the effects of wood on stream habitat and native fish assemblages in new zealand

14
The effects of wood on stream habitat and native fish assemblages in New Zealand Brenda R. Baillie 1,2 , Brendan J. Hicks 2 , Michael R. van den Heuvel 3 , Mark O. Kimberley 1 , Ian D. Hogg 2 1 Scion, Private Bag 3020, Rotorua, 3046, New Zealand 2 Centre for Biodiversity and Ecology Research, University of Waikato, Private Bag 3015, Hamilton, 3240, New Zealand 3 Department of Biology, Canadian Rivers Institute, University of Prince Edward Island, 550 University Avenue, Charlottetown, PEI C1A 4P3, Canada Accepted for publication March 19, 2013 Abstract Historic deforestation has deprived many river systems of their natural wood loadings. To study the effects of the loss of wood from waterways, a field trial was conducted in three small forested streams in New Zealand. The objectives were to (i) examine differences in fish assemblages among wooded pools (where wood provided cover), open pools and riffles and (ii) measure the effects of wood removal on channel morphology and fish assemblages. In the first part of the study, no significant differences were found in total fish density among the three habitats. However, total fish biomass was significantly higher in wooded pools (64% of total fish biomass) compared with open pools and riffles. Mean density and biomass of banded kokopu (Galaxias fasciatus) and mean biomass of longfin eel (Anguilla dieffenbachii) were highest in wooded pools, whereas the density and biomass of bluegill bully (Gobiomorphus hubbsi) and torrentfish (Cheimarrichthys fosteri) were highest in riffles. In the second part of the study, wood was removed from a 200-m section (treatment) in each stream, significantly reducing pool area and increasing the proportion of channel area and length in riffles. At the habitat scale, banded kokopu and large longfin eel were the two species mostly affected by wood removal. At the reach scale, banded kokopu biomass was significantly lower in the treatment sections. Although wooded pools were a small portion of total habitat, they provided important habitat for two of New Zealands larger native fish taxa. Key words: wood; streams; habitat; pools; fish; New Zealand Introduction Deforestation, and conversion of land to agriculture, has led to significant modification of many of the worlds river systems (McGlone 1989; Montgomery et al. 2003; Harding et al. 2004; Williams 2008). Complicit with deforestation is the loss of riparian forests and the reduction in natural wood loadings in waterways (Montgomery et al. 2003). Large wood (LW) provides a significant structural and functional role in streams by increasing hydrological and geo- morphological complexity and controlling the reten- tion and movement of organic matter and sediment (Bilby 1981; Mosley 1981; Montgomery et al. 2003; Mutz 2003) influencing biological processes in stream systems (Anderson 1982; Benke & Wallace 2003; Dolloff & Warren 2003). One of the more important functions of LW for fish is the creation of pool habitat and overhead cover. The diversity and complexity of habitat created by LW also provides refuge for fish in extreme hydrological conditions, protection from predators, isolation from competitors and facilitates coexistence of competitive species (Sedell et al. 1990; Fausch & Northcote 1992; Dolloff & Warren 2003). As a result, fish abundance and species richness are usually higher and fish popu- lations are typically larger in streams with high LW loadings (House & Boehne 1987; Fausch & Northcote 1992; Inoue & Nakano 1998). Wood removal experiments provide insight into the implications of reduced wood sources on stream environments and fish assemblages. Marked changes in channel morphology often occur as sediment stored behind debris dams is mobilised and redistrib- uted throughout the system (MacDonald & Keller 1987; Smith et al. 1993a; Diez et al. 2000). The most significant effect of wood removal on fishes is the degradation of aquatic habitat, through loss of wood cover, and pools. The magnitude of fish community response to wood removal varies both spatially and Correspondence: B. R. Baillie, Scion, Private Bag 3020, Rotorua 3046, New Zealand. E-mail: [email protected] doi: 10.1111/eff.12055 1 Ecology of Freshwater Fish 2013 Ó 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd ECOLOGY OF FRESHWATER FISH

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Page 1: The effects of wood on stream habitat and native fish assemblages in New Zealand

The effects of wood on stream habitat and nativefish assemblages in New ZealandBrenda R. Baillie1,2, Brendan J. Hicks2, Michael R. van den Heuvel3, Mark O. Kimberley1, Ian D. Hogg21Scion, Private Bag 3020, Rotorua, 3046, New Zealand2Centre for Biodiversity and Ecology Research, University of Waikato, Private Bag 3015, Hamilton, 3240, New Zealand3Department of Biology, Canadian Rivers Institute, University of Prince Edward Island, 550 University Avenue, Charlottetown, PEI C1A 4P3, Canada

Accepted for publication March 19, 2013

Abstract – Historic deforestation has deprived many river systems of their natural wood loadings. To study theeffects of the loss of wood from waterways, a field trial was conducted in three small forested streams in NewZealand. The objectives were to (i) examine differences in fish assemblages among wooded pools (where woodprovided cover), open pools and riffles and (ii) measure the effects of wood removal on channel morphology andfish assemblages. In the first part of the study, no significant differences were found in total fish density among thethree habitats. However, total fish biomass was significantly higher in wooded pools (64% of total fish biomass)compared with open pools and riffles. Mean density and biomass of banded kokopu (Galaxias fasciatus) and meanbiomass of longfin eel (Anguilla dieffenbachii) were highest in wooded pools, whereas the density and biomass ofbluegill bully (Gobiomorphus hubbsi) and torrentfish (Cheimarrichthys fosteri) were highest in riffles. In the secondpart of the study, wood was removed from a 200-m section (treatment) in each stream, significantly reducing poolarea and increasing the proportion of channel area and length in riffles. At the habitat scale, banded kokopu andlarge longfin eel were the two species mostly affected by wood removal. At the reach scale, banded kokopubiomass was significantly lower in the treatment sections. Although wooded pools were a small portion of totalhabitat, they provided important habitat for two of New Zealand’s larger native fish taxa.

