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Research Document - 2013/063

The biology and relative abundance of Shortjaw Cisco (Coregonus zenithicus) in Lake Nipigon

By T.C. Pratt

Abstract

Shortjaw Cisco (Coregonus zenithicus) were originally described in Lake Nipigon in the 1920s, and have been consistently identified in the limited number of subsequent cisco surveys of the lake. With the exception of ecomorphological and genetic research conducted on the cisco community of Lake Nipigon in the late 1990s, little attention has been focused on any cisco species from this lake. DFO Science was charged with conducting a pre-COSEWIC assessment for Shortjaw Cisco, and this manuscript assesses the current:

  1. life history characteristics;
  2. the characteristics or elements of the species habitat;
  3. threats to the species; and
  4. population trends with a limited time series for Shortjaw Cisco from Lake Nipigon.

The relative abundance of Shortjaw Cisco has declined more than 50% in Lake Nipigon from the 1998-99 through 2008-09 period, and no recent strong year-classes were observed. Mean age of the population is ~12 years, and annual mortality estimates are low (11.7%). The Lake Nipigon Shortjaw Cisco population is dominated by females (sex ratio is 65% female), which appear to have greater longevity than males. Shortjaw Cisco in Lake Nipigon prefer shallower depths (mean depth of capture 30.2 m) than their conspecifics from the Great Lakes. The Shortjaw Cisco population in Lake Nipigon is a Mysis spp. specialist, with the majority of individuals consuming only this prey species. Anthropogenic threats are limited on Lake Nipigon, as only a small fishery is prosecuted on the lake. Observed declines are more likely due to food web changes due to invasive species, or competition with other native fishes.

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