1998 Recreational Fishery Stock Assessment Final Report

Click image to view 129K JPG

Introduction

This report presents the results of a stock assessment of Shere Lake, completed by Ted Zimmerman and Don Cadden on May 21, 1998. Shere Lake is small and shallow (Table 1), and is located approximately 33 km NW of Valemount (Figure 1). The lake is accessible from the Croydon-Dunster road (Figure 2), and a small day-use Forest Recreation Site permits access to the lake shore. The lake receives moderate use by the angling public in the Robson Valley area. This assessment was the first to be conducted since the lake was initially stocked with eastern brook trout (Salvelinus confluentus) in 1984, following a reconnaissance-level inventory in 1981 (Miller, 1981).

FIGURE 1. Map of Shere Lake Click image to view 139K JPG

To ensure that hatchery-raised brook trout are not accidentally released into the wild, the Ministry of Environment maintains a 'closed system' stocking policy, which requires 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 Shere Lake and surrounding country Click image to view 281K GIF

This assessment concluded that the risk of brook trout escapement in Shere Lake was low, however a follow-up assessment was recommended to document the degree of channelization of portion of the outlet located on private land, east of the lake. Therefore, the objectives of the 1998 assessment were to :

1. Assess the current status of brook trout in Shere Lake;

2. Examine the portion of the lake's outlet flowing through private land and;

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

Methods

An experimental, 91.2m sinking monofilament gill net was set in Shere Lake on May 20,1998 according to the methods described in "Fish Collection Methods and Standards" (Resource Inventory Committee, 1997). The net was deployed May 20 at 8:10 PM and retrieved on May 21 at 10:48 AM, for a total soak time of 14 hours, 38 minutes. The net was set in a NE-SW direction from the west shore, approximately 200m ENE of the Forest Recreation Site boat launch (Figure 3). The net was placed across depths ranging from the surface (near shore) to approximately 3.5 m.

FIGURE 3. Shere Lake bathymetric map, showing 1998 net set location Click image to view 35K GIF

All brook trout captured were sampled for fork length, weight,sex, and maturity. Fin rays were taken from brook trout for ageing purposes, and otoliths were removed from a subset of fish for verification of age techniques. Due to limited financial resources, a stratified random sample of fin rays selected from 10 cm size categories was sent to be aged. The structures were delivered to Darlene Gillespie of TimeMark Consulting Ltd. (Nanaimo, B.C.), aged, and returned to the Ministry.

The portion of Shere Creek lying on private land was surveyed by vehicle on May 21, and an interview was held with the land owner to discuss the drainage and to document any observations that he had with respect to brook trout presence in the system.

Results and Discussion

CATCH SUMMARY

The net catch yielded 86 eastern brook trout (EB) and 8 lake chub (LKC), for a catch per unit effort of 5.9 EB per net-hour. One fish was destroyed in the net, and 3 others escaped. 32 female and 5 male fish were identified, while the remaining 44 were classified as 'unknown.' All but one fish examined were classified as 'immature'. See Appendix 1 for individual brook trout sample data.

ANALYSIS

Size and Age Distribution

FIGURE 4. Length frequency distribution of brook trout sampled in Shere Lake, May 21, 1998. Arrows indicate mean length by age class Click image to view 4K GIF

The brook trout sampled in Shere Lake were dominated by two-year-olds, exhibiting a mean length of 254 mm (Figure 4). Three- and four-year olds were also well represented, however there was a size class gap between two- and three-year-old fish. This gap is likely attributable to the difference in mean weights at stocking of the 1996 and 1997 releases: those stocked in 1996 averaged 10.95 g, while those in 1997 averaged 3.01 g. The 1996 yearlings would therefore have grown relatively larger at the end of their first growing season, compared to the 1997 yearlings.

The presence of yearlings in our sample may be due to an error in the ageing process, since the lake was not stocked with yearlings until after our survey. A secondary explanation for this finding may be that some degree of natural recruitment is occuring in Shere Lake, however all fish stocked since 1996 have theoretically been triploid, or sterile stock. The yearlings observed in our sample would therefore have to be progeny of the fertile stock that include the surviving 1993 and 1994 cohorts (Figure 5).

FIGURE 5. Age frequency distribution of brook trout sampled in Shere Lake, May 21, 1998 Click image to view 3K GIF

The presence of numerous four-year-olds, weighing up to 1080 g, suggests that Shere Lake is receiving sustainable levels of angling pressure and that larger fish continue to be available to the recreational angler. However, it may be that Shere Lake produces much larger fish and that our results reflect a remnant population that is not retained by anglers. Indeed, a 1988 article in B.C. Sport Fishing reported a 2325 g, 489 cm brook trout which was caught in the winter fishery on Shere Lake, suggesting that angling efforts may target 'trophy' sized brook trout during the winter months. If this is the case, then the larger cohorts observed in our sample may contribute to this trophy fishery, after they have grown through their fourth year.

FIGURE 6. Length vs. weight of brook trout sampled in Shere Lake, May 21, 1998 Click image to view 3K GIF

Condition

The weight of Shere Lake brook trout increased according the equation W = 8X10^-6 x L^3.1028 (R²=0.98), where W = weight in grams and L = length in mm (Figure 6). There were no anomolies noted amongst the fish sampled, and in general the fish appeared healthy for their respective size. Two fish were very deep for their length, and exhibited condition factors in excess of 2.0.

The mean condition factor of the brook trout population was identical to that observed in Lasalle Lake (West), which received releases on a bi-annual basis. This supports the argument that Shere Lake is a highly productive lake and can therefore support higher densities of fish without compromising their condition (see below).

Management Recommendations

The good growth and abundance of brook trout observed in Shere Lake is likely due to the lake's shallow mean depth (Table 1). The littoral area extends throughout the entire lake basin (Miller 1981), thereby increasing the lake's productive capacity (Wetzel 1975).

FIGURE 7. Shere Lake outlet, 1997 Click image to view 80K JPG

Our investigation of the lake's outlet (Figure 7) yielded little tangible evidence of brook trout escapement. When viewed from the lake, the outlet was obstructed by a floating vegetative mat and no discernible channel could be found. When viewed from a road adjacent to the lake, an outlet channel was observed that had negligible flow. The owner of the property east of the lake, on which a portion of the outlet flows, indicated that during high water brook trout may be able to navigate downstream into a secondary pond which lies on his property. From this pond, trout could potentially escape into a modified channel that flows adjacent to the CN Rail line. However, the likelihood of brook trout escaping via this route is highly unlikely, as the modified channel does not flow in a contiguous manner, and the habitat quality in the channel is very poor.

Since 1996, all brook trout released into Shere Lake have been sterile, or triploid, stock. Since these fish will not develop reproductive products, it is expected that their growth will exceed that of the fertile stock that were released prior to this time. In addition, the mean weight of the stock released in 1996 was in excess of 10 g, and it is therefore likely that this cohort will be relatively larger at a given age than the other fish released in prior and subsequent years. We therefore expect that the future brook trout fishery on Shere Lake should yield larger fish than it currently does.

In summary, Shere Lake is currently producing a robust brook trout fishery which is expected to produce good results in the foreseeable future. Since the risk of brook trout escapement is low, the Ministry will continue to stock the lake with 2,500 fish on an annual basis.

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.

Miller, C. 1981. Shere Lake Inventory Report. Ministry of Environment, Lands and Parks, Prince George.

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

Wetzel, R.G. 1975. Limnology. Philadelphia, W.B. Saunders Company, 743 p.

For More Information:

Contact :Ted Zimmerman
Sr. Fisheries Biologist, Omineca sub-Region
Prince George, B.C.
250-563-3860