Recreational Fishery
Stock Assessment

1999 Final Report

PHOTO 1. Grizzly Lake West Aerial Photo, June 2001 Click image to view 72K JPG

PHOTO 2. Grizzly Lake Forest Recreation Site Click image to view 40K JPG

This report presents the results of a stock assessment of Grizzly Lake (West), completed on July 20, 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 and Nechako Fisheries Council.

Margo French and Lawrence Ward of the CSTC delivered the fieldwork component of the assessment; the author conducted the analysis and reporting of the field results. Inquiries pertaining to this report should be directed to the author at the email and address located at the bottom of this page.

Grizzly Lake (West) is located 65 km southwest of Prince George and is accessed via the Bobtail Forest Service road. A 10 vehicle-unit B.C. Forest Recreation Site is located adjacent to the lake on an abandoned mill site, which provides access to the lake's shore for car-top boats. Click here for specific road directions to Grizzly Lake (West), and here to view photographs of the lake.

FIGURE 1. Number and mean mass of trout stocked in Grizzly Lake (West), 1976-1999.

A complete reconnaissance survey of Grizzly Lake (West) was first performed by D.M. Coombes in June 1984, eight years after the lake had been initially stocked. However, four separate creel surveys had been completed prior to the 1984 survey : a one-day angler check was performed by the Conservation Officer Service in 1975 (Nivison), and a series of surveys were completed between 1979 and 1981 (Aitken 1980, Dixon 1983) as part of the development of a regional fisheries management plan. The 1975 check revealed that a significant number of rainbow trout were being removed from the lake, a situation which likely influenced the decision to initiate supplemental stocking in 1976.

The number of fish released into Grizzly Lake (West) was variable (Figure 1 and Appendix 2) until 1988, when the number was reduced and subsequently maintained at 5,000 fish per year. The mean mass of released yearlings was generally larger throughout the 1990s than in previous years, a reflection of improvements in hatchery technology and fish culture policy. However, no systematic approach had been undertaken to evaluate the performance of the Grizzly Lake fishery in recent years, and the revised stocking strategy had not been tested. The lake was accordingly targeted for assessment in 1999, the results of which are presented here and compared against previously collected data.

Methods

FIGURE 2. Location of Grizzly Lake (West) gill net set, July 19-20, 1999. Click image to view detailed 69K map.

A 91.4 m sinking monofilament gill net of experimental mesh sizes was set in Grizzly Lake (West) at 1:10 PM on July 19, 1999, according to the methods specified in the 1997 Resource Inventory Committee document Fish Collection Methods and Standards. The net was deployed in a SW-NE orientation from the north shore (Figure 1) and was retrieved on July 20 at 11:00 AM, for a total soak time of 21.2 hours.

All trout collected were sampled for fork length (mm) and weight (to 25 g). A subsample was assessed for sex, maturity, and stomach contents, which were examined in the field. Scales were collected for age structure analysis from a subsample of the total number of fish captured. The age analysis was performed by Darlene Gillespie of TimeMark Consulting Ltd. (Nanaimo, B.C.).

Results and Discussion

CATCH SUMMARY

The net catch yielded 78 rainbow trout (RB) for a catch per unit effort (CPUE) of 3.68 RB per net-hour, and 159 lake chub. No other species were captured. See Appendix 1 for rainbow trout sample attribute data.

Of the total catch, 55 rainbow trout were assessed for sex, maturity, and age determination. Of this subsample, 34 (62%) were female, 13 (24%) were male, and 8 (14%) could not be classified. Six trout (11%) were classified as immature, 48 (87%) were maturing, and one (2%) could not be determined. Eighteen trout had fish remains in their stomachs, indicating that a portion of the population had shifted to a piscivorous diet.

WEIGHT-LENGTH and CONDITION

The weight of the sampled population increased according to the equation W = 2 x 10^-8 x L ^4.05 (R²=0.58), where W = weight in grams and L = fork length in millimeters. The functional regression value b = 4.05 suggests that the population exhibited allometric growth and that larger fish were more rotund.

Since b > 3, the mean condition (K) of the entire population is not necessarily indicative of the health of the population, since K increases with length in this case. The mean condition of fish larger than 350 mm was 0.80, which is relatively low when compared to similarly sized fish from other lakes in the region. Interestingly, the mean condition of fish >350 mm sampled in 1984 was higher (K=0.97, n=4) than those sampled in this survey; whether this is an artefact of sample timing, sample size, or due to differences in weighing precision between the two studies, is unknown. If none of these factors are responsible for the difference noted, then the data would suggest that larger trout have become less robust in recent years.

