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Effects of oxygen, temperatureand light gradients on the verticaldistribution of rainbow trout,Oncorhynchus mykiss, in two NorthIsland, New Zealand, lakes differing introphic statusD. K. Rowe a & B. L. Chisnall aa National Institute of Water & Atmospheric Research Ltd,P.O.Box 11 115, Hamilton, New Zealand

Available online: 30 Mar 2010

To cite this article: D. K. Rowe & B. L. Chisnall (1995): Effects of oxygen, temperature and lightgradients on the vertical distribution of rainbow trout, Oncorhynchus mykiss, in two North Island,New Zealand, lakes differing in trophic status, New Zealand Journal of Marine and FreshwaterResearch, 29:3, 421-434

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New Zealand Journal of Marine and Freshwater Research, 1995, Vol. 29: 421-4340028-8330/95/2903-0421 $2.50/0 © The Royal Society of New Zealand 1995

421

Effects of oxygen, temperature and light gradients on the verticaldistribution of rainbow trout, Oncorhynchus mykiss, in two NorthIsland, New Zealand, lakes differing in trophic status

D. K. ROWEB. L. CHISNALL

National Institute of Water & AtmosphericResearch Ltd

P.O.Box 11 115Hamilton, New Zealand

Abstract Vertical distributions of adult rainbowtrout (> 25 cm fork length, FL) were determinedwith a SIMRAD ES470 split-beam echosounder intwo 80-90 m deep lakes differing in water quality.Between November 1993 and February 1994, mosttrout (> 80%) were between 10 and 40 m, within orclose to the thermocline. However, a small groupof fish occupied colder waters, deeper than 50 m.In February, surface water temperatures > 21.0°Cand hypolimnetic oxygen levels < 2.5 g m -3

compressed the habitable depth range for trout inLake Rotoiti to 12-35 m compared with 12-80 min Lake Rotoma. Deeper-dwelling trout inhabitingwaters over 50 m in Lake Rotoiti would have beenforced into shallower waters at this time. However,the vertical distribution of the remaining trout inLake Rotoiti was not compressed. In March 1994,adult trout were still present in waters 10-40 mdeep in both lakes, but many of the smaller fishhad moved into shallower waters (< 10 m deep),probably because of declining water temperaturesin the epilimnion and increased densities of theirpreferred prey. At this time, the lower depth rangefor trout in Lake Rotoiti was limited to 28 m by the2.5 g m-3 oxygen level and trout occupied warmerwaters than they did in Lake Rotoma. When oxygenwas not limiting, water temperature was the main

M94033Received 12 July 1994; accepted 4 May 1995

variable determining the depth of the trout, andmonthly changes in the mean depth of trout betweenboth lakes and months were explained by a thermo-regulatory model for trout movement.

Keywords rainbow trout; Oncorhynchus mykiss;Lake Rotoiti; Lake Rotoma; target strength;echosounding; vertical distribution; depth selection;oxygen; temperature; light; habitat squeeze;thermoregulation

INTRODUCTION

The concept of physical habitat as a limiting factorhas proved useful for predicting changes in fishdistribution and abundance in rivers (Fausch et al.1988). However, this concept has not been widelyused to obtain an understanding of pelagic fishdistribution and abundance in lakes. Fish densitiesin lakes are generally expressed in terms of surfacearea, implying that fish are homogeneouslydistributed throughout lakes and that lake depthhas little effect on fish abundance. This approachassumes that water depth is relatively unimportantfor fish habitat; however, the reality is far differentand Olsen et al. (1988) recommended that moreattention should be focused on the habitatlimitations for fish in lakes.

In lacustrine environments, pelagic fish oftenoccupy discrete depth strata (e.g., Northcote &Rundberg 1970; Dembinski 1971; Engel &Magnuson 1976; Rudstam & Magnuson 1985) and,in New Zealand's central North Island lakes, boththe mean depth and depth range of native fishspecies varied from month to month as well asbetween lakes (Rowe 1994). Although suchstratified vertical distributions of fish are believedto be caused mainly by fish preferences fortemperature and light (Brett 1971; Rudstam &Magnuson 1985; Levy 1990), several studies haveshown that fish cannot tolerate, and will activelyavoid, certain high water temperatures and low

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422 New Zealand Journal of Marine and Freshwater Research, 1995, Vol. 29

oxygen levels (McCauley & Pond 1971; Davis1975; Cherry et al. 1977; Alabaster & Lloyd 1980;Kazakov & Khalyapina 1981; Forsyth et al. 1990).Furthermore, the feeding efficiency of visualpredators, such as trout, decreases below certainlight levels (Robinson & Tash 1979). The functionaldepth habitat for fish in lakes can therefore bedecreased when a reduction in water qualityincreases the gradient in light, temperature, oroxygen with depth, thereby narrowing the depthrange of acceptable levels for fish.

As a narrowing of depth habitat may haveimplications for fish density, mortality, and lakecarrying capacity, it is important to determine thephysical variables defining functional fish habitatin lakes. Such information is important forunderstanding biotic interactions, and will assistwater managers to define the water qualityparameters needed to protect fish life and to predictthe consequences of changes in water quality onfisheries. In particular, knowledge of the factorsdetermining functional depth habitat for fish inlakes will lead to a better understanding of "habitatsqueeze" (sensu Coutant 1985).

