ABSTRACT
Skeena Fisheries Commission (SFC) conducted a hydroacoustic survey of Owikeno Lake in March 2016. The main objective of the survey was to enumerate and sample the sockeye fry population in Owikeno Lake. The results of the survey are contained in this report.
Hydroacoustic sampling was conducted using a DT X echosounder with a downward pointing split beam 200 kHz transducer. Fish samples were captured with a mid water trawl. The trawl sample was used to determine the species composition of the pelagic “small” size fish.
The 2016 hydroacoustic estimate of the juvenile sockeye population at Owikeno Lake appears to be low considering the 2014 sonar escapement estimate to the Owikeno Watershed of approximately 300,000 spawners. The relatively large size of the juvenile sockeye captured during the trawl tows corroborates the low juvenile sockeye density estimated through hydroacoustic in Owikeno Lake in March 2016.
TABLE OF CONTENTS
ABSTRACT ...... 2 TABLE OF CONTENTS ...... 3 LIST OF TABLES ...... 4 LIST OF FIGURES ...... 4 INTRODUCTION ...... 5 METHODS ...... 8 Hydroacoustic Survey ...... 8 Hydroacoustic data analysis ...... 9 Fish Sampling ...... 9 Temperature and Dissolved Oxygen ...... 10 RESULTS AND DISCUSSION ...... 11 CONCLUSION ...... 13 ACKNOWLEDGEMENTS ...... 13 REFERENCES ...... 14
3 LIST OF TABLES
Table 1. Physical characteristics of Owikeno Lake ...... 5 Table 2. 2016 Owikeno Lake hydroacoustic survey trawl summary ...... 15 Table 3. 2016 Owikeno Lake trawl fish sample summary ...... 15 Table 4. 2016 Owikeno Lake hydroacoustic integration estimate ...... 16
LIST OF FIGURES Figure 1. Location of Owikeno Lake on British Columbia’s Central Coast and survey map of Owikeno Lake West...... 6 Figure 2. Location of Owikeno Lake on British Columbia’s Central Coast and survey map of Owikeno Lake East...... 7 Figure 3. Photo of the Biosonics biofin towing body to which the transducer was mounted and towed beside the survey vessel...... 8 Figure 4. Photo of the trawl net and winch set up for the Owikeno Lake survey. March 12, 2016...... 10 Figure 5. Photo of seven juvenile sockeye captured in trawl tow #7. March 12, 2016...... 11 Figure 6. Photo of 24 threespine stickleback and 2 juvenile sockeye caught in the second trawl tow at Owikeno Lake. March 11, 2016...... 12 Figure 7. Temperature profiles for Owikeno Lake on March 12 2016...... 17 Figure 8. Dissolved oxygen profiles for Owikeno Lake on March 12 2016...... 17
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INTRODUCTION
In March of 2016, the Skeena Fisheries Commission (SFC) conducted a hydroacoustic survey of Owikeno Lake (Figures 1 and 2). The main objectives of this survey were to estimate the juvenile sockeye (Oncorhynchus nerka ) population size and the relative proportions of juvenile sockeye and competitor limnetic species of Owikeno Lake.
Owikeno Lake is a large glacial fjord lake with a surface area of 9,450 ha (Table 1) located on the Central Coast of British Columbia (Figure 1), at an elevation of only 15 m (Table 1). The lake is 56 km long and is the source of the Wannock River, a 6 km long river draining an area of 4,140 km 2, which flows into Rivers Inlet. Owikeno Lake is deep with an average depth of 172 m (Stockner and Shortreed, 1979) and a maximum depth of 370 m (Table 1), and is composed of a chain of four distinct basins, each of which is separated by a relatively shallow narrows. The two lower basins (Basins 1 and 2 on Figures 1 and 2) comprise over 90 percent of the total lake area and are characterized by extreme depths, exposure to strong winds and high turbidity. In contrast, the upper two basins (Basins 3 and 4 on Figure 2) comprise less than 10 percent of the lake area and are shallow, sheltered from the prevailing winds, and are considerably less turbid (Ruggles, 1965).
