Blackjack Lake Assessment To Evaluate Stocking of 3N vs 2N Taylor Cutthroat Yearlings, 2004

Region 1, Nanaimo

Nanaimo/Cowichan Planning Unit

Prepared by: Georgina Fosker and Lucas Philp

Ministry of Water, Land and Air Protection 2080-A Labieux Road, Nanaimo, BC, V9T 6J9

November, 2004

1.0 Introduction 1.1 Objective Blackjack Lake, in the Nanaimo/Cowichan planning unit, was assessed in the fall of 2004 as part of a study to evaluate performance of 3N vs 2N Taylor cutthroat yearlings. The primary objective was to determine if there is a difference in growth and/or survival (relative abundance) between 3N and 2N yearlings stocked the previous spring as yearlings in the same numbers, at the same size and in the same lake. The second objective of this assessment was to determine if the current stocking plan for this lake is meeting needs for the trout fishery. The Blackjack Lake assessment will focus on:

differential growth and survival of 2N and 3N Taylor cutthroat trout effectiveness of the current stocking plan in providing adequate numbers

of trout of suitable size to sustain the fishery current level of natural recruitment of trout in the lake condition factor, size at age and age structure of trout and char in the

samples presence and relative abundance of other species of fish in the lake future considerations for the Blackjack Lake fish stocking and

management program 1.2 Background Sterile or triploid (3N) cutthroat trout do not undergo sexual maturation or reproduce, thus energy that would otherwise go into gonad development can benefit body growth. Non-sterile or diploid (2N) fish do undergo sexual maturation and reproduce, thus energy going to gonad development and spawning detracts from body growth. Triploid fish, released from the stresses of reproduction, can be expected to live longer and grow larger than their diploid counterparts. Although additional expense is involved in producing triploids, they have the potential to produce a higher quality fishery. Production of triploid Taylor cutthroat yearlings is in the developmental stage at Vancouver Island Trout Hatchery and this study is designed to evaluate their performance in their first year after stocking, when compared with the diploid yearlings normally used. The study was carried out in four Vancouver Island lakes, chosen to represent a variety of small lakes where Taylor cutthroat yearlings are presently stocked. Cutthroat trout were the only salmonids recorded in these lakes from previous assessment reports found in Regional files, and spiny-rayed fishes and other non-native species were absent.

roughly perpendicular to the contours. Both nets were set late in the day, left overnight, and were retrieved the following morning. A summary of the gillnet sets is given in Appendix B.

Figure 1. A bathymetric map of Blackjack Lake showing the survey site location and the positions of the floating and sinking gill nets. Data recorded from the gillnet catch included length (cm), weight (g), sex, stage of gonad maturity, stomach contents and parasite presence, for all trout captured. The catch record is listed in Appendix C, and photographs of the catch are in Appendix D. Char taken in the samples were weighed and measured then handed over to Tracy Michalski for further examination, as part of her Dolly Varden studies. Scales were sampled from all unclipped cutthroat trout. Scales were removed from the area between the posterior edge of the dorsal fin and the lateral line, approximately 2 scale rows above the lateral and placed between labelled glass microscope slides. A contractor, Bob Hamaguchi, read the scales to determine age. Printed copies of each scale sample, with contractor’s determination of annuli indicated, are presented in Appendix E. Condition factor, length-age, and length-weight relationships were calculated separately for unclipped, AD clipped and RM clipped trout. Condition factor was calculated using the following equation: K = 100000W L3

where W represents weight in grams and L represents length in millimetres. 3.0 Results 3.1 Temperature-Oxygen Profile Blackjack Lake has a maximum depth of 6 m. The temperature-oxygen profile shows the surface layer is well mixed, with the thermocline starting at a depth of 3 meters (Figure 2). Temperature values above the thermocline were uniform, ranging from 13.7oC, at the surface, to 13.1oC at 3 meters. Temperatures within the thermocline dropped rapidly from 12.2oC at 4 m to 10.9 oC at 5.5 m. Oxygen levels were at or near saturation above the thermocline, but dropped rapidly within the thermocline to near anoxic conditions below 5 m. Oxygen levels above the thermocline ranged from 9.2 mg/L, at the surface, to 8.3 mg/L at 3 m. Oxygen levels within the thermocline ranged from 2.5 mg/L, at 4 m, to 0.4 mg/L at 5.5 m.

