Fish fauna of dry tropical and subtropical estuarine floodplain wetlands

CSIRO PUBLISHING

www.publish.csiro.au/journals/mfr Marine and Freshwater Research, 2007, 58, 931–943

Fish fauna of dry tropical and subtropical estuarinefloodplain wetlands

Marcus SheavesA,B, Ross JohnstonA and Kátya AbrantesA

ACoastal and Estuary Ecosystem Ecology Laboratory, School of Marine and Tropical Biology,James Cook University, Townsville, Qld 4811, Australia.

BCorresponding author. Email: [email protected]

Abstract. Estuarine floodplain wetland pools occur adjacent to marine coasts and estuaries throughout the world. InAustralia’s dry tropics and sub-tropics, low and irregular rainfall means estuarine wetland pools are isolated for muchof the time, resulting in varied within-pool conditions, with chemistry ranging from fresh to hypersaline, depending onthe balance between freshwater and marine inputs and the time between connections. Varied physical conditions andirregular connectivity provide the potential for substantial faunal difference among pools. The present study compares thecompositions and structures of the fish fauna of a broad cross section of estuarine wetland pools adjacent to the estuaryof the Fitzroy River, one of the largest rivers in Australia’s dry tropical/subtropical zone. Ten pools were sampled betweenFebruary 2004 and May 2005. The total species richness was low, with the 6123 fish recorded over the study, comprisingonly 44 species. This low species richness was reflected at the individual pool level, with a maximum total richness of 25species in a single pool. Different pools had faunas that differed as a function of the proportion of the community comprisedof marine spawned, compared with freshwater spawned, species. This was a reflection of the extent of connectivity tofreshwater and marine systems, which determined both the physical nature of pools and the sources of faunal supply.Despite faunal differences among pools, at a functional level pool fish faunas were dominated by detritivores, regardlessof pool type, size, season or connectedness.

Additional keywords: connectivity, estuary, faunal composition, functional composition, physical conditions.

Introduction

Estuarine floodplain wetlands occur adjacent to marine coastsand estuaries throughout the world, and function as linksbetween terrestrial and marine ecosystems (Levin et al. 2001).They comprise temporarily flooded lowlands, palustrine habi-tats (Kingsford and Norman 2002), and permanent and semi-permanent pools, lakes and lagoons (Allanson and Whitfield1983), and possess vibrant and diverse flora and fauna (e.g.Russell and Garrett 1988; Kingsford and Norman 2002;Sheridan and Hays 2003). Species that use estuarine wetlands arethemselves special. They can be seen in one sense as generalists,able to thrive in and use a range of conditions (Gelwick et al.2001), and in another as specialists, that require great phenotypicplasticity (Ray 2005) to prosper under various environmentalregimes.

Although relatively few fish that use estuarine wetlands areobligatory users, estuarine wetlands are often critical nurs-ery grounds for fish and invertebrates (Secor and Rooker2005). Although these nursery ground values are not completelyunderstood for tropical and subtropical estuaries (Sheaves2001, 2005), abundant food supplies (Javonillo et al. 1997)and/or reduced predation (Paterson and Whitfield 2000) leadto enhanced storage pools of energy and genetic material, ulti-mately resulting in more productive and less variable adult stocksthan would otherwise be the case (Kraus and Secor 2005).

Beyond their nursery ground value, tropical and subtropicalestuarine wetlands harbour a diversity of commercially, tra-ditionally and recreationally valuable fishes and invertebrates(Barbier and Strand 1998; Paterson and Whitfield 2000; Leyet al. 2002), and have considerable traditional (Anon 2004), aes-thetic (Anon 2005), and conservation value (Barbier et al. 2002).The harsh and variable environment in which these pools exist,coupled with anthropogenic pressures (water extraction, farmingand development), incipient climate change and their extensiveecological, fisheries and aesthetic values, make detailed under-standing an important research priority. Despite this, Australia’stropical and subtropical estuarine wetland pools are poorly stud-ied, with the few detailed studies (Russell and Garrett 1983,1985; Hyland 2002) concentrating mainly on their value for thecommercially important centropomid fish, Lates calcarifer.