Sockeye escapement to Owikeno Lake tributaries has been depressed since the 1970s, and has not rebounded despite significant restrictions on commercial fishing since the 1980’s. Brood year 2014 escapement to the Owikeno Watershed, which produced the juvenile sockeye being estimated during the March 2016 hydroacoustic survey, was estimated at 300,000 spawners, using a sonar method in the Wannock River. Table 1. Physical characteristics of Owikeno Lake Elevation Average Maximum Surface Lake Watershed Clarity (m) Depth (m) Depth (m) Area (ha) Owikeno Wannock 15 172 370 9,450 Turbid
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Figure 1. Location of Owikeno Lake on British Columbia’s Central Coast and survey map of Owikeno Lake West.
Figure 2. Location of Owikeno Lake on British Columbia’s Central Coast and survey map of Owikeno Lake East. METHODS
Hydroacoustic Survey
The Owikeno Lake hydroacoustic survey was conducted using similar methods and technology as in previous hydroacoustic surveys conducted by the SFC, Fisheries and Oceans Canada, Cultus Lake division, and the Great Lakes Fishery Commission (Hall 2007, Hall and Carr ‐Harris 2008, MacLellan and Hume 2010 and Parker ‐Stetter et. al. 2009). Transects were sampled using a Biosonics DT ‐X echosounder with a 200 kHz split ‐beam transducer producing a 6 degree beam. The single downward ‐pointing transducer was mounted to a Biosonics biofin towing body (Figure 3) which was towed beside the survey vessel, a 23’ landing craft, at a depth of 1.1 m. Hydroacoustic data were collected to an acoustic threshold of ‐100 dB using Biosonics Visual Acquisition software as the vessel proceeded along transects at a constant speed of 1 m/sec.
Figure 3. Photo of the Biosonics biofin towing body to which the transducer was mounted and towed beside the survey vessel.
The hydroacoustic survey at Owikeno Lake was conducted along transects within basins 1 and 2 only (Figures 1 and 2). The transects were established for previous surveys conducted by the Department of Fisheries and Oceans. These include thirteen transects in Basin 1, and eight transects in Basin 2. The hydroacoustic system was calibrated prior to the survey by suspending a standard tungsten carbide sphere (36 mm diameter) in the acoustic beam. The observed target strength was compared to the predicted target strength at that temperature for the standard target. The difference between the observed and predicted target strength produced a 0.9 dB calibration offset, which was applied prior to post processing of the data.
Hydroacoustic data analysis
Post processing of hydroacoustic data was performed using Echoview software (v. 7.0.77). Acoustic targets below 65 decibels were eliminated from analysis using the Parker Stetter (2009) method of linking the Sv threshold to a TS threshold of 71 decibels, in order to include off axis sub threshold targets that would exceed the 65 dB threshold once compensation for their position is applied by the ST, or single target detection algorithm.
Following the general guidelines of MacLellan and Hume 2010, population estimates were calculated using the integration estimation method for down ‐looking acoustic data only because the total estimated fish densities was above 500 fish/ha. The integration method scales the average area backscattering coefficient (Sv) for each depth layer using the average target strength for the respective depth layer to calculate the areal fish density for the stratum (n/m 2).
Primary analysis outputs from Echoview were processed in Excel (2010) to calculate estimates of total juvenile sockeye for Basin 1 and Basin 2 of Owikeno Lake. Population estimation procedures were consistent with a stratified random transects sampling technique described by MacLennan and Simmonds (2005), and previously used by the Department of Fisheries and Oceans for hydroacoustic surveys at Owikeno Lake. Data from each transect were analyzed in 5m depth layers from 5m to 20m, and 10m depth layers from 20m to 70m. The areal fish densities calculated for each transect layers are averaged for each layers across transects, and are multiplied by the total area for the respective layers. Basin wide estimates are produced from the sum of the population estimates for each layer. The basin population estimates were summed to provide a total population estimate for the whole lake.
Confidence intervals (95%) for fish densities and population estimates are determined by using each transect as a separate sample. The variability between transects within the lake basins determines the error estimate around the average density or population estimate.
Fish Sampling Pelagic fish were sampled using a 2 x 2 m midwater trawl, which was deployed to a maximum depth of 21 m. The net was towed behind the survey vessel at a constant speed of approximately 1 m/s, and retrieved with a portable winch (Figure 4). The depth of each tow varied according to the length of the line that was deployed, which was calibrated and marked prior to sampling. Small fish were sorted by species and stored in 10% formaldehyde, and weighed and measured after at least 30 days of preservation.