Key words: wood; streams; habitat; pools; fish; New Zealand

Introduction

Deforestation, and conversion of land to agriculture,has led to significant modification of many of theworld’s river systems (McGlone 1989; Montgomeryet al. 2003; Harding et al. 2004; Williams 2008).Complicit with deforestation is the loss of riparianforests and the reduction in natural wood loadings inwaterways (Montgomery et al. 2003). Large wood(LW) provides a significant structural and functionalrole in streams by increasing hydrological and geo-morphological complexity and controlling the reten-tion and movement of organic matter and sediment(Bilby 1981; Mosley 1981; Montgomery et al. 2003;Mutz 2003) influencing biological processes instream systems (Anderson 1982; Benke & Wallace2003; Dolloff & Warren 2003).One of the more important functions of LW for fish

is the creation of pool habitat and overhead cover. Thediversity and complexity of habitat created by LW

also provides refuge for fish in extreme hydrologicalconditions, protection from predators, isolation fromcompetitors and facilitates coexistence of competitivespecies (Sedell et al. 1990; Fausch & Northcote 1992;Dolloff & Warren 2003). As a result, fish abundanceand species richness are usually higher and fish popu-lations are typically larger in streams with high LWloadings (House & Boehne 1987; Fausch & Northcote1992; Inoue & Nakano 1998).Wood removal experiments provide insight into

the implications of reduced wood sources on streamenvironments and fish assemblages. Marked changesin channel morphology often occur as sedimentstored behind debris dams is mobilised and redistrib-uted throughout the system (MacDonald & Keller1987; Smith et al. 1993a; Diez et al. 2000). The mostsignificant effect of wood removal on fishes is thedegradation of aquatic habitat, through loss of woodcover, and pools. The magnitude of fish communityresponse to wood removal varies both spatially and

Correspondence: B. R. Baillie, Scion, Private Bag 3020, Rotorua 3046, New Zealand. E-mail: [email protected]

doi: 10.1111/eff.12055 1

Ecology of Freshwater Fish 2013 � 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

ECOLOGY OFFRESHWATER FISH

Page 2: The effects of wood on stream habitat and native fish assemblages in New Zealand

temporally. While several studies measured adecrease in the abundance, size and biomass of bothwarmwater and coldwater fish species, some speciesshowed little or no response to wood removal(Angermeier & Karr 1984; Dolloff 1986; Fausch &Northcote 1992; Warren & Kraft 2003).Most of the research on wood and fish interactions

has focused on salmonid and warmwater fish speciesin North America (Dolloff & Warren 2003). New Zea-land’s native fish fauna is dominated by Galaxiidae,the most speciose cool-temperate freshwater fish inthe Southern Hemisphere (McDowall 2006) with bothgalaxiids and freshwater eels (Anguilla spp.) compris-ing most of the larger fish species among New Zea-land’s freshwater fish assemblages (McDowall 2000).New Zealand’s larger freshwater eels and several gal-axiid species are known to exploit or prefer in-streamor overhead cover, and low-velocity habitat such aspools and backwaters (Taylor 1988; Glova et al.1998; Jowett et al. 1998; Baker & Smith 2007).In this study, influence of wood on New Zealand’s

native fish assemblages was studied over a 2-yearperiod in three small headwater forest streams. Datacollected in the first year were used to test thehypothesis that fish assemblages in wooded poolswould differ from pools without wood (open pools)

and riffles. At the end of the first year, wood wasartificially removed from a section of stream channelin each of the three streams to test the hypothesesthat (i) wood removal would alter channel morphol-ogy reducing the number, size and type of pools and(ii) wood removal would reduce fish abundance andalter fish assemblages. It was further hypothesisedthat the greatest effect of those habitat changes wouldbe on those species where cover and low velocity areimportant components of their habitat requirements.

Methods

Study area

The study area was located in three small tributariesof the Waiopoahu Stream in the Bay of Plenty regionof New Zealand (Fig. 1, Table 1). The study areawas selected to maximise the likelihood of capturinga full range of diadromous native fish species andwas therefore located within 6 km of the coast, atlow elevation (50 m a.s.l.), with no downstreamobstructions or exotic fish species. The three streams(sites) were in close proximity to each other (Fig. 1)to minimise background variation in climate, hydrol-ogy, geology and soils. Mean annual rainfall for the

Site1

Site2

Site 3

N1 km0

Flow

Control

Treatment

Buffer

Fig. 1. Location of the study area in the Bay of Plenty region of New Zealand and inset showing treatment and control sections withineach stream.

2

Baillie et al.

Page 3: The effects of wood on stream habitat and native fish assemblages in New Zealand

area is 1400 mm (Quayle 1984). The area is in steep(20–35°) hill country underlain by greywacke andoverlain by Brown and Pumice soils (Ministry ofWorks & Development 1975; Hewitt 1998). Thecatchment was previously a mix of indigenous scruband low-productivity pasture land (Ministry of Works& Development 1975) and at the time of the studywas in mature first-rotation Pinus radiata forest (age24–25 years). Riparian buffers of predominantlyindigenous vegetation (Dealbata sp., Cyathea sp.,Schefflera digitata, Melicytus ramiflorus, Aristoteliaserrata) varied in width (approximately 5–50 m)along the stream margins.

Study design

At each of the three sites, an upstream 200-m controlsection and a downstream 200-m treatment section(Fig. 1) were selected with similar channel morphol-ogy characteristics. Control sections were situatedabove the treatment sections to remove the possibilityof downstream effects of wood removal impacting onthe control sites (Feltmate & Williams 1991). A 10-mbuffer was left between each control and treatment sec-tion (Fig. 1) to minimise any morphological changesto wood removal in the treatment sections extendingupstream and influencing the control sections.In the first year (2006), fish assemblages were com-

pared between wooded pools, open pools and rifflesusing data from both the control and treatment sections(combined) in each of the three sites. The sites weremeasured in autumn (March/April) and again in spring(October). For the second part of the study, at the endof the first year (December 2006), wood was removedfrom the downstream treatment sections in each of thethree sites. The sites were remeasured in autumn andspring 2007 to assess the effects of wood removal onchannel morphology and fish assemblages. Measure-ments were carried out in low-flow conditions.

