Recreational Fishery Stock Assessment 1999 Final Report

PHOTO 1. Sawmill Lake Aerial Photo. June 2001. Click image to view 62K JPG

Introduction

This report presents the results of a stock assessment of Sawmill Lake, completed on August 19, 1999 under a partnership arrangement between the Ministry of Environment, Lands and Parks and the Carrier Sekani Tribal Council (CSTC), with funding obtained from Fisheries Renewal B.C. through the Upper Fraser-Nechako Fisheries Council. Field activities were carried out by Margo French and Lawrence Ward of the CSTC. Analysis and reporting of the field results were conducted by the author. Inquiries pertaining to this report should be directed to the author at the email and address located at the bottom of this page.

Sawmill Lake was initially surveyed in 1985 and was found to support a modest population of rainbow trout as well as lake chub at that time. The lake was subsequently stocked in 1986 with rainbow trout and has received 2,500 yearlings per year since that time. No follow-up surveys had been conducted on the lake to determine whether the stocking program was meeting its objective to produce a viable recreational fishery. Sawmill Lake was therefore assigned a high priority for assessment in 1999. Click here for directions to Sawmill Lake.

Methods

FIGURE 1. Location of Sawmill Lake gill net sets, August 17-19, 1999. Click image to view full bathymetric map. Click image to view 49K GIF

An experimental, 91.2m sinking monofilament gill net was set in Sawmill Lake on August 17, 1999 according to the methods specified in the Resource Inventory Committee document titled Fish Collection Methods and Standards. The net was deployed at 1:00 PM and retrieved on August 18 at 10:00 AM. No fish were captured after this period, so the net was redeployed at 10:30 AM at a different site (Figure 1). This second net set was retrieved on August 19 at 9:45 AM, for a total combined soak time of 44.25 hrs.

Set 1 was positioned from the north shore and extended in a SSE orientation, to a maximum depth of 16.25 m. Set 2 was positioned off the southeast shore in a N-S orientation, east of the first set. The second set also ranged in depth from the surface to 16.25 m.

All trout collected were sampled for fork length, weight, sex, maturity, and stomach contents. The general condition of the fish sampled was also recorded. Scale samples were collected for age structure analysis. This analysis was performed by Darlene Gillespie of TimeMark Consulting Ltd. (Nanaimo, B.C.).

Water temperature, weather conditions, and a Secchi disk reading were also reported.

Results and Discussion

CATCH SUMMARY

The net catch yielded 36 rainbow trout (RB) and 13 lake chub, for a catch per unit effort (CPUE) of 0.8 RB per net-hour. The complete data set can be viewed in Appendix 1. Twenty-eight of the 36 rainbow trout sampled were classified as "immature" and their sex could therefore not be determined. Of the remaining 8 fish, 3 were male and 5 were female.

CONDITION

The weight of sampled Sawmill Lake rainbow trout increased according to the equation W = 1x10^-5 x L^2.9761 (R²=0.9926); this relationship is expressed in Figure 2. The small number of adults sampled precludes an in-depth analysis of the health of Sawmill Lake rainbow trout. There were no anomolies noted in the length-weight relationship, however 2 of the 8 adults sampled exhibited an egg bound condition. This suggests that spawning habitat may be limited or nonexistent in Sawmill Lake, which concurs with the findings of the original lake survey conducted in 1985.

FIGURE 2. Length vs. weight of rainbow trout captured in Sawmill Lake, August 19, 1999. Click image to view 5K GIF

FIGURE 3. Length frequency distribution of rainbow trout captured in Sawmill Lake, August 19, 1999. Click image to view 7K GIF

FIGURE 4. Age vs. length of rainbow trout captured in Sawmill Lake, August 19, 1999 Click image to view 7K GIF

LENGTH FREQUENCY DISTRIBUTION

Figure 3 illustrates the length-frequency distribution of rainbow trout sampled in 1999. The majority of trout were 2-year-olds in the 110 - 140 mm range, which were stocked as yearlings into Sawmill Lake on May 29, 1998. A gap is evident in the 150 - 230 mm size range, and this gap is also reflected by the absence of three-year-olds in the sample (see Appendix 1), a finding which concurs with the stocking release records (Appendix 2) which indicate that Sawmill Lake was not stocked in 1997.

The remainder of the sample is comprised of 4- to 9- year old trout (see below), with most catchable trout falling within 230 - 330 mm.

GROWTH

Figure 4 shows the length-at-age relationship for rainbow trout in Sawmill Lake. This population exhibits very slow growth, with 5 year old fish attaining a mean length and weight of 287 mm and 213 g respectively. Furthermore, the 1998 yearlings sampled in 1999 were stocked at a mean weight of 7.8 g (Appendix 2). At the time of the survey, these 2 year old fish averaged 18.5 g, representing a mean growth rate of only 11 g over a 64 week period.

Sawmill Lake has been stocked at a rate of 2,500 yearlings per year, however the provincial stocking formula recommends that it be stocked at 1,700 per year. The current stocking density is likely inducing downward pressure on growth rates due to intra-cohort competition for limited food resources. The provincial formula has generally been found to produce small fish when used in the lakes of the Omineca Region.

Management Recommendations

While Sawmill Lake is producing catchable trout (i.e. 25 cm and larger) at present stocking levels, the length of time required to produce these fish is at least 4 years. This fishery would be more efficiently managed by reducing the stocking density to 1000 fish per year to reduce intra-cohort competition. The effect of this reduction would result in more rapid growth rates, thereby creating catchable fish in a shorter time period with the potential for larger fish overall. A target of 3 year old catchables is reasonable given the productive capacity seen in lakes of similar size and TDS in the region.

The presence of older, larger fish in the sample suggests low exploitation by anglers, a situation which may be due to a number of factors such as the lake's distance from Prince George, lack of knowledge of the lake, or poor local conditions which may not appeal to the angling public. Increasing the profile of this lake may reduce pressure on nearby, wild systems while taking advantage of an actively managed fishery. A brief review of the lake's amenities and aesthetic value is also warranted.

The current fishery consists of well conditioned, healthy individuals with no obvious deformities or health problems. Six of the fish captured were in spawning, or ripe, condition. It is recommended that triploid rainbow trout be utilized in Sawmill Lake, as they become available from throuth the provincial Fish Culture program, to alleviate any spawnbound problems and to promote growth rather than egg development in females.

Literature Cited

Philip, D.F. 1985.  A reconnaissance survey of Sawmill Lake. Report prepared for the Fisheries Branch, Ministry of Environment. Prince George.

Appendix 1.
Appendix 2.

For More Information:

Contact :Ted Zimmerman
Sr. Fisheries Biologist, Omineca sub-Region
Prince George, B.C.
250-565-6852