Habitat squeeze occurs in summer months whenhigh water temperatures in surface waters createan upper limit to the depth range for fish and whenlow oxygen levels in the hypolimnion set a lowerlimit. Fish are confined to a middle stratum ofwater and if the temperature and oxygen limits tohabitable depth move closer together, habitatsqueeze may affect the vertical distribution of fish.Habitat squeeze has been recorded for both stripedbass Morone saxatilis, and northern pike Esoxlucius, in southern lakes in North America (Coutant1985, 1990; Headrick & Carline 1993; Zale et al.1990). Concern has also been expressed thatrainbow trout {Oncorhynchus mykiss) populationsin some North Island, New Zealand lakes could besimilarly affected (Rowe & Scott 1989).

In rivers, adult rainbow trout have been reportedto avoid water temperatures over 21-23°C(Hokanson et al. 1977). However, when free toroam within a thermal plume entering a lake,rainbow trout generally avoided temperatures over21°C (Spigarelli & Thommes 1979). Several studiesin lakes have indicated that 21°C sets the upperlimit to the vertical distribution of trout (Table 1).Coutant (1977), in a review of temperaturepreference data on fish, indicated that rainbowtrout occur in waters up to 22°C. However, inlakes very few (< 5%) adult trout have been foundin waters above the 21°C isotherm (Spigarelli &

Thommes 1979; Jones 1982; Stables & Thomas1992). Consequently, 21.0°C is taken as theeffective temperature threshold determining theupper limit to trout habitat in lakes. Similarly, troutare rarely found in waters where oxygen levels aremuch below 3.0 g m"3 (Table 1). Notwithstandingthe ability of rainbow trout to make brief foraysinto deoxygenated water (Luecke & Teuscher1994), trout generally avoid oxygen levels below2.5 g m"3 (Table 1). Consequently, 2.5 g m~3 istaken as the level of oxygen which can set thelower depth limit to trout distribution.

These thresholds in temperature (21°C) andoxygen (2.5 g m~3) can be expected to set theupper and lower boundaries to the depth rangewhich trout occupy in lakes, provided that otherphysical variables with vertical gradients, such aspressure and light, are within acceptable limits andare not of overriding importance. For example,Cryer (1991) determined the vertical distributionof rainbow trout in Lake Taupo (maximum depthabout 150 m) and found that, although troutoccurred throughout the water column in all months,they were more abundant in shallower waters (0-50 m) in summer, with smaller fish in shallowerwaters (0-20 m) than larger ones (20-50 m). Cryer(1991) indicated that the shallower distribution oftrout in this lake in summer months may haveresulted from a preference for light. However,increased densities of prey species in shallow watersat this time may also have caused this. Thus,although high water temperatures and low oxygenlevels may set limits to the habitable depth rangefor trout in lakes, habitat squeeze may notnecessarily constrict the vertical distribution oftrout, as other factors may determine the actualdepth range for trout within the range of habitablewater.

For habitat squeeze to affect fish populations itmust be shown that fish distributions are actuallycompressed when habitat squeeze occurs. Wetherefore determined whether habitat squeeze(owing to the incidence of limiting watertemperatures and oxygen levels for rainbow troutin lakes) occurred for trout in Lake Rotoiti duringsummer months when the hypolimnion deoxy-genates. We tested the null hypothesis that suchhabitat squeeze would not result in a constrictionin the depth range for trout in Lake Rotoiti relativeto that in a control lake (Lake Rotoma). In addition,we examined the role of oxygen, temperature, andlight levels in determining the daytime restingdepths of rainbow trout in these two lakes.

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Rowe & Chisnall—Rainbow trout depth distribution in lakes 423

STUDY SITES

Lakes Rotoiti and Rotoma are both moderate-sizedlakes (surface area 34.3 and 11.1 km2, respectively)at altitudes of 278 and 313 m, respectively (Irwin1975). They are located in the Central North IslandPlateau of New Zealand and were both formed as aresult of volcanic activity in the region over 7000years ago (Healy 1975). They are morphologicallysimilar in that they are elongate and contain large,deep (80-90 m) basins. Both lakes contain self-recruiting populations of rainbow trout, smelt(Retropinna retropinna), and common bullies(Gobiomorphus cotidianus). Goldfish (Carassiusauratus) are also present in both lakes, but they arerare, as is the koaro (Galaxias brevipinnis) whichoccurs only in Lake Rotoiti (Smith 1959; Rowe1994).

Jolly (1968) described their basic limnology:both lakes are monomictic and stratify duringsummer months. Water quality has deteriorated inLake Rotoiti and the hypolimnion now deoxy-genates between March and May each year (Gibbs1992). By contrast, Lake Rotoma is oligotrophicand oxygen saturation levels 1 m above the bottomare generally greater than 50% throughout the year(McColl 1972).

METHODS

The size and depth of individual rainbow trout inLakes Rotoiti and Rotoma was determined using aSIMRAD ES470 split-beam echosounder, with a

beam width of 11.5° and an operating frequency of70 kHz. Time-varied gain was 40 Log R, and pulseduration 0.5 ms. Data output included an echogramwith echoes colour coded according to targetstrength and a digital computer file that includedthe data triplet for each echo (i.e., its ping number,depth, and an arbitrary value related to targetstrength). The range of arbitrary values of 1-80corresponded linearly with the target strength range-44 to -14 decibels (SIMRAD 1986). As time andresources precluded the development of arelationship for target strength and rainbow troutsize, trout size is expressed in terms of acousticenergy (decibels, dB).

