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Fisheries Research 82 (2006) 176–185

Hydroacoustic separation of rainbow smelt (Osmerus mordax)age groups in Lake Champlain

S.L. Parker Stetter a,∗, L.G. Rudstam a, J.L. Stritzel Thomson b,1, D.L. Parrish c

a Department of Natural Resources, Cornell University Biological Field Station, 900 Shackelton Point Road, Bridgeport, NY 13030, USAb Vermont Cooperative Fish and Wildlife Research Unit, The Rubenstein School of Environment and Natural Resources,

University of Vermont, Burlington, VT 05405, USAc U.S. Geological Survey, Vermont Cooperative Fish and Wildlife Research Unit, The Rubenstein School of Environment and Natural Resources,

University of Vermont, Burlington, VT 05405, USA

Received 18 August 2005; received in revised form 10 May 2006; accepted 16 June 2006

bstract

Separate assessment of young-of-year (YOY) and yearling-and-older (YAO) fish is desirable from both ecological and management perspectives.coustic assessments provide information on fish population size structure in the target strength (TS) distribution, but interpretation of TSistributions must be done carefully, as single age groups can produce multiple TS modes. We assessed the ability of in situ TS distributions todentify Lake Champlain rainbow smelt (Osmerus mordax) age groups in June, July, and September of 2001 using mobile and stationary surveys,nowledge of vertical distribution preferences, and predicted TS from trawl catches. YAO rainbow smelt (93–179 mm total length) had wide TSistributions between −60 and −35 dB in all 3 months with two modes at approximately −50 and −40 dB. Most stationary survey single-fishracks attributed to YAO had targets in both TS modes and a wide TS range often over 15 dB. Between June and September, YOY rainbow smelt TSncreased, but single-fish tracks were unimodal, and the TS range was smaller (6 dB). Overlap in TS attributed to YOY and YAO increased from noverlap in June (YOY TS −76 to −61 dB, 15–25 mm) to moderate overlap in July (−76 to −50 dB, 25–63 mm) to considerable overlap in September

−68 to −45 dB, 33–80 mm). In June and July, the TS distribution changed abruptly at the thermocline, indicating almost complete separation of thewo groups. A more gradual TS transition was evident in September, indicating substantial overlap between YOY and YAO. Separate estimates cane obtained in September by decomposing TS overlap into components attributed to YOY and YAO rainbow smelt. However, this decompositionntroduces additional uncertainty and an assessment in July or possibly August is preferable to obtain separate abundance estimates of YOY and YAO.

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2006 Elsevier B.V. All rights reserved.

eywords: Rainbow smelt; Hydroacoustics; Age-group separation; Target stren

. Introduction

Separate assessments of fish age groups are needed to addressoth ecological and management questions. The assessment ofoung-of-year (YOY) provides an index of future fish recruit-

ent, while an assessment of yearling-and-older (YAO) is

eeded for an estimate of fishable stock biomass. Ecologicalnteractions, such as competition or cannibalism, also requirenowledge of the abundance of individual age groups.

∗ Corresponding author. Present address: School of Aquatic and Fishery Sci-nces, University of Washington, P.O. Box 355020, Seattle, WA 98195-5020,SA. Tel.: +1 206 221 5459; fax: +1 206 685 7471.

E-mail addresses: slps@u.washington.edu (S.L. Parker Stetter),gr1@cornell.edu (L.G. Rudstam), Jennifer.S.Thomson@state.ma.usJ.L. Stritzel Thomson), Donna.Parrish@uvm.edu (D.L. Parrish).

1 Present address: Annisquam River Marine Fisheries Station, Massachusettsivision of Marine Fisheries, 30 Emerson Avenue, Gloucester, MA 01930, USA.

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165-7836/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.fishres.2006.06.014

Modes in acoustic in situ target strength (TS) distributionsave been used to identify different species, or age groupsithin a species, and to calculate separate density estimates

or these groups (Brenner et al., 1987; Rudstam et al., 1987;arange et al., 1994; Appenzeller, 1998). Even though fish

ength is an important predictor of TS (e.g., Love, 1971), factorsuch as tilt angle, gonadal development, and degree of stom-ch fullness also influence observed TS (Ona, 1990; Nielsennd Lundgren, 1999; Gauthier and Rose, 2001; Ona et al.,001; Horne, 2003). Some species can give rise to TS dis-ributions with two or more modes even from narrow lengthistributions or within single-fish tracks (e.g., Williamson andraynor, 1984; Knudsen et al., 2004). These species include

ainbow smelt (Osmerus mordax) (Burczynski et al., 1987;ppenzeller, 1998; Rudstam et al., 1999, 2003). Therefore, a

areful examination is needed to determine whether modes inhe TS distribution are a result of the presence of multiple size

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lasses or variability within the TS distribution of individualsh.

