Recreational Fishery
Stock Assessment

1998 Final Report

PHOTO 1. Air Photo of La Salle Lake West, July 2000 Click image to view 72K JPG

PHOTO 2. La Salle Lake West Click image to view 40K JPG

Introduction

This report presents the results of a stock assessment of La Salle Lake (West), completed by Ted Zimmerman and Don Cadden on May 20, 1998. La Salle Lake (West) is a small, oligotrophic 12 ha lake (Table 1) located just west of La Salle Lake (East), and approximately 48 km west of McBride (Figure 1). Both lakes are easily accessible from Highway 16 and receive moderate use by the angling public in the Robson Valley area. La Salle Lake (East) was assessed in 1998, the results of this survey can be viewed by clicking here.

The Ministry of Environment conducted a reconnaissance survey of La Salle Lake (West) in 1987 (Westcott), and concluded that the lake could support a viable eastern brook trout (Salvelinus confluentus) fishery if an annual stocking program was implemented. Based on this finding, the lake was initially stocked with brook trout in 1988, and then subsequently received intermittent releases of brook trout until 1996 (Appendix 2). A follow-up stock assessment was conducted in 1992 (Van Schubert), which confirmed that the lake was producing brook trout up to 1000g.

FIGURE 1. Map of La Salle Lakes Click image to view 139K JPG

To ensure that brook trout are not accidentally released into the wild, the Ministry of Environment maintains a 'closed system' stocking policy, which states that all lakes stocked with brook trout must be completely isolated from the watershed in which they are located. In 1996, occasional reports of brook trout presence in provincial streams led fisheries managers to conclude that some lakes were not completely isolated. To better understand the drainage structure of brook trout lakes in the Omineca region, each lake currently stocked with brook trout was surveyed by Barry in 1997.

FIGURE 2. Annotated view of La Salle Lakes Click image to view 275K GIF

This assessment concluded that the outlet of La Salle Lake (West) may not be entirely closed to brook trout migration, because its channel could become free-flowing during a high water year (see below and Figure 2). It was recommended that the lake's outlet be examined during the spring freshet of the following year to further assess the risk associated with stocking brook trout into La Salle Lake (West). Therefore, the objectives of the 1998 assessment were to :

1. Assess the current status of brook trout in La Salle Lake (West);

2. Examine the lake's outlet to assess the risk of brook trout escapement and;

3. Adjust or eliminate the brook trout stocking program, if required.

FIGURE 3. La Salle Lake (West) bathymetric map, showing 1998 net set location Click image to view 40K GIF

Methods

An experimental, 91.2m sinking monofilament gill net was set in La Salle Lake (West) on May 20,1998 according to the methods described in "Fish Collection Methods and Standards" (Resource Inventory Committee, 1997). The net was deployed May 19 at 12:20 PM and retrieved on May 20 at 9:10 AM, for a total soak time of 20 hours, 50 minutes. The net was set in a NE-SW direction from the west shore, approximately 670 m SSE of the Forest Recreation Site boat launch (Figure 3). The net was placed across depths ranging from the surface (near shore) to approximately 6.0 m.

All trout captured were sampled for fork length, weight,sex, and maturity. Fin rays were taken from all brook trout, however they were not sent for age analysis (see below).

Approximately 200m of the lake's outlet was surveyed by foot on May 20, and the entire drainage was briefly surveyed by helicopter on June 1, 1998, as part of a separate project. The outlet was examined for flow and channel characteristics.

Results

CATCH SUMMARY

The net catch yielded 21 eastern brook trout (EB) and 8 lake chub (LKC), for a catch per unit effort of 1.0 EB per net-hour. No juvenile fish were captured. 5 female and 11 male fish were identified, while the remaining 5 were classified as "unknown." The gonads of 6 of the male fish were difficult to assess as they were rated as "immature" or "maturing," and their assigned gender may be in error. See Appendix 1 for individual brook trout sample data.

ANALYSIS

FIGURE 4. Length vs. weight of brook trout sampled in La Salle Lake West, May 20, 1998 Click image to view 5K GIF

Condition

The weight of La Salle Lake (West) brook trout increased according the equation W = 3x10^-6 x L^3.2923 (R²=0.79), where W = weight in grams and L = length in mm (Figure 4). There were no anomolies noted amongst the fish sampled, and in general the fish appeared healthy for their respective size.

FIGURE 5. Length frequency distribution of brook trout in La Salle Lake West, comparing 1992 and 1998 results. Click image to view 5K GIF

Size Distribution

The length-frequency distribution of brook trout captured in 1998 was similar to that reported in 1992, with the majority of the population falling in the 350 to 430 mm size categories (Figure 5). The 1992 sample appears to exhibit a bimodal distribution, with no fish represented in the 400 mm category; these two groups may have represented the 1987 and 1988 cohorts. In 1998 the distributions were less distinct, and should represent the 1992, 1993 and 1995 cohorts based on the stocking history of La Salle Lake (West) (Appendix 2).

Discussion and Management Recommendations

While the growth and condition of brook trout in La Salle Lake (West) appears to be similar to that observed in 1992 (Figure 6), and is likely sufficient to support a recreational fishery, our investigation of the lake's outlet revealed that there is indeed a potential risk of brook trout escapement.

PHOTO 3. La Salle Lake (West) brook trout, captured in 1992 Click image to view 93K JPG

Our examination of the lake's outlet (Figure 7) by air on June 1 revealed that the outlet channel loses its definition within a few hundred metres of the lake, perhaps flowing sub-surface before entering the Fraser River (see Figure 2). However, during a higher-than-normal spring freshet, the flow from the lake's outlet may breach any barriers that presently exist, resulting in brook trout escapement into the Fraser River. This occurred in 1997 at the outlet of Barton Lake (Barry, 1997), a finding which demonstrated the risk of brook trout escapement in seemingly 'closed' systems.

Given this potential risk, it was decided that the Ministry would cease stocking the lake with brook trout, commencing immediately. The age and growth analysis that normally accompanies a stock assessment was not performed in this analysis, since the cessation of stocking in La Salle Lake (West) will result in the elimination of the remaining brook trout, either through angling or from natural mortality. Any statistics that would have been obtained would therefore not have been used for the management of the fishery.

PHOTO 4. La Salle Lake (West) outlet, 1997 Click image to view 101K JPG

The Robson Valley lacks a variety of recreational lake angling opportunities due to the paucity of small lakes in the district. Given the reasonable growth observed for brook trout in La Salle Lake (West), it is likely that rainbow trout will exhibit similar growth rates and should provide a viable recreational fishing experience to anglers in the district. The Ministry has therefore elected to replace the brook trout stocking program with rainbow trout, commencing in the spring of 1999. This stocking program will also provide fisheries managers with valuable data by which to compare stocking procedures in the adjacent La Salle Lake (East).

Appendix 1.
Appendix 2.

Literature Cited

Barry, S. 1997. Channel assessment of brook trout lakes, 1997. Omineca sub-Region. Ministry of Environment, Lands and Parks. Prince George.

Resources Inventory Committee. 1997. Fish Collection Methods and Standards, V. 4.0 Victoria.

Van Schubert, R. 1992. A stocking assessment of La Salle Lake (West). Ministry of Environment, Lands and Parks. Prince George.

Westcott, B. 1987. A reconnaissance survey of La Salle Lake (West). Ministry of Environment, Lands and Parks. Prince George.

For More Information:

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