Shortjaw cisco COSEWIC assessment and status report: chapter 4

• Previous page
• Table of contents
• Next page

Introduction

The shortjaw cisco (Coregonus zenithicus) is a widespread species in the salmonid subfamily, Coregoninae. Originally described from Lake Superior at Duluth, Minnesota (the “Zenith” city) by Jordan and Evermann (1909), it was subsequently discovered in most of the Laurentian Great Lakes and many smaller lakes in central North America (Scott and Crossman 1973; Clarke 1973; Clarke and Todd 1980). Elliptical in body shape, shortjaw ciscoes, like other ciscoes, are laterally compressed and are covered with large, smooth scales (Figure 1). Generally silver in color with little pigmentation on the paired fins, these fish are olive or tan dorsally shading to white ventrally. The mouth is small and toothless, and the lower jaw is generally even with the upper jaw or shorter and included within the gape of the upper jaw (Eddy and Underhill 1978; Becker 1983). The lower jaw may occasionally extend beyond the premaxillaries in some populations, and, indeed, Jordan and Evermann (1909) typify populations in Lake Superior in this way, although this is inaccurate. The premaxillaries generally make a distinct angle on the snout in contrast to most other cisco species where the premaxillaries are generally in line with the slope of the head or make only a very minor angle at the snout. The gill rakers on the first branchial arch generally number less than 40, and are often in the mid-30s in contrast to most other cisco species that have counts of not only more than 40 but even 45-55.  In addition, the gill rakers tend to be moderate or short in length compared to those of most other cisco species (Becker 1983). Unfortunately, no single diagnostic character exists with which to identify the species, but rather an association of characters must be used, of which the single most important is gill raker number. Considerable variation in size exists across the range of the species, and adults of some populations (e.g., George Lake, Manitoba and White Partridge Lake, Ontario) measure less than 150 mm standard length (SL) while adults of other populations reach lengths greater than 300 mm SL up to a maximum of about 400 mm (e.g., Lake Nipigon, Ontario). Large specimens generally approach 300 g in mass, and exceptionally large fish can reach 1.0 kg. The biology is best known in the Great Lakes (including Lake Nipigon) where the species was once a major component of vigorous food fisheries, occupying intermediate depths between 20-180 m.