Lake Champlain is a large inland lake on the border of Nework (USA), Vermont (USA), and Quebec (Canada). As rainbowmelt constitute up to 99% of pelagic trawl catches (Pientka andarrish, 2002), Lake Champlain is ideal for investigating varia-

ion in rainbow smelt TS. Yearling-and-older rainbow smelt areargely restricted to the meta- and hypolimnion during periodsf stratification, while YOY remain in epilimnetic waters duringhe summer and begin migrating into cooler metalimnetic watersuring the late summer and early fall (Ferguson, 1965; Tin andude, 1983; Nellbring, 1989).

In this study, we tested the ability of in situ TS distributions todentify Lake Champlain rainbow smelt age groups (YOY andAO) in June, July, and September of 2001, using data fromobile and stationary surveys, knowledge of vertical distribution

references, and predicted TS based on the length distribution inrawl catches. Stationary data were used to obtain many echoesrom single-fish. We used the seasonal change in vertical dis-ribution and length of rainbow smelt to establish appropriateS ranges for both YOY and YAO from June to September. Weesigned a protocol for separating YOY and YAO rainbow smeltn all 3 months, including the periods of spatial overlap, and pro-ide recommendations on survey timing for assessing rainbowmelt in northern temperate lakes using acoustic techniques.

. Methods

.1. General

Hydroacoustic and trawl data were collected in 2001 in theortion of Lake Champlain known as the Main Lake. Surveysere conducted over three nights in the periods 17–21 June,2–26 July, and 16–20 September. All work was performed onhe University of Vermont R/V “Melosira” (13.7 m length, 275orse power engine, 5.7 m s−1 cruising speed). Both mobile andtationary acoustic data were collected. Mobile acoustics andrawling were conducted 1 h or later after sunset and ended 1 hr earlier before sunrise. Stationary acoustics were performedetween sunset and the beginning of mobile acoustics each nightnd also during one to two 15 min intervals during each night.

A Simrad EY500 split beam echosounder (70 kHz, 11.1◦ half-ower beam width, 0.2 ms pulse length, 2 pings s−1) was usedo collect acoustic data on Lake Champlain. The transducer was

ounted on a towed body suspended from a winch on the star-oard stern of the boat. Towing speed was approximately 3 m s−1

nd total length of each survey approximately 115 km. The unitas calibrated with a standard copper sphere within a few weeksf each survey. During stationary acoustics, the horizontal posi-ion of the towed body was maintained by attaching stabilisingines. Thermal profiles were taken one or two times per nightsing a lowered depth–temperature recorder.

.2. Fish collections

Two trawl types were used to collect rainbow smelt for targetdentification in 2001. Yearling-and-older rainbow smelt were

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Research 82 (2006) 176–185 177

argeted with a mid-water trawl with a 6 mm square-mesh codnd and 6 m × 6 m average fishing dimensions. A 2 m × 2 mucker trawl with a 1 mm cod end was used to sample YOYainbow smelt. Both nets were fitted with a netsonde onhe headrope to measure tow depth. Total length (mm) was

easured in the field for all YAO rainbow smelt and for YOYn September. As YOY specimens in June and July were toomall to measure in the field, they were preserved in ethanolnd measured in the laboratory.

We calculated TS (dB) of rainbow smelt in trawl catches usingTS-length equation for rainbow smelt at 70 kHz (Rudstam et

l., 2003):

S = 19.9 × log 10 (length) − 67.8

here length is rainbow smelt total length in centimetres. Aon-target species, cisco (Coregonus artedii), was detected inome mid-water trawls. The TS (dB) of cisco was calculatedsing Love’s equation evaluated for 70 kHz (TS = 19.1 log10length) − 63.85, length in cm, Love, 1971).

.3. Hydroacoustic analyses

All hydroacoustic data were processed and analysed withchoview 3.25.55 (SonarData, 2004). Transects were divided

nto 100 m segments in June and July and 400 m segments ineptember. As finer vertical segments were needed to separateOY and YAO in September, it was necessary to use longer

400 m) horizontal segments to ensure that >100 single targetsere found in each cell. All transect files were inspected forottom identification, vessel noise spikes, and electrical interfer-nce prior to processing. Spikes and interference were removedanually. We excluded data within 0.5 m of the bottom and dataithin 3 m from the surface.

.4. Mobile survey TS distributions

For TS distribution analyses, we divided the water columnased on the thermal preferences of YOY and YAO rainbowmelt in June and July. The YOY zone was defined from 3 mepth to the top of the thermocline. The YAO zone was defined ashe top of the thermocline to 0.5 m from the bottom. In Septem-er, when YOY and YAO rainbow smelt were mixed within thehermocline, we exported single targets in 5 m depth bins.

