Lake whitefish (Coregonus clupeaformis) in Lake Simcoe COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

Total catch of lake whitefish during their spawning run at two shoals in Lake Simcoe decreased from the late 1970s to the late 80s, then increased to 1991 (Figure 4). The increase in catch in the 1990s was largely the result of hatchery-reared fish, which have been planted in Lake Simcoe annually since 1982. It is interesting to note that when hatchery-reared lake whitefish began showing up on the spawning shoals in large numbers (1990 and 1991) that the catch of wild fish also increased. Since 1991, total catch has been variable and has not indicated any changing trend. However, the catch of wild fish has decreased while the catch of hatchery-reared fish has increased.

Figure 4. Catch of lake whitefish during the fall index trap netting program at the North Georgina and Strawberry Island spawning shoals in Lake Simcoe, 1978 to 2000.

Fall index trap netting methodology was inconsistent with respect to net size, sampling period and location prior to 1978 making comparisons of total catch at the North Georgina and Strawberry Island sites difficult. However, historical comparisons can be made to a portion of the catch at Strawberry Island. A season of October 15th to the 26th was netted at Strawberry Island for many years dating back to 1959. This time represents only the early portion of the lake whitefish spawning run. Figure 5 indicates that catch of lake whitefish at Strawberry Island was much greater during the early years of the program, especially during the mid-1960s, than it was over the past two decades. The only other explanation would be that the timing of the spawning run has changed over time (i.e., lake whitefish moved to spawning shoals earlier in the year during the 1960s and 70s). However, the timing of the spawning run has not changed since 1977 (Amtstaetter 2002) and MacCrimmon and Skobe (1970) reported similar results with respect to timing of the spawning run during the 1960s. Although the catch data in Figure 5 includes a short period of time (11 days) during a very early portion of the spawning run, it sheds insight into the historical magnitude of the spawning run.

Figure 5. Estimated catch of lake whitefish during fall index trap netting on Lake Simcoe from the period of October 15th to the 26th at StrawberryIsland, 1959 to 2000.

Estimated catch of lake whitefish during the winter fishery on Lake Simcoe was high throughout the 1960s and then decreased to its lowest level by 1977 (Figure 6). Since then, estimated catch has increased, but levels are still much lower than those recorded early in the program. The number of hatchery-reared fish in the catch has increased since stocking began in 1982 and the catch of wild fish has remained consistent since the early 1980s. However, it is important to note that the amount of fishing effort exerted during the winter fishery has increased for all anglers as well as for anglers specifically targeting lake whitefish (Figure 7). The catch rate of wild lake whitefish for anglers targeting the species has had a 15 year decline of 60%.

Figure 6. Estimated catch of lake whitefish during the winter fishery on Lake Simcoe adjusted to a 50-day season, 1961 to 2001.

Figure 7. Estimated angling effort during the winter fishery on Lake Simcoe adjusted to a 50-day season, 1961 to 2001.

The estimated catch of lake whitefish during the summer fishery is much lower than that of the winter fishery. The highest estimated catch during the summer for which data are available (1981 to 1998) was close to 3500 fish in 1998 (Figure 8). Due to differences in summer creel survey methodology prior to 1981, including creel survey duration and the type of information collected, comparisons of catch to earlier surveys are difficult. The only comparable statistic throughout the history of the summer creel survey is observed catch per unit effort (CPUE) of all anglers. Comparisons of observed CPUE of all anglers was much higher during the 1960s and decreased sharply by 1970. A gradual decreasing trend continued until 1993 and in 1998, CPUE increased to values recorded in the 1970s (Figure 8). This may have been due in part to an increase in summer angling effort specifically targeting lake whitefish in 1998 (Figure 9). The contribution of hatchery-reared and wild fish to the catch was not recorded, but the presence of stocked fish most certainly contributed to the increase in lake whitefish catch in 1998.

Figure 8. Observed catch per unit effort and estimated catch of lake whitefish during the summer fishery on Lake Simcoe, 1960 to 1998.

Figure 9. Estimated angling effort during the Lake Simcoesummer fishery, 1977 to 1998.

The mean fork length of Lake Simcoe lake whitefish captured on spawning shoals and during the winter fishery has increased over time (Figures 3 and Figure10). The mean fork length of wild fish has increased at a relatively consistent rate of 2.5 to 3 mm per year from 1976 to 1997 and at approximately 10 mm per year from the mid-1960s to the mid-1970s. Since 1992, when the first large stocking events (>150,000 fish per year) contributed significantly to the catch, the mean size of hatchery-reared fish has increased at a rate of 7 to 10 mm per year. Total length values measured during the 1960s were converted to fork length using the equation: FLEN=0.951(TLEN)-19.12 mm. This equation was determined using data from the 1969 fall index trap netting program, which included measurements of both fork and total length.