Sampling was carried out during summermonths, specifically in November and December1993, and February and March 1994. Transectswere run from one side of each lake to the other,and because of the generally low counts (1-5) oftrout per transect, 15-20 transects were needed perlake. Transects were located to cover the mainbasins of each lake, and sampling was carried outwithin a 5-6 h period during the middle of the dayto minimise effects of diel changes in the verticaldistribution of fish.

Trout were identified by matching the echoesfrom each individual fish, on each echogram, withthe corresponding data triplets. In general, anindividual trout was characterised by a series of3-7 echoes with consecutive ping numbers andslightly decreasing depths, reflecting the backwardtilt of the V-fin containing the transducer. Troutwere discriminated from the much smaller smelt

Table 1 Water temperature levels reported to limit the depth distribution of large (> 1 kg) rainbow trout in lakes,and oxygen levels below which trout cannot survive.

Level Source Description

Water temperatures21 °C Horak & Tanner ( 1964)

21°C

21°C21°C

Oxygen levels

2.6 g m"3

2.5 g m"3

3.1 g m"3

Overholtz et al. (1977)

May & Gloss (1979)Stables & Thomas (1992)

Anon (1957)inMcKee&Wolf (1963)

Aylesetal. (1976)

Jones(1982)

Upper limit to the depth of rainbow trout in HorsefootReservoir, Colorado

Upper limit to the depth distribution of rainbow trout ina pond

Upper limit for rainbow trout in Lake PowellLevel below which nearly all rainbow and cutthroat trout

occurred in Spada Lake

Minimum value at which rainbow trout survived 3 days at 20°C

Level at which 95% mortality of rainbow trout occurred inan experimental lake

Level below which no rainbow trout occurred in LaurelRiver Lake

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424 New Zealand Journal of Marine and Freshwater Research, 1995, Vol. 29

(30-100 mm fork length, FL) and juvenile bullies(4-20 mm FL)—the only other fish in the pelagiczone, on the basis of a maximum target strength(MTS) values greater than -31 dB. This thresholdwas selected because we found that large individualsmelt produced MTS values up to -32 dB (unpubl.data).

Cry er (1991) estimated the size of rainbow troutin Lake Taupo from fish size-target strengthrelationships developed for gadoids. This wasbecause the limited data available for trout wassimilar to that for gadoids. However, the truerelationship for trout size-target strength is notknown, and Cryer's (1991) approach may haveoverestimated the size of smaller fish. As -29 dBwas used to distinguish trout over 30 cm FL inLake Taupo (Cryer 1991), a threshold of-31 dBwould be expected to select only trout over about25 cm FL in Lakes Rotoma and Rotoiti. As aresult, relatively small (FL < 25 cm) trout,producing echoes below -31 dB, would have beenexcluded from our study. This approach will nothave affected the measurement of habitat squeezeas larger rainbow trout have lower temperaturepreferences than smaller ones (McCauley &Huggins 1979; Spigarelli & Thommes 1979), andwarm surface water temperatures will affect largertrout first.

Schools of relatively large (FL > 60 mm)common smelt occurred between 30 and 40 m insome transects in Lake Rotoiti and the MTS valuesfrom these schools often exceeded -31 dB. Theschools of smelt could not be mistaken for trout,because they had a much larger echo trace thanindividual trout. However, trout close to suchschools (i.e. within 1 m) could not be reliablydistinguished from them on the basis of either echotrace or target strength data. Fortunately, thisproblem occurred only in Lake Rotoiti, as the largesmelt were more dense here than in Lake Rotomaand they only occurred in certain regions of a fewtransects in February and March.

All trout between 0 and 3 m depth will havebeen missed because the transducer was towed at 3m, and some trout between 3 and 5 m will alsohave been missed because the transducer may havedisturbed fish directly below it. Trout were presentin surface waters as we observed the occasionaltrout feeding at the lake surface but, except inMarch, no trout were detected (acoustically)between 5 and 10 m. We were therefore confidentthat in November, December, and January mosttrout were either feeding in shallow surface (0-4

m) waters, or were below 10 m and thereforeamenable to acoustic sampling. In Lake Rotoiti inMarch, trout were often recorded from water asshallow as 4 m.

Oxygen and temperature levels were recordedin both lakes at 1 m intervals down to 70 m using aYellow Springs Instruments (YSI) Model 54Aoxygen/temperature probe. Light levels weremeasured at 1 m intervals down to 50 m with a 4n,QSP Model 200, scalar irradiance probe, with aspectral bandwidth of 400-700 nm (BiosphericalInstruments, Lahoya, California). Levels below 0.01quanta m"2 s"1 could not be measured directly, sowere estimated from the extrapolated light-depthcurve. Monthly changes in the depth habitatavailable to rainbow trout in each lake weredetermined by comparing the depths of the 21.0°Cisotherm and the 2.5 g m~3 oxygen concentrationbetween months.