We used the manufacturers single target detection equationith the following settings: lower threshold −76 dB, echo lengthithin 0.5 and 1.8 times the original pulse length, maximum gain

ompensation of 8 dB (two-way), and maximum phase devi-tion of 4◦ (Simrad, 1996). We calculated background noisedB) at 1 m range by extrapolating from noise measurementst depth given a time varied gain (TVG) function (Watkins andrierley, 1996; Korneliussen, 2000). Noise at any depth can thene calculated from the noise level at 1 m and the TVG func-ion. Variability in the return signal causes some noise spikes

o pass the Simrad single-fish detection criteria. Therefore, wenly accepted targets that were at least 14 dB larger than thealculated noise level at the deepest section of the vertical bineing considered. We chose 14 dB because noise levels varied

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78 S.L. Parker Stetter et al. / Fish

6 dB and we needed to exclude noise spikes that may be con-idered fish at the outer edge of the beam (we used a maximumwo-way beam compensation of 8 dB, 6 + 8 = 14 dB). We refero this level as our “noise-biased threshold”.

We calculated the value of Nv, the average number of fish percoustic reverberation volume, for each cell (Sawada et al., 1993)ssuming a TS of −45 dB for YAO rainbow smelt and −60 dBJune), −55 dB (July) and −50 dB (September) for YOY rain-ow smelt. Cells with an Nv > 0.10 were considered to havesh densities too high to allow accurate determination of initu TS (Warner et al., 2002; Rudstam et al., 2003) and werexcluded.

In situ TS values were used to generate mobile survey TSistributions for both YOY and YAO rainbow smelt in June anduly and for 5 m depth bins in September. If the TS distributionas bimodal, we defined the inflection point as the TS with the

owest proportion of targets between the two modes.

.5. Stationary survey TS distributions

Target strength measurements were obtained from single-sh tracks identified by the Echoview single-fish 4-D tracketection algorithm (SonarData, 2004). Each track was visuallynspected for consistent movement through the beam to ensurehat all TS measurements from a track could be attributed tone fish. Tracks with sudden changes in location were excludednd assumed to represent more than one fish. We only includedracks with ≥25 detections. We selected YAO rainbow smeltracks from ≥15 m in June and July and used these to gen-rate a mean TS distribution for YAO. In September, duringhermal overlap between YAO and YOY rainbow smelt, weelected single-fish tracks from 5 m depth layers between 5 and0 m. We generated a mean TS distribution for YAO rainbowmelt by pooling single tracks from 30 to 50 m and for YOYainbow smelt by pooling single tracks from 5 to 15 m. Meanepth and the number of detections were recorded for each fishrack.

To evaluate whether single YAO rainbow smelt could appears either large or small targets, we examined the inflection pointn the bimodal TS distributions and determined the proportion ofchoes from single YAO rainbow smelt tracks that occurred onoth sides of this inflection point (see target strength distributionn Section 2). We present the average of these proportions forach month to give equal weight to each track.

.6. Separating YOY and YAO during period of spatialverlap

YOY and YAO rainbow smelt overlapped spatially through-ut the meta- and upper hypolimnion during September (seeection 3.5). The target strength distributions of YAO and YOYainbow smelt also overlapped in September (between −60 and45 dB, see Section 3.5). Therefore, we developed a simple

ethod to decompose the TS distribution into the contributions

rom YOY and YAO within the zone of TS overlap. As the TS ofOY rainbow smelt was not larger than −45 dB, the presencef targets ≥−44 dB can be used as an index of YAO density.

3

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Research 82 (2006) 176–185

he number of smaller targets expected from the YAO rainbowmelt can then be calculated from the number of targets largerhan −44 dB if the shape of the TS distribution expected fromAO rainbow smelt is known. We identified depths, generally0–50 m, at which only YAO rainbow smelt were present basedn trawling and the shape of the TS distribution. We dividedhese TS distributions into small targets (TS, −60 to −45 dB)nd large targets (TL, −44 to −35 dB). For each depth interval,argets smaller than our noise-biased threshold (see Section 2)ere excluded. The expected number of small targets given thebserved number of large targets E(TS|TL) was calculated forach depth interval as:

(TS|TL) = TS

TL

e calculated the number of YAO targets (TYAO) within TSistributions containing both YAO and YOY rainbow smelt fromhe observed number of targets too large (TL) to represent YOYainbow smelt:

YAO = TL + TL × E(TS|TL)

f more small targets (TS) were predicted for YAO than wereresent in a given layer, all small targets within the −60 to45 dB bin were assigned to the YAO component.The number

f YOY targets was then calculated from the total number ofargets in the range expected from YOY rainbow smelt (−68 to

45 dB) by subtracting the number of targets within that rangettributed to YAO rainbow smelt (TL × E(TS|TL)). All targetumbers were converted to proportions at depth. No estimatesf YAO were made for the epilimnion and we corrected forOY rainbow smelt only to 40 m depth. We do not expect YOY

ainbow smelt deeper than 40 m. Discrete depth trawling withstacked closing 1 m × 1 m 500 �m mesh Tucker trawl duringeptember 2002 found no YOY rainbow smelt deeper than0 m (Parker Stetter, 2005).

