour with a pink or purple lustre and a green or blue back. It spawned in August and September, earlier than most other ciscoes. Not much is known about its specific behaviors. Its diet consisted of Mysis relicta, fingernail clams, and various aquatic insects. Because of its large size, the deepwater cisco was heavily fished commercially. The main reasons for its extinction was a combination of competition from the invasive alewife, predation by the introduced sea lamprey, and commercial fishing in the Great Lakes.
Part of the reason of the lack of information of this subspecies exists could be attributed to the commercialfishing during the early 1930s, before deep water species analysis could really take place. The deepwater cisco population was abundant before the 1900s as it roamed around Lake Huron and Michigan. Even before the large-scale commercial fishing commenced in Lake Michigan, larger ciscoes like the deepwater cisco had no natural predators. It was not until the time period around 1930 to 1932 that the subspecies was subject to commercial fishing exploitation. The deepwater cisco, also known as chubs in fishing terms, was prized for its larger-than-normal size. During this exploitation, the numbers of this subspecies population declined dramatically. The last specimens of deepwater ciscoes were recorded in Lake Huron in 1952 and Lake Michigan in 1951, being declared extinct shortly after. When a re-evaluation was conducted in several states that border with Lake Michigan during 1960 to 1961, there was no identifiable subspecies of the deepwater cisco of the catching of samplings, thus they were still extinct. It was after the excessive fishing that the invading sea lamprey started to finish off the remaining population in the 1940s.