Wood characteristics

At each of the four measurements, all large pieces ofwood (LW) (� 10 cm diameter and � 1 m length)along each 200-m section and within the bankfullwidth of the stream channel were measured forlength, small-end, mid-stem and large-end diameter,and whether the piece was influencing pool forma-tion. A piece of LW was considered to be influencingpool formation where it either created a step in thechannel profile, deflected flow, initiated scouring ofthe channel bed or was a key contributor to dammingof the stream channel to create a pool (Hawkins et al.1993; Rosenfeld & Huato 2003). The volume ofeach piece was calculated using Newton’s formula(Harmon & Sexton 1996):Ta

ble1.

Characteristicsof

thethreestream

sitesin

theWaiopoahu

catchm

ent.

Site

and

treatm

ent

Catchment

area

(ha)

Flow

(l�s�

1)

Meanwater

temperature

(°C)

Meanbankfullwidth

(m)

Gradient(%

)Woodvolume(m

3�ha

�1)

Pre

autumn

Pre

spring

Post

autumn

Post

spring

Pre

autumn

Pre

spring

Post

autumn

Post

spring

Pre

autumn

Pre

spring

Post

autumn

Post

spring

Pre

autumn

Pre

spring

Post

autumn

Post

spring

Site

1Control

916.5

13.3

9.9

13.9

12.4

4.8

4.7

4.4

4.5

3.3

ND

3.8

3.7

5876

7273

Treatm

ent

3.9

3.9

3.7

3.6

3.2

ND

4.0

3.3

6952

52

Site

2Control

178

14.6

13.9

10.3

14.6

12.3

4.8

4.9

5.0

5.4

2.2

ND

2.4

2.4

5560

6750

Treatm

ent

4.6

4.8

4.6

4.8

2.2

ND

2.5

2.8

6046

2027

Site

3Control

179

13.8

13.5

10.1

14.2

ND

4.0

4.2

4.2

4.4

2.4

ND

2.6

2.9

5341

4536

Treatm

ent

4.1

4.4

4.2

4.3

2.6

ND

2.3

2.3

7654

126

Pre

refers

tothetwomeasurements

priorto

woodremoval,andPostto

thetwomeasurements

afterwoodremoval;ND=no

data.

3

Effects of wood on New Zealand’s native fish assemblages

Page 4: The effects of wood on stream habitat and native fish assemblages in New Zealand

Vpiece ¼ ðLðAb þ 4Am þ AtÞ=6Þ10; 000

(1)

where Vpiece = volume of piece (m3); L = length ofpiece (m); Ab = area at the base of the piece (cm2);Am = area at the mid-point of the piece (cm2); andAt = area at the top of the piece (cm2). The width,height and depth of root wads were summed to givean approximate volume. Wood volumes in each sec-tion were expressed as m3�ha�1 using bankfull chan-nel width and transect length to calculate streambedarea.After the first year of measurements, all stems,

branches and associated accumulations of organicmatter were manually removed from the stream chan-nel in the three treatment sections. Pieces suspendedabove the channel and not influencing channel mor-phology were retained. Pieces embedded in the bankswere cut flush with the bank edge. Treatment sectionswere regularly maintained (1 to 2 month intervals) inthe year following wood removal, particularly follow-ing high-rainfall events, and any additional pieces orlarge accumulations of organic matter entering thesystem or exposed by channel down-cutting wereremoved.

Stream characteristics

Bankfull width was measured to the nearest 0.1 m at10-m intervals along each control and treatment sec-tion. Surficial substrate was systematically sampled atthe same locations (10 samples spaced out across thechannel width) using Leopold’s (1970) pebble countprocedure (200 samples per 200-m section) and clas-sified into nine inorganic classes based on Gordonet al. (1992) and two organic substrate classes [LWand small wood (<10 cm diameter)]. The habitat ineach section was classified as pool, riffle or run usingthe definitions in Hawkins et al. (1993) and measuredfor length and width (m) to determine the percentagelength and percentage area of stream channel in eachhabitat type for each section. Channel gradient (%)was measured using a clinometer and an averagegradient, weighted by distance, calculated for eachcontrol and treatment section. These measurementswere repeated twice before and twice after woodremoval with the exception of channel gradient,which was not remeasured in the second measure-ment prior to wood removal. Water temperature waslogged at 15-min intervals over a 4-day period duringeach measurement period using Onset StowAwayTidbit temperature loggers (Onset Computer Corpora-tion, Bourne, MA, USA) deployed at the downstreamend of each site to calculate average temperature(Table 1). Single baseflow measurements were takenat the downstream end of each site using a Flo-Mate

water velocity metre, Model 2000 (Marsh-McBirneyInc., Frederick, MD, USA) (Table 1).

Fish sampling

In Year 1, three wooded pools (pools where woodand associated debris provided cover), three openpools (minimal or no wood) and three riffles wererandomly selected from each of the control and treat-ment sections at each site for fish sampling. The totalnumbers in each section from which these sampleswere selected ranged from 4 to 14 for wooded pools,3 to 13 for open pools and 8 to 16 for riffles. In Year2, as wood removal eliminated wood pools in thetreatment sections, only open pools and riffles couldbe sampled. Therefore, where possible six pools weresampled in the treatment sections to match the sixpools sampled in the control sections although actualnumbers ranged from two to six. Runs were omittedfrom fish sampling as they formed a small percentageof the habitat in these streams.To determine the number of passes to be used,

prior to the main study, three riffles, open pools andwooded pools were electro-fished using multipasselectric fishing, up to a maximum of six passes.Working in an upstream direction, each habitat wasblocked off at the downstream end with a 10-mmmesh net and fished using a battery-powered back-pack electric fishing machine (Kainga EFM 300,NIWA, Christchurch, New Zealand). Each fish cap-tured was identified in the field, measured for lengthto the nearest millimetre and weighed to the nearest0.1 g. Each habitat fished was measured for length,width and depth. Results showed that 58% of the totalnumber of fish, similar to Jowett & Richardson(1996), and 75% of the fish biomass were captured inthe first pass. One-pass electric fishing was thereforeused for the remainder of the trial in order to sample alarger number of habitats to capture the range of fishassemblages present in these streams, rather thanusing multipass fishing of fewer habitat units.