The final step was the application of YOY and YAO propor-ions, calculated from the TS distributions within each depthayer, to the total density calculated for the same depth layers.ny targets smaller than <−68 dB were not considered to beOY, and any targets >−35 dB were classified as larger fish

cisco, Atlantic salmon (Salmo salar), or lake trout (Salvelinusamaycush)).

. Results

.1. Temperature

Temperature profiles for June, July, and September showedariation in the strength and depth of the thermocline. June hadhe weakest thermal structure, July the widest range of tem-eratures, and September had the sharpest transition betweenpilimnetic and metalimnetic waters (Fig. 1).

.2. Fish collection

During June, July, and September we collected >1500 rain-ow smelt in 27 trawls (Table 1). Total length (mm) of YOY

S.L. Parker Stetter et al. / Fisheries

Fig. 1. Temperature profiles for June (solid black), July (solid grey), and Septem-ber (dashed black) showing variation in the strength and depth of the thermoclinein the Main Lake of Lake Champlain, 2001.

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Table 1Trawl results for Main Lake 2001

Trawl Depths (m) Prop (YOY) Prop (YAO) Prop (

JuneTT-1 50, 30, 10 0.99 0.01TT-2 10, 5 1.00TT-3 40, 30, 20 0.24 0.76TT-4 20, 15, 10, 5 1.00TT-5 5, 3 0.57 0.43TT-6 10 1.00TT-7 6 1.00MW-1 45, 35, 25 0.82 0.18MW-2 40, 30, 25 1.00MW-3 27, 20 0.04 0.91 0.04MW-4 18 1.00

JulyTT-8 6 1.00TT-9 10 1.00MW-5 15, 10, 6 1.00MW-6 12, 9, 6 0.83 0.16MW-7 22 0.83 0.03 0.14

MW-8 16 1.00MW-9 36 1.00MW-10 19 0.87 0.13

SeptemberTT-10 15, 7 1.00TT-11 15 1.00MW-11 25, 18 0.42 0.57 0.01MW-12 30, 20 0.37 0.50 0.13

MW-13 15 1.00MW-14 30, 25 0.95 0.04 0.01MW-15 30, 25 0.83 0.17 0.00

MW-16 30, 20 1.00

Depths are the stepped depths at which the trawl was towed. Trawling took place ontrawl, TT: Tucker trawl. TT-1 and TT-3 were 10 min in duration; all others lasted 20 m

Research 82 (2006) 176–185 179

ainbow smelt increased throughout the season from a maxi-um of 25 mm in June to a maximum of 80 mm in September

Table 1). The TS distribution of YOY predicted from trawlatches using the TS-length regression was unimodal in all 3onths, and increased through the season (Fig. 2). Maximum

ength of YAO rainbow smelt did not increase through the sea-on (181 mm in June and 180 mm in September, Table 1) andhe predicted TS distribution of trawl captured YAO rainbowmelt was similar and unimodal in all 3 months (Fig. 3). CiscoOY and YAO were captured in mid-water trawls mainly in Julyut also in September and a few in June (Table 1) and was thenly non-target species captured in more than one trawl dur-ng each survey. Predicted TS for YOY cisco in July (−49.5 to

47.4 dB) and September (−45.2 to −43.7) and for YAO ciscon July and September (−37.0 to −35.1 dB) overlapped with

easured TS of YAO rainbow smelt in both July and Septembersee below).

Cisco) YOY lengthrange (mm)

YAO lengthrange (mm)

Cisco length range(mm)

10–17 101–10415–2213–16 93–17014–1811–13 45–8713–1714–19

96–172 291–324133–181

25 116–165 314122–165

19–2928–34

111–17121–31 96–210 295, 32825–37 109–160 66–85

320, 327

24–4298

27–45 79–172 337, 365

34–5638–5433–80 100–179 110–12038–77 119–168 110–131

320–336

35–56 140, 15736–61 108–160 290–33042–72 110–180 118

325–32646–69 125, 165

between 17 and 21 June, 22–26 July, and 16–20 September. MW: midwaterin.

