Lake whitefish (Coregonus clupeaformis) in Lake Simcoe COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

The decline of lake whitefish, lake trout (Salvelinus namaycush) and cisco (C. artedi) has been attributed to nutrient loading and accelerated eutrophication and its impacts on spawning and hypolimnetic habitat in Lake Simcoe (Evans 1978; Evans et al. 1988; Evans et al. 1996). There has been a threefold increase in phosphorus loading from pre-settlement rates (Johnson and Nicholls 1989) which has affected water quality. Several species of zooplankton indicative of eutrophic states are now common in Lake Simcoe. The scarcity of one oligotrophic indicator coupled with the absence of another, suggest the impairment of the deep cold-water habitat of Lake Simcoe (Nicholls and Tudorancea 2001). Evans et al. (1996) reported that the volume-weighted temperature-corrected hypolimnetic dissolved oxygen concentration from August 30th to September 19thdeclined from approximately 4.5 mg/L in 1975 to 2.0 mg/L by 1993. Recruitment failure of lake trout, lake whitefish and cisco occurred in the 1960s, 1970s and 1980s, respectively. More recently, declines in the abundance of burbot (Lota lota) have also been observed. This order is the same as the order in which these fish spawn in Lake Simcoe. Lake trout, lake whitefish, cisco and burbot spawn in October, November, December and January, respectively. Smith (1972) reported a similar sequence in which cultural eutrophication adversely affected groups of fish in the Great Lakes. However, he indicated that declines in cisco took place before declines in lake whitefish.

Rainbow smelt (Osmerus mordax) was first documented in Lake Simcoe in 1961 and by the early 1970s had become very well established. The timing of rainbow smelt expansion in the late 1960s coincided very closely with recruitment failure of lake whitefish. Evans and Waring (1987) suspected that the decline in lake whitefish recruitment in Lake Simcoe was probably caused by competition between young rainbow smelt and lake whitefish or predation by adult rainbow smelt on young lake whitefish, and that predation appeared to be of lesser importance. It has been documented that rainbow smelt prey upon larval lake whitefish and cisco (Loftus and Hulsman 1986; Evans and Loftus 1987). However, rainbow smelt piscivory on lake whitefish has not been observed in Lake Simcoe (Day and DesJardine 1975; MacCrimmon and Pugsley 1979). Evans and Loftus (1987) found that in 13 of 24 case studies of rainbow smelt introduction to Ontario lakes that there was a reduction in lake whitefish recruitment documented (Lake Simcoe was included in their study). Reckahn (1970) found that intra-specific competition between young lake whitefish in South Bay, Lake Huron was likely much more significant than competition with other species including rainbow smelt. The role that rainbow smelt may have played in lake whitefish recruitment failure in Lake Simcoe is unclear. However, there is little doubt that any potential effect would have acted only on juvenile fish less than six months of age given the high survival of hatchery-reared lake whitefish in the presence of a large rainbow smelt population throughout the 1980s and early 1990s.

The continued success of cisco recruitment during the rapid expansion of rainbow smelt provides insight into the potential effects of the introduction of rainbow smelt on lake whitefish. Given the overlap in distribution of larval cisco and lake whitefish (DesJardine 1979; Cucin and Faber 1985), any effects of rainbow smelt on lake whitefish would also be expected on cisco at this life stage. There is also a large degree of overlap in the diet items of young lake whitefish including copepods and cladocerans. As a result, any competitive effect that could affect lake whitefish and not cisco would be the result of spatial segregation between the two species. Post-larval lake whitefish and cisco do occupy different habitat types, benthic and pelagic, respectively. However, given the diel behaviour exhibited by rainbow smelt during the summer period, they share habitat with both lake whitefish and cisco. For example, at night, adult rainbow smelt disperse from the lake bottom into the water column and often into the epilimnion (Ferguson 1965; Heist and Swenson 1983) while young-of-year rainbow smelt appear to move from the epilimnion to the hypolimnion at night (Brandt et al. 1980).

The catch of rainbow smelt during the winter fishery on Lake Simcoe has declined since 1989. By 1999, catch decreased to levels observed during the mid-1960s and has remained low through to 2001. To date, a response in the success of lake whitefish recruitment to the decrease in rainbow smelt abundance has not been detected.

However, Lake Simcoe fish monitoring programs rarely capture juvenile fish and, as a result, a lag time of several years exists between a potential change in the success of natural recruitment and observations of results. Continued sampling over the next few years may provide more evidence relating to the potential interactions between rainbow smelt and lake whitefish.

Zebra mussels (Dreissena polymorpha) and spiny waterflea (Bythotrephes sp.) were first observed in Lake Simcoe in 1992 and 1994, respectively. Given their recent introduction, these species could not have affected lake whitefish recruitment failure in the late 1960s. There is no evidence that these species have negatively affected growth or survival of hatchery-reared Lake Simcoe lake whitefish. However, the effect, if any, that zebra mussels or spiny waterflea would have on juvenile lake whitefish less than six months of age is unknown. Since the introduction of these species has the potential to alter the structure of the zooplankton community (MacIsaac 1996; Yan and Pawson 1997), it could possibly alter prey availability for juvenile lake whitefish. Evans (pers. comm.) has also suggested these species along with lake trout predation may have contributed to recruitment failure in cisco, which is now evident and the decline in smelt by altering their prey abundance. He recently hypothesized that a decline in smelt abundance and the relatively low numbers of young-of-the-year smelt occupying early juvenile whitefish habitat may help explain the continuation of low levels of lake whitefish recruitment and the resurgence of slimy and spoonhead sculpins.