Data analyses

Data were initially examined using Proc Univariate(SAS statistical software, version 9.0, SAS InstituteInc., Cary, NC, USA), and where necessary, variableswere logarithmically transformed in order to satisfyassumptions of normality [log (x) or log (x + 1)when x contained zero values]. Results of all statisti-cal tests conducted in the study were considered sig-nificant if P < 0.05.To test for differences in channel characteristics

(bankfull width, channel gradient, substrate, number,length and area of pools, riffles and runs) betweenthe control and treatment sections of each site in Year

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Page 5: The effects of wood on stream habitat and native fish assemblages in New Zealand

1 prior to wood removal, two-way analyses of vari-ance (ANOVAs) were conducted using SAS ProcMixed, including site as a random replicate effect(n = 3) and treatment (control and treatment) as afixed effect.Fish data collected in Year 1 were used to test for

differences in species richness and fish attributes (fishdensity, biomass, length and weight) for total fishand key fish species among the three habitats(wooded pools, open pools and riffles). ANOVAs wereperformed using SAS Proc Mixed, with site includedas a random replicate effect, habitat as a fixed effectand season as a repeated measures fixed effect. Anal-yses were performed on the mean values of eachattribute averaged for each site by habitat and season.Pairwise differences between habitats were testedusing the Tukey–Kramer test.To test the effects of wood removal, ANOVAs were

used with year (Year 1 before treatment and Year 2after treatment), treatment (control vs. wood removal)and season included as fixed effects and sites as arandom replicate effect. The treatment X year interac-tion provided a BACI (before–after control impact)test of whether wood removal had a significant effecton a range of attributes. Attributes tested in this waywere bankfull width, substrate, habitat (pools, rifflesand runs) and fish density and biomass per squaremetre of stream bed for total fish and key fish spe-cies. Data from the fished sections of wooded pools,open pools and riffles were weighted by total habitatareas when calculating fish density and biomass ineach section.Nonmetric multidimensional scaling (MJM Soft-

ware Design, Gleneden Beach, OR, USA) was used toexamine differences in fish community compositionbetween habitats (wood pools, open pools and riffles)and season (autumn and spring) using Year 1 data.MDS is an ordination technique that arranges samplesin multidimensional space so that the distancebetween samples reflects the difference in communitystructure (McCune & Grace 2002). Sørenson’s dis-tance (McCune & Grace 2002) was used to measuredissimilarity between samples based on log-trans-formed mean fish abundance data, including rare spe-cies. The preliminary 50 runs identified threedimensions, as the optimal solution, and 500 iterationswere used in the final run giving a final stress of12.64 and a final instability of 0.00044. Coefficientsof determination (r2) were then used to express theproportion of variation represented by each axis. ANO-

VA followed by the Tukey–Kramer test was used tocompare axis scores in relation to habitat, season(autumn and spring) and site, and Pearson correlations(r) were used to examine relationships between trans-formed abundance of fish species, a range of environ-mental variables and the ordination axes.

Results

Effects of wood removal on wood loadings and channelmorphology

Prior to wood removal, wood loadings in the con-trol and treatment sections of the three sites rangedfrom 41 to 76 m3�ha�1 (Table 1). On average, 65%of pieces were located in debris dams (range 55–74%). After removing wood from the stream chan-nel, any remaining wood in the treatment sectionswas mainly attributable to pieces embedded in thebank or substrate, and in the case of Site 2, severallarge tree fern root wads embedded in the bankcontributed to the remaining wood volumes in thetreatment section.There were no significant differences in bankfull

width and channel gradient between the control(upstream) and treatment (downstream) sections ateach site, either prior to or after wood removal(Table 1). Gravels dominated substrate compositionin all three sites throughout the trial period, rangingfrom 50% to 84% of substrate composition, followedby fines (10–28%) and cobbles (5–21%). Medium–large gravels comprised the median substrate size atmost sites. Prior to wood removal, the percentage ofsubstrates in most classes were not significantly dif-ferent between the control and treatment sections atthe three sites. After wood removal, the percentageof large gravels increased significantly in all threetreatment sections (ANOVA BACI: F1,4 = 36.23,P = 0.004).The streams in this study were composed of a

pool–riffle–run morphology, dominated by riffles(Fig. 2). Prior to wood removal, there were no signif-icant differences in the proportions of channel area,length and numbers in pools, riffles and runs betweenthe control and treatment sections. The proportion ofchannel area in pools significantly declined (ANOVABACI: F1,4 = 9.44, P = 0.037) in the treatment sec-tions after wood removal (Fig. 2). However, the ini-tial decline and subsequent partial recovery of poolnumbers after wood removal resulted in no signifi-cant change in pool numbers. Wood removal alsohad no significant effect on mean pool length (Fig. 3)and maximum pool depth. Wood contributed to for-mation of 59–67% of pools before wood removal,and wooded pools comprised 13–14% of the channelarea. Wood influence on pools was similar afterwood removal in the control sections. In the treat-ment sections, remaining wood pieces embedded inthe bank influenced 8–21% of pool formation. How-ever, none of these pieces provided wood cover.Scour processes formed most of the remaining pools.Wood removal resulted in a significant increase in

the proportion of channel area (ANOVA BACI: F1,4 =

5

Effects of wood on New Zealand’s native fish assemblages

Page 6: The effects of wood on stream habitat and native fish assemblages in New Zealand

22.46, P = 0.009) and channel length (ANOVA BACI:F1,4 = 19.46, P = 0.012) in riffles (Figs 2 and 3)with an overall decline in riffle numbers (ANOVABACI: F1,4 = 7.24, P = 0.055). Runs formed a minorcomponent of channel composition and were leastaffected by wood removal, showing no significantresponse.