The depth and MTS value of each trout wererecorded for each transect and the number of troutin each 2 m depth strata summed for all transects ineach lake on each sampling occasion. These datawere then corrected for increases in samplingvolume with depth, using a wedge-shaped modelto approximate the sampling volume, and relativedensities of trout were calculated for each 2 mdepth stratum in each lake for each month.Relationships between trout size and depth wereexamined for each month and lake by determiningthe frequencies of large (MTS > -24.5 dB) andsmall (MTS < -24.5 dB) trout above and below themedian depth for each data set. A test ofindependence based on y} square (Sokal & Rohlf1973) was used to determine whether significantdifferences in the size of fish occurred with depthbetween months or lakes.

We determined whether habitat squeezeoccurred for trout in Lakes Rotoiti and Rotomaduring summer, and whether the vertical distri-bution of trout in Lake Rotoiti was compressed bysuch habitat squeeze, by contrasting the depth rangefor trout in Lake Rotoiti with that in Lake Rotomawhere there was no hypolimnetic deoxygenation.In addition, the mean depth for all rainbow troutwas calculated for each month for both lakes andwe determined whether the vertical distribution oftrout in these lakes could be predicted by simplemodels based on preference or avoidance oftemperature, oxygen, or light levels. We tested thehypothesis that temporal (between months) andspatial (between lakes) variation in the mean depthof trout was related to a constant level of oxygen,

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Rowe & Chisnall—Rainbow trout depth distribution in lakes 425

Fig. 1 Temperature, oxygen andlight gradients in Lakes Rotoiti(solid line) and Rotoma (dashedline) on the days when trout depthdistributions were determined inNovember and December 1993,and in February and March 1994.

20

40aa

60

Temperature (°C)10 15 20 10 15 20 10 15 20 10 15 20

20

?£ 40a0JQ

60

i)

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i

i

I {

J i

Jt \

J p

I !

y<~—••

^ ¡

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i B

0 4 8 0 4 8 0 4 8 0 4 8

Oxygen (mg I'1)

c

| 2 0

.cÖ-40Q

60

— /

!

y (no data) y

c

0.1 1 10 100 0.1 1 10 100 0.1 1 10 100 0.1 1 10 100

Light (quanta m'2s'1)NOV DEC FEB MAR

temperature, or light (i.e. preference level).Similarly, we determined whether variation in theminimum and maximum depths at which trout werefound in each month was related to a constant levelof oxygen, temperature or light (i.e. avoidancelevels).

RESULTS

Depth habitat for troutWater temperatures > 21 °C or hypolimnetic oxygen

levels < 2.5 g m~3 did not occur in November orDecember in either lake (Fig. 1). However, inFebruary, water temperatures > 21°C set an upperlimit to the depth of water which trout could inhabitin both lakes (Fig. 2). At this time oxygen levels< 2.5 g m~3 also set a lower depth limit (35 m) forhabitable water in Lake Rotoiti, but not in LakeRotoma (Fig. 2). In March, low oxygen levels hadraised the lower depth limit for trout in Lake Rotoitito 28 m (Fig. 2), but water temperatures in theepilimnion of both lakes were below 20.0°C (Fig. 1)so there was no temperature constraint on the

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426 New Zealand Journal of Marine and Freshwater Research, 1995, Vol. 29

NOV DEC FEB MAR Fig. 2 Changes in the depthdistribution of adult rainbow trout(solid black) in relation to theirhabitable depth range (dashedbox) and the depth range for themetalimnion (shaded) in: A, LakeRotoiti and B, Lake Rotomaduring summer months.

10 0 10 10 0 10 10 0 10

Relative abundance (%)10 0 10

minimum depth for trout at this time. Consequently,habitat squeeze (sensu Coutant 1985), implying asimultaneous upper and lower constraint on thehabitable depth range for fish in lakes, occurredfor rainbow trout in Lake Rotoiti only in February.

Vertical distribution of troutIn November, December, and February no troutwere recorded between 4 and 10 m in either lake,and the highest densities of rainbow trout occurredbetween 10 and 40 m (Fig. 2). By March, manytrout were still present between 10 and 40 m;however, relatively high densities of trout alsooccurred in water < 10 m deep in both lakes (Fig.2). A close examination of the size of trout aboveand below 10 m in March showed that 89% of troutabove 10 m were relatively small fish (MTS valuesbetween -25 and -32 dB: Fig. 3). In comparison,trout below 10 m in March included about equalnumbers (53 versus 47%, respectively) offish withMTS values higher and lower than -24.5 dB.Although there were significantly more larger troutbelow the mean depth than above it in November

and March in Lake Rotoiti (Table 2), sample sizefor large trout was relatively small in Lake Rotoitiin November. Furthermore, we could detect nodifference in the proportion of large trout above orbelow the mean depth for trout in Lake Rotoma forany month (Table 2). There was therefore littleevidence of a relationship between trout size anddepth in either lake, except in March, when smalltrout predominated in waters above 10 m. Echoesfrom smelt concentrations in the top 10 m wereclearly visible on certain echograms from LakeRotoiti at this time, so the shallower Marchdistribution of many adult trout was associatedwith an increased density of smelt, particularly inparts of Lake Rotoiti.

In Lake Rotoma, a small group of trout (17.3%of total) occurred in waters deeper than 50 m in allmonths (Fig. 2). In Lake Rotoiti such trout wereonly present in November and December. Thesedeeper-dwelling trout were > 10 m deeper than theclosest trout in shallower waters. No trout wereobserved below 56 m in Lake Rotoiti or 72 m inLake Rotoma between November and March.

