Recreational Fishery Stock Assessment 1999 Final Report

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Introduction

This report presents the results of a stock assessment of Butterfly Lake, completed on August 25, 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-Nechako Fisheries Council. Gary George and Clayton Charlie of the CSTC delivered the fieldwork component of the program, and 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.

Butterfly Lake is one of five stocked lakes that are managed within Eskers Provincial Park, located 33 km northwest of Prince George. The lake was initially surveyed in 1985 and was determined to be barren of fish based on gill net and minnow trap surveys at that time. The lake was subsequently stocked in 1988 and 1989 with eastern brook trout fingerlings. A brief follow-up assessment was conducted in 1991, to determine the status of the brook trout population.

Stocking was ceased after 1991 at the request of the Parks Branch. The cessation of stocking was brought about due to concerns regarding uncontrolled angler access to the lakes, which was occurring in response to the stocking program but in advance of the completion of adequate infrastructure to support the increased traffic.

After the stocking program ceased, unconfirmed reports were submitted to Fisheries and Parks staff that some of the lakes were continuing to produce brook trout, presumably through natural recruitment. Reports were also received that indicated that fish may have been transferred between lakes, a situation which, if true, would have a direct bearing on the management objectives for each of the lakes initially stocked. In 1996, Parks Branch requested that the stocking program be reinvoked, as it was felt that angler use could now be controlled given the state of the park's infrastructure. The lake was accordingly stocked in 1997 at an alternate-year stocking frequency.

Since no assessments had been done since stocking was ceased, and since the renewed stocking program had been in effect for two years, Butterfly Lake was assigned a high priority for assessment in 1999.

FIGURE 1. Location of Butterfly Lake gill net set, August 25, 1999. Click image to view full bathymetric map. Click image to view 40K GIF

Methods

A 91.4 m sinking monofilament gill net with experimental mesh sizes was set in Butterfly Lake on August 24, 1999, according to the methods specified in the Resource Inventory Committee document Fish Collection Methods and Standards. The net was deployed at 11:30 AM and retrieved August 25 at 11:00 AM, for a total soak time of 23.5 hours. The net was set from the northwest shore of the lake, and extended in a SE orientation. The net ranged in depth from the surface to approximately 5 m. (Figure 1).

All trout collected were sampled for fork length, weight, sex, and maturity. Weight was measured to the nearest 25 g. Fin rays were collected for age structure analysis. This analysis was performed by Darlene Gillespie of TimeMark Consulting Ltd. (Nanaimo, B.C.).

Results and Discussion

CATCH SUMMARY

The net catch yielded 31 eastern brook trout (EB), for a catch per unit effort (CPUE) of 1.3 EB per net-hour. See Appendix 1 for specific fish attribute data. The sex ratio of the sample was 12 females : 18 males, with 1 unknown. The sampled population was dominated by maturing and mature fish (Figure 2), with only 4 fish classified as "immature" or "unknown". Since 1997, Butterfly Lake has received triploid brook trout, which remain permanently immature throughout their life cycle so as to prevent natural recruitment of this introduced species. The presence of mature fish in the sample therefore suggests that the brook trout population is being propagated by progeny of the stock released in the late 1980s.

FIGURE 2.  Maturity of brook trout sampled in Butterfly Lake, August 25, 1999.

CONDITION

With a reported mean brook trout condition of 0.87 (Table 2), it is possible that the level of precision applied to the weighing procedure was low, or that the weighing apparatus was not correctly calibrated prior to conducting the survey. This is supported by the fact that the field crew that conducted this survey also reported highly variable weight data for Byers Lake. Other lakes in the region support brook trout with higher and less variable condition factors (e.g. Shere Lake), further suggesting that the Butterfly Lake fish weight data is suspect.