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60 2 4 6 8 10 12 14 16 18 20

Temperature (oC) or Oxygen (mg/L)

Dept

h (m

)

Temperature (C)

Oxygen (mg/L)

Figure 2. Temperature-oxygen profile of Blackjack Lake, October 7, 2004. 3.2 Netting Data A total of 60 cutthroat trout, 5 rainbow trout, 5 coho salmon, and 4 Dolly Varden were captured in the floating and sinking gill nets in Blackjack Lake (Table 2). The size distribution of cutthroat trout varied between 15.1 cm and 34.8 cm. The size distribution of rainbow trout captured ranged from 17.3 cm to 26.0 cm. Coho salmon size distribution ranged between 15.1 cm and 17.9 cm, while Dolly Varden size distribution ranged between 23.2 cm and 31.5 cm. The 3N and 2N cutthroat trout stocked in the spring of 2004 comprised 1.4 % and 6.8 % of the total gill net catch, respectively. Unclipped cutthroat comprised 73%.

Table 2. Summary of catch from the floating and sinking gill nets in Blackjack Lake. Species Sample

Size % of Catch

Size Range Mean Length (cm)

Mean Weight (g)

Mean K Value

K Value Std. Dev.

Sex Ratio (M:F:U)

Cutthroat Trout (3N)

1 1.4 --- 19.4 58 0.7944 --- 0:1:0

Cutthroat Trout (2N)

5 6.8 19.0 cm – 17.3 cm 72 g – 48 g

18.5 60.4 0.9541 0.0893 0:0:5

Cutthroat Trout (Unclipped)

54 73 34.8 cm – 16.1 cm 468g – 40g

22.5 116.9 0.9581 0.0918 16:34:3

Rainbow Trout (Wild)

5 6.8 26.0 cm – 17.3 cm 188 g – 58 g

21.4 110.4 1.0824 0.0503 0:1:4

Coho Salmon (Wild)

5 6.8 17.9 cm – 15.1 cm 62 g – 48 g

16.4 53.2 1.2156 0.1193 1:3:1

Dolly Varden (Wild)

4 5.4 31.5 cm - 23.2 cm 324 g – 118 g

28.9 240 0.9578 0.0534 3:0:1

The length-frequency distribution, by age class, of cutthroat and rainbow sampled in Blackjack Lake is shown in Figure 3. Of the 54 cutthroat sampled, 14 were aged as 1+, 36 were aged as 2+, 6 were aged as 3+, 1 was aged as 4+ and 3 could not be aged due to scale regeneration. Of the 5 rainbow sampled, 1 was aged as 2+, 3 were aged as 3+ and 1 could not be aged due to scale regeneration. Length at age appears normal for both cutthroat and rainbow in Vancouver Island lakes, as does the length-frequency distribution. Overlap between the cutthroat 1+ and 2+ age classes are apparent from the length-frequency distributions as no separation in length exists between these age classes.

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14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Length (cm)

# of

Fis

h

Cutthroat Trout 1+ (n=14)Cutthroat Trout 2+ (n=36)Cutthroat Trout 3+ (n=6)Cutthroat Trout 4+ (n=1)Rainbow Trout 2+ (n=1)Rainbow Trout 3+ (n=3)

Figure 3. Length-frequency distribution, by age, of cutthroat and rainbow in Blackjack

Lake, October 8, 2004. Figure 4 illustrates the length/weight relationships and mean condition factors of all cutthroat trout captured in Blackjack Lake. The cutthroat trout were separated by strain in

order to compare growth rates. The length/weight curve for unclipped cutthroat trout in the samples indicates size increased according to the formula W = 6.9353e0.1207L, where W represents weight in grams and L represents length in centimetres. The closeness of fit or R2 value is equal to 0.9704. The mean condition factor for unclipped cutthroat trout was 0.9581. The 2N cutthroat trout have a length/weight curve equal to W = 4.2315e0.1434L, with an R2 value of 0.6514. The mean condition factor for the 2N cutthroat trout was 0.9541. The 3N cutthroat were represented by a single individual. The rainbow trout have a length/weight curve equal to W = 5.7214e0.1362L, with an R2 value of 0.9869. The mean condition factor for the rainbow trout was 1.0824. The Dolly Varden have a length/weight curve equal to W = 8.0818e0.1153L, with an R2 value of 0.9895. The mean condition factor for the Dolly Varden was 0.9578. The coho have a length/weight curve equal to W = 10.705e0.0976L, with an R2 value of 0.9495. The mean condition factor for the coho was 1.2156.

Cutthroat 2Ny = 4.2315e0.1434x

R2 = 0.6514Ave. K = 0.95Coho

y = 10.705e0.0976x

R2 = 0.9495Ave. K = 1.22

Rainbowy = 5.7214e0.1352x

R2 = 0.9869Ave. K = 1.08

Dolly Vardeny = 8.0818e0.1153x

R2 = 0.9895Ave. K = 0.96

Cutthroat No Clipy = 6.9353e0.1207x

R2 = 0.9704Ave. K = 0.96

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Length (cm)

Wei

gth

(g)

Cutthroat No Clip (n=54)Cuthroat 2N (n=5)Cutthroat 3N (n=1)Rainbow (n=5)Coho (n=5)Dolly Varden (n=4)

Figure 4. Length/weight relationships and average condition factors of all cutthroat,

rainbow, coho and Dolly Varden sampled in Blackjack Lake, October 8, 2004. Cutthroat trout separated by strain.