180 S.L. Parker Stetter et al. / Fisheries Research 82 (2006) 176–185

Fig. 2. Target strength distributions for YOY rainbow smelt from model predic-tions of TS from trawl specimens (solid grey, based on Rudstam et al., 2003),mobile acoustic surveys (solid black), and from stationary tracked single-fish(dashed black) in June (a), July (b), and September (c). No single-fish trackswere analysed for YOY in June and July. TS distributions are from 3 to 15 m inJune, from 3 to 20 m in July and from 5 to 15 m in September. Sample size for thecurves were June: 93 trawl specimens, 115,462 mobile survey targets; July: 109tit

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Fig. 3. Target strength distributions for YAO rainbow smelt from model pre-dictions of TS from trawl captures (solid grey, based on Rudstam et al., 2003),mobile acoustics surveys (solid black), and from stationary tracked single-fish(dashed black) in June (a), July (b), and September (c). TS distributions weretaken from 15 to 50 m in June and from 20 to 50 m in July. In September,both the TS distribution and single-fish data were taken from 30 to 50 m depth.Sample sizes for the curves were June: 135 trawl specimens, 24,236 mobilesurvey targets, 18 stationary tracked fish; July: 198 trawl specimens, 24,138mobile survey targets, 10 stationary tracked fish; and September: 242 trawl spec-imens, 13,383 mobile survey targets, 33 stationary tracked fish. Targets smallertn

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rawl specimens, 68,399 mobile survey targets; and September: 208 trawl spec-mens, 138,616 mobile survey targets, 45 stationary tracked fish. Noise-biasedhresholds are shown (vertical line) if above our −76 dB collection threshold.

.3. Mobile survey TS distributions

The in situ TS distribution for YOY rainbow smelt fromur mobile surveys was unimodal in June, July, and Septem-er (Fig. 2). All observed targets larger than −76 dB in Junend July and all targets above −75 dB in September were abovehe noise-biased threshold (Fig. 2). Plots of TS by depth andchograms (Fig. 4, June presented) show discrete YOY layerso 15 m in June and 20 m in July. In September, YOY and YAOainbow smelt overlapped at the edges of their depth distribu-ions and distinct YOY aggregations were observed down to 15r 20 m (Fig. 5).

Yearling-and-older rainbow smelt TS distributions from ourobile surveys were wide and bimodal in all 3 months (Fig. 3).ased on echograms and TS-depth plots (e.g., Fig. 4), YAOere found immediately below the lower limit of YOY dur-

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han the −60 dB noise-biased threshold (vertical line) are considered to beoise.

ng June and July. Targets smaller than −60 dB (the noise-iased threshold at 50 m) were considered to be from back-round noise (e.g., Fig. 4) and were excluded in all 3 monthsFig. 3).

Target strengths distributions in 5 m depth layers during oureptember mobile survey show the transition from a TS distri-ution dominated by YOY to a TS distribution dominated byAO rainbow smelt (Fig. 5). Most YOY targets in Septemberere above −65 dB (Fig. 5), so noise did not add targets in theOY size range until the 25–30 m depth layer. Smaller targets

n deep water may also represent mysids (Fig. 5). Up to 8%f YOY and YAO cells had Nv values > 0.10 during June, July,nd September, and TS from these cells were excluded from thenalysis.

S.L. Parker Stetter et al. / Fisheries Research 82 (2006) 176–185 181

Fig. 4. (a) Mobile survey single target detections from June 2001 showing dis-tribution of hydroacoustic targets with −76 dB collection threshold. (b) Targetstrength vs. depth plot for the same area shows physical and/or hydroacousticseparation of YOY and YAO rainbow smelt and mysids. The increase in noiseaE

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Fig. 5. September TS distribution in 5 m depth layers from mobile survey (solidblack) and stationary tracked single-fish tracks (dashed black). Sample size forthe curves are 5–10 m: 12,602 mobile survey targets, 17 stationary tracked fish;10–15 m: 16,515 mobile survey targets, 29 stationary tracked fish; 15–20 m:11,206 mobile survey targets, 57 stationary tracked fish; 20–25 m: 9583 mobile

TS

A

JJ

S

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ppearing as single targets at depth is apparent in both figures. Produced inchoview (SonarData, 2004).

.4. Stationary survey TS distributions

We detected 104 single-fish tracks with more than 25 TSeasurements per track (Table 2). Yearling-and-older rainbow

melt tracks consisted of TS values ranging over 10–15 dBTable 2). These tracks show both gradual and rapid changesn TS over time (Fig. 6), and some of the tracks from indi-idual fish were bimodal. The combined TS distributions fromtationary tracked YAO rainbow smelt were also bimodal andimilar to that observed in the mobile survey data (Fig. 3). Aarge proportion of the measured TS within each track were

elow the inflection point of the bimodal distributions (averagef 39% in June, 42% in July and 59% in September, Table 2).hus, larger fish produced many of the small echoes observed

n deeper water, likely with changes in orientation relative to

survey targets, 67 stationary tracked fish; 25–30 m: 12,882 mobile survey targets,80 stationary tracked fish; 30–35 m: 10,016 mobile survey targets, 19 stationarytracked fish; and 35–50 m: 3366 mobile survey targets, 14 stationary tracked fish.Noise-biased thresholds for each depth bin are shown (vertical line) if above our−76 dB collection threshold.

able 2tationary tracked single-fish results for June, July, and September

ge/month No. of single-fishtracks

Depth range (m) Echoes/track TSmin, TSmax (dB) TS range (dB)(mean ± 1 S.E.)

