PHOTO 1. Air Photo of Tsitniz Lake, July 2000 Click image to view 82K JPG

Recreational Fishery Stock Assessment 1998 Final Report

Introduction

This report presents the results of a stock assessment of Tsitniz Lake, completed by Ted Zimmerman and Sean Barry on June 18, 1998. Tsitniz Lake was initially stocked with rainbow trout in 1963 and subsequently received alternate year releases, until a 1980 creel census revealed that angler interest in the lake had diminished. After an 11 year hiatus from 1983 to 1993, the lake was placed back on the stock schedule in 1994 to address both renewed angler interest and the recreational needs of Camp McInnis, which is located on the lake. The current assessment is the first to examine the growth and abundance of stocked rainbow trout in Tsitniz Lake.

Methods

FIGURE 1. Location of 1998 Tsitniz Lake gill net set. Click image to view 19K GIF

An experimental, 91.2m sinking monofilament gill net was set in Tsitniz Lake on June 18,1998 according to the methods outlined in the Resource Inventory Committee document Fish Collection Methods and Standards. The net was deployed at 10:29 AM and retrieved the same day at 8:54 PM, for a total soak time of 10 hours, 25 minutes. The net was set in a E-W direction from the west shore (Figure 1), and ranged in depth from the surface to approximately 7.5m.

All trout collected were sampled for fork length. Due to the large numbers of yearlings captured, only a portion of the smaller fish were sampled for weight, sex, and maturity (Appendix1). Scale samples were collected from all fish larger than 200 mm, and from a sub-sample of those smaller than 200 mm. The scales were sent to Darlene Gillespie of TimeMark Consulting Ltd. (Nanaimo, B.C.) for ageing. Upon return to the Ministry, scale prints were subsequently checked and ages were adjusted by the author to account for anomalies between the ages reported by the contractor and the known stock schedule and postulated age composition of rainbow trout for Tsitniz Lake.

Results and Discussion

CATCH SUMMARY

The net catch yielded 42 rainbow trout (RB), for a catch per unit effort (CPUE) of 4.0 RB per net-hour. The CPUE for fish greater than 200 mm (i.e. 1995 cohorts and older) was 1.4 RB per net-hour. 3 of the yearling rainbow trout were damaged during retrieval, resulting in a sample size of 39 fish (Appendix 1). No other fish were captured. Of the 39 rainbow trout captured, 11 (28%) were female, 3 (8%) were male, and 25 (64%) could not be classified.

FIGURE 2. Length vs. weight of Tsitniz Lake rainbow trout, June 18, 1998 Click image to view 5K GIF

FIGURE 3. Length frequency distribution of Tsitniz Lake rainbow trout, June 1998 Click image to view 7K GIF

FIGURE 4. Age vs. length of Tsitniz Lake rainbow trout, June 1998 Click image to view 7K GIF

CONDITION

The weight of sampled Tsitniz Lake rainbow trout increased according to the equation W = 4x10^-5 x L^2.9946; this relationship is expressed in Figure 2. There were no anomolies noted in the length-weight relationship, suggesting that Tsitniz rainbow trout remain healthy throughout their growth period up to the maximum age recorded in this survey. See Appendix 1 for individual fish data.

LENGTH FREQUENCY DISTRIBUTION

Figure 3 illustrates the length-frequency distribution of the sample and includes all fish captured. The 1998 sample is dominated by 110 - 160 mm yearlings which were stocked into Tsitniz Lake on May 29, at a mean weight of 7.8 g. The 3- and 5- year olds do not exhibit a tight class affinity, however most 5-year-olds are represented by the 350 mm and larger size classes.

The alternate year stock schedule is reflected in the size class gaps observed in the length frequency distribution. With only 2500 fish stocked every other year, densities are low and this likely results in low catch rates per angler-hour. Indeed, our net surveys yielded only 1.4 adult trout per net-hour.

GROWTH

Figure 4 shows the length-at-age relationship for rainbow trout in Tsitniz Lake. It is assumed that natural recruitment is inhibited due to a lack of spawning and rearing sites, and age classes have therefore been adjusted to 1997, 1995 and 1993 cohorts. Five-year-old trout averaged 380 mm and 546 g (1.2 lbs), however one fish attained a length of 451 mm and a weight of 925 g (2 lbs). The small sample size of this age class (n=5) precludes any analysis of the variability in these figures, however errors in the ageing and age adjustment process may account for some of the variance.

Management Recommendations

Tsitniz Lake appears to produce moderate sized fish at present stocking levels. The current fishery consists of well conditioned, healthy individuals with no obvious deformities or health problems. While fish densities are presently low, it is unlikely that Tsitniz Lake can support higher trout densities without compromising the size of the fish captured.

Six of the fish captured were in spawning, or ripe, condition. It is recommended that triploid rainbow trout be utilized in Tsitniz Lake, as they become available from the Fish Culture section of the Ministry of Fisheries, to alleviate any spawnbound problems and to promote growth rather than egg development in females. In all other respects the existing stock schedule appears to be sufficient to support a moderate recreational fishery.

Appendix 1.
Appendix 2.

For More Information:

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