Lake whitefish (Coregonus clupeaformis) in Lake Simcoe COSEWIC assessment and status report: chapter 6

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Biology

Reproduction

Lake Simcoe lake whitefish abundance declined dramatically in the 1970s, largely as a result of recruitment failure. Annual stocking of Lake Simcoe strain lake whitefish began in 1982 and stocked fish now constitute the majority of the Lake Simcoe lake whitefish population. However, wild lake whitefish continue to be present, three decades after recruitment problems began. There are several possible explanations for the continued presence of wild lake whitefish including: some successful natural reproduction still takes place, hatchery reared fish being mistaken for wild fish, or the fish are extremely long-lived.

The age of wild lake whitefish is a key factor in determining whether successful natural reproduction has taken place during the past three decades. Unfortunately, the accuracy of scale age assessment can be very poor, particularly for older fish. There is a high degree of confidence in scale age assessment of hatchery-reared fish less than 7 years of age, when using fin clips to identify possible ages of the fish. All lake whitefish stocked into Lake Simcoe since 1982 have been fin clipped with one of nine possible fin clips or fin clip combinations. The proportion of wild fish assessed as less than 7 years of age in the catch on spawning shoals and during the winter fishery has generally been very low. There were several peaks indicating the potential for occasional successful year classes of wild fish (1976 and 1999 in fall index trap netting as well as 1982, 1987, 1990 and 1992 in the winter fishery). However, none of the peaks were observed in both the catch on spawning shoals and during the winter fishery or in successive years, suggesting that they may not be the result of sporadic successful recruitment of wild year classes.

Other evidence indicates that Lake Simcoe lake whitefish did reproduce naturally, even during times of recruitment problems. Larval lake whitefish were captured during larval surface trawling conducted from 1975 to 1981, indicating that viable gametes were deposited and were capable of incubating and hatching. Unfortunately, larval trawling records for Lake Simcoe do not exist prior to the decline of the lake whitefish population for comparative purposes.

Survey work conducted by the Lake Simcoe Fisheries Assessment Unit (LSFAU) in 2002 found that lake whitefish are still reproducing naturally in Lake Simcoe, although the magnitude or significance of these events is still unknown. Larval lake whitefish were captured in May 2002 during an equipment testing exercise by the LSFAU. In July 2002, 13 one-year-old and 1 three-year-old wild lake whitefish were captured in small mesh gillnets. It is unlikely that the 13 unclipped one-year-old specimens were unclipped hatchery-reared fish given the low incidence (0.9%) of observed clip error in the 2001 hatchery-reared year class. 

Rainbow smelt, an exotic species first introduced to Lake Simcoe in 1961, has also been implicated as a factor contributing to recruitment failure of Lake Simcoe lake whitefish. Evans and Waring (1987) suspected that the decline in lake whitefish recruitment was probably caused by competition between young rainbow smelt and lake whitefish and predation by adult rainbow smelt on young lake whitefish, and that predation appeared to be of lesser importance. The role that rainbow smelt may have played in lake whitefish recruitment failure in Lake Simcoe remains unclear.

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 since 2001. To date, a response in the success of lake whitefish recruitment has not been detected. However, LSFAU long-term monitoring programs did not 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.  Preliminary survey work conducted in 2002 cannot be used to draw conclusions about the extent of natural reproduction because there are no comparable surveys during times of recruitment failure. Continued sampling over the next few years may provide insight into the possible interaction between rainbow smelt and lake whitefish. However, drawing firm conclusions will be difficult given the multitude of other changes Lake Simcoe has experienced (e.g., changes in nutrient loads, and the introduction of zebra mussels and spiny water flea).

Lake Simcoe lake whitefish begin to reach sexual maturity at approximately 4 to 5 years of age, full maturity being reached by 8+ years for both sexes (Evans et al. 1988). The relative fecundity of Lake Simcoe lake whitefish was estimated at 21,662 eggs/kg in 1966 (Semple 1968), 18,498 eggs/kg in 1977 (Evans 1978) and 25,425 eggs/kg in 2001.

Nutrition

Lake whitefish diet shifts from a dominance of plankton to benthic organisms during their first summer. Reckahn (1970) observed that major food items of young whitefish in South Bay, Lake Huron consisted of copepods in May, cladocerans in June and early July, dipteran larvae and ostracods in late July and August, ostracods and cladocerans in September and pelecypods and dipteran larvae in October and November.