Inflectionvalue (dB)

Proportion< inflection

une 16 18–48 31–82 −62.1, −32.0 14 ± 1.2 −48 0.42uly 10 18–41 28–186 −58.2, −30.5 15 ± 1.2 −47 0.59

eptemberYOY 45 6–15 25–58 −68.6, −45.8 6 ± 0.5 n/aYAO 33 30–42 53–200 −65.2, −35.6 10 ± 0.5 −46 0.39

epth range is mean depth for single tracks. Echoes/track is the range of the number of echoes in single tracks. TSmin, TSmax are the lowest and highest single echo inll tracks. TS range is the mean ± 1 standard error for all single-fish tracks. Inflection value (dB) is the inflection point in mobile survey TS distributions (Fig. 3a–c).roportion < inflection is the proportion of targets from YAO rainbow smelt < inflection value.

182 S.L. Parker Stetter et al. / Fisheries Research 82 (2006) 176–185

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tsTsSaddt(

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ig. 6. Variation in TS in stationary tracked single YAO with >80 observations.uly (2–9), and September (10–21) are presented.

he beam. Note that TS from tracked fish did not produce mea-urements below −60 dB (Fig. 3), confirming that the smallerS than −60 dB in deep water are from noise spikes or pos-ibly mysids. The TS distribution from single YOY tracks ineptember (Fig. 2c) was also relatively wide and complex,lthough the TS distribution of an individual track was sel-

om bimodal. Similar to the mobile survey, changes in TSistribution with depth from the stationary survey illustratehe transition from YOY to YAO rainbow smelt in SeptemberFig. 5).

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the first 150 pings of long tracks are shown. Yearling-and-older from June (1),

.5. Target strength recommendations

The range of TS that can be attributed to YAO rainbow smelthowed little variation among months, falling between −60nd −35 dB in all months (Fig. 3). Young-of-year TS, how-ver, changed throughout the season, ranging between −76 and

61 dB in June, −76 and −50 dB in July, and −68 and −45 dB

n September (Fig. 2). TS versus depth profiles for June (e.g.,ig. 4) suggest that our −76 dB lower collection threshold mayave been too high for YOY rainbow smelt. Setting the threshold

S.L. Parker Stetter et al. / Fisheries

Fig. 7. The proportion of targets at depth for YAO (−61 to −35 dB, open circles),YOY (−68 to −45 dB, solid circles), small targets (−76 to −68 dB, open trian-gles), and predators/large targets (−34 to −20 dB, solid triangles) as producedby the ratio technique to separate overlapped age-classes in September 2001.Estimated noise-biased thresholds for the bottom of each depth bin were −87(5–10 m, below collection threshold), −80 (10–15 m, below collection thresh-old), −75 (15–20 m), −73 (20–25 m), −68 (25–30 m), −65 (30–35 m), −64(od

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35–40 m), −62 (40–45 m), and −60 dB (45–50 m). Targets below these thresh-lds were not used in this analysis. Note that small targets are excluded in watereeper than 30 m due to changing noise-biased thresholds.

oo high would exclude smaller in situ TS values and lead to anverage in situ backscattering cross-section that is biased high,esulting in an underestimation of YOY abundance in June.

.6. Separating YOY and YAO in areas of overlap

Results from our ratio technique suggest that separation ofOY and YAO is possible when the depth distributions and TS

anges overlap (Fig. 7). Within the 30–50 m depth layers for alleptember transects (N = 20), E(TS|TL) values were 0.77 ± 0.06mean ± 1 standard error, N = 14 transects) in the open lake and.17 ± 0.01 (mean ± 1 standard error, N = 6 transects) in bayegions. Targets below the noise-biased threshold were excludedor each of the 5 m depth bins (Fig. 7).

. Discussion

We used in situ TS distributions from mobile surveys, single-sh tracks from stationary surveys, and TS-length regressions,ombined with depth distribution in trawl samples and knowl-dge of age specific temperature preferences, to assess whetherOY and YAO rainbow smelt can be acoustically separated dur-

ng June, July, and September. Our results indicate that modes inhe in situ TS distributions should not be interpreted as differentainbow smelt age groups because variability and/or bimodalityn YAO TS distributions will lead to TS from YAO rainbow smelthat overlap with the TS expected from YOY rainbow smelt. It isnly in June, when YOY rainbow smelt are smaller than 25 mm,hat TS distributions are easily separated with large rainbowmelt having TS > −60 dB and YOY rainbow smelt having TS

elow −60 dB.