In many parts of the world tropical and subtropical estu-arine wetlands comprise highly connected, lushly vegetatedswamps. However, much of Australia’s tropical and subtrop-ical zone receives low (<1200 mm year−1), highly variable,highly seasonal rainfall (Bureau of Meteorology), leading towetlands situated in dry schlerophyl forests, salt marshes or saltpans. For much of the time such dry tropical and subtropicalestuarine wetlands comprise isolated pools that may remain dis-connected from the estuary and freshwater systems for yearsat a time (Hyland 2002). This irregular connectivity results in

© CSIRO 2007 10.1071/MF06246 1323-1650/07/100931

932 Marine and Freshwater Research M. Sheaves et al.

Keppel Bay

Fitzroy River

Frogmore Lagoon

Woolwash Lagoon

Little German Jack’s Lagoon

12 Mile Creek

Gonong Creek

Munduran Creek

Rockhampton

N

23°30�S

151°00�E

10 km

The ‘N

arrows’EASTERN

AUSTRALIA

Fig. 1. Locations of the Fitzroy Estuarine Wetland pools.

varied conditions of pool chemistry ranging from fully fresh tohypersaline, depending on the balance between fresh and marineinputs and the period of time between connections. Such varia-tion in physical conditions, and irregular connectivity, providesthe potential for substantial faunal difference between wetlandpools.

To test the extent to which the variable physical conditions indry tropical and subtropical estuarine wetlands are reflected inbiological variability, the present study investigates the degreeto which salinity, connectivity and pool size influence the tax-onomic and trophic compositions of the fish fauna of a broadcross section of estuarine wetland pools adjacent to the estu-ary of the Fitzroy River, in north-east Queensland, Australia. Itshould be noted that the present study was conducted during adrought period, with no substantial wet season flows during theproject. Consequently, the results of the study are not necessarilyrepresentative of the situation during wet years.

Materials and methodsStudy areaThe Fitzroy River (Fig. 1) is the largest coastal river system incentral and southern Queensland, Australia, draining a catch-ment of more than 140 000 km2. The Fitzroy River estuary isintersected by the Tropic of Capricorn, and is ∼65 km in length,

with its upper boundary defined by a barrage constructed in 1970at the city of Rockhampton to prevent ingress of saltwater toareas further upstream (Kowarsky and Ross 1981). Interspersedacross the floodplain adjacent to the estuary are a variety ofsemi-permanent wetland pools that are intermittently connectedto the estuary, and that are the focus of the present study. Thesepools represent persistent remnants of extensive shallow-waterwetland nursery habitats that cover much of the Fitzroy Riverfloodplain during major flooding.

Seven pools adjacent to the Fitzroy River estuary, that couldbe accessed reliably (depending on road conditions and will-ingness of land owners), and without major human-constructedimpediments (dams, bund walls, etc.), were included in the study.In addition, sampling was extended to include three pools in theMunduran Creek, which flows into the ‘Narrows’, a body ofwater connecting the Fitzroy River with Port Curtis (Fig. 1).These pools provided spatial replication of a highly connected,‘marine’ pool, as well as spatial replication of pools in anupstream sequence within one system.

Connectivity varied greatly among the 10 pools (Table 1),with some pools directly connected to the estuary on the highesttides, and others only connected to the estuary indirectly duringmajor flooding or as a result of local flooding following storms.Consequently, the set of pools provided: (1) a variety of salinityregimes owing to differences in regularity of freshwater input and

Fish fauna of dry subtropical wetlands Marine and Freshwater Research 933

Tab

le1.