The comparison of mean length and mean weight of 3N and 2N cutthroat trout is given in Figure 5. The mean length and weight of all 3N cutthroat trout captured was 19.4 cm and 58 g, respectively. The mean length and weight of all 2N cutthroat trout captured was 18.48 cm and 60.4 g, respectively. 95% confidence intervals are shown for each mean.

60.418.48 5819.40

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Length Weight

Mea

n Le

ngth

(cm

) / M

ean

Wei

ght (

g)Mean CT YE 2NLength/Weight (N=5)

Mean CT YE 3NLength/Weight (N=1)

Figure 5. Comparison of mean length and mean weight of 3N and 2N cutthroat trout in

Blackjack, October 8, 2004. Graph displays 95% confidence intervals.

3.3 Presence/Absence of Other Species The presence of cutthroat trout, rainbow trout, coho salmon and Dolly Varden char in the gill nets in Blackjack Lake is consistent with the BC Fisheries database. Stickleback were not taken in the nets; however, they were present in the stomach contents of some cutthroat. No other species, including non-native species, were found in the samples. 3.0 Discussion 4.1 Temperature-Oxygen Profile The temperature-oxygen profile shows a well mixed surface layer with oxygen values near saturation above the thermocline. Conditions in the upper 3 m of the water column are suitable for fish health and development. Below 4 m, conditions are near anoxic and not suitable for fish. 4.2 Fish Sampling Data Comparison of mean length and weight of the 2N and 3N cutthroat shows the cutthroat stocked as 3N are slightly larger than the 2N. The length difference is very small (19.4 cm vs 18.5 cm and is not significant). The mean weight of the 2N is 2.4 g greater than

the 3N (60.4 g vs 58 g, but the difference is not significant). The relative abundance of the 2N is higher than the 3N by a factor of 5 (5 vs 1). The length-frequency distributions of 2N, 3N and unclipped cutthroat trout appear normal, with normal growth for Vancouver Island small lakes. The mean condition factor also appears normal for Vancouver Island cutthroat trout. Mean condition factor of the unclipped cutthroat was higher than both 2N and 3N strains. The length-frequency distribution of rainbow trout appears normal, with normal growth for Vancouver Island small Lakes. The mean condition factor also appears normal for Vancouver Island rainbow trout. The natural recruitment level of cutthroat trout in Blackjack Lake appears high. The age 1+ year class in the sample comprised 6 clipped cutthroat of known hatchery origin and 8 unclipped cutthroat which would be inferred to be of wild origin. Natural recruitment of rainbow trout, coho salmon, and Dolly Varden char is occurring in Blackjack Lake as none of these species have been stocked in the lake. 4.3 Netting Data in Relation to Stocking History A total of 60 cutthroat trout, including 6 clipped fish of known hatchery origin, were captured in the gill nets in Blackjack Lake. The absence of cutthroat trout in the 0+ age class (younger than the clipped fish stocked) might be interpreted that there is little natural recruitment of cutthroat trout in this lake; however, a more likely interpretation is that 0+ cutthroat remain in their natal stream and recruit to the lake when older. The presence of unclipped age 1+ cutthroat trout (wild origin) suggests there is natural recruitment of cutthroat trout in this lake. The origin of cutthroat trout in the 2+, 3+ and 4+ age classes cannot be distinguished as hatchery trout released before 2004 were unclipped. 5.0 Conclusions In Blackjack Lake, no clear difference in the performance of the two strains is evident because of the small sample size (only one 3N cutthroat in the sample). Growth of the 3N cutthroat was equal to the 2N. Survival of the 2N cutthroat appeared higher than 3N; however, the sample size was too small to draw a conclusion from. The assessment of Blackjack Lake indicates current conditions are satisfactory for sustaining the fishery. Continued annual stocking of cutthroat trout will be necessary to sustain the fishery. The present stocking rate of 500 cutthroat yearlings is satisfactory. 6.0 Recommendations The recommendation by the Regional Lakes Biologist is that the stocking rate of 500 yearling cutthroat annually in Blackjack Lake should remain unchanged.

Appendix A – Temperature-Oxygen Data

Appendix D – Photos

Picture 1. Looking northwest from survey site.

Picture 2. Looking southeast from survey site.

Picture 3. Looking southeast towards outlet (Blackjack Creek).

Picture 4. Looking northwest from outlet at southeast end of lake.

Picture 5. Looking north from boat launch site.

Picture 6. Looking east towards floating gillnet from boat launch site.

Picture 7. Floating gillnet catch.

Picture 8. Sinking gillnet catch.

Appendix E – Scale Readings