Based on both mobile and stationary surveys, YOY had uni-odal and relatively narrow TS distributions in all 3 months.hese observed TS distributions were considerably wider than

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Research 82 (2006) 176–185 183

he one predicted from a TS-length regression based on trawlatches. A narrow TS range is expected due to the small spheri-al nature of YOY swimbladders (Medwin and Clay, 1998) andas been observed in other species such as YOY yellow perchPerca flavescens) and YOY Eurasian perch (Perca fluviatlis)Rudstam et al., 2002; Frouzova and Kubecka, 2004). The dif-erence between the mobile and stationary results for YOY ineptember suggests that our selected YOY stationary tracks mayave been biased small. However, the large differences betweenhe observed TS distribution and the one predicted from TS-ength regression of Rudstam et al. (2003) may indicate thathis regression is not applicable to rainbow smelt larvae or thatrawl catches were biased against smaller fish. Although smallainbow smelt were included in the derivation of the TS-lengthegression, the best-fit line tended to overestimate the TS ofmall rainbow smelt (see Fig. 2 in Rudstam et al., 2003).

The TS distributions for YAO from mobile and stationaryurveys were bimodal in June, July, and September. Tracks fromingle-fish exhibited wide ranges in TS and the proportion ofS measurements within the smaller TS mode of a single-fish

rack were on the average between 40 and 60%. In contrast,he predicted TS distributions from YAO trawl samples werenimodal in all months. The mode of the predicted YAO TSistributions was at, or slightly higher than, the inflection pointn all 3 months. This is because the TS-length regression is basedn average backscattering cross-section, not maximum values orodes (Rudstam et al., 2003).Our observations suggest that bimodality results from signifi-

ant variation within single-fish tracks and not from the presencef different YAO size classes. Ranges in TS distributions of0–20 dB have been found in other studies of single-fish trackse.g., Horne et al., 2000; Gauthier and Rose, 2001; Knudsen etl., 2004), and agree well with our average ranges between 10nd 15 dB for our survey months. The higher TS mode is proba-ly representative of dorsal aspect. Results from a Kirchoff–Rayode scattering model (Clay and Horne, 1994) for rainbow

melt suggest that 120 mm YAO can generate targets between62 and −38 dB within ±30◦ tilt off their dorsal aspect (Horne,niversity of Washington, personal communication). This range

s similar to our observations from slightly larger YAO rain-ow smelt, and suggests that the two TS modes originate fromhanges in tilt angle (see also Nielsen and Lundgren, 1999;authier and Rose, 2001; Ona et al., 2001). Rainbow smelt feed

ctively during the night (Parker et al., 2001) and should bexpected to change tilt angle during attack and pursuit of mobilerey such as mysids or small fish. Additional variability in TSay result from changes in swimbladder shape associated with

ifferent degrees of stomach fullness (Ona, 1990; Horne, 2003).ertical vessel avoidance, causing a reduction in individual fishcattering strength (Foote, 1980), is less likely as we did notbserve changes in vertical distribution in stationary and mobileight surveys. Also, the range and general shape of the TS dis-ributions from the mobile and stationary surveys were similar,

ven though the proportional distribution of targets between thewo modes differed, particularly in July. Given the large sampleize in the mobile survey, we believe that mobile survey resultsre more representative of the nighttime TS distribution in the

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ake. Many of the stationary tracks were collected at during late-usk when remaining ambient light may have influenced YAOigrations and movements (Appenzeller and Leggett, 1995),

nd the tracked single-fish in July may have been oriented fur-her off their dorsal aspect than during the later mobile survey.

The September TS distributions from mobile and stationaryurveys show the transition from YOY in upper waters to YAO inhe deeper waters. The epilimnetic waters were characterised by

ore unimodal YOY distributions while deeper waters showedimodal YAO distributions. The TS distributions from singleOY tracks had a smaller TS mode than results from the mobile

urvey. This suggests that our tracked single YOY may haveeen biased towards smaller fish, or changes in YOY tilt angle,ossibly associated with active feeding or vessel avoidance, mayave influenced stationary data more than mobile results. Thereere also differences between stationary and mobile TS dis-

ributions in the 20–25 and 25–30 m depth ranges. This results likely due to differences in YAO rainbow smelt depth dis-ribution at different site depths during mobile and stationaryurveys. Trawling results suggest that YOY are still found inhe next depth bin (25–30 m), and the dominant lower moden the mobile survey TS distribution supports this observation.n the lower depth bins (30–35, 35–50 m), the relative shapes,nflection points, and ranges of the TS distributions from mobileurveys and stationary tracked single YAO are similar.

