| Citation |
Sebastian, D., R. Dolighan, H. Andrusak, J. Hume, P. Woodruff, and G. Scholten (2003) Summary of Quesnel Lake kokanee and rainbow trout biology with reference to sockeye salmon, Stock Management Report No. 17, 2003. Province of British Columbia. |
| Abstract/Description or Keywords |
All available data on Quesnel Lake kokanee and rainbow trout are summarized to provide an updated assessment of the status of the lakes’ populations. Kokanee data suggests there are two distinct populations that may well be reproductively isolated. The stream spawning component historically spawned in reaches of Wasco Creek, the Horsefly River, little Horsefly River and the Mitchell River from mid-September to mid-October. Data from the 1950s and 1960s suggest the stream spawner numbers may have ranged from 50,000 - 100,000 but today there are less than 1000. Shore spawner numbers could not be quantified but they continue to spawn in a few known locations identified in the early 1950s from mid-October to early November. There does not appear to be much difference in size and age at maturity between the stream and shore spawners. Studies on kokanee in the 1950s and 1960s by the International Pacific Salmon Fisheries Commission (IPSFC) focused on the hypothesis that kokanee competed with sockeye juveniles for food and were the cause of cyclical dominance in Quesnel lake sockeye. This work concluded that kokanee did not compete for food, interspecific competition was not a factor causing cyclical dominance and some data indicated that large sockeye numbers could adversely impact kokanee. There is evidence from the hydroacoustic and trawl survey data that suggests a decline in total numbers of kokanee during the 1990s and early 2000s. The total kokanee population in the 2000s has been estimated at only 1.5-2.2 million compared with 3.5 million in the early 1990s. Based on a large lake kokanee biostandard of 5-7% of the total population size a crude estimate of total spawners ranges from 75,000-150,000. Compared to other similar sized lakes such as Kootenay, Okanagan, Arrow and Shuswap the estimated number of kokanee spawners in Quesnel Lake is exceptionally low. In sharp contrast, the Quesnel Lake sockeye population has increased dramatically during the last decade with annual acoustic estimates ranging from 4-65 million fry. It is speculated that the decline in kokanee is in response to increased competition for food due to significantly increased numbers of sockeye fry. There is no evidence that kokanee spawning habitat has deteriorated to any degree to account for the decline in numbers. It is further speculated that the stream spawning component has declined while the shore spawners have not because stream spawner progeny emerge much later than sockeye fry and shore spawning kokanee fry. Rainbow trout spawn in the Horsefly and Mitchell rivers with the former system appearing to support the largest numbers. These trout grow to a large size and are the target of a very popular sport fishery. Some crude estimates of escapement numbers place the Horsefly River numbers between 500-800 and the Mitchell River numbers likely less than half of this estimate. While these trout reside in the lake for most of their adult life some also undertake migrations into the two rivers to feed on emerging sockeye fry in the spring and on eggs later in the summer and early fall. These migrations are well known to anglers and some excellent river fishing is experienced throughout the summer and fall. Quesnel Lake rainbow trout life history is similar to other piscivorous rainbow trout found in Shuswap, Kootenay and Okanagan lakes. They are highly piscivorous, grow to large size (7-10 kg) and generally mature at age 6 or 7. The juvenile fish appear to be highly reliant on stream rearing for one or two years in the Horsefly and Mitchell rivers prior to entering the lake. A decrease in the average size of spawners may have been in part due to some over fishing in the past, however recent age analyses indicates that not only size but rainbow growth rates have declined over the past decade. Stomach sampling confirmed that their primary food source is age 1-3 kokanee which have been declining in numbers over the same period. Diet analysis showed that as the numbers of sockeye fry increased following dominant and subdominant (i.e. high) return years, the numbers of age 1 and 2 kokanee eaten declined and the total biomass of O. nerka consumed (per stomach) declined by up to 70%. It follows then that recent increases in the numbers of sockeye and concurrent declines in abundance of kokanee may be the primary cause of declining size and size-atage of the Quesnel Lake rainbow trout. Recommendations for further study include continuation of the annual kokanee trawl and acoustics survey in cooperation with DFO. This data provides the best insight into the status of the kokanee population. Improvements to kokanee escapement estimates are required and some index sites for both stream and shore spawners are proposed. The Horsefly River rainbow trout spawning population should continue to be monitored using a resistivity counter. Rainbow trout spawner enumeration work in 2002 and 2003 on the Horsefly system needs to be summarized and incorporated in an updated version of this report. Better biological data for both kokanee and rainbow trout is required, especially in view of the prospect of continued decline in kokanee and lower rainbow trout growth rates. Harvest estimates are required for the large rainbow trout and some consideration should be given to implementing a restricted tag or stamp system similar to those in place on Shuswap and Kootenay lakes. Concern expressed by fisheries biologists and the public regarding the current status (declining) of the rainbow trout and kokanee populations is supported by biological data in this report. Finally, it is clear that kokanee are a significant prey item that influences the growth of predator populations in Quesnel Lake and therefore their abundance is of paramount importance. A workshop should be held involving researchers and biologists who work on lakes with similar fish assemblages and problems. The collective experience at the workshop should greatly assist in development of an action plan for future work on Quesnel Lake. |