Fish capture

A total of 2183 fish comprising 11 native specieswere caught over the study period. Bullies (Gobio-morphus spp.) comprised the largest component oftotal fish captured: common bully (Gobiomorphuscotidianus), 27%; redfin bully (Gobiomorphus hut-toni), 21%; and bluegill bully, 19%. The remainingspecies caught included longfin eel, banded kokopu,shortfin eel (Anguilla australis), torrentfish and com-mon smelt (Retropinna retropinna). Inanga (Galaxias

maculatus), koaro (Galaxias brevipinnis) and short-jaw kokopu (Galaxias postvectis) were rarely caught.No salmonids were caught during this study.

Influence of wood on fish assemblages

The 779 fish caught in Year 1 were used to analysefish community characteristics between woodedpools, open pools and riffles. Species richness wassignificantly higher in riffles (average, 3.4 species)than open pools (average, 2.1 species), but there wasno significant difference between riffles and woodedpools (average, 2.8 species) or between the two pooltypes. There was no significant difference in total fishdensity (fish 100 m�2) between the three habitats,but habitat had a significant effect on total fish bio-mass (g 100 m�2) (ANOVA: habitat: F2,4 = 23.99,P = 0.0059) with wood pools containing significantlyhigher fish biomass than open pools and riffles

0

20

40

60

80

100

C T C T

Mea

n %

cha

nnel

are

a (m

2 )

Pool Riffle Run

Before AfterFig. 2. The proportional average areas of the control (C) and treatment (T) sections in pools, riffles and runs before and after woodremoval (n = 3 sites).

0

2

4

6

8

10

12

14

16

18

Mea

n ha

bita

t len

gth

(m)

C T C T C T C TBefore After Before After

Pools Riffles

b

aa aa

aa a

Fig. 3. The average lengths (�SE) of pools and riffles in the control (C) and treatment (T) sections before and after wood removal (n = 3sites). Runs were excluded as they were a minor component of channel morphology. Means with the same letters are not significantlydifferent.

6

Baillie et al.

Page 7: The effects of wood on stream habitat and native fish assemblages in New Zealand

(Table 2). Pairwise comparisons showed differencesat the species level (Table 2). Bluegill bully densityand biomass were significantly higher in riffles thanin open or wooded pools. Torrentfish density andbiomass differed significantly between the three habi-tats with highest densities also occurring in riffles.Banded kokopu density was significantly higher inwooded pools than riffles, but no significantdifferences were detected between the two pooltypes. Banded kokopu biomass was also highest inwooded pools, but differed significantly between thethree habitats. There were no significant differences

in longfin eel [total, large (� 300 mm in length) andsmall (<300 mm in length)] densities between thethree habitats. Total longfin biomass was highest inwooded pools and differed significantly from riffles,but differences with open pools were not significant.No significant differences in common bully and red-fin bully density and biomass were detected betweenthe three habitats.Longfin eel was the only species showing physical

differences between habitats (Table 2). Longfin eelmean length and mean weight were significantlygreater in wooded pools and open pools compared

Table 2. Density, biomass and size of key fish species in wooded pools, open pools and riffles before wood removal (� represent one standard error for fishdensity, biomass, length and weight; means with the same letters are not significantly different).

Bluegill bully Banded kokopu Common bully Longfin eel Redfin bully Torrentfish† Total

Mean fish density (no 100 m�2)Wooded pool 5.2 � 2.1b 11.7 � 2.7a 13.9a � 2.6a 23.7 � 3.6a 19.3 � 4.3a 3.1 � 1.7a 9.5 � 10.0aOpen pool 4.7 � 2.5b 4.1 � 1.4ab 22.9a � 6.3a 11.6 � 2.7a 12.3 � 2.6a 1.2 � 1.2#b 60.9 � 7.2aRiffle 48.1 � 8.0a 0.2*b 7.1a � 1.9a 18.6 � 3.7a 28.4 � 4.2a 8.6 � 1.9c 111.9 � 10.0a

Mean fish biomass (g 100 m�2)Wooded pool 5.7 � 2.5b 432.3 � 136.7a 61.0 � 12.8a 1817.1 � 450.9a 53.6 � 12.8a 15.1 � 8.1b 2574.4 � 495.3aOpen pool 4.1 � 1.9b 56.3 � 24.7b 81.6 � 19.6a 597.8 � 275.5ab 34.1 � 9.1a 3.5 � 3.5#c 875.3 � 289.9bRiffle 46.2 � 7.7a 2.1*c 21.3 � 6.0a 183.7 � 58.7b 60.5 � 10.9a 39.2 � 9.8a 356.2 � 59.7b

Fish geometric mean length (mm) � SEWooded pool 51 � 2a 120 � 19a 68 � 3a 194 � 13a 60 � 2a 72 � 5Open pool 49 � 2a 90 � 17a 63 � 2a 164 � 16a 60 � 2a 61 � 8#

Riffle 49 � 0.4a 90*a 62 � 3a 140 � 9b 56 � 1a 71 � 3

Fish geometric mean weight (g) � SEWooded pool 1.1 � 0.1a 18.1 � 11.0a 3.5 � 0.5a 12.2 � 2.8a 2.2 � 0.2a 4.3 � 1.0Open pool 0.9 � 0.1a 6.4 � 4.8a 2.9 � 0.4a 6.8 � 2.3a 2.0 � 0.3a 3.0 � 1.3#

Riffle 0.9 � 0.1a 7.0*a 2.5 � 0.4a 4.0 � 0.8b 1.7 � 0.1a 3.9 � 0.5

*n = 1; #n = 2.†Insufficient numbers of torrentfish in wooded and open pools for ANOVA of mean lengths and weights.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Wooded pool Open pool Riffle Wooded pool Open pool Riffle

Other Longfin eel Common bully Redfin bully Bluegill bully Torrentfish Banded kokopu

Fish numbers BiomassFig. 4. Comparison of fish community composition and biomass distribution between wooded pools, open pools and riffles before woodremoval. Other = inanga, koaro, shortfin eel, shortjaw kokopu and smelt. N = 191, 144 and 444 fish for wooded pools, open pools andriffles, respectively. Sample size is the same for fish numbers and biomass.

