Shortjaw cisco COSEWIC assessment and status report: chapter 11

Limiting Factors

No single factor is known to be responsible for the decline of the shortjaw cisco in the Great Lakes. In Lake Erie, for example, profound ecological changes have occurred that have shifted the lake to a more mesotrophic condition (although the lake has rebounded from eutrophy because of phosphorus controls and the effects of zebra mussels), to the detriment of the deep-water, more oligotrophic community that historically existed there (Hartman 1972). While the physical conditions of Lakes Michigan and Huron have not changed much, with the notable exception of Saginaw Bay, the biological community has become considerably altered (Smith 1972). Undoubtedly, the vigorous food fisheries had a negative impact earlier in the 1900s, especially on the larger individuals, at first, then on smaller individuals as mesh sizes were reduced to maintain catch levels. However, competition and predation from rainbow smelt and alewives have certainly had more of an impact during the last 30 years--years in which the food fisheries for chubs have been much less extensive than historical levels (Crowder 1980; Rice et al. 1987; Fleischer 1992). Sea lamprey predation continues to take a toll on Great Lakes species, and affects smaller species such as chubs in addition to larger species such as lake trout, burbot, and lake whitefish (U.S.G.S., Great Lakes Science Center, unpublished data). Abiotic factors such as weather and thermal changes in the lakes have also been suspected to play a role in population destabilization (Brown et al. 1987; Eck and Wells 1987; Taylor et al. 1987). Such destabilization can favor one species over another, resulting in competitive displacement or hybridization (Smith 1964; Todd and Stedman 1989; Davis and Todd 1998). The overwhelming preponderance of work on Great Lakes chubs has been with adults or relatively large young-of-the-year fish, primarily because of their vulnerability to capturing gear in both food fisheries and scientific assessment programs. The growing knowledge of how biotic and abiotic changes in the system have influenced populations has revealed that the larval and juvenile stages are the most vulnerable, and factors limiting survival at these stages need better understanding.

Factors limiting populations in smaller lakes are essentially unknown. Rainbow smelt have been introduced into many cisco lakes, including Lake Nipigon, Ontario, and have had a noticeable effect on the ecological makeup of those systems (Wain 1993; Franzin et al. 1994). The effects that rainbow smelt have on reducing and altering zooplankton populations have apparently not resulted in the extirpation of populations of shortjaw ciscoes, but alarm should be taken in situations of sympatric occurrence of rainbow smelt and shortjaw ciscoes in small lacustrine systems because of the potential for ecological destabilization.

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