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Figure 4. Photo of the trawl net and winch set up for the Owikeno Lake survey. March 12, 2016.
Temperature and Dissolved Oxygen Temperature and dissolved oxygen data were collected at one sampling station (Figure 1) using a hand held Oxyguard Polaris meter with a maximum cable length of 30 m. The meter was calibrated to the nearest 100’ elevation and allowed to stabilize for at least 15 minutes before data were recorded.
10 RESULTS AND DISCUSSION
Owikeno Lake was surveyed on the nights of March 9th to 12th, 2016. The surface temperature was 5.8ºC degrees. The water temperature was stable throughout the water column down to 29m (Figure 7). The dissolved oxygen was also mostly stable throughout the water column at approximately 13.7 mg/L.
We captured 16 juvenile sockeye during seven trawl tows in Basin 1, with a combined towing time of 140 minutes (Figures 1, 2, 5 and Table 2). Juvenile sockeye were captured during every trawl tows but three. Trawl tow #4 was conducted at the surface and did not catch any juvenile sockeye. There were no catches from trawl tows #5 and 6, most likely because of a tangled trawl net, and complications during trawl retrieval. The average length of sockeye fry captured by trawl in Basin 1 was 64.7 mm, with an average weight of 2.8 grams (Table 3).
We captured 8 juvenile sockeye and 63 threespine stickleback (Gasterosteus aculeatus ) during four trawl tows in Basin 2, with a combined towing time 43 minutes (Figures 2, 6 and Table 2). Juvenile sockeye and threespine stickleback were captured during every trawl tows in Basin 2. The average length of sockeye fry captured by trawl in Basin 2 was 68.6 mm, with an average weight of 3.3 grams (Table 3).
Figure 5. Photo of seven juvenile sockeye captured in trawl tow #7 in Basin #1 of Owikeno Lake. March 12, 2016. The juvenile sockeye population for basins 1 and 2 in March 2016 are estimated at approximately 3.12 x 10 6 ± 18%, and 5.09 x 10 5 ± 34%, respectively (Table 4). The total juvenile sockeye population for Owikeno Lake is estimated at approximately 3.63 x 10 6 ± 19% calculated using the integration method (Table 4). Considering the estimated brood year 2014 escapement of approximately 300,000 spawners, the 2016 sockeye fry population estimate for Owikeno Lake is equivalent to a production of just over 10 fry per spawner. The relatively large size of the juvenile sockeye captured during the trawl tows corroborates the low juvenile sockeye density estimated during this hydroacoustic survey. The total juvenile sockeye biomass in Owikeno Lake in March 2016 is estimated at 10,436 kg.
Figure 6. Photo of 24 threespine stickleback and 2 juvenile sockeye caught in the second trawl tow in Basin #2 of Owikeno Lake. March 11, 2016.
12 CONCLUSION
Hydroacoustic surveys allow us to gauge trends in juvenile sockeye populations in lakes that represent ongoing or potential conservation concerns. Regular hydroacoustic surveys provide a baseline that we can use to compare estimates across years. Where escapement is known, hydroacoustic data provides an indicator of freshwater survival.
The March 2016 juvenile sockeye population estimate at Owikeno Lake appears to be low compared to the 2014 brood year escapement. The relatively large size of the juvenile sockeye captured during the trawl tows corroborates the low juvenile sockeye density estimated through hydroacoustic in Owikeno Lake in March 2016.
ACKNOWLEDGEMENTS
Funding for this project was provided by the Wuikinuxv First Nation. The field work was carried out by Dave Rolston, Billie Johnson, and Chris McConechy, under the supervision of the author. Thanks to Rick Ferguson and Fisheries and Oceans Canada for sharing their data and transect designs in addition to assisting with data analysis. Analysis of the data and report preparation were by the author, with mapping by Gordon Wilson.
13 REFERENCES
Hall, P. and Carr Harris C. 2008. Skeena & Nass Sockeye Lakes Hydroacoustic Surveys Report 2007. Skeena Fisheries Commission. Hazelton, B.C. Report to the Pacific Salmon Commission.
