Hydroacoustic and Gill Net Assessment: Banks Lake, Washington

Hydroacoustic and Gill Net Assessment

Annual Report 2001 - 2002 February 2007 DOE/BP-00005860-3

This Document should be cited as follows:

Baldwin, Casey, Matt Polacek, "Hydroacoustic and Gill Net Assessment; Banks Lake, Washington", 2001-2002 Annual Report, Project No. 200102800, 34 electronic pages, (BPA Report DOE/BP-00005860-3)

Bonneville Power Administration P.O. Box 3621 Portland, OR 97208

This report was funded by the Bonneville Power Administration (BPA), U.S. Department of Energy, as part of BPA's program to protect, mitigate, and enhance fish and wildlife affected by the development and operation of hydroelectric facilities on the Columbia River and its tributaries. The views in this report are the author's and do not necessarily represent the views of BPA. Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002

Casey Baldwin and Matt Polacek

Washington Department of Fish and Wildlife Science Division Ecological Investigations Unit Banks Lake Fishery Evaluation Project

A Supplemental Technical Report to the 2001-2002 Banks Lake Annual Report; (DOE/BP-00005860-1).

Project # 2001-02800 Contract # 00005860

July 2004

Abstract

The Banks Lake Fishery Evaluation Project (BLFEP) was created in 2001 with funds from the Bonneville Power Administration. The overall objective of the project was to maximize the Banks Lake fishery by determining the factors that limit the recruitment of hatchery-stocked kokanee, rainbow trout, and panfish. A series of objectives were identified to test hypotheses related to the decline of the kokanee and panfish fisheries in Banks Lake. The methodologies selected for monitoring the populations and determining limiting factors included surveys of both the littoral and limnetic zones of the reservoir, where species composition can vary dramatically. Factors for decline being examined include predation, competition, over-harvest, entrainment, productivity, and habitat quality and quantity. The purpose of this survey was to estimate the species composition, distribution, and abundance of the limnetic fish community of Banks Lake using hydroacoustics and gill nets. Surveys were conducted in the spring and summer to test for differences in seasonal distribution and efficacy of the survey gear to establish a time period for conducting annual surveys for trend monitoring. Whitefish dominated the limnetic gill net species composition by weight (95%, both months) and number (83% July and 88% May). In May, relatively few fish were distributed deeper than 8 m in transects 12-18 at the South end of the reservoir and low densities were common for the deepest depth strata (16-24 m). In July, most fish were in the deepest depth strata available, 16-24 m throughout most of the reservoir. However, in transect 7 (Devils Lake Embayment) where deeper depths were available (54 m), many acoustic fish targets were also suspended mid-water in the 24-32 m depth strata. Acoustic target distribution varied by depth and location within and between surveys; however, the reservoir-wide mean fish density was not significantly different in May and July. The abundance estimate for acoustic fish targets between 100-800 mm was 1.3 x106 fish (± 0.45-0.65 x106) during both months. The middle portion of the reservoir (near Steamboat Rock) had the highest density of fishes during both seasons. Additionally, in July, fish were concentrating near the North Dam (transect 1) where cool water was being pumped in from Lake Roosevelt and in Devil’s Lake Embayment (transect 7) where deeper bottom depths offered a larger volume of cool water. These areas represent potential cool water refuge for salmonids. At the North end, near limnology site LIM1 and hydroacoustic transects 1 and 2, there was only one gill net set in July and it caught a rainbow trout near the surface. The rainbow trout net pens are released in this area in May-June each year, so it is possible that high abundance of acoustic targets near the surface were recently released rainbow trout. Unfortunately, the water temperature at LIM1 increased from 16 oC in July, to 19 oC in August, thereby eliminating this area as a potential thermal refuge for kokanee in late-summer. Due to the deeper vertical distribution of fish in July, we recommend that the mid-summer period continue to be used for assessing limnetic fish distribution and abundance.

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 ii Acknowledgements

We thank the Bonneville Power Administration for funding the project, and the Northwest Power Planning Council, Columbia Basin Fish and Wildlife Authority, Independent Scientific Review Panel, and R. Morinaka (BPA) for approving and providing input and suggestions during development of the research plan. We thank D. Burgess (WDFW) and the Moses Lake research team for assistance during fish sampling. Dr. R. Black of Eastern Washington University for assistance with the limnology sampling plan and K. Knuttgen for research and summary of historical surveys of Banks Lake. Additional fieldwork or planning assistance was contributed by K. Knuttgen, J. Korth, J. McLellan, A. Smith, J. Kiesel, and H. Woller.

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 iii Table of Contents

Abstract...... ii Acknowledgements...... iii Table of Contents...... iv List of Tables ...... v List of Figures...... vi

Introduction...... 1

Study Area ...... 1

Methods...... 2 Hydroacoustic Density and Distribution...... 2 Gill Net Surveys...... 5 Limnetic Fish Abundance...... 5 Abiotic Conditions...... 6

Results...... 6 Hydroacoustic Density and Distribution...... 6 Gill Net Surveys...... 10 Limnetic Fish Abundance...... 10 Abiotic Conditions...... 18

Discussion...... 18

References...... 25

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 iv List of Tables

Table 1. Species composition by number, weight and the minimum and maximum lengths of fish captured in offshore gill nets on Banks Lake, WA for May and July 2002 ...... 11

Table 2. Effort (net nights), catch and catch per unit effort by gear type for fish captured in the main lake basin during May and July and in the Devil’s Lake embayment in July...... 12

