Recreational Fishery
Stock Assessment

1999 Final Report

PHOTO 1. Portal Lake, August 1999. Click image to view 76K JPG

PHOTO 2. Portal Lake Day Use Site, August 1999. Click image to view 56K JPG

Introduction

This report presents the results of a stock assessment of Portal Lake, completed on August 11, 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. Field activities were carried out by Margo French and Lawrence Ward of the CSTC. Analysis and reporting of the field results were conducted by the author. Inquiries pertaining to this report should be directed to the author at the email and address located at the bottom of this page.

Portal Lake was initially surveyed in 1981 and was determined to be barren based on gill net and minnow trap surveys at that time. The lake was subsequently stocked in 1982 with rainbow trout fall fry, and has since received intermittent fall releases. A follow-up survey was conducted in 1985 (Philip) to investigate the status of the stocked fishery and to resample certain biophysical attributes initially measured during the 1981 survey. Since this survey, a variety of stocking alternatives (i.e. fall fry vs. yearlings, annual vs. alternate year stock schedules) have been employed on Portal Lake, however to date these alternatives have not been systematically evaluated. Portal Lake was therefore assigned a high priority for assessment in 1999.

Methods

FIGURE 1. Location of Portal Lake gill net sets, August 10 & 11, 1999. Click image to view 26K GIF

An experimental, 91.2m sinking monofilament gill net was set in Portal Lake on August 10, 1999 according to the methods specified in the Resource Inventory Committee document Fish Collection Methods and Standards. The net was deployed at 11:15 AM and retrieved the same day at 3:30 PM. Only ten fish were captured after this period, so the net was redeployed on August 11 at 9:30 AM at a different site (Figure 1). This second net set was retrieved on August 11 at 12:15 PM, for a total combined soak time of 7.0 hrs. Set 1 was positioned from the north shore and extended in a ENE orientation, to a maximum depth of 4.0 m. Set 2 was positioned off the southeast shore and extended in a N-S orientation, east of the first set. The second set ranged in depth from the surface to approximately 3.0 m.

All trout collected were sampled for fork length, weight, sex, maturity, and stomach contents. The general condition of the fish sampled was also recorded. Scale samples were collected for age structure analysis. This analysis was performed by Darlene Gillespie of TimeMark Consulting Ltd. (Nanaimo, B.C.).

Water temperature, weather conditions, and a Secchi disk reading were also reported.

Results and Discussion

CATCH SUMMARY

The net catch yielded 29 rainbow trout (RB), for a catch per unit effort (CPUE) of 4.1 RB per net-hour. The complete data set can be viewed in Appendix 1.Twenty-five of the 29 rainbow trout sampled were classified as "immature" and their sex could not be determined. Of the remaining 4 fish, 3 were male and 1 was female.

CONDITION

The length-weight relationship of the Portal Lake rainbow trout sample is described by the equation W = 4x10^-5 x L^2.72(R²=0.99); this relationship is expressed in Figure 2. The small number of adults sampled precludes an in-depth analysis of the health of PortalLake rainbow trout, however there were no anomolies noted in the length-weight relationship. All fish appeared to be well fed: the stomach contents of the larger fish sampled contained snails, clams, and Gammarus, while the smaller yearlings (i.e. < 125 mm) sampled were feeding exclusively on insects and zooplankton.

LENGTH FREQUENCY DISTRIBUTION

Figure 3 compares the length frequency distributions of rainbow trout sampled during the 1999 and 1985 surveys. In 1999, the catch was dominated by 100 - 150 mm yearlings which were stocked into Portal Lake as fall fry on October 6, 1998. The remainder of the sample is made up of the 1995 cohort, which averaged 350 mm in length. The small number of 4-year-olds is likely explained by a combination of the short set time (7 hrs.), and to angling and fishing mortality, which would significantly reduce the quantity of catchable trout by 1999.

The 1985 sample distribution represents the 1982 cohort only, which were stocked as fall fry. No other cohorts were released until the sample was obtained in 1985. The mean length of these 3-year-old trout was 261 mm.

FIGURE 4. Age vs. length of rainbow trout captured in Portal Lake, comparing 1985 and 1999 results. Click image to view 3K GIF

GROWTH

Figure 4 compares the length-at-age relationship for rainbow trout in Portal Lake between 1985 and 1999. The growth rate exhibited by the 1982 cohort is not indicative of the growth demonstrated by subsequently released fish, as density dependent factors were likely responsible for reduced growth of that year class due to its high initial stocking rate. Since Portal Lake was not likely fished by anglers in the early 1980s (since it had only been recently stocked), exploitation of the stock would have been minimal, thereby compounding the density dependent effects on growth.

It is difficult to interpret current growth rates in the context of an alternate year stock schedule, since a stocking event was missed in 1997. However the observed growth rate of 4-year-old trout suggests that Portal Lake rainbow are recruiting to the fishery (assuming 25 cm minimum size for angler satisfaction) at about age 2.

The yearlings sampled in this survey were stocked as fall fry at a mean weight of 0.8 g (Appendix 2) in 1998. By August 1999, these yearlings averaged 23.2 g, representing a mean growth rate of 22.6 g over a 43 week period. This growth rate is slightly less than that produced by the Fish Culture program for hatchery yearlings, suggesting that Portal Lake rainbow exhibit adequate growth in their first year at current stock densities.

Management Recommendations

PHOTO 3. Portal Lake rainbow trout sampled from Set #2 August 11, 1999. Click image to view 61K JPG

The growth rates observed for yearlings and 4-year-olds in the present study suggest that current stocking rates are adequate to produce catchable trout at age 2. However, these rates may not be indicative of the longer term performance of the fishery since a true alternate-year stock schedule has not been evaluated. Before any firm conclusions can be reached regarding the status of the Portal Lake fishery, several issues must first be addressed :

• accurate data on current angling pressure should be obtained;

• stocking rates should remain consistent for at least a four year period in order to effectively evaluate stock rates and fishery performance; and

• a followup assessment should be completed once consistent stocking has occurred over a four year period

In the interim, the Ministry will continue to stock 5,000 fall fry in alternate years into Portal Lake. A recommended window for followup sampling is August 2001, at which time the 1998 and 2000 cohorts will be available to the sample. Sampling during this time will provide data on the mean growth rate of three-year-old fish at a consistent stocking level. This data can then be used as the basis to make decisions regarding stocking options and/or harvest rates should they be deemed necessary.

Literature Cited

Miller, C. and B. Tupniak. 1981.  Portal Lake : Lake Survey Data. Report prepared for the Fisheries Branch, Ministry of Environment. Prince George.

Philip, D.F. 1986.  Portal Lake - Stock Assessment August 13,14, 1985. 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