Recreational Fishery Stock Assessment 1999 Final Report

PHOTO 1. Trapping Lake Aerial Photo. June 2001. Click image to view 55K JPG

Introduction

This report presents the results of a stock assessment of Trapping Lake, completed by Ted Zimmerman and Sean Barry on June 4, 1998. Trapping Lake received assessments in 1961, 1968, and 1987 prior to it being stocked in 1988 with rainbow trout. Since 1988, no population surveys had been performed to determine the growth and abundance of the stocked rainbow trout relative to the wild population. In addition, forest road construction severely damaged Trapping Creek in 1986, which eliminated the capability of the lake to sustain its wild trout population. A follow-up survey was therefore required to assess the status of the existing fishery in Trapping Lake, and to document any changes to the outlet habitat that had occurred over the last decade. Trapping Lake was therefore assigned a high priority for sampling as part of a region-wide stock assessment program in 1998.

Methods

FIGURE 1. Location of Trapping Lake gill net set, June 1998. Click image to view 45K GIF

An experimental, 91.2m sinking monofilament gill net was set in Trapping Lake on June 3,1998 according to the methods outlined in the Resource Inventory Committee document Fish Collection Methods and Standards. The net was deployed at 11:30 AM and retrieved on June 4 at 11:05 AM, for a total soak time of 23 hours, 35 minutes.

The net was set from the west shore in an E-W direction, originating just south of the lake's outlet (Figure 1). The net ranged in depth from the surface to approximately 7.5m.

All trout collected were sampled for fork length and weight, and the sex and maturity of each fish was recorded. (Appendix 1). Scale samples were collected from all rainbow trout, and 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 in Trapping Lake.

Data obtained during the 1998 assessment was compared with data from the 1987 survey to determine the relative growth and condition of wild vs. stocked fish.

Results and Discussion

CATCH SUMMARY

The net catch yielded 58 rainbow trout (RB), for a catch per unit effort of 2.5 RB per net-hour. No other species were captured. See Appendix 1 for individual rainbow trout sample data. Of the 58 rainbow trout captured, 41 (71%) were female, 11 (19%) were male, and 6 (10%) could not be classified.

ANALYSIS

FIGURE 2. Length vs. weight of rainbow trout captured in Trapping Lake, June 3, 1998 Click image to view 5K GIF

Condition

The weight of Trapping Lake rainbow trout increased according the equation W = 4x10^-5 x L^3.1655; this relationship is expressed in Figure 2. Fish greater than 400 mm in length tended to be somewhat lighter for their length than their smaller counterparts. Of these three fish, two exhibited stomach parasitism which were unique to these fish only (see Appendix 1). This parasitism likely explains the reduced condition and may be symptomatic of the larger Trapping Lake rainbow trout in general, as other anglers that were encountered during the survey indicated that they had observed parasitism in their catch as well.
Length-Frequency Distributions

FIGURE 3. Length frequency distributions of rainbow trout captured in Trapping Lake, comparing 1968, 1987 and 1998 results. Numbers denote median lengths of available age classes. Click image to view 10K GIF

Sampling bias

Figure 3 illustrates the length-frequency distributions of the 1968, 1987 and 1998 rainbow trout populations sampled in Trapping Lake, and the median lengths of each age class of the latter two sample events. The discrepancy between the number of length classes observed in 1998, compared to the 1987 and 1968 samples is likely attributable to the short set times (1987=3.25 hr, 1968=not avail.), and small sample sizes (1987 N=20, 1968 N=17) of the first two surveys.

Aside from the difference in sample intensity, the absence of smaller size classes in the first two surveys may be due to the fact that wild yearlings remained in their natal streams until such time as they were large enough to compete effectively in a lake environment. If the yearlings sampled in 1998 are of hatchery origin, their occurrence in the main body of the lake may be attributable to an inability to seek out suitable rearing sites. Another explanation is that the 1998 yearlings had yet to adjust to the lake environment and seek cover, after having been released only 5 days prior to the survey date. A third possibility may be that, given the destruction of the lake's rearing habitat (see above),these yearlings were left exposed in the main body of the lake, with no suitable rearing habitat available to them.

Recruitment

The presence of fish in the 190 - 220 mm size classes in 1998 suggests that the lake may support limited natural recruitment, since these size ranges are too large to be the yearlings stocked 5 days prior to the survey, but too small to be 3-year-old fish. This argument is supported by the fact that 2-year-olds were aged in the 1998 sample (Figures 3 and 4, see below). Interestingly, 4-year-olds were not sampled in 1998, and there appears to be a break in the length-frequency distribution for this age class. This result would be expected if the lake did not support natural recruitment, since no cohorts of that year class were stocked. This suggests that recruitment events occur sporadically, perhaps during years with higher than normal spring freshets, when high inlet flows would provide sufficient habitat to support both spawning and rearing.

Growth

While fish in age classes 2 and 3 exhibited slightly reduced growth in 1998 relative to the 1987 sample (Table 2), the change in growth is not so great as to affect the quality of the fishery. Indeed, age 3 fish were on average only 7 mm smaller in 1998, and this difference was not significant (t=0.65 p=0.52). If Trapping Lake rainbow trout were exhibiting successful reproduction (i.e. contributing recruits to the fishery), it would be expected that augmenting the age 1 cohort through stocking would result in slower growth due to density dependent effects. The fact that this has not occurred supports the argument that natural recruitment is limited and does not contribute substantially to the fishery.

Inlet/Outlet Observations

There has been no systematic investigation of the impacts to spawning and rearing sites since 1986 (Photo 1), when the lake's outlet was severely damaged. A follow-up survey took place in 1987 (Photo 2, and Photo 3), however this was based on visual observations only. While on site in 1998, we also conducted a cursory examination of the inlet and outlet for spawning and rearing potential. The outlet was blocked by a series of beaver dams, with little flow downstream, while the inlet did not appear to provide adequate flow. At that time, neither system was deemed suitable for spawning or rearing, and our observations concur with Westcott (1987) who rated the production potential in Trapping Lake as low to negligible.

PHOTO 1. Outlet of Trapping Lake, 1986 Click photo to view 95K JPG

PHOTO 2. Forestry road crossing outlet of Trapping Lake, 1987 Click photo to view 95K JPG

PHOTO 3. Outlet of Trapping Lake, 1987 Click photo to view 101K JPG

Management Recommendations

The current structure of the Trapping Lake rainbow trout population appears to be very similar to pre-stocked conditions, as trout growth rates and size distributions have not shown significant change over the last decade. While the CPUE data might suggest that trout were more abundant in 1987 (1987=5.7 fish/hr, 1998=2.5 fish/hr), this data should be viewed with caution as net catch rates can be influenced by many extraneous factors which can vary in different years. Assuming that fishing effort remains relatively constant, the lake should continue to support a moderate recreational fishery with catches of up to 700g (1.5 lbs). The potential for the lake to produce larger fish is likely constrained by its low productivity (TDS=37 ppm), and any attempts to manipulate stocking rates in order to produce larger sized fish would probably prove to be futile.

The current state of Trapping Creek continues to be a cause for concern. The restoration of this creek to pre-impact conditions would provide spawning and rearing habitat for the current rainbow trout population. This work would ultimately result in a self-sustaining fishery, and would thereby eliminate the stock's reliance on hatchery production.

Literature Cited

Westcott, R. 1988. Note to file regarding the 1988 stock assessment of Trapping Lake. 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