Table 3. Abundance and Density (fish/ha) of all fish targets and specific size classes of fish targets in Banks Lake Washington, 2002. Length estimates were generated by converting target strength (dB) to total length (mm) using an equation from (Love 1971) ...... 16

Table 4. Species-specific abundance estimates of limnetic fishes (~100- 800 mm TL) in Banks Lake, Washington in May and July 2002. Estimates were generated through mobile hydroacoustics and gill net surveys; error bounds were not estimated for percent species composition (% Spp) so the 2 SE only incorporates variance based on the acoustic estimate...... 19

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 v List of Figures

Figure 1. Map of Banks Lake, WA showing locations of hydroacoustic transects and limnology sites where data was collected in July of 2002. The southern most transect was not surveyed due to insufficient nighttime hours ...... 3

Figure 2. Density of target-tracked fish for 16 hydroacoustic transects in May 2002 on Banks Lake, Washington. Transects three and four were not analyzed due to shallow depths for the vertical transducer and excessive noise for the horizontal transducer. Transect 1 began at the North Dam (near Electric City) and transect 18 ended several kilometers before Dry Falls Dam (near Coulee City). The horizontal transducer observed fish between 1 and 8 m depth, whereas the vertical transducer observed fish from 8m to the bottom of the reservoir ...... 7

Figure 3. Density of target-tracked fish in 8 m depth bins for 16 acoustic transects on Banks Lake, Washington in May of 2002. Transect 1 began at the North Dam (near Electric City) and transect 18 ended several kilometers before Dry Falls Dam (near Coulee City). The horizontal transducer observed fish between 1 and 8 m depth whereas the vertical transducer observed fish from 8 m to the bottom of the reservoir. Transect 7 (Devils Lake Embayment) was the only transect where bottom depth exceeded 24 m ...... 8

Figure 4. Density of target-tracked fish in 8 m depth bins for 18 acoustic transects on Banks Lake, Washington in July of 2002. Transect 1 began at the North Dam (near Electric City) and transect 18 ended several kilometers before Dry Falls Dam (near Coulee City). The horizontal transducer observed fish between 1 and 8 m depth whereas the vertical transducer observed fish from 8 m to the bottom of the reservoir. Transect 7 (Devils Lake Embayment) was the only transect where bottom depth exceeded 24 m ...... 9

Figure 5. The vertical depth distribution of all fish, whitefish and rainbow trout in the limnetic zone of Banks Lake in May, 2002. Only one kokanee was collected. Catch was corrected proportional to the vertical and horizontal netting effort in each depth bin ...... 13

Figure 6. The vertical depth distribution of whitefish, rainbow trout and kokanee in the limnetic zone of Banks Lake in July, 2002. Catch was corrected proportional to the vertical and horizontal netting effort in each depth bin ...... 14

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 vi

Figure 7. The vertical depth distribution of whitefish and rainbow trout in Devils Lake Embayment of Banks Lake in July, 2002. Catch was corrected proportional to the vertical and horizontal netting effort in each depth bin. No kokanee were captured ...... 15

Figure 8. Length frequency of acoustic targets, converted to fish length (Love 1971), for Banks Lake, Washington in May and July of 2002...... 16

Figure 9. Length Frequency of fish captured in the limnetic zone during various months and locations on Banks Lake, Washington, 2002. Gear used to collect fish included both horizontal and vertical gill nets with varying mesh sizes ...... 17

Figure 10.Temperature and dissolved oxygen profiles from Banks Lake, Washington in May and July, 2002. Lim1 was the northern most site, Lim3 was the central site (offshore west of Steamboat Rock), and Lim5 was the southern most site ...... 20

Figure 11.Temperature and dissolved oxygen profiles from the Devil’s Lake embayment in Banks Lake, Washington July, 2002...... 21

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 vii Introduction

The Banks Lake Fishery Evaluation Project (BLFEP) was created in 2001 with funds from the Bonneville Power Administration. The overall objective of the project was to maximize the Banks Lake fishery by determining the factors that limit the recruitment of hatchery-stocked kokanee, rainbow trout, and panfish. Generally, the historic fishery was comprised primarily of kokanee (Oncorhynchus nerka) and yellow perch (Perca flavescens) (Duff 1973); however, with the introduction of walleye (Sander vitreus) in the early 1960’s and smallmouth bass (Micropterus dolomieu) in the early 1980’s, the fishery has since shifted primarily to bass, walleye and rainbow trout (Oncorhynchus mykiss) (Lewis et al. 2002; Polacek et al. 2003b). Creel surveys conducted in 2002 and 2003 indicated that few anglers targeted and caught kokanee (or panfish) in Banks Lake, even though over one million kokanee are stocked annually (Lewis et al. 2002; Polacek et al. 2003b). A series of objectives were identified to test hypotheses related to the decline of the kokanee and panfish fisheries in Banks Lake. The methodologies selected for monitoring the populations and determining limiting factors included surveys of both the littoral and limnetic zones of the reservoir, where species composition can vary dramatically. Factors for decline being examined include predation, competition, over- harvest, entrainment, productivity, and habitat quality and quantity. The purpose of this survey was to estimate the species composition, distribution, and abundance of the limnetic fish community of Banks Lake using hydroacoustics and gill nets. Hydroacoustics uses sound impulses transmitted through water to determine fish size, depth, and population density (Traynor and Ehrenberg 1979; Brandt 1996; Cryer 1996). Abundance and distribution can then be determined by expanding results from individual transects to the entire system (Thorne 1979; Levy et al. 1991; Beauchamp et al. 1997). Hydroacoustics is most effective for suspended limnetic species, such as kokanee, when surveyed with a vertically oriented transducer. However, recent advances in technology using a horizontally oriented transducer allows for fish detection within 1.5 m of the surface (Yule 2000). Hydroacoustics cannot determine species composition, so alternative methods must complement a hydroacoustic survey. Common methods for verifying acoustic targets include trawling, purse seining, and gill netting (Parkinson et al. 1994; Bean et al. 1996; Yule 2000). Homogeny in species composition and length distribution results in increased confidence in hydroacoustic estimates.

