Recreational Fishery Stock Assessment 1999 Final Report

PHOTO 1. Cobb Lake Aerial Photo. June 2001. Click image to view 57K JPG

Introduction

This report presents the results of a stock assessment of Cobb Lake, completed by Ted Zimmerman and Sean Barry on May 28, 1998. Cobb Lake was initially stocked with brook trout in 1984 and rainbow trout in 1986. No stock assessments had been performed prior to this assessment, however brook trout creel surveys were conducted in 1986 (Euchner) and 1988 (Labach). Cobb Lake was therefore targeted as a high priority for sampling as part of a region-wide stock assessment program in 1998.

Methods

An experimental, 91.2m sinking monofilament gill net was set in Cobb Lake on May 28,1998 according to the methods prescribed in the Resource Inventory Committee document Fish Collection Methods and Standards. The net was deployed at 14:15 and retrieved the same day at 17:43, for a total soak time of 3 hours, 28 minutes.

FIGURE 1. 1998 Net set location Click image to view 45K GIF

The net was set in a NW-SE direction from the south shore, originating near the lake's primary inlet (Figure 1). The net ranged in depth from the surface to approximately 8 m.

All trout captured were sampled for fork length and weight. Fin rays were removed from brook trout for ageing, and otoliths were removed from a subset of brook trout for verification of age techniques. Scale samples were taken from all rainbow trout. Age structures were sent to Darlene Gillespie of TimeMark Consulting (Nanaimo, B.C.) on June 17, 1998. During the survey, two groups of anglers were interviewed and their catch was also sampled for length and weight. Scale samples were also obtained.

Results and Discussion

CATCH SUMMARY

The net catch yielded 7 eastern brook trout (EB), 32 rainbow trout (RB), 50 suckers (unid.) and 1 lake chub, for a catch per unit effort of 2.0 EB and 9.2 RB per net-hour. All suckers were released alive from the net. See Appendix 1 for individual rainbow trout sample data, and Appendix 2 for individual brook trout sample data. Of the 32 rainbow trout captured in the gill net, 22 (69%) were female, 9 (28%) were male, and 1 (3%) could not be identified.

A total of 8 rainbow trout were sampled from anglers. The following table describes the attributes of the angled fish.

	FIGURE 2. Length vs. weight of rainbow trout captured in Cobb Lake, 1998
	FIGURE 3. Length frequency distribution of rainbow trout captured in 1998
	FIGURE 4. Age vs. length of rainbow trout captured in 1998.
	FIGURE 5. Length frequency distribution of brook trout sampled by creel, Jan/Feb 1988.
	FIGURE 6. Length frequency distribution of brook trout sampled by gill net, May 1998.

RAINBOW TROUT ANALYSIS

Condition

The weight of Cobb Lake rainbow trout (all samples combined) increased according the equation W = 6E-5 x L^2.7132, this relationship is expressed in Figure 2. In general, Cobb Lake rainbow trout were slightly lighter for their length than those sampled in other lakes in the region during the same time period. This phenomenon is known to occur in lakes in the Omineca region in which both brook trout and rainbow trout have been co-stocked, and may reflect some degree of interspecific competition.

Size Distribution

Figure 3 shows that the catch composition of the gill net was biased towards larger rainbow trout. This is an expected result since the net was intentionally placed near the inlet stream and therefore targeted mature fish (see Appendix 1) that were attempting to spawn. It is likely that the smaller cohorts were not available to the catch due to displacement by their larger counterparts. The angling size distribution reflects the gill net size distribution, which is likely due to anglers targeting the high-use area at the mouth of the inlet stream as well.

Growth

Cobb Lake rainbow trout appear to exhibit good growth rates (Figure 4), with age 3 fish attaining a mean length of 372 mm (+/- 25mm SD) and a mean weight of 509 g (+/- 96g SD). The length-at-age data may be biased due to the sample having targeted spawners, however 1998 stock assessments in Eena, Chubb, and Trapping lakes similarly targeted spawning populations which did not exhibit the growth rates observed in the Cobb Lake sample. The asymptotic growth rates are consistent with other stock assessments.

BROOK TROUT ANALYSIS

Since only seven brook trout were captured in the current study, few inferences can be made regarding their status in Cobb Lake. Cobb Lake brook trout exhibited a slightly reduced condition relative to other lakes stocked exclusively with brook trout. This phenomenon is consistent with that observed for rainbow trout and is likely attributable to interspecific competition, as discussed above.

Figure 5 shows the length frequency distribution of rainbow trout sampled during a winter creel survey in 1988 (Labach). By way of comparison, Figure 6 shows the results from the May 1998 survey. The size of fish captured in 1998 appears to be within the range of those captured by anglers 10 years earlier. While it is likely that the brook trout in Cobb Lake today are exhibiting similar growth and abundance to those sampled in 1988, a comparative winter creel survey should be performed to confirm the status of brook trout in Cobb Lake.

Management Recommendations

While the condition of both species of trout is slightly lower than that observed in other monoculture lakes within the region, Cobb Lake continues to support a viable fishery, with good sized trout available to the recreational angler. The current stocking rate for both species appears to be meeting angler needs, and anglers that were encountered during the assessment expressed satisfaction in their fishing experience.

To better understand the status of brook trout in Cobb Lake, a winter creel survey should be completed, preferably during the same time period as those performed in 1986 and 1988. This would provide fisheries managers with a larger sample of trout by which to base management decisions, as well as information on winter angling pressure and its relative pressure on the resource.

Appendix 1.
Appendix 2.

Literature Cited

Euchner, T. 1986. Creel survey of the winter 1985/86 Cobb Lake brook trout ice-fishery. Ministry of Environment, Lands and Parks. Prince George.

Labach, M. 1988. Creel survey of the winter 1987/88 Cobb Lake ice-fishery. 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