LENGTH FREQUENCY DISTRIBUTION and GROWTH

FIGURE 3. Length frequency distribution of rainbow trout sampled in Grizzly Lake (West), comparing 1984 and 1999 results.

The fork length frequency distribution of trout sampled in 1999 was right-shifted relative to the population sampled in 1984 (Figure 3), suggesting that the 1999 population consisted of larger individuals overall. This difference may be partly attributable to sample timing, as the 1999 survey took place 6 ½ weeks later than the 1984 assessment. The improvement in growth may also reflect a response to lower stocking densities, however since the level of natural recruitment in Grizzly West is unknown, this cannot be directly assessed. Nevertheless, with fewer hathery fish to contribute to interspecific competition, it is reasonable to expect that Grizzly West trout have exhibited a moderately positive growth response.

The level of angler interest in Grizzly West can be indirectly inferred by comparing historical creel data with gill net data from recent years (Figure 4). No fish > 350 mm were captured by anglers in 1980, and a low percentage were captured in 1975.

FIGURE 4. Relative catch of rainbow trout by length class, comparing creel (1975,1980) and gill net (1984,1999) results. Numbers in brackets indicate sample size.

This is in sharp contrast to the 1984 and 1999 gill net surveys, where 7% and 18% of the catch was made up of this size class, respectively. While an improvement in growth rate might account for part of this difference, it is also possible that the level of retention by anglers in the late 1970s and early 1980s was significant enough to prevent fish from reaching the larger sizes that have been observed more recently. If fishing pressure has been reduced in recent years, then it would be expected that larger fish would turn up in the sample. While the separation between growth differences and angling pressure cannot be directly quantified, it is likely that reduced fishing pressure is partly accountable for the presence of larger fish in 1999.

The trout sampled in this study exhibited a typical asymptotic growth pattern, as seen in Figure 5, consisting of individuals between 1 and 9 years old. In general, trout from the Omineca region do not live past 6 years of age in lakes that receive moderate angling pressure.

FIGURE 5. Age vs. fork length of rainbow trout sampled in Grizzly Lake (West), July 20, 1999.

The fact that Grizzly West contains fish up to 9 years old lends additional support to the argument that angler exploitation has diminished in recent years, since these older cohorts would have been harvested under heavier use.

The rainbow trout sampled in 1984 were not aged, therefore a direct comparison of the age class composition between the two sample events cannot be made. At present growth levels, Grizzly West trout enter into the recreational fishery at Age 3, assuming a minimum catchable size of 250 mm. This is on par with growth rates observed for trout in other regional lakes, and is likely sufficient to sustain moderate angling pressure.

Management Recommendations

Grizzly Lake (West) was stocked in order to create a high yield fishery (note to file, 1989), and has been managed accordingly since it was initially stocked in 1976. The reduction in stocking from approximately 10,000 to 5,000 fish per year in 1988 appears to roughly coincide with a reduction in angling pressure, resulting in a fishery which today will provide recreational anglers with good quantities of trout up to 750 g.

It would be worthwhile to resolve changes to trout growth rates and angling pressure over time in order to better understand the factors that drive fish production and yield in Grizzly West. This could be accomplished by conducting an age analysis of the scales collected in 1984, and by repeating the coarse creel surveys that were completed in the early 1980s. This would provide a useful statistic on the recreational interest in a particular lake over time, especially when one considers that the fishery performance appears to be better today than it was 20 years ago. In the meantime, continued stocking at present rates should provide a sustainable and good quality fishery that meets the "high yield" management objective for Grizzly West.

Literature Cited.

Aitken, D. 1980.  Creel Survey 1980 : Prince George Area. Fisheries Branch. Ministry of Environment. Prince George.

Dixon, B.M. 1983.  A Review of Creel Data for Region 7, Omineca. Fisheries Branch. Ministry of Environment. Prince George.

Coombes, D.M.V. 1984.  A Reconaissance Survey of Grizzly Lake. Fisheries Branch. Ministry of Environment. Prince George.

Nivison, C.J. 1975.  Memorandum to G. Chislett, Regional Fisheries Biologist. 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