Phy

sica

lnat

ure

ofth

eF

itzr

oyE

stua

rine

Wet

land

pool

sA

ppro

xim

ate

max

imum

pool

dim

ensi

ons

(len

gth

×w

idth

×de

pth)

inpa

rent

hese

sbe

low

pool

nam

e

Sit

eP

hysi

cals

etti

ngL

and-

use

and

bank

side

Reg

ular

ity

ofR

egul

arit

yof

tida

lU

sual

sali

nity

regi

me

vege

tati

onfr

eshw

ater

inpu

tco

nnec

tion

toes

tuar

y

12M

ile

Cre

ekbr

acki

shPa

rtof

ase

mi-

Gra

zing

,sal

tmar

shR

egul

arst

ream

flow

Hig

hest

spri

ngti

des

Bra

ckis

h(8

00×

15×

4m

)pe

rman

ents

trea

m(S

poro

boli

svi

rgin

icus

)(l

ocal

stor

ms)

(tw

oto

four

tim

esye

ar−1

)(r

ange

0.7–

36.0

L)

(mea

n10

.9±

3.1

s.e.

)

12M

ile

Cre

ekdo

wns

trea

mPa

rtof

ase

mi-

Gra

zing

Reg

ular

stre

amfl

owIn

dire

cton

lyFr

esh

(1km

×10

m×

3m

)pe

rman

ents

trea

m(p

astu

re,E

ucal

yptw

oodl

and)

(loc

alst

orm

s)(r

ange

0–3.

9L

)(m

ean

1.8

±0.

6s.

e.)

12M

ile

Cre

ekup

stre

amPa

rtof

ase

mi-

Gra

zing

Reg

ular

stre

amfl

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dire

cton

lyFr

esh

(330

×7

×4

m)

perm

anen

tstr

eam

(pas

ture

,Euc

alyp

twoo

dlan

d)(l

ocal

stor

ms)

(ran

ge0–

3.8

L)

(mea

n1.

5±

0.5

s.e.

)

Gon

ong

Cre

ekm

arin

ePa

rtof

ase

mi-

Gra

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estr

yR

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flow

Mos

tspr

ing

tide

sM

arin

e(4

50×

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2m

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rman

ents

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m(E

ucal

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oodl

and,

(loc

alst

orm

s)(∼

25ti

mes

year

−1)

(ran

ge0.

5–47

.7L

)m

angr

ove)

(mea

n31

.0±

4.2

s.e.

)

Mun

dura

nC

reek

mar

ine

Part

ofa

sem

i-G

razi

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try

Reg

ular

stre

amfl

owM

osts

prin

gti

des

Mar

ine

(350

×18

×2

m)

perm

anen

tstr

eam

(Euc

alyp

twoo

dlan

d,m

angr

ove)

(loc

alst

orm

s)(∼

25ti

mes

year

−1)

(ran

ge0.

2–54

.0L

)(m

ean

20.8

±4.

2s.

e.)

Mun

dura

nC

reek

dow

nstr

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Part

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i-G

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ng,f

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try

Reg

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stre

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lyFr

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(60

×20

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perm

anen

tstr

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(Euc

alyp

twoo

dlan

d)(l

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(ran

ge0–

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Part

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(10

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perm

anen

tstr

eam

(Euc

alyp

twoo

dlan

d)(l

ocal

stor

ms)

(ran

ge0–

1.4

L)

(mea

n0.

3±

0.1

s.e.

)

Frog

mor

eL

agoo

nIs

olat

edL

agoo

nG

razi

ngN

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duri

ngst

udy

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rect

only

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h(2

km×

80m

×5

m)

(Euc

alyp

twoo

dlan

d,pa

stur

e)(r

ange

0–0.

6L

)(m

ean

0.4

±0.

1s.

e.)

Woo

lwas

hL

agoo

nIs

olat

edL

agoo

nG

razi

ngN

one

duri

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Indi

rect

only

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h(2

km×

80m

×3

m)

(Euc

alyp

twoo

dlan

d,pa

stur

e)(r

ange

0–2.

1L

)(m

ean

0.9

±0.

6s.

e.)

Lit

tle

Ger

man

Jack

’sL

agoo

nIs

olat

edL

agoo

nG

razi

ng,s

altm

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Non

edu

ring

stud

yH

ighe

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ring

tide

sH

yper

sali

ne(4

00×

20×

1m

)(S

poro

boli

svi

rgin

icus

,man

grov

e)(f

our

toei

ghtt

imes

year

−1)

(ran

ge9.