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with riffles (Table 2). No significant seasonal effectsin density and biomass were detected for total fish orfor the key fish species with the exception of redfinbully biomass, which was significantly higher inspring (ANOVA: season: F1,8 = 10.07, P = 0.013).Season had no significant effect on fish mean lengthsand weights. Small sample sizes precluded analysisof other fish species.Fish assemblages were similar between the two

pool types (Fig. 4). Both were dominated by commonand redfin bully and longfin eel with wooded poolscontaining a higher proportion of banded kokopu andlongfin eel and open pools containing a higher pro-portion of common bully. A high proportion of com-mon smelt were also found in open pools (includedunder other in Fig. 4). Common smelt were caught inthe autumn sampling period only. The fish assem-blage in riffles was dominated by bluegill and redfinbully followed by longfin eel and torrentfish. Sixty-four per cent of the total fish biomass was located inwood pools, with longfin eel comprising most of thebiomass in all three habitat types (Fig. 4).The total variation in fish assemblages explained

by all three axes in MDS ordination was 88% (Axis1, 21%; Axis 2, 51%; Axis 3, 16%) (Fig. 5). Analy-sis of MDS scores in relation to habitat, season andsite showed that season had a significant influence onAxis 1 (ANOVA: F1,8 = 7.16, P = 0.03). The main fishspecies influencing this result were common smelt,which were negatively correlated with Axis 1, havinghigher abundances in autumn (r = �0.55) and com-mon bully, longfin eel and redfin bully having higherabundances in spring (r = 0.56, 0.47 and 0.39,respectively). Habitat had a significant influence onAxis 2 (ANOVA: F2,4 = 45.39, P = 0.002) with fish

assemblages in riffles differing significantly fromwooded and open pools. The main fish species differ-entiating fish assemblages in riffles from pools werebluegill bully, redfin bully, torrentfish and longfineel. These species were significantly negatively corre-lated with Axis 2 (r = �0.81, �0.75, �0.73 and�0.42, respectively), all having higher abundances inriffles compared with pools. Common bully, bandedkokopu and common smelt were significantly posi-tively correlated with Axis 2 (r = 0.42, 0.37 and0.33, respectively), all having higher abundances inpools compared with riffles. Neither habitat, season,nor site significantly influenced Axis 3.To examine the effect of other environmental vari-

ables on fish assemblages, we calculated correlationsbetween mean habitat length, depth, area and volumewith the axis scores. Average habitat depth was posi-tively correlated, and average habitat length and areanegatively correlated with Axis 2 scores.

Effects of wood removal on fish assemblages

Although the total area of stream channel fished inthe first year before wood removal (976 m2) and inthe second year after (910 m2) wood removal wassimilar, total catch increased by approximately 80%in the second year. The increase occurred in both thecontrol and treatment sections (Fig. 6a) and could notbe directly attributed to the wood removal process.Common bully accounted for about half the increase,which occurred mainly in the pools (Fig. 6a).Because most of the increase in fish abundance inYear 2 was attributable to the smaller sized fish, totalfish biomass increased by only 18%. Thirty-eight percent of the fish biomass was located in the wooded

–1.5

–1

–0.5

0

0.5

1

1.5

2

–1.5 –1 –0.5 0 0.5 1

Axi

s 2

Axis 1

Open pool Riffle Wooded pool

Spring

Riff

les

Pool

s

AutumnSM CB, LF, RF

BG, R

F, T

F, L

FCB

,BK,

SM

Fig. 5. Location of samples on the two main axes of a three-dimensional MDS ordination of fish communities to determine differencesbetween riffles, open pools and wooded pools, based on log-transformed average abundance data collected in Year 1 (stress value = 12.64;instability = 0.00044). BG = bluegill bully, RF = redfin bully, TF = torrentfish, CB = common bully, LF = longfin eel, SM = commonsmelt, BK = banded kokopu.

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pools in the control sections with longfin eel stillcomprising most of the total fish biomass at 58%.Prior to wood removal, mean banded kokopu den-

sity was highest in wooded pools in both the controland treatment sections (Fig. 6b) (73% of all bandedkokopu caught). Following wood removal, althoughthe number of banded kokopu doubled (Fig. 6b),most of the fish (84%) were caught in pools in thecontrol sections (47% in wooded pools; 31% in openpools). Similar to banded kokopu, mean large longfineel density was highest in wooded pools in both thecontrol and treatment sections prior to wood removal(Fig. 6c) (70% of all large longfin eel caught). After

wood removal, mean large longfin eel density washighest in the wooded pools in the control sections(50% of all large longfin eels caught) (Fig. 6c).At the reach scale, there were no significant

changes in total fish density or biomass attributableto wood removal and no significant changes wereobserved for the key fish species with the exceptionof banded kokopu where biomass declined signifi-cantly in the treatment sections following woodremoval (ANOVA BACI: F1,4 = 8.98, P = 0.04)(Fig. 7). No significant seasonal effects were detectedfor total fish density and biomass and the key fishspecies at the reach scale.

0

50

100

150

200

250

300

C T C T C T C T C T C T

Mea

n no

. fis

h 10

0 m

–2

no WP

All fish

before wood removal after wood removal(N = 779) (N = 1404)

(a)

Mea

n no

. fis

h 10

0 m

–2

05

1015202530354045

C T C T C T C T C T C T

no WP

Banded kokopu

before wood removal after wood removal(N = 40) (N = 86)

(b)

Mea

n no

. fis

h 10

0 m

–2

0

2

4

6

8

10

12

14

C T C T C T C T C T C T

no WP

Large longfin eels ≥ 300 mm

Wooded pools Open pools Riffles

before wood removal after wood removal(N = 20) (N = 24)

(c)

Fig. 6. Average fish density for (a) all fish, (b) banded kokopu and (c) large longfin eel in the control (C) and treatment (T) sections, foreach of the three habitats before and after wood removal (WP = wooded pools). Error bars indicate �SE.

