WALLEYE : suggested factors for decline

WALLEYE, Factors suggested for Decline

From: The Status of Walleye in Nipigon Bay Area of Concern: 2012

Prepared for: Environment Canada

By: Terry Marshall

March 31, 2013

There has been much debate over what was responsible for the dramatic decline in the walleye population in Nipigon Bay.

A number of factors have been suggested, including overfishing and habitat degradation [Ryder 1968, Wilson et al 2007].  Sea Lamprey control may also have played a role.  Kelso and Cullis [1996] provide a detailed timeline of these various perturbations.

EXPLOITATION

At the time of collapse of the walleye population in Nipigon Bay, less was understood about the dangers of overfishing, with the feeling that fish populations were able to compensate for large reductions in their abundance.  In 1956, in a review of walleye dynamics in the Nipigon River during the peak of commercial harvest, Ryder [1956] reported “… the commercial catch has increased immensely over the past two years, thus reducing the competitive factor among pickerel themselves.  The drastic reduction of Lake Trout … removes a competitive factor making more food available to the pickerel.  It might be concluded then, that the present rate of exploitation is far below the maximum catch that could be taken to improve the quality of  the population… the harvest has not yet approached the point where optimal benefits to the pickerel population and subsequently to the angler are received.”

This proved to be false, as the walleye population rapidly declined over the next few years.

In Nipigon Bay, the commercial harvest was a classic example of fishing a stock down to insignificance.

Within this Bay, walleye were very concentrated post-spawn, and gillnet and poundnet operations targeted them very effectively.  The gillnet catch-per-unit-effort [CUE] remained extremely high through the latter period [1959-63] of reduced abundance, revealing the efficiency of the fishermen as they became more attuned to the fish’s seasonal movements [Ryder 1968]. In addition , a substantial angling fishery also existed [Schram et al 1991]

The walleye catch in Nipigon Bay from all commercial gear during the eight peak years of harvest prior to the collapse [1951-1959, 1956 excluded]  totalled 97,245 kg.  The average weight of walleye in the catch can be assumed to be similar to that reported for the Black Bay harvest , which was 0.87kg [P. Addison, pers. Comm.].  This then translates into an annual harvest of about 14,000 walleye which when related to the estimated population size of 41,000 mature fish [Ryder 1968] implies an annual exploitation rate of 34% [or higher, including the angling harvest].  While this high of an exploitation rate may arguably be sustainable in more southern locales [ Schmalz et al 2011], it has never proven to be the case in the colder waters of Ontario [Baccante and Colby 1996]

Nipigon Bay and the Nipigon River were closed to commercial fishing for walleye in 1984 and to angling in 1989, along with the Jackfish River.