Acoustic separation of YOY and YAO rainbow smelt is pos-ible in June and July due to differences in thermal preferencesetween the two age groups. In June, YOY and YAO also haveon-overlapping TS distributions and can be separated by tar-ets smaller and larger than −60 dB. Therefore, we can obtaineparate YOY and YAO rainbow smelt abundance estimates inune, even in the presence of a weak thermocline and mixing ofhe two age groups. Although the TS distribution of YOY (−76o −50 dB) and YAO (−60 to −35 dB) overlap in July, sepa-ate density assessments should be possible based on the strongpatial separation of the two age groups by a well-developedhermocline at that time. In September, both the TS and verti-al distribution of YOY and YAO overlap. Our ratio technique,or partitioning YOY and YAO targets within the metalimnionnd upper hypolimnion, produces proportion-at-depth profilesor YOY and YAO that agree with trawling results and YAOhermal preferences. This method can also be used in July andugust in areas where the two age groups overlap spatially. Vari-

bility in the return signal would have added background noiseo our estimates of YOY rainbow smelt in deeper waters if weid not exclude TS below our noise-biased thresholds (−65 dBt 30 m and −60 dB at 50 m depth). This limitation intro-uces additional uncertainty and may result in underestimatesf YOY in density calculations. Results from the ratio techniqueill be sensitive to variation in the YAO TS ratio used, and

ssumes that the ratio calculated from immediately below theone of overlap is appropriate for YAO found within the zone ofverlap.

A non-target species, cisco, was caught in nine mid-waterrawls. In most instances, the TS of both cisco size classes fellithin the TS range of YAO rainbow smelt. The inclusion ofon-target species may inflate our hydroacoustic estimates of

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Research 82 (2006) 176–185

AO rainbow smelt and potentially bias the calculation of meann situ back-scattering cross-section. We did not modify analy-es to compensate for this bias as only two trawls caught >10f YOY cisco and other trawls suggest that these catches wereot representative of lake-wide conditions. Trawling by Vermontepartment of Fish and Wildlife (VTFW) in August 2001 found7% of the total Main Lake trawl catch to be YOY cisco. During

he VTFW survey, single trawls with high YOY cisco numbersere also encountered (Staats, VTFW, personal communica-

ion). Additional mid-water trawling performed in 2002 did notnclude significant numbers of cisco in any area (Parker Stet-er, unpublished data). It is likely that large targets (>−35 dB)vident in September ratio calculations are due to YAO cisco orredatory salmonids that are also present in the lake and mayave avoided the trawl.

There is good agreement between the range of in situ TSor YOY and YAO rainbow smelt produced by mobile andtationary surveys. The TS range of YAO rainbow smelt was60 to −35 dB in June, July, and September, and this agreesith analyses in other areas of Lake Champlain (Parker Stetter,npublished data). The TS recommendation for YOY, based onhese and other areas of Lake Champlain (Parker Stetter, 2005),s −76 to −61 dB in June, −76 to −50 dB in July, and −68o −45 dB in September. As we collected data with a −76 dBS threshold, it is possible that the appropriate lower TS limit

s lower, especially for YOY rainbow smelt in June. The smallower limit for YOY TS in both July (−76 dB) and September−68 dB) was due to the appearance of unexpected, small YOYuring both months. Main Lake trawls detected the presence ofOY rainbow smelt during September that were within the size

ange for YOY in July. If spawning was more synchronized,he lower limit for the YOY TS distribution might need largereasonal adjustment.

. Conclusion

Our examination of TS distributions from mobile and station-ry surveys suggests that separate hydroacoustic assessments ofOY and YAO rainbow smelt can be obtained from a single

urvey. The optimal timing of rainbow smelt surveys in Lakehamplain, and possibly elsewhere, is July and August. During

his time, a strong thermal structure should still be present thatill lessen the effect of overlapping YOY and YAO TS ranges.e developed a ratio technique to separate YOY and YAO

sing the mobile survey TS distribution, but caution that thisntroduces additional uncertainty into the analyses. Our analy-es confirm that individual YAO rainbow smelt have complexS distributions with modes that do not correspond to individ-al size classes. Additional sampling, such as targeted trawling,ust therefore be used to gain insight on the size distribution ofYAO population.

cknowledgements

Special thanks to the crews of the University of Vermont RVMelosira,” the Vermont Fish and Wildlife “Dore,” and the Stateniversity of New York at Plattsburg RV “Monitor.” Thanks

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lso to Nicholas Staats of the Vermont Department of Fish andildlife for access to trawl catch data. We are grateful to two

nonymous reviewers for comments that increased the qualitynd presentation of this manuscript. This work was sponsoredn part by a grant from the National Sea Grant College Pro-ram, National Oceanic and Atmospheric Administration, USepartment of Commerce, to Lake Champlain Sea Grant underrant number NA16RG2206. The views expressed here are thosef the authors and do not necessarily reflect the views of theponsors. Mention of brand names does not constitute productndorsement by the U.S. federal government. Additional fund-ng was received from the National Science and Engineeringesearch Council of Canada (NSERC Post-Graduate Scholar-

hip to S.L.P.S.). This is publication number LCSG-01-06 ofake Champlain Sea Grant and number 228 of the Cornell Bio-

ogical Field Station.

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