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Effects of wood on New Zealand’s native fish assemblages

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Discussion

Effects of wood removal on channel morphology

In this study, wood removal resulted in major changesin channel morphology. It instigated rapid destabilisa-tion and redistribution of sediment and organic matterstored behind debris dams, initiating channel scourimmediately upstream of debris dams and in-fillingpools further downstream. These processes resulted ina simplified channel morphology dominated by longsections of riffles, fewer pools and coarsening of thechannel substrate. Similar processes and channelresponse have been observed in wood removal studiesin Europe and America (e.g., Bilby 1981, 1984;MacDonald & Keller 1987; Lisle 1995) illustratingthe important morphological role of wood in smallforested streams. While some authors have reportednew sediment deposition sites developing behindobstructions such as point bars (MacDonald & Keller1987; Smith et al. 1993b), this process was notobserved in our study. Overall, wood removal reducedthe sediment storage capacity of these streams.Following wood removal, the area of pools

declined with remaining pools formed primarily byscouring process in unconsolidated sediments, on theouter edge of bends and where there were changes insubstrate such as bedrock protrusions. Other woodremoval studies have also reported a reduction in areaand number of pools although a wide range of pool-forming processes have been observed including theelimination, partial in-filling or deepening of pre-existing pools or the creation of new and sometimessmaller pools (Bilby 1984; MacDonald & Keller1987; Lisle 1995; Diez et al. 2000). However, this isnot always the case; experimental removal of woodin an Alaska stream had marked effects on sediment

distribution and channel morphology, but producedno obvious changes in pool characteristics (Smithet al. 1993b). This may have been the result of newlydeveloped point bars partially replacing some of thefunctions associated with wood such as sedimentstorage and pool formation.

Influence of wood on fish assemblages

The hypothesis that fish assemblages in woodedpools differ from those in open pools and riffles wassupported by the results of this study. Wooded pools,although a small proportion of total habitat, were themain habitat provider for the two largest fish species(banded kokopu and the large longfin eel) and sup-ported most of the fish biomass, whereas open poolscontained higher proportions of both common bulliesand smelt. Bluegill bullies and torrent fish were themain species differentiating fish assemblages in rif-fles from wooded and open pools. It has been shownfor both salmonid and warmwater fish that the struc-turally complex habitat created by wood is a key fac-tor contributing to habitat partitioning of fishassemblages, often supporting larger fish and signifi-cant proportions of the total fish biomass (Monzyket al. 1997; Dolloff & Warren 2003). This study andothers (Langford et al. 2012) suggest that wood playsa similar role for fish assemblages dominated byfreshwater eels and galaxiids.The volumes of LW in our streams were low rela-

tive to volumes in old-growth indigenous forests(Meleason et al. 2005; Baillie et al. 2008), meaningthat this study may have underestimated the potentialeffect of wood on fish assemblages. Also, there areinherent difficulties of electric-fishing wooded pools,compared with open waters which may have resultedin underestimates of fish numbers. The one-pass fish

0

200

400

600

800

1000

1200

1400

C T C T C T C T C T C T

Mea

n fis

h bi

omas

s (g

100

m–2

)

After

a

a

b

a

a

a

a

a aa

aa

Total fishBefore After Before After Before

Banded kokopu Large longfin eelFig. 7. Estimated mean biomass of total fish, banded kokopu and large longfin eel in the control (C) and treatment (T) sections at the reachscale, before and after wood removal. Error bars indicate �SE. Means with the same letters are not significantly different.

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sampling capture rate (approximately 60%) may alsohave influenced these results.Although several species showed strong habitat

preferences, our study also found considerable com-monalities in fish assemblages among the three habi-tats, dominated by a core group of species includingcommon bullies, longfin eels (all sizes) and redfinbullies (Table 2). Other variables apart from wood,such as hydrology, food resources, habitat require-ments, competition and predation pressures, and life-history requirements, all potentially influence fishcommunity composition (Braaten & Berry 1997;Prenda et al. 1997; Reichard 2008; Crow et al.2010), and a number of these may have been influ-encing fish assemblages in our streams.While pools are generally defined as areas of

relatively deeper and still- or slower-flowing water(Hawkins et al. 1993), some pool types such asplunge pools contain areas of high turbulence. Wheresuch pools were located adjacent to downstream rif-fles, species such as bluegill bully and torrentfish,commonly associated with higher energy habitats,were occasionally found. In addition, while themajority of banded kokopu and large longfin eel werefound in wooded pools, they also occurred in openpools where they exploited alternative sources ofcover such as large cobbles or rock crevices. Conse-quently, fish communities associated with the threehabitats showed some spatial overlap, particularlybetween the two pool types.Many of New Zealand’s small streams, including

those in our study, are hydrologically and morpho-logically unstable (Winterbourn 1995; Duncan &Woods 2004). The ability to exploit a wide range ofhabitats is thought to be an adaptive strategy to copewith the large changes in environmental conditionsassociated with hydrologically unstable streams(Matthews & Hill 1980). These systems often containa wide range of generalist fish species occupying avariety of habitats, with considerable habitat overlap(Matthews & Hill 1980; Braaten & Berry 1997).Therefore, stream instability may be a contributingfactor to the core of generalist fish species occupyinga diverse range of habitats, and fewer habitat special-ists encountered in this and similar studies (Taylor1988; Jowett & Richardson 1995; Chadderton &Allibone 2000).

