Montana Field Guide

Cisco is Montana's newest fish species. They were introduced by the Montana Department of Fish, Wildlife and Parks into Fort Peck Reservoir in 1984, from Saskatchewan, to act as a forage fish for walleye and lake trout. They have produced a rapidly expanding self-reproducing population. Studies are ongoing to determine the value of Cisco as a forage fish. Cisco, also known as lake herring, are widespread across eastern and central Canada. They spawn in the fall in shallow water. Cisco are an open water or pelagic species, forming large schools at medium depths where they feed mostly on plankton.

These fish are silvery and somewhat darker on the back. The lower jaw often protrudes slightly with the mouth closed, but the jaws may be equal or the upper may be slightly longer. They have 38 or more gill rakers on the first arch, two flaps of skin between the nostrils (see Lake Whitefish for illustration), and the membrane surrounding the eye has a distinct notch below the pupil (see Mountain Whitefish for illustration).

Habitat includes deep lakes and large rivers. Pelagic species are usually found in open, deep water. They spawn over shoreline substrate.

Cisco are largely plankton feeders. Some terrestrial and emergent aquatic insects are taken, and rarely fish.

Summer kills can be a major threat to populations in inland lakes.

Successful reproduction in the state started in the fall of 1985. Spawning occurs in mid-November at 40 degrees F. in Fort Peck Reservoir. They broadcast spawn over the shoreline substrate. Eggs hatch in late April-early May and most fish mature at ages 1-4 years.