Adult lake whitefish are benthivores and their diet consists primarily of insect larvae, molluscs and amphipods (Scott and Crossman 1973). The prominent food items found in adult Lake Simcoe lake whitefish during several diet investigations were molluscs and insect larvae (Rawson 1930; Burns 1985; Amstaetter 1999, 2000; Johanson 2001). The main difference between stomach contents from earlier studies and those conducted from 1999 to 2001 was the presence of zebra mussels and spiny water flea (Bythotrephes sp.). These species were introduced into Lake Simcoe in the early 1990s. Zebra mussels were the most dominant and spiny waterflea were the fourth most abundant prey item by weight during recent summer and spring diet investigations (Amtstaetter 1999, 2000; Johanson 2001). It is important to note that the weight of zebra mussels included the shell, which does not contribute energetically to fish diet (Pothoven et al. 2001). The weights of spiny waterflea included the spine which were found by Parker et al. (2001) to have slower evacuation rates than other prey items and as a result, overestimate predation rates. Although fish and fish remains in lake whitefish stomachs were relatively rare in number, they were one of the top three items when ranked by weight (Burns 1985; Amtstaetter 1999, 2000; Johanson 2001). Lake Simcoe lake whitefish also feed on items such as salted minnows, grain, sago and macaroni which are placed in the water by winter anglers attempting to attract fish (MacCrimmon and Skobe 1970; DesJardine and Lawrence 1977).

There is no evidence that food availability is limiting the abundance or growth of Lake Simcoe lake whitefish. The size of Lake Simcoe lake whitefish is much larger than historical values (Figure 3). Since the early 1960’s, the mean weight and length of wild Lake Simcoe lake whitefish has increased by approximately 360% and 60% respectively. Possible explanations of the increase in size include an increase in the predominance of old individuals in the population and decreased intra-specific competition resulting in increased growth rate.

Figure 3.  Mean fork length of Lake Simcoe lake whitefish captured during the winter fishery on Lake Simcoe, 1976 to 2001.

Fish health and contaminants

All Lake Simcoe lake whitefish used for egg collection purposes since 1981 have been screened for disease. Very few infections have been found (S. Lord, Fish Health Laboratory, University of Guelph, pers. comm.). One case of enteric redmouth disease was found in 1989. The causative agent of bacterial kidney disease has been found sporadically in Lake Simcoe lake whitefish and is considered by the Fish Health Laboratory to be endemic in the province. A harmless parasite (Tetracotyle sp.) has been found in the hearts of almost 100% of the lake whitefish screened.

Lake whitefish are collected by the LSFAU on an ongoing basis for analysis by the Ontario Ministry of the Environment (OMOE) as part of the provincial Sport Fish Contaminant Monitoring Program. Lake Simcoe lake whitefish are tested for mercury, PCBs, mirex/photomirex, pesticides, dioxins, and furans (Ontario Ministry of the Environment 2001). Results of contaminant sampling indicate that Lake Simcoe lake whitefish have always had very low contaminant levels.

Stocking history

Lake whitefish fry were stocked into Lake Simcoe periodically from 1888 to 1955. MacCrimmon and Skobe (1970) reported that lake whitefish stocked into Lake Simcoe as fry were, for many years, from Georgian Bay stock reared at the provincial fish hatchery in Collingwood. It is possible that fry of Georgian Bay origin that were released into Lake Simcoe survived to contribute to the spawning population, but the probability of such an event or the contribution that these fish have made to Lake Simcoe’s lake whitefish population is unknown. Millar (1946), Dymond (1956), Christie (1963), MacCrimmon and Skobe (1970), Tuunainen (1982) and Salojärvi (1992b) suggested that planting fry in a lake with a naturally reproducing population does not have an affect on adult abundance.

In the early 1980s, lake whitefish recruitment failure had become evident and efforts to conserve the stock had become a major priority for the OMNR. Through the combined efforts of fish research and culture, the OMNR pioneered the development of rearing techniques that allowed the production of sufficient numbers of yearling whitefish to maintain a viable population.

In 1982, an intensive stocking program began where advanced life stages (yearling and fall fingerling) of Lake Simcoe lake whitefish were stocked into Lake Simcoe. From 1982 to 2002, 2,538,657 lake whitefish were stocked into Lake Simcoe (Table 1). To maintain the genetic strain, only fish captured in Lake Simcoe were used as parent stock since 1982. This stocking program was initiated as a rehabilitation action in response to the decline in lake whitefish abundance detected in the 1970s. The primary objective of the Lake Simcoe lake whitefish stocking program is to maintain the native stock until such time that natural reproduction can be restored while also maintaining a large recreational fishery for this species.