Study Area

The Bureau of Reclamation created Banks Lake in 1951 to function as a water storage reservoir for the Columbia Basin Irrigation Project. It occupies the Upper Grand Coulee, formerly a channel of the Columbia River, located in the high scrub desert of Grant County, Washington. Banks Lake is contained within two earth-fill dams, the North Dam to the north (47.9407712°N, 119.0163637°W) and Dry Falls Dam to the south (47.6217523°N, 119.3133534°W). The North Dam, near Electric City, WA is 44 m high and 442 m long. Dry Falls Dam, close to Coulee City, WA is 37 m high and 2987 m long and supports a two-lane highway. Banks Lake is 43 km long, contains 1.6 billion

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 1 m3 of water and has a wetted surface area of 10,881 hectares. At an elevation of 479 m above sea level (1,570 ft) (full pool), the average depth is 14 m with a maximum depth of 25 m throughout most of the reservoir, and 52 m in Devil’s Lake Embayment (a 99 ha embayment Northwest of Steamboat Rock State Park). Water is pumped up 85 vertical meters from a pumping plant at the left forebay of Grand Coulee Dam (Lake Roosevelt) to a feeder canal 2.6 km in length, which delivers water to Banks Lake at the North Dam. Additionally, Banks Lake is used as a pumped storage / power generating reservoir. The project includes six reversible pump-generating units (P/G) at the North Dam where water can be pumped back into FDR for power generation. Dry Falls Dam also houses a power plant. Water for irrigation is withdrawn from the south end of Banks Lake through a turbine at Dry Falls Dam. Water levels fluctuate minimally (1-2 m) during the irrigation season, from late March until late October. Banks Lake is an oligotrophic, complex river-run reservoir during the irrigation season and reverts to thermal characteristics of a typical northern latitude lake thereafter. In late spring a thermal stratification occurs; however, once pumping through the feeder canal begins, stratification at the north end disappears due to mixing with cool water pumped up from Lake Roosevelt. Water temperatures can reach over 20°C at the surface and 15°C near the bottom during early mid-summer to early-fall, and dissolved oxygen levels can fall below 5 mg/L at the bottom by late summer (Polacek et al. 2003a; Polacek et al. 2003b). The current fish assemblage of Banks Lake is comprised of both warm and cool water fish species, including kokanee, rainbow trout, whitefish, small and largemouth bass, walleye, yellow perch, pumpkinseed (Lepomis gibbosus), black crappie (Pomoxis nigromaculatus), carp (Cyprinus carpio), burbot (Lota lota), Sculpin (Cottus sp.), and others.

Methods

Hydroacoustic Density and Distribution

Banks Lake was surveyed in May and July 2002 with an HTI model 241 echosounder with two 200 kHz transducers; a 15° split-beam transducer in vertical orientation and a 6° x 10° elliptical split-beam transducer in horizontal orientation. The transducers were clamped to a pole and mounted to the starboard side of 6.7 m vessel 1 m below the surface. Data were logged directly into a computer and unprocessed echoes were backed up using digital audiotapes. A pulse repetition rate of 8 pings per second was multiplexed between the transducers at a pulse width of 1.25 ms and a 10 kHz pulse width chirp. The horizontal transducer was offset by 7° and sampled fish targets from 1.5- to 8 m below the surface. Data within 12 m of the horizontal transducer and 8 m of the vertical transducer were excluded from analysis due to the narrow beam width reducing detectability and potential boat avoidance by fish in the near field (Mous and Kemper 1996; Yule 2000). Transects were conducted in an elongated zigzag pattern across the limnetic zone of Banks Lake by navigating from predetermined global positioning system (GPS) waypoints (Figure 1). On May 27-28, we surveyed 4 transects between 2313 and 0024 before high winds kept us from finishing the survey. On May 30-31, we surveyed

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 2 another 12 transects between 1026 and 0337. On July 29-30, we surveyedN 18orth transects Dam and feeder between 1018 and 0529. Each transect ranged from 1.4 to 3.7 km (mean 2.7)canal for to a FDR total

Electric City

Northrup Creek

State Route

155

Hydroacoustic transects

Limnology sampling sites

U.S. Route 2

Dry Falls Dam Coulee City

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 3 survey distance of 44.6 km in May and 48.2 km in July. Boat speed averaged 8.0 and 7.6 km/hour in May and July, respectively. A GPS logged the latitude and longitude into the data files and transects distances were calculated using Terrain Navigator software version 4.05 (Maptech 1999). A series of acoustic echoes were considered a fish if tracked for at least 3 consecutive pings, within 0.3 m/ping, a maximum velocity of 5 ms/ping, and target strengths between –55 and –27.7 dB (approximately 30-800 mm) . Target strengths were converted to fish lengths using a formula generated by Love (1971, 1977), where TL was the fish total length (mm) and TS (dB) was the mean target strength of each tracked fish.