Figure 10. Mean fork length of Lake Simcoe lake whitefish captured during fall index trap netting. Data from 1976 to 2000 include fish captured at the North Georgina and Strawberry Island spawning shoals. Data from 1964, 1969, and 1974 include fish captured from all sites.

The mean weight of lake whitefish captured during the winter fishery on Lake Simcoe in recent years was much greater than historical records (Figure 11). Mean weight of wild fish has increased by 35 g per year since 1978 and the mean weight of hatchery-reared fish has increased by 105 g per year since 1992. MacCrimmon and Skobe (1970) reported the mean weight of fish captured during the winter fishery was between 340 and 570 g. It is interesting to note that Rawson (1930) reported the mean weight of several thousand lake whitefish harvested in 1928 was 510 g. The mean weight recorded in 2001 was more than four times the historical measure. Furthermore, fish greater than 910 g were considered rare in historical catches and during the 2001 winter fishery, the smallest fish measured was 975 g.

There are several possible explanations for the increase in the size of Lake Simcoe lake whitefish since the mid-1960s. These possibilities include an increase in the predominance of old individuals in the population and decreased intra-specific competition. As a result of recruitment problems in the 1960s, the predominance of older and larger lake whitefish in the lake increased. Although an increase in the mean size of the fish in the population would be expected as a result of an older population, it does not explain the observed increase in growth rates (Amtstaetter 2002). Many studies have noted an inverse relationship between whitefish growth and population size (Healy 1980; Jensen 1981; Salojärvi 1992a; Salonen et al. 1998). Rawson (1930) noted the relatively small size of Lake Simcoe lake whitefish and suggested that it was likely the result of overcrowding and a resultant competition for food. Colby et al. (1972) reported that salmonid communities may exhibit an increase in growth rates as a result of eutrophication in oligotrophic lakes.

Figure 11. Mean weight of lake whitefish captured during the winter fishery on Lake Simcoe, 1978 to 2001.

The dramatic decrease in the abundance of lake whitefish in the early 1970s was the result of recruitment failure. However, wild (unclipped) fish continue to be captured in Lake Simcoe three decades later. There are several possible explanations for their continued presence including: some successful natural recruitment still takes place, hatchery-reared fish being mistaken for wild fish, or the fish are extremely long lived. Determining the age of these unclipped fish, and whether they are of hatchery origin, is important in determining the status of the wild lake whitefish population.

The age of wild lake whitefish is a key factor in determining whether successful natural recruitment has taken place during the last three decades. Most age assessment of Lake Simcoe lake whitefish has depended on scales. Typically, scale age assessment underestimates the age of fish, especially with slow growing individuals (Mills and Beamish 1980; Casselman 1983). Scale age assessment for hatchery-reared lake whitefish in Lake Simcoe has been more accurate than for wild fish because fin clip information was used to identify possible year classes to which the fish could belong. Difficulties in assessing the age of hatchery-reared lake whitefish begin when the fish reach an age of seven years and increase as the fish increase in age. There is no validation of scale age assessment of wild Lake Simcoe lake whitefish less than seven years of age. However, given the relatively young age and the success of age assessment of hatchery-reared fish, scale age assessment should identify unclipped fish less than seven years of age. To investigate whether successful natural recruitment has taken place, the proportion of unclipped fish, less than seven years of age in the catch was examined.

The proportion of wild fish assessed as less than seven years of age in the catch on spawning shoals and during the winter fishery has been very low (Figures 12 and Figure13). There were several peaks indicating the potential for occasional successful year classes of wild fish (1976 and 1999 fall index trap netting and 1982, 1987, 1990, and 1992 winter fishery). However, none of the peaks were observed in both programs or in successive years, indicating that they were probably not the result of sporadic successful recruitment of wild year classes. The possibility remains that some of these fish assessed as young may actually be older individuals whose ages were underestimated. During periods of successful natural recruitment (1960s), age frequency distributions indicate that 44 to 50% of the wild fish were assessed as less than seven years of age. There is little doubt that a shift in the age structure of wild Lake Simcoe lake whitefish has occurred.