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Rowe & Chisnall—Rainbow trout depth distribution in lakes

12

10

8

6

Ü 2

I 0

I"10

8

427

Fig. 3 Size-frequency distri-bution for rainbow trout in LakeRotoiti in March: A, above 10 mand B, below 10 m.

-30 -28 -26 -24 -22

Fish size (dB)

-20 -18 -16

The mean depth for all trout (Table 3) wasdeepest in both lakes in December, and shallowestin March. When trout present in shallow (< 10 m)waters in March (Fig. 3, Table 2) were excluded,the mean depth of trout was greater in both lakes(Table 3), but was still less than the mean depth for

other months. Similarly, when the small number offish below 50 m were excluded, the qualitativechanges in monthly mean depth persisted in bothlakes (Table 3). Trout therefore occurred in deeperwater in Lake Rotoma than in Lake Rotoiti in allmonths (Table 3).

Table 2 Proportions of large (MTS -17 to -24 dB) versus small (MTS -25to -32 dB) adult trout, above and below the median depth for all trout, in LakesRotoiti and Rotoma during summer months. *** P < 0.01, jl test ofindependence between depth and size of trout; NS, not significant.

NovemberL. RotoitiL. RotomaDecemberL. RotoitiL. RotomaFebruaryL. RotoitiL. RotomaMarchL. RotoitiL. Rotoma

n

126

2214

3117

6012

Small trout

% above °k

58.366.7

50.050.0

48.441.2

61.750.0

? below

41.733.3

50.050.0

51.658.8

38.350.0

n

42

263

3611

357

Large trout

% above

0.00.0

46.233.3

44.463.6

25.757.1

% below

100.0100.0

53.866.7

55.536.4

74.342.8

***NS

NSNS

NSNS

***NS

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428 New Zealand Journal of Marine and Freshwater Research, 1995, Vol. 29

Effects of habitat squeeze on trout verticaldistributionStratification resulted in the presence of athermocline in both lakes in all months (Fig. 1)and, in general, the distribution of trout straddledthis thermocline (Fig. 2). In Lake Rotoiti, highestdensities of trout occurred within the thermoclinein November, December, and February (Fig. 2),whereas in Lake Rotoma, highest densities of troutoccurred within the thermocline in all months (Fig.2). The vertical distribution of the trout thereforetraversed the depth stratum where gradients intemperature were greatest. In Lake Rotoiti, oxygengradients were also greatest in the thermocline;however, this was not the situation in Lake Rotoma.Here, peak oxygen levels occurred near the top ofthe thermocline and oxygen only began to declinenear the bottom of the thermocline (Fig. 1).

Although the 21°C isocline at 10 m providedan upper limit to the habitable depth range forrainbow trout in Lake Rotoma in February, thetrout in this lake were below 18 m at this time, so thevertical distribution of the trout in Lake Rotomawas not affected by the warm surface waters in theepilimnion in February. However, in Lake Rotoitiin February, the minimum and maximum depthsfor the actual depth distribution for trout exactlymatched the upper and lower limits to their habitabledepth range set by limiting temperature and oxygenlevels. Habitat squeeze could therefore haveaffected trout distribution in Lake Rotoiti inFebruary.

It was apparent that the small group of troutpresent below 50 m in Lake Rotoma in all months,

and in Lake Rotoiti in November and December,were absent in Lake Rotoiti in February and March.Limiting oxygen levels therefore restricted thelower depth limit for these trout in Lake Rotoiti inFebruary and March (Fig. 2). However, this effectwas limited to the few fish that occupied watersdeeper than 50 m in each lake. If these deeper-dwelling fish are excluded, the depth range fortrout in Lake Rotoiti in February was 12-35 mcompared with 18-36 m for trout in Lake Rotoma(Table 3). The majority of trout in Lake Rotomatherefore occupied a similar depth range to themajority of trout in Lake Rotoiti and there was noevidence that the vertical distribution of these fishwas vertically compressed in Lake Rotoiti inFebruary. Only the few, deeper-dwelling troutbelow 50 m were affected.

Effects of oxygen, temperature, and lightThe monthly mean depth, and the monthlyminimum and maximum depths for the depth rangeof trout in each lake were not related to any constantlevel of water temperature, oxygen concentration,oxygen saturation, or light (Table 4). Light levelscan influence the vertical distribution of fish inlakes (Brett 1971 ; Levy 1990) and light penetratedto greater depths in Lake Rotoma than in LakeRotoiti (Fig. 1). Trout occurred in deeper water inLake Rotoma than in Lake Rotoiti (Table 3), butlight levels at the mean depth, and at the minimumand maximum depths for trout varied between bothlakes and months (Table 4). There was thereforeno relationship between light levels and the depthof trout, even in Lake Rotoma.

Table 3 Mean depths and the depth range for rainbow trout in Lakes Rotomaand Rotoiti during summer months for all fish, all fish excluding those presentabove 10 m, and all fish excluding those above 10 m and below 50 m.