TL = [2252.1*[EXP(0.1204*TS)]

Estimated lengths were then corrected based on a calibration conducted with a Tungsten Carbide calibration ball were TLc was the corrected total length, TL1 was the original estimate of total length and TL2 was the estimate of total length of the standard Tungston Carbide calibration ball. TLc = TL1 + (TL1 / TL2)

Hydroacoustic fish density.—Density (fish/10,000 m3) was calculated for each transect and transect densities were averaged together for a reservoir wide estimate of fish density. For each transect, individual tracked fish were verified as real within the post-processing software Echoscape 1.52 (HTI 2001). Raw fish counts were adjusted to the effective beam width within each 2 m depth strata by the equation:

⎛ EBW ⎞ = [FFF ⎜1001 −•+ ⎟] ⎝ NBW ⎠

where F1 was the adjusted fish count, F0 was the original fish count, EBW was the effective beam width for that stratum and NBW was the nominal beam width for the transducer. Density was calculated by dividing the adjusted fish count by the total swept volume for each transect. Swept volume (V) was calculated as:

V = ½ * b * h * l

where l was the distance (m) of the transect, h was the distance (m) from the transducer to the end of the stratum (mean bottom depth), and b was the beam diameter calculated by:

⎛ NBW ⎞ = Rb tan2 ⎜ ⎟ ⎝ 2 ⎠ where R is the range (m) to the end of the stratum (mean bottom depth). Swept volume was adjusted by subtracting the un-surveyed near-field volume (0-8 m vertical transducer; 0-12 m horizontal transducer) from the total volume.

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 4 Gill Net Surveys

Limnetic gill net surveys were used to provide species verification, depth distributions, and length frequencies of acoustic targets larger than 100 mm. The night of the survey, and for two nights following the survey, 7-14 vertical gill nets and one floating and one sinking horizontal gill net were fished in the limnetic zone of Banks Lake. The 14 vertical nets consisted of replicate samples of seven nets that were 2.6 m wide and 26.2 m deep, consisted of one mesh size throughout (25, 38, 51, 64, 76, 89, or 102 mm stretch). Horizontal nets were 2.6 m deep and 46 m long with seven panels (same mesh array as the verticals) that were 6.5 m long. Terrain Navigator (Maptech 1999) software was used to spatially segregate (~500 m diameter) the limnetic sampling sites by placing a point near the center of each quadrant of each section in their respective township and range. Additional points were added along the North, South, East, and West borders of each section, as well as the center point. This method provided uniform embaymentrage of potential offshore sites throughout the lake, and a GPS point to navigate to for net deployment. Netting locations were then randomly selected using Statview version 5.0.1 (SAS 1998). Our goal was to sample 20 % (51 of 252) of the potential limnetic sampling sites that were deep enough (at least 12 m) and far enough from shore (~ 200 m) to be considered in the limnetic zone.

July Supplemental Netting.—Additional vertical gill nets were set in the Devil’s Lake Embayment to provide supplemental information due to observations of an unusually dense distribution of acoustic targets in this area. The six supplemental gill nets were configured the same as the previously described vertical nets except they were 46 m deep and we did not set a net with 25 mm mesh. These nets were indiscriminately set throughout the 99 ha embayment. Random site selection consistent with previous methods was not possible due to the limited area in this embayment. All but one of the supplemental nets fished for 2 consecutive nights due to extremely windy conditions prohibiting retrieval after the first night. The results of this supplemental netting effort were not included with the random- reservoir wide netting effort when calculating CPUE, distribution, and species composition due to the potential bias of this supplemental effort.

Limnetic Fish Abundance

Mean fish density was multiplied by reservoir volume to estimate abundance. Two standard errors were used to estimate the 95 % confidence interval of the acoustic abundance estimate. Size-specific abundance estimates were determined by applying the percent frequency of each size class from the down-looking transducer to the total abundance estimate. We applied the length frequency from the vertical transducer to the horizontal acoustic targets because fish target echoes in horizontal aspect do not relate to fish length as they do in vertical aspect (Kubecka 1994; Yule 2000). The assumption that fish species composition and size distribution was the same from 1- to 8 m (horizontal acoustics) and from 8- to 24 m (or 54 m in Devil’s Lake Embayment) was validated with netting data. The coefficient of variation from the total abundance estimate was applied to size-specific abundance estimates. Species-specific abundance estimates were calculated

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 5 by multiplying the species composition from the gill net survey by the acoustic abundance estimates. We did not capture fish less than 100 mm long in the offshore zone so no estimate was made for acoustic targets corresponding to this size class.

Abiotic Conditions

Abiotic conditions were measured on May 22 and July 23, 2002 at 3 limnetic locations in the main basin and on July 23, 2002 in the Devil’s Lake embayment. Temperature, dissolved oxygen, conductivity, turbidity, and pH were recorded in 3 m increments, beginning at 0.5 m with a Hydrolab Minisonde 4a. The northern limnology station [Lim1; (N47o 56.106; W119o 04.072)] was located offshore from the mouth of Osborne Bay, approximately 4 km from the North Dam (Figure 1). The middle limnology station [Lim3; (N47o 53.059; W119o 08.308)] was located offshore, west of the North end of Steamboat Rock (Figure1). The southern limnology station [Lim5; (N47o 43.791; W119o 15.715)] was located offshore of the Million Dollar Mile, approximately 12.5 km from Dry Falls Dam.