Figure 12. Proportion of lake whitefish assessed as less than seven years of age in the catch at the North Georgina and Strawberry Island spawning shoals during fall index trap netting, 1976 to 2001.

Figure 13. Proportion of lake whitefish assessed as less than seven years of age in the catch during the winter fishery, 1976 to 2000.

Not all of the unclipped lake whitefish captured and assessed as less than seven years of age can be explained by underestimation of the age of fish. There were nine fish captured during the past five years that were less than 430 mm in fork length. Comparisons with growth rates of hatchery-reared lake whitefish suggest that these unclipped fish were truly less than seven years of age. However, the extent of the age assessment difficulties for larger fish can still not be determined. Therefore, our best estimation must rely on scale age assessment which indicates that since 1976, young (< seven years of age) unclipped fish contribute an average of 2.5% and 3.6% of the catch of unclipped fish during the spawning run and winter fishery, respectively.

The possibility remains that young, unclipped Lake Simcoe lake whitefish may actually be hatchery-reared fish. There are several factors that could contribute to this result including fin clips not being applied to all hatchery-reared fish, regeneration of fin clips, and fin clips not being identified at time of recapture. Fin clip assessment of hatchery-reared fish prior to stocking indicates that missed or poor fin clips existed. Since the 1987 year class, approximately 0.78% of the fish stocked had no fin clip applied and 0.99% of the fish stocked had poor clips applied. Poor fin clips were defined as less than 50% complete. The presence of these young, unclipped hatchery-reared fish could artificially inflate the calculated value of the proportion of young, wild fish in the catch.

Although fin clipping error and scale age assessment difficulties can explain the presence of young, unclipped fish in the catch, it does not prove the complete absence of natural recruitment to maturity. In fact, a few small, young individuals were captured during both the spawning run and the winter fishery before juveniles were stocked into Lake Simcoe. If a small number of wild fish survived to be captured during the early 1980s, then it is possible that some wild individuals currently survive to maturity.

Since natural recruitment problems of lake whitefish in Lake Simcoe began in the late 1960s and only a small proportion of unclipped fish captured can be attributed to clipping error or recent natural recruitment to maturity, most of the unclipped fish must be very old. There are several pieces of evidence that suggest this could be the case. Firstly, a lake whitefish tag recapture shows that an individual tagged as a mature adult was recaptured 28 years later. There is no doubt that this fish was greater than 33 years of age at time of recapture. Secondly, otolith samples from 56 unclipped Lake Simcoe lake whitefish collected in 1990, 1999 and 2000 were prepared using an acid etching technique and assessed for age. Results indicated that 54 of the 56 fish were from the 1969 year class or earlier, these fish ranged in age from 30 to 48 years. Finally, it appears that some lake whitefish which had their adipose fin removed as part of a mark recapture study in 1972 (N>10,000) and 1975 (N unknown) still exist in Lake Simcoe (Amtstaetter 2002).

It is interesting to note that if most of the wild fish in Lake Simcoe are very old fish, that such a large number were captured during the fall spawning run when hatchery-reared lake whitefish were first captured in large numbers (1990 and 1991). This was probably not the result of high catch due to one or two years of favourable spawning conditions (e.g., weather) attracting more fish to the shoals because the decline in catch over the following years was gradual. It appears that the presence of hatchery-reared fish on the spawning shoals may have affected the magnitude of the catch of wild fish.

Summary

Strong corroboration relating to adult lake whitefish abundance and size in Lake Simcoe exist between all programs. Generally, catches were high during the 1960s and decreased sharply by the early 1970s. Since then, catch has remained relatively low compared to catches during the 1960s, but has increased somewhat as a result of the annual stocking of hatchery-reared fish. Wild lake whitefish continue to contribute to the recreational fisheries as well as to trap net catches on spawning shoals during the fall. However, catch rates of wild fish during the winter fishery have decreased since 1986 and catch has decreased during the fall index trap netting program since 1992. The size of Lake Simcoe lake whitefish is much larger than historical values. Since the early 1960s the mean weight and mean length of wild fish 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. The mean size of wild and hatchery-reared fish are currently very similar.

Currently, the Lake Simcoe lake whitefish population is made up largely of hatchery-reared fish along with a smaller population of wild fish. Catch rates of wild fish are extremely low compared with data from the 1960s. Evidence suggests that, currently, the wild lake whitefish population is made up mostly of old individuals that were the result of successful recruitment in the 1960s. While it is possible that some successful natural recruitment to maturity still takes place, the magnitude of such events appears to be small and has little bearing on the size and age structure of the population.