Mean

All fish >

Lake RotomaNovemberDecemberFebruaryMarchLake RotoitiNovemberDecemberFebruaryMarch

29.230.226.317.9

24.927.221.49.9

depth

Fish10 m

29.230.226.326.0

24.927.221.418.8

(m)

Fish10-50 m

25.328.423.322.5

24.026.621.418.8

Depth range

All fish

17-5514-6318-718-68

14-5712-6312-354-28

Fish> 10m

17-5514-6318-7116-68

14-5712-6312-3510-28

(m)

Fish10-50 m

17-4014-4218-3616-34

14-3612-4412-3510-28

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Rowe & Chisnall—Rainbow trout depth distribution in lakes 429

Low oxygen levels limited trout depthdistribution in Lake Rotoiti in February and March,but the only other discernable relationship betweenthe maximum and minimum depths for trout andthe physical variables measured occurred for watertemperature. Although the minimum depth of troutvaried between both lakes and months, monthlywater temperatures at the minimum depth showedlittle variation between lakes, except in February(Table 4). At this time the water temperature at theminimum depth for rainbow trout in Lake Rotoitiwas 21.0°C, compared with 19.4°C in LakeRotoma. This 1.6°C difference between lakes wasmuch greater than the 0.0-0.2°C difference thatcharacterised differences in other months, and wasprobably related to the effect of low hypolimneticoxygen levels on the depth distribution of trout inLake Rotoiti. Water temperatures at the maximumdepth for trout varied by only 0.4°C between monthsin Lake Rotoma but ranged from 11.2 to 13.3°C inLake Rotoiti (Table 4). This variation in watertemperatures at the maximum depth for trout inLake Rotoiti was caused by high values in Februaryand March (Table 4), when low oxygen levels set alower limit to the trout depth distribution.

DISCUSSION

Habitat squeeze and troutThe depth stratum containing rainbow trout in LakeRotoiti in February was bounded by watertemperatures > 21.0°C and oxygen levels < 2.5 g

m -\ Although the habitable depth range for troutwas clearly restricted in Lake Rotoiti in February,the evidence for consequent compression of thevertical distribution of trout was less certain.

The few trout which inhabited deeper waters(> 50 m) in these lakes would have been affectedby the low oxygen levels in Lake Rotoiti inFebruary, but such fish may not be typical of themajority of trout. Depth selection by these deeper-dwelling fish could well be anomalous and underthe control of a different mechanism to thatinfluencing the majority of trout. Whereas there isno direct evidence for this, anomalous depth distri-butions of rainbow trout do occur; the occasionalrainbow trout making brief excursions into hypoxicwaters (Luecke & Teuscher 1994—cited in Rahel& Nutzman 1994). Moreover, rainbow trout areknown to select cooler (hence deeper) waters whenstarved (Javaid & Anderson 1967), so fish that arenot eating because of injury or disease can beexpected to occur in cooler and hence deeper waterthan other fish. The relative isolation of the deeperdwelling trout from the majority that occurred inshallower waters close to the thermocline, their fewnumber in both lakes, and their presence in rela-tively deep water where there is little light, impliesthat their presence was anomalous and that theirdepth selection mechanism was not typical of themajority offish. When the majority of trout whichoccupied waters mainly within the thermocline areconsidered alone, there was no evidence that theirvertical distribution in February was squeezed.

Table 4 Temperature, oxygen and light levels at the mean, minimum, and maximum depths for all rainbow troutin Lakes Rotoma and Rotoiti during summer months.

Mean depthTemperature (°C)Oxygen (g nr3)Oxygen(%)Light (quanta m~2 s"1)Minimum depthTemperature (°C)Oxygen (g nr3)Oxygen(%)Light (quanta m"2 s*1)Maximum depthTemperature (°C)Oxygen (g nr3)Oxygen (%)Light (quanta m~2 s"1)

Nov

11.59.4

89.52.1

14.910.2

105.111.5

10.69.1

85.10.1

Lake

Dec

11.29.7

91.70.6

16.89.9

105.88.4

10.18.8

80.9<0.01

Rotoma

Feb

13.09.7

95.6-

19.49.4

106.3-

10.17.7

70.8-

Mar

19.39.5

107.06.0

19.49.4

108.125.0

10.37.3

67.6<0.01

Nov

12.98.8

86.30.2

14.910.2

105.11.6

11.27.6

71.8<0.01

Lake

Dec

13.47.3

72.6<0.01

17.010.1

108.54.0

11.44.6

43.7<0.01

Rotoiti

Feb

16.34.8

50.8-

21.08.8

102.6-

12.72.5

24.5-

Mar

19.39.7

109.22.6

19.610.1

114.712.0

13.32.6

25.8<0.01

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430 New Zealand Journal of Marine and Freshwater Research, 1995, Vol. 29

This variation in the depths of rainbow troutserves to highlight the difficulties involved inmeasuring the vertical distribution of fish in lakes.It is apparent that different groups of fish canselect different depths at the same time because ofindividual differences in activity patterns andbehaviour. For example, most trout were present atdepths of 10-40 m within or close to thethermocline, and in most instances were spatiallyisolated from their main prey species, the commonsmelt. As they were not foraging at this depth theywere probably resting. However, we observed sometrout within 1 m of the lake surface and these fishwere clearly feeding. Similarly, many of the smalleradult trout that were present in waters shallowerthan 10 m in March, when large aggregations ofsmelt occurred in these waters, were probablyfeeding. The smelt present in these shallow watersare smaller than those inhabiting deeper waters(Rowe 1994), and although such small smelt aretaken by all adult trout, they form nearly 100% ofthe diet of trout in the 20-35 cm FL size range(Smith 1959; Rowe 1984; Cryer 1991). It istherefore probable that the generally smaller adulttrout in waters < 10 m in March were feeding onsmelt, whereas other similar-sized trout remainedin deeper waters. Such temporal differences inbehaviour can be expected to lead to splitdistributions of fish (e.g. Narver 1970; Hamrin1986) and unless compensated for will confoundattempts to determine the reasons why fish inhabitcertain depth strata in lakes.