Results

Hydroacoustic Density and Distribution

Acoustic target distribution varied by depth and location within and between surveys; however, the reservoir wide mean fish density (fish / 1x104 m3) was not significantly different in May [10.4 (mean) + 5.1 (2SE)] and July (10.3 + 3.4)(t-test; p =0.97, df =26). In May, fish density was not significantly different for the sidelooking (11.2 + 3.5) and downlooking (9.6 + 8.1) transducers (t-test; p=0.73, df=20); however, the similarity was primarily due to a very high density from 8-16 m on the downlooking transducer in transect 5 (Figure 2). Relatively few fish were distributed deeper than 8 m in transects 12-18 at the South end of the reservoir and low densities were common for the deepest depth strata (16-24 m) (Figure 3). In July, there was significantly lower density for the sidelooking transducer (3.1 + 3.2) than the downlooking transducer (17.7 + 6.8)(t-test; p<0.01, df=24). In July, most fish were in the deepest depth strata available, 16-24 m throughout most of the reservoir (Figure 4). However, in transect 7 (Devils Lake Embayment) where deeper depths were available (54 m), many acoustic fish targets were also suspended midwater in the 24-32 m depth strata (Figure 4).

In May, fish densities were lowest near the North Dam and Dry Falls Dam with the highest densities occurring in the middle transects (North and West of Steamboat Rock State Park) (Figure 2). In July, fish densities were also high in transects 5-12 (near Steamboat Rock); however, densities were also relatively high in transects 1 and 18 (Figure 2).

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 6 100 ) 3 80 May 2002

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Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 9 the vertical transducer observed fish from 8 m to the bottom of the reservoir. Transect 7 (Devils Lake Embayment) was the only transect where bottom depth exceeded 24 m.

Gill Net Surveys

May.— A total of 48 of 252 (19%) limnetic sites were sampled with a combination of vertical and horizontal gill nets. Limnetic nets caught 127 fish during three nights and lake whitefish dominated the species composition by weight (95%) and number (88%) (Table 1). Vertical gill nets accounted for 46% of the total catch, primarily catching whitefish (93%) (Table 2). The floating horizontal net only caught whitefish (53%) and rainbow trout (47%), and the sinking horizontal net caught several species including whitefish (96%), burbot (2%) and kokanee (2%)(Table 2). Catch rates were the highest in the sinking gill net (16.3 fish/net night) and lowest in the vertical gill nets (1.40 fish/net night)(Table 2). In May, fish were distributed throughout the water column. Whitefish were captured at all depths, with the highest frequency occurring between 18 and 22 meters (Figure 5). Rainbow trout were generally caught near the surface, with several fish caught at 8-10 meters (Figure 5). July. – A total of 127 fish were collected in 3 nights with vertical and horizontal gill nets at 48 limnetic sites within the main lake basin (excluding the Devil’s Lake embayment). Vertical gill nets accounted for 76% of the total catch, primarily catching whitefish (83%). The floating horizontal net only caught a total of one whitefish and one kokanee. The sinking horizontal also caught only whitefish (90%) and kokanee (10%). One 2002 net pen stocked kokanee was caught during the July survey. Catch rates were highest in the vertical gill nets (1.95 fish/net night) and lowest in the floating gill net (0.50 fish/net night) (Table 1). In July the majority of whitefish were captured below 16 m, with nearly 50% deeper than 21 meters (Figure 6). Rainbow trout were sampled at the surface and midway in the water column. Kokanee (n = 18) collected in July were mainly distributed from 10 to 24 meters (Figure 6). A total of 219 fish were collected in 2 nights with vertical gill nets in the Devil’s Lake embayment. One of the gill nets was pulled after one night; however, five nets remained in the lake for two nights due to inclement weather. Whitefish accounted for 94% of the total catch, with catch rates near 19 whitefish per net night. Catch rates for rainbow trout (n = 9), smallmouth bass (n = 1), and walleye (n = 3) were 0.8, 0.1, and 0.3 fish per net night, respectively. No kokanee were caught in the Devil’s Lake embayment. Whitefish were distributed between 16 and 42 meters, with the highest density in the 22-24 meter depth bin. Rainbow trout were generally caught between 8 and 16 meters (Figure 7); however, sample sizes were too low to determine depth preferences.

Limnetic Fish Abundance

For both May and July, the total limnetic fish abundance estimate was 1.5 x106 fish [mean; ± 0.50-0.76 x106 (2 SE)] with target strengths between –55 and –27.7 dB (~30- 800 mm) (Table 3). For fish that were better represented by our gill net survey (100-800 mm), the estimate was 1.3 x106 fish (± 0.45-0.65 x106) (Table 3). There were only two

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 10

Table 1. Species composition by number, weight and the minimum and maximum lengths of fish captured in offshore gill nets on Banks Lake, WA for May and July 2002.