Hall, 2007. Skeena Sockeye Lakes Hydroacoustic Surveys Report 2006. Skeena Fisheries Commission. Prepared for Pacific Salmon Commission.
MacLellan, S.G. and Hume, J.M.B. 2010. An evaluation of methods used by the freshwater ecosystems section for pelagic fish surveys of sockeye rearing lakes in British Columbia. Can. Tech. Rep. Fish. Aquat. Sci. 2886: v + 67 p.
Parker Stetter, S.L., Rudstam, L.G., Sullivan, P.J. and Warner, D.M. 2009. Standard operating procedures for fishery acoustics in the Great Lakes. Great Lakes Fisheries Commission Special Publication 09 01. 180 pp. Available at: http://www.glfc.org/pubs/SpecialPubs/Sp09_1.pdf. Accessed April 22, 2009.
Ruggles, C.P. 1965. Owikeno Lake Juvenile Sockeye Studies 1960 63. Department of Fisheries and Oceans. 37 pp.
Stockner, J.G., and K.S. Shortreed. 1979. Limnological studies of 13 sockeye salmon (Oncorhynchus nerka) nursery lakes in British Columbia, Canada. Fish. Mar. Serv. Tech. Rep. 865: 125 p.
14 Table 2. 2016 Owikeno Lake hydroacoustic survey trawl summary Lake Date Trawl # Time Time End Easting Northing Depth SX TS Notes Start Start Start (m) Owikeno – Basin 2 11 Mar 16 1 2211 2221 664356 5732160 9.5 3 25 Good trawl tow Owikeno – Basin 2 11 Mar 16 2 2253 2304 664332 5732124 21 2 24 Good trawl tow Owikeno – Basin 2 12 Mar 16 3 0034 0044 662493 5728884 14.5 2 6 Good trawl tow Owikeno – Basin 2 12 Mar 16 4 0314 0326 661053 5726549 11.5 1 8 Good trawl tow Owikeno – Basin 1 12 Mar 16 1 0359 0414 657872 5725805 20.5 3 0 Good trawl tow Owikeno – Basin 1 12 Mar 16 2 0429 0444 656664 5726199 20 3 0 Good trawl tow Owikeno – Basin 1 12 Mar 16 3 0457 0512 656197 5726663 8 3 0 Good trawl tow Owikeno – Basin 1 12 Mar 16 4 0532 0547 655318 5726806 2 0 0 Good trawl tow Owikeno – Basin 1 12 Mar 16 5 2012 2042 644433 5726626 18 0 0 Trawl caught in motor during retrieve Owikeno – Basin 1 12 Mar 16 6 2100 2130 642657 5726694 0 0 Trawl tangled Owikeno – Basin 1 12 Mar 16 7 2219 2239 636990 5727767 19 7 0 Good trawl tow SX: Sockeye, TS: Threespine stickleback
Table 3. 2016 Owikeno Lake trawl fish sample summary Mean Max. Min. Std. Dev Mean Max. Min. Std. Dev Lake Species N Length Length Length Length (mm) Weight (g) Weight (g) Weight (g) Weight (g) (mm) (mm) (mm) Owikeno – Basin 2 SX 8 68.6 80 55 8.3 3.29 5.33 1.89 1.2 Owikeno – Basin 2 TS 63 n/a n/a n/a n/a n/a n/a n/a n/a Owikeno – Basin 1 SX 16 64.7 71 54 5.3 2.8 4.05 1.54 0.7
Table 4. 2016 Owikeno Lake hydroacoustic integration estimate Density Population 95% C.I. Lake Estimate Method Size Class n/ha n % Juvenile sockeye 472 3,124,031 18 Owikeno – Basin 1 Integration Other Small 0 0 Juvenile sockeye 260 509,238 34 Owikeno – Basin 2 Integration Stickleback 2039 3,997,291 34 Juvenile sockeye 423 3,633,268 19 Owikeno – Total Integration Stickleback 466 3,997,291 19
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Figure 7. Temperature profiles for Owikeno Lake on March 12 2016.
Figure 8. Dissolved oxygen profiles for Owikeno Lake on March 12 2016.