Fish response to wood removal

At the reach scale, banded kokopu biomass declinedsignificantly in the treatment sections following woodremoval. At the habitat scale, banded kokopu andlarge longfin eel densities declined in the treatmentsections compared with the controls (Fig. 6), indicat-ing suboptimal habitat for these fish. Highest densi-

ties were located in the remaining wooded pools inthe control sections. Some of this increase was likelydue to fish movement from the treated sections. Aswood removal not only reduced the area of pools buteliminated all the wooded pools, it is not surprisingthat these fish were most affected by wood removal.Longfin eel, however, can exploit a wide range ofhabitats (Taylor 1988; Glova et al. 1998) and may bemore adaptable to the loss of wooded pools thanbanded kokopu, which have more constrained habitatrequirements (Taylor 1988; McDowall 2000; Rowe& Smith 2003). Even so, wooded pools were the pre-ferred habitat of large longfin eel in this study andthe lower density of large longfin eel in the treatmentsections after wood removal is most likely due to theloss of wooded pools. In Southern England streams,densities of large eel (Anguilla anguilla) were alsopositively correlated with woody debris and woodedpools (Langford et al. 2012), indicating the value ofthis medium for other Anguilla species.The importance of wood in providing in-stream

habitat varies between fish species (Dolloff & Warren2003), and this influences the magnitude of fish com-munity response to wood removal. In our study, fishresponse was evident for two species, but woodremoval had little impact on the remaining fish spe-cies, indicating that the habitat provided by woodwas not a critical requirement, although changes introphic structure and food webs associated with lossof wood from river systems could still indirectlyaffect these species. In a number of other studies,wood removal has decreased the abundance, size andbiomass of both warmwater and coldwater fishspecies, reducing fish productivity in these systems(Angermeier & Karr 1984; Dolloff 1986; Zalewskiet al. 2003). Spatial and temporal response of fish towood removal was also evident in high-gradientstreams of the Adirondack Mountains in USA(Warren & Kraft 2003). Long-term impacts of woodremoval on channel morphology and fish assem-blages can sometimes last up to decades (House &Boehne 1987; Klein et al. 1987; Hicks et al. 1991;Fausch & Northcote 1992). Long-term recovery ofthe treatment sections in our streams, now that woodis no longer being removed, will depend on the avail-able wood supply both upstream and in riparianareas, wood delivery mechanisms, fluvial processeswithin the stream system and future management ofthe catchment.

Potential confounding factors

This study was designed to minimise variationbetween Sites, and remove confounding factors asso-ciated with riparian removal, harvesting and exoticfish species. Using a BACI (sensu Stewart-Oaten

11

Effects of wood on New Zealand’s native fish assemblages

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et al. 1986) design, which included both spatial andtemporal replication, we were able to incorporate nat-ural variation occurring among sites and the stronginterannual variation in fish abundance, although timeconstraints limited this study to two measurementsover the course of 1 year before and again followingwood removal. It also highlighted the importance ofisolating the effects of wood removal from other dis-turbances associated with land-use change and forestharvesting that can affect fish assemblages (Hartman2004; Hicks et al. 2004). Selection of scale needs tobe carefully considered. Langford et al. (2012) foundvarying and sometimes conflicting results whenexamining fish community characteristics at both thehabitat and reach scale. Similarly in this study,changes in fish community composition from woodremoval at the habitat scale were not always evidentat the reach scale as wooded pools, although contain-ing most of the large fish and fish biomass compriseda small portion of total habitat at the reach scale. Asthe home range of some fish species is likely to begreater than the control and treatment reaches, coinci-dental catches may have also influenced results.

Implications of historic wood loss on fish assemblages

Historical reduction and alteration of forest coverhave seriously reduced recruitment of wood tostreams, particularly larger trees that are critical informing complex, longer-lasting habitat (Montgom-ery et al. 2003). Although the effectiveness of rein-troducing wood into streams to improve fish habitatvaries (Reich et al. 2003; Nagayama & Nakamura2010; Ant�on et al. 2011), it is an option worthy offurther research in New Zealand for streams lackingnatural wood recruitment from forested riparian mar-gins, particularly as a number of studies includingours show that deep water habitat and cover providedby wood support larger fish and much of the fish bio-mass (e.g., Greenberg 1991; Chadderton & Allibone2000; Dolloff & Warren 2003; Langford et al. 2012).As these fishes are at the top of the food chain andconstitute most of the fish biomass, they are impor-tant determinants of community structure, influencingthe underlying trophic structure, food webs, produc-tivity and carrying capacity in streams.Other Southern Hemisphere galaxiid are also likely

to be affected by depleted wood resources and maybenefit from the re-introduction of wood (Jowettet al. 1998; Chadderton & Allibone 2000; McDowall2006; Baker & Smith 2007). In one such Australianstudy, experimental introduction of wood resulted ina short-term increase in abundance of the Mountaingalaxias (Galaxias olidus) (Bond & Lake 2005).Globally, eels are declining due to commercial andcustomary fishing, physical barriers to migration and

habitat degradation (Doole 2005; Jellyman 2009).Widespread historic loss of forests and associatedin-stream wood (Montgomery et al. 2003) have con-tributed to the deterioration in habitat for these spe-cies. Wood is currently undervalued as a resourceand rehabilitation tool for freshwater eel habitat glob-ally and could play a greater role in their conserva-tion and recovery.While a number of studies have demonstrated the

ecological role of wood for salmonids and warmwaterfish species (Dolloff & Warren 2003), our study high-lights the importance of wood for two other fishgroups: the galaxiids, which occur throughout theSouthern Hemisphere, and the more widespread fresh-water eels (Anguilla spp.). Overall, the study confirmsthe importance of wood in the protection, manage-ment and restoration of small forested streams.

Acknowledgements

We would like to thank Robin Black, Tony Evanson, Dud-ley Bell, Alex Ring, Warrick Powrie, JJ Cornwall and othersfor their assistance with the fieldwork. Max Butler, JamesMcPhee and Junior Ngaro from Maungawaru Logging car-ried out the chainsaw work required to remove wood fromthe treatment sections. We would also like to thank theHoupoto TePua committee of management for their supportfor this project. Mike Joy, Jon Harding and Andrew Dolloffreviewed earlier versions of this manuscript, and we wouldlike to thank them and two anonymous reviewers for theirhelpful and constructive comments. Funding for this projectwas provided by the Ministry of Science and Innovation,Bright Futures Scholarship, Scion and Hancock ForestManagement Ltd.

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