Main Lake Basin May 2002 Length (mm) Species Number % n Weight (g) % Wt Minimum Maximum Burbot 1 0.8% 1,025 0.9% 565 565 Kokanee 1 0.8% 55 0.0% 193 193 Whitefish 112 88.2% 108,695 94.9% 282 529 Rainbow trout 12 9.4% 3,581 3.1% 220 387 Smallmouth bass 0 0.0% NA 0.0% NA NA Walleye 1 0.8% 1,200 1.0% 516 516 Total 127 100.0% 114,556 100.0%

Main Lake Basin July 2002 Length (mm) Species Number % n Weight (g) % Wt Minimum Maximum Burbot 0 0% NA 0% NA NA Kokanee 18 14% 1,858 2% 109 390 Whitefish 105 83% 90,935 95% 307 540 Rainbow trout 3 2% 1,540 2% 275 387 Smallmouth bass 0 0% 0 0% NA NA Walleye 1 1% 1,050 1% 525 525 Total 127 100.0% 95,383 100%

Devil’s Lake July 2002 Length (mm) Species Number % n Weight (g) % Wt Minimum Maximum Burbot 0 0% NA 0% NA NA Kokanee 0 0% NA 0% NA NA Whitefish 206 94% 165,165 96% 340 540 Rainbow trout 9 4% 3,261 2% 282 417 Smallmouth bass 1 1% 100 0% 206 206 Walleye 3 1% 2,735 2% 395 536 Total 219 100.0% 171,261 100%

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 11 Table 2. Effort (net nights), catch and catch per unit effort by gear type for fish captured in the main lake basin during May and July and in the Devil’s Lake embayment in July.

Floating Horizontal Sinking Horizontal Vertical Sample Period/Spp Effort Catch CPUE Effort Catch CPUE Effort Catch CPUE May 3 0 0.0 3 1 0.3 42 0 0.0 Burbot 3 0 0.0 3 1 0.3 42 0 0.0 Kokanee 3 0 0.0 3 47 15.7 42 0 0.0 Whitefish 3 10 3.3 3 0 0.0 42 55 1.3 Rainbow trout 3 9 3.0 3 0 0.0 42 3 0.1 Smallmouth bass 3 0 0.0 3 0 0.0 42 0 0.0 Walleye 3 0 0.0 3 0 0.0 42 1 0.0 Total 3 19 6.3 3 49 16.3 42 59 1.4

July Burbot 4 0 0.0 5 0 0.0 48 0 0.0 Kokanee 4 1 0.3 5 3 0.6 48 14 0.3 Whitefish 4 1 0.3 5 26 5.2 48 78 1.6 Rainbow trout 4 0 0.0 5 0 0.0 48 3 0.1 Smallmouth bass 4 0 0.0 5 0 0.0 48 0 0.0 Walleye 4 0 0.0 5 0 0.0 48 1 0.0 Total 4 2 0.5 5 29 5.8 48 127 2.6

July (Devil’s Lake) Burbot NA NA NA NA NA NA 11* 0 0.0 Kokanee NA NA NA NA NA NA 11* 0 0.0 Whitefish NA NA NA NA NA NA 11* 206 18.7 Rainbow trout NA NA NA NA NA NA 11* 9 0.8 Smallmouth bass NA NA NA NA NA NA 11* 1 0.1 Walleye NA NA NA NA NA NA 11* 3 0.3 Total NA NA NA NA NA NA 11* 219 19.9 NA- No horizontal nets were set in Devil’s Lake. *Six nets were set, but five of the nets remained in the water for 2 consecutive nights.

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 12 4

12 WHITEFISH n = 108 16

Depth (m) 4

16 RAINBOW TROUT n = 12 20

0.0 0.2 0.4 0.6 0.8 Frequency

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 13 4

12 WHITEFISH n = 103 16

16 RAINBOW TROUT Depth (m) 20 n = 3

8 KOKANEE 12 n = 18 16

28 0.0 0.1 0.2 0.3 0.4 0.5

Frequency

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 14

4 8 12 16 WHITEFISH 20 n = 206 24 28 32 36 40 44 48 52 56 60

4 0.1 0.2 0.3 0.4

Depth (m) 8 12 16 20 24 28 32 36 Rainbow Trout 40 n = 9 44 48 52 56 60

0.1 0.2 0.3 0.4

Frequency

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 15 Table 3. Abundance and Density (fish/ha) of all fish targets and specific size classes of fish targets in Banks Lake Washington, 2002. Length estimates were generated by converting target strength (dB) to total length (mm) using an equation from (Love 1971).

Size Class (mm) Month Parameter 30-800 100-800 30-100 100-200 200-400 400-800 % Frequency 100% 86% 14% 19% 50% 17% Abundance 1,539,719 1,317,679 218,400 296,660 764,400 256,620 May 2 SE 761,386 651,588 107,998 146,697 377,993 126,898 Fish/ha 140 120 20 27 69 23

% Frequency 100% 90% 10% 15% 44% 31% Abundance 1,523,887 1,375,712 148,175 229,285 670,651 475,776 July 2 SE 500,671 451,988 48,683 75,331 220,341 156,315 Fish/ha 139 125 13 21 61 43

0.08

Banks Lake 2002

0.06 May July

0.04 Percent Frequency Percent 0.02

0.00 0 100 200 300 400 500 600 700 800

Estimated Length (mm)

Figure 8. Length frequency of acoustic targets, converted to fish length (Love 1971), for Banks Lake, Washington in May and July of 2002.

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 16 40 35 30 25 May 20 15

Number of Fish of Number 10 5

40 35 Kokanee 30 Whitefish 25 Other Fish July 20 15

Number of Fish Number 10 5

90 80 70 July 60 Devil's Lake 50 40 30 Number of Fish 20 10

0 100 200 300 400 500 600 700 800

Length (mm)

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 17 distinct modes in the length frequency of acoustic targets and the largest mode encompassed the size range of all fish captured in the gill nets (Figures 8, 9). However, we divided up the fish targets into size classes (100-200; 200-400; 400-800), where the average fish length was 2x the previous size class to give some indication of how the limnetic fish abundance is distributed among various sizes of fish (Table 3). We then applied the total species composition to the acoustic estimate for all fish targets from 100- 800 mm, which provided abundance estimates of over 1x106 sub-adult and adult whitefish (Table 4).