LENGTH FREQUENCY DISTRIBUTION

FIGURE 3. Length distribution of Butterfly Lake brook trout, comparing 1991 and 1999 results. Click image to view 5K GIF

The length frequency distribution (Figure 3) of the 1999 catch was bimodal, with five age classes represented. According to the age results (see Appendix 1), the 1997 cohort is made up of only mature fish; these fish are likely of naturalized origin, since the 1997 and 1999 releases were triploid stock. The 1991 sample was not aged, but it is known that only two age classes were present in the lake at that time. The unimodal size distribution may include both age classes, but it is more likely that it represents the 1988 (i.e. 3+) cohort.

The discrepancy between the length and age frequency distributions reported here (see below) may be attributable to ageing error. The ageing contractor indicated that fin ray sections were not consistently cut at the base of the fin rays, and in some circumstances this may have lead to incorrect age intrepretation. It is likely that most of the fish in the 220 to 260 mm size range represent 2-year-old brook trout.

AGE DISTRIBUTION

The 1999 population is primarily represented by the 1995 and 1996 year classes (Figure 4), while sample gear bias or ageing error likely accounts for the few numbers of 2-year-olds seen. Assuming that relatively constant fishing mortality has occurred in Butterfly Lake over the last decade, the 1999 age distribution can be explained by the naturalization of Butterfly Lake brook trout that were initially stocked in 1988.

The 1988 cohort was released into a barren environment, a condition which usually results in rapid growth and maturity.

FIGURE 4. Age frequency and growth of brook trout sampled in Butterfly Lake, 1999. Click image to view 5K GIF

Assuming that at least some of the fish released in 1988 and 1989 reached full maturity by age 2, their progeny (F1) would have entered the spawning population by 1993. By 1994, the spawning population consisted of the 1988, 89, 91 and 92 year classes, which contributed to a strong 1995 cohort, as observed by the large number of 4-year-old fish captured in 1999.

This explanation assumes that cohort strength is a function of parental population size and that spawning success was not density dependent leading up to 1995. Age class strength from 1995 onwards would no longer be limited by spawning population size, as parental age classes approached full representation. If the assumptions of this explanation are valid, it is reasonable to expect that the Butterfly Lake population will approach carrying capacity within two to three years from present, as the 1995 cohort reaches its maximum age, and limiting factors (such as the amount of available spawning habitat) constrain additional production.

GROWTH

Currently, Butterfly Lake brook trout reach a 'catchable' size of 250 mm by about age 3. However the rate of growth observed in 1999 may not be sustainable, since cohort strength will increase over time and density dependent factors will begin to affect growth performance. This reduction in growth will be compounded by the stocking events which occurred in 1997 and 1999. The predicted outcome of this decrease in growth performance includes a delay of recruitment into the fishery until age 4 or later, and a smaller maximum size attained at age 6.

Management Recommendations

Butterfly Lake appears to support a naturalized stock of brook trout. The supplementation of this stock with triploid fish may have an adverse effect on the growth of subsequent year classes, with consequences to the recreational fishery. The relative performance of naturalized and hatchery-origin stocks should be investigated by employing a mark-recapture study to assess the growth and survival of the two groups, and to test the predictions laid out in the previous section. This test could be further enhanced by eliminating the stocking of Bow Lake, in order to compare the fishery performance between the two lakes.

Attention should also be given to the status of the naturalized stock, as the Province has now adopted a non-reproductive policy for eastern brook trout. Since the naturalized stock has managed to sustain itself over a ten year period, the likelihood of its extirpation through fishing or natural events is low. The introduced triploid stock may have a negative effect on natural recruitment, since triploid trout develop secondary sexual characteristics (even though they are not mature) and could potentially compete for limited spawning habitat. The population should therefore be carefully monitored to determine whether the sterile stocking policy reduces natural recruitment, contributes to overall recruitment, or has no effect on the population. If the naturalized stock continues to thrive despite the stocking program, then additional action may needed to reduce the risk of escapement of reproductively viable brook trout beyond Butterfly Lake.

Literature Cited.

Philip, D.F. 1985.  A Reconnaissance Survey of Butterfly Lake. Report prepared for the Fisheries Branch, Ministry of Environment. Prince George.

Van Schubert, R. 1991.  A Stocking Assessment of Butterfly Lake. Report prepared for the Fisheries Branch, Ministry of Environment. 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