Although there was no evidence of a contractionin the depth range for the majority of trout in LakeRotoiti, the warmer water at the maximum andminimum depths for trout in Lake Rotoiti inFebruary and March indicates that trout inhabitedwarmer waters in Lake Rotoiti than in Lake Rotomain these months. As water temperature declinedwith depth, the vertical distribution of trout inLake Rotoiti was therefore shallower than expected.Thus habitat squeeze affected some trout in LakeRotoiti but not all, and although there was noevidence for compression of the depth range forthe majority of trout, their vertical distribution wasshifted upwards into warmer waters.

Habitat squeeze can be expected to occur forrainbow trout in Lake Rotoiti only during summermonths when limiting levels of temperature andoxygen occur. However, limiting oxygen levelswere shallowest in March, whereas limitingtemperature levels occurred in February. This timelag between maximum water temperatures and

minimum oxygen levels meant that trout depthhabitat in Lake Rotoiti was not compressed asmuch as was expected. Nevertheless, habitatsqueeze was much greater in Lake Rotoiti than inLake Rotoma, and any further habitat squeeze inLake Rotoiti can be expected to constrict the verticaldistribution of trout, increasing densities within anarrower depth stratum.

Increased packing of fish within a narrowerdepth stratum could affect fish production andfishery management, but would only be expectedto occur in Lake Rotoiti in February if either the2.5 g m"3 oxygen threshold level rose above 35 m,or if the 21.0°C isocline descended below 12 m.Historical records of oxygen levels in Lake Rotoitiindicate that the 2.5 g m™3 oxygen level occurred atabout 50 m in February 1957-68 (Jolly 1968; Fish1969). Gibbs (1992) reported it between 35 and 40m in February 1982, and we recorded it at 35 m inFebruary 1994. As the depth of the thermocline inlakes can be raised by increasing nutrientconcentrations (Tanner 1960), and the depth of theepilimnion can be decreased by the effects ofdecreased water clarity on thermal structure(Mazumder & Taylor 1994), any future addition ofnutrients to Lake Rotoiti can be expected to raisethe depth of the thermocline, and the deoxygenatedhypolimnion, pushing trout further towards the lakesurface during summer. Alternatively, increasedwarming of the epilimnion by climate changeeffects (Rowe & Scott 1989) could also reduce thehabitable depth range for trout in this lake.

Factors influencing trout vertical distributionsThe changes in the vertical distribution of rainbowtrout in Lakes Rotoiti and Rotoma during summermonths were qualitatively similar to those recordedin Lake Taupo by Cryer (1991). Cryer (1991) alsofound that trout occupied deeper waters duringwinter and spring months, reaching their greatestdepth in December. After December, the troutconcentrated in relatively shallower waters, withsmaller trout predominating closest to the surface.

Although the oxygen and temperature limitswhich determined the depth of available habitatfor trout could account for the vertical distributionof trout in Lake Rotoiti in February, these limits donot explain the changes in vertical distribution oftrout between months for either lake, nor betweenlakes for any month. Furthermore, it was apparentthat the seasonal changes in the depth of trout inLakes Rotoiti and Rotoma were not related tosimple models of trout behaviour based on either

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Rowe & Chisnall—Rainbow trout depth distribution in lakes 431

preferences for, or avoidance of, particular light,temperature or oxygen levels. Nevertheless, theclose association between the depth range of troutand the position of the thermocline, and thegenerally low variation in water temperatures atthe maximum depth of trout in each lake betweenmonths, and at the minimum depth for trout ineach month between lakes, all indicate that watertemperature is important for determining the verticaldistribution of trout in these lakes in summermonths.

The increase in water temperatures at both theminimum depth and at the mean depth for troutbetween November and March indicates thattemperature acclimation may have been occurring.If so, the monthly changes in trout mean depthcould be related to temperature preferences basedon acclimation to surface water temperatures inthe epilimnion. However, this model assumes thattrout rest, or hold position in the colder deeperwaters of the thermocline where preferredtemperatures occur, and undertake periodic foraysto the shallower warmer waters of the epilimnionto feed.

Movements of trout between warmer epi-limnetic water and colder waters in the thermoclinecan be inferred from both the known feedingpatterns of rainbow trout in these lakes and thethermoregulatory behaviour of rainbow trout. Forexample, in summer months, trout in these lakesfeed mainly on small-sized (30-50 mm FL) smelt(Smith 1959; Rowe 1984; Cryer 1991), which occurin the epilimnion (Rowe 1994). However, summerwater temperatures in the epilimnion of these lakesare too warm (>19°C) for optimal growth ofrainbow trout (Hokanson et al. 1977; Wurtsbaugh& Davis 1977) so, according to the thermo-regulatory hypothesis (Brett 1971; Biette & Geen1980), trout would be expected to move to colderand hence deeper waters after feeding, in order toreduce their body temperature.