Abiotic Conditions

In May, the vertical temperature profiles from the three limnology stations gradually climbed from 9-13 oC between the bottom and the surface (Figure 10). Likewise, there was very little change to the dissolved oxygen (DO) concentration at any of the stations in May (Figure 10). Temperature was more variable in July, with temperatures as high as 23 oC at 0.5 m, and generally around 14 oC near the bottom (Figure 10). Dissolved oxygen conditions were also more variable in July, with near bottom DO dropping to 6.2 in July (Figure 10). The remaining water quality parameters were reported in Polacek et al. (2003a). Station LIM1 showed a different thermal profile in July than either the middle or southern stations, with near isothermic conditions at 16 oC from 2 m below the surface to 2 m off the bottom (Figure 10). Stations LIM3 and LIM5 had a gradual thermal gradient that did not reach 16 oC until a depth of 15 to 19 m, respectively. In the Devil’s Lake embayment in July, water temperatures ranged from 24.5° C at the surface to 4.8° C at 53 meters, with the thermocline at 20 meters. Dissolved oxygen levels reached a high of 10.7 mg/L at 6 meters and low of 0.6 mg/L near the bottom (53 meters (Figure 11).

Discussion

Whitefish were the predominant fish in the limnetic zone of Banks Lake, and the combination of hydroacoustics and gill net surveys revealed that whitefish inhabited all depths and areas of the reservoir in high abundance. Whitefish were commonly captured throughout the water column in May, when temperatures were favorable regardless of depth. In July, when temperatures commonly exceeded 16 C in the upper 16 m, whitefish were rarely captured in the upper portion of the water column and hydroacoustic density was significantly lower in the near surface strata (Figures 2, 6, 10) We had intended to use acoustics and netting to assess the kokanee population in Banks Lake, however, kokanee were such a minor proportion of the catch that we had little confidence that our survey accurately represented kokanee. Kokanee prefer temperatures in the 10-15 °C range and therefore much of Banks Lake was thermally stressful to kokanee during the warm summer months, even without the overwhelming abundance of whitefish already occupying these habitats. The middle portion of the reservoir (near Steamboat Rock) had the highest density of fishes during both seasons. Additionally, in July, fish were concentrating near the North Dam (transect 1) where cool water was being pumped in from Lake Roosevelt and in Devil’s Lake Embayment (transect 7) where

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 18

Table 4. Species-specific abundance estimates of limnetic fishes (~100-800 mm TL) in Banks Lake, Washington in May and July 2002. Estimates were generated through mobile hydroacoustics and gill net surveys; error bounds were not estimated for percent species composition (% Spp) so the 2 SE only incorporates variance based on the acoustic estimate.

% Spp Abundance 2 SE Acoustic estimate 1,317,679 651,588

Whitefish 0.88 1,159,558 573,397 May Kokanee 0.01 13,177 6,516 Rainbow Trout 0.09 118,591 58,643 Other Fish 0.02 26,354 13,032

Acoustic estimate 1,375,712 451,988

Whitefish 0.81 1,067,320 527,786 July Kokanee 0.15 197,652 97,738 Rainbow Trout 0.03 39,530 19,548 Other Fish 0.01 13,177 6,516

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 19 Station: Lim1 Station: Lim3 Station: Lim5 1

Depth (m) Depth 15 17 Temp. 19 DO

21 23 May 22 May 22 May 22

Depth (m) 15

21 23 July 23 July 23 July 23 25 5 101520255 101520255 10152025 Temperature (oC) and Dissolved Oxygen (mg/L)

Figure 10. Temperature and dissolved oxygen profiles from Banks Lake, Washington in May and July, 2002. Lim1 was the northern most site, Lim3 was the central site (offshore west of Steamboat Rock), and Lim5 was the southern most site.

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 20 0

20 Temperature Disolved Oxygen 30 Depth (m) Depth 40

0 5 10 15 20 25

Temperature (oC) and Disolved Oxygen (mg/L)

Figure 11. Temperature and dissolved oxygen profiles from the Devil’s Lake embayment in Banks Lake, Washington July, 2002.

deeper bottom depths offered a larger volume of cool water. These areas represent potential cool water refuge for salmonids. At the North end, near limnology site LIM1 and hydroacoustic transects 1 and 2, there was only one gill net set in July and it caught a rainbow trout near the surface. The rainbow trout net pens are released in this area in May-June each year, so it is possible that high abundance of acoustic targets near the surface were recently released rainbow trout. Unfortunately, the water temperature at LIM1 increased from 16 oC in July, to 19 oC in August, thereby eliminating this area as a potential thermal refuge for kokanee in late-summer. In Devils Lake Embayment, the gill nets with 89 and 102 mm mesh sizes were so satiated with large whitefish (65 and 110 fish, respectively) that the net efficiency may have been compromised. If other species were present at lower densities they may not have had a chance to encounter the net before it “filled-up” with whitefish. We do not know if the catch rate of whitefish would have been higher based on attraction of struggling fish or lower based on avoidance.

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 21 Therefore, the Devil’s Lake Embayment species composition and catch rate information should be used with caution, and we did not use this data in or reservoir wide-species composition expansion of the acoustic abundance estimate. We assumed equal probability of gill net capture between species; however, this assumption could have overestimated abundance, if a species was more vulnerable to the gill nets. For example, if kokanee were more active than lake whitefish, but just as likely to be retained by the net once it was encountered, then kokanee abundance was overestimated while lake whitefish abundance was underestimated. The gill nets only captured fish greater than 100 mm, and larger fish have greater capture probabilities in gill nets (Hamley 1975; Rudstam et al. 1984; Henderson and Wong 1991). We applied the species composition from all fish captured in the gill nets to all acoustic targets greater than –45 dB (~100 mm). If species composition of the smaller fish (<200 mm) was different, then our acoustic estimates would be biased for the smaller size classes. This was the primary reason we did not generate species-specific estimates for acoustic targets smaller than 100 mm. Our species-specific abundance estimates were not valid for species other than whitefish because they were such a small proportion of the catch. For example, when rainbow relative abundance fluctuated from 9% in May to 3 % in July our estimate of abundance decreased 3-fold. We do not believe that change was real, but a function of small sample size, distribution, activity rates, net pen release patterns, and variance of the sampling gear. Although there was substantially lower density of fish in transect 5 in July, compared to May, this change was less important because there was considerably less volume due to shallow bottom depths (mean bottom depth 12 m). In transect 5 our acoustic gear only sampled 11% as much volume as transect 7. The downlooking transducer was only sampling a small volume of water in transect 5 in May, and the density estimate of 65 fish / 10,000 m3 was generated from detecting 126 fish targets. Conversely, the estimated density of 61 fish / 10,000 m3 in transect 7 in July was generated from detecting 2032 fish on the downlooking transducer. We could not determine the volume of water in the limnetic zone independently from the littoral zone. Mean density was extrapolated to reservoir-wide volume; therefore, we assumed that fish density in the littoral zone was equal to the limnetic zone for the species composition observed in limnetic gill nets. We recognize that species composition was much different in the littoral zone and included many more species than we observed in the limnetic zone (Polacek et al. 2003a; Polacek et al. 2003). We recognized this in the results section and emphasized that the abundance estimates for species other than whitefish do not represent a complete reservoir wide abundance. Additionally, if nearshore densities of whitefish were higher than offshore densities then we underestimated reservoir wide abundance for whitefish; however, the relatively small volume of water in the littoral zone minimized the potential bias from this assumption. The effect of this potential bias was further minimized during the July survey, due to the high epilimnetic temperatures that were well above thermal optimum for whitefish. The horizontal transducer could not differentiate target strength, so we could not determine the density of specific size classes for near-surface targets. We assumed that the size distribution of fish was the same from 1.5-8 m and from 8 m to the bottom. Similar mean lengths for each species and depth interval from the gill nets verified this assumption.

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 22 An important aspect of these surveys was that we did not encounter high catch rates of walleye in the offshore nets, indicating that spatial overlap of distribution was limited, or that walleye densities were very low. However, predation may still be a limiting factor if newly stocked kokanee are vulnerable to smallmouth bass and walleye in littoral areas. Future studies can focus on acute predation during stocking events without the confounding factor of missing a large proportion of the predators on a reservoir-wide basis (Baldwin et al. 2000; Baldwin et al. 2003). This study evaluated the limnetic fish community of Banks Lake, as one portion of a larger project with the goal of evaluating the Banks Lake Fishery and determining limiting factors for stocked kokanee and naturally recruiting panfish. We have identified two potential limiting factors for stocked kokanee; 1) competition with a high abundance of whitefish and 2) limited refuge from warm water temperatures in late summer. Initial limnological studies have shown relatively high abundance of zooplankton forage and the presence of large bodied Daphnia spp., indicating that Banks Lake is not food limited (Polacek et al. 2003a; Polacek et al. 2003). Future efforts may need to determine the carrying capacity of Banks Lake for kokanee based on the available late-summer habitat. Managers may be grossly overestimating the ability of Banks Lake to yield kokanee if they base stocking rates on total hectares and not usable volume during the most limiting time period. Additionally, the stress of sharing that habitat with a highly abundant whitefish population may reduce survival even more. Little is known about competition for space in lentic ecosystems and knowing the tolerance limits for density of competing species may be critical for the success of kokanee in Banks Lake. Substantial reservoir- wide reductions in whitefish abundance could be necessary to allow success for the hatchery kokanee program. Sport fishing for whitefish is a very low proportion of the total angling effort in Banks Lake, and, since its not popular in other Washington lakes and reservoirs its not likely that promotional efforts will yield a fishery that could reduce the population significantly. Another alternative would be to commercially harvest whitefish, if it could be conducted in a manor that would not affect the current fisheries for bass, walleye, and rainbow trout. It is premature to make final conclusions about how to use the information provided in this report, given the substantial effort that is underway to evaluate various limiting factors on a reservoir-wide basis. However, the information in this report should provide a foundation for making these or other management decisions in the future. Finally, these surveys indicate that July-August provided the more precise estimate of limnetic fish abundance and that future monitoring should continue during this period when fish are distributed deeper thereby making them more susceptible to detection by the vertically oriented transducer. Future efforts should also evaluate the distribution and life history of juvenile whitefish, to better understand recruitment and spatial and temporal overlap that could be influencing competition with more desirable sport fishes.

Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 23 References

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Hydroacoustic and Gill Net Assessment of Banks Lake, Washington 2002 25