Increased acclimation temperatures raise thepreferred temperature for rainbow trout (Cherry etal. 1977; McCauley et al. 1977) and there is a closerelationship between the final temperaturepreferendum for many fish and optimal temper-atures for growth (McCauley & Casselman 1981).This relationship exists for rainbow trout asjuveniles grew fastest when daytime temperatureswere between 17 and 19°C (Hokanson et al. 1977),which is close to their final preferendum of 18-19°C (McCauley & Pond 1971; Cherry et al. 1977).Thus if trout in lakes feed in shallow waters that

are too warm for optimal growth, they can beexpected to move to cooler waters after feeding inorder to reduce their body temperature.

Such thermoregulatory movements have beenrecorded for rainbow trout. For example, rainbowtrout regulated their body temperatures by movingto areas in a thermal plume in a lake where watertemperatures were within the preferred range(Spigarelli & Thommes 1979). Furthermore,individual rainbow trout moved between warmsurface and cooler bottom waters in a verticaltemperature gradient (Sutterlin & Stevens 1992).Such movements were periodic, occurring at about12-hour intervals, and were probably related tobiotic factors such as feeding and growth ratherthan to physical factors as the movements were notin phase. Rainbow trout in Lakes Rotoma andRotoiti can therefore be expected to undertakeperiodic vertical movements between the warmwaters of the epilimnion where they feed and thecooler waters of the thermocline where preferredtemperatures occur.

To test this thermoregulatory model for depthselection by rainbow trout in Lakes Rotoiti andRotoma, we estimated the preferred temperaturesfor adult trout given acclimation temperatures inthe epilimnion. We then compared these predictedtemperatures with the actual temperatures at themean depth for such fish in both lakes. Althoughthe preferred temperatures for adult rainbow troutfor given acclimation temperatures are not known,Cherry et al. (1977) provided data for small (50-100 mm FL) trout which can be extrapolated tolarger fish. For example, Spigarelli & Thommes(1979) reported a difference of 5°C between thepreferred temperature for small (0.2 kg) and large(5.0 kg) rainbow trout. As trout in Lakes Rotomaand Rotoiti generally range from 2 to 5 kg (unpubl.data), a correction factor of -5°C was thereforeapplied to the data provided by Cherry et al. (1977).

The predicted and actual water temperatureswere surprisingly close, particularly in Lake Rotoiti(Table 5). In general, differences between predictedand actual temperatures were < 2.6°C in LakeRotoma and, except in March, < 0.3 °C in LakeRotoiti (Table 5). The somewhat higher thanexpected difference (+3.6°C) between preferredand actual temperatures at the mean depth of troutin Lake Rotoiti in March was due to low oxygenlevels which forced trout into shallower waters atthis time. The thermoregulatory model wastherefore useful for predicting changes in the depthdistribution of rainbow trout in these lakes. In

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432 New Zealand Journal of Marine and Freshwater Research, 1995, Vol. 29

Table 5 Differences between predicted preferred temperatures for large (2-5 kg) rainbow trout acclimated to water temperatures in the epilimnion ofLakes Rotoma and Rotoiti and actual temperatures at their mean depth.

(A) Predicted preferredtemperature (°C)

Lake RotomaNovemberDecemberFebruaryMarchLake RotoitiNovemberDecemberFebruaryMarch

12.813.815.514.5

12.713.616.014.5

(B) Temperatureat mean depth (°C)

11.511.213.113.9

12.913.516.314.5

Difference (B-A)(°C)

-1.3-2.6-2.4-0.6

+0.2-0.1+0.3+3.6

particular, it explained both the monthly changesin the mean depth of trout and the deeperdistribution of trout in Lake Rbtoma. It shouldtherefore be formally tested to determine whetherit provides a good working model for predictingtrout depth distributions in lakes.

We have provided evidence that the thermalstructure of Lakes Rotoiti and Rotoma influencesthe vertical distribution of rainbow trout in summermonths. The population size and density of trout insuch lakes may therefore be related to the amountof thermal habitat available rather than to lakesurface area, or to total lake volume. The importanceof thermal habitat for the production of fish wasdemonstrated by the successful establishment ofAtlantic salmon (Salmo salar) in several NorthAmerican lakes. This depended on the presence offorage fish within the habitable depth range forthese salmon (Kircheis & Stanley 1981). Rainbowsmelt (Osmerus mordax) proved to be the onlyforage species capable of supporting Atlanticsalmon because both species were stenotherms,and occupied similar depth ranges. Knowledge ofthe factors influencing vertical distributions of fishin lakes is therefore essential for understandingfish dynamics in lakes and for f isheriesmanagement.

ACKNOWLEDGMENTS

We are grateful to the Department of Conservation(Tongariro/Taupo Conservancy) for the use of theSIMRAD ES470 echosounder and thank Wayne Bonessfor his assistance with data collection. We also thank DrJohn Hayes for critically reviewing an earlier version ofthe MS, and Dr Martin Cryer and an anonymous refereefor their helpful comments. This study was funded by

the New Zealand Foundation for Research Science andTechnology, Contract no. CO 1322.

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