1–13

3.4

L)

(mea

n44

.6±

19.1

s.e.

)


Table 2. Number of 18 mm cast net samples per site, per trip

Site Sampling time

Feb 2004 May 2004 Jul 2004 Nov 2004 Feb 2005 May 2005

12 Mile brackish 52 50 53 82 103 6012 Mile downstream 41 28 13 50 3712 Mile upstream 22 20 23 18 31 16Frogmore 31 76 206 120 120 60Gonong marine 35 51 52 51 48Little German Jack’s 20 50Munduran marine 44 55 64 72 62Munduran downstream 5 5 5Munduran upstream 4 4Woolwash 50 66

tidal connection (Table 1); (2) two sets of pools in an upstreamsequence (12 Mile and Munduran Creeks); and (3) pools thatwere part of semi-permanent streams as well as those that wereisolated lagoons. Additionally, the nature of connections to theestuary varied among pools with direct marine connections.Gonong and Munduran marine were separated from the estu-ary by short (<10 m) rock and rubble bars that were overtoppedon most spring tides. Little German Jack’s was connected by ashort (50 m), poorly defined channel during the highest springtides (four to eight times per year) and by sheet flow acrossthe salt pan during extreme high tides. 12 Mile brackish had nodefined connecting channel, only connecting to the estuary bysheet flow across a broad (∼3 km) salt pan during the highestspring tides (two to four times per year).

SamplingSampling was conducted between February 2004 and May 2005(Table 2). Only three sites were sampled during the initialsampling trip in February 2004 when field techniques werebeing evaluated and refined. Most sites were sampled repeat-edly from May 2004 to May 2005, with sampling time alignedwith the ‘wet’(February), ‘post-wet’(May), ‘dry’(July) and ‘pre-wet’ (November) seasons. Extremely shallow water depths andextremely muddy bank conditions prevented sampling fish atMunduran upstream and downstream, and Little German Jack’son several occasions, although water quality samples could becollected. Sampling was conducted less frequently at WoolwashLagoon because of the similarity of its fauna to that of the adja-cent Frogmore Lagoon over the early part of the study. On eachsampling occasion, salinity, water temperature, turbidity, pH andmaximum depth were recorded for each pool.

Many sampling gears were considered for the project but fewwere appropriate for comparative quantitative sampling acrossthe range of salinities and variety of habitat types encountered.Three gear types (gill-nets (four mesh sizes), cast nets (twomesh sizes), fish traps and scoop nets), likely to be useful acrossthe broad range of habitats present, were trialled at three sites(Frogmore Lagoon, 12 Mile Creek brackish, 12 Mile Creekupstream) during the initial sampling trip.

Large (18-mm) mesh cast nets were clearly the superior sam-pling gear of those trialled (Sheaves et al. 2006). Although

gill-nets had the potential to capture large fish that might bepoorly represented by cast nets, catch rates of such fish werevery low in gill-nets. Additionally, for all three sites large-meshcast nets captured more species than the four gill-net mesh sizes(two nets of each of four mesh sizes (25, 50, 100, 200 mm)with three 2-h daytime soaks and a single night time soak) com-bined. With one exception, profiles of relative abundance, forthe species captured by the two gears, were very similar for bothgears. At Frogmore Lagoon the highly abundant Nematalosaerebi (Clupeidae) was caught in much higher relative abundancein 18 mm cast nets, apparently the result of N. erebi aggregatingin the deep central areas of the lagoon where deep water madegill-net sampling ineffective. Small mesh (6 mm mesh) cast netsdid catch fish below the lower size limit of the larger 18 mm meshcast net but added no additional species, represented larger sizeclasses poorly, and were much more time consuming to use.Although fish traps are useful for sampling a broad range ofhabitats, including many inaccessible to other gears (Sheaves1992) catch rates were low relative to cast nets, and they had thepropensity to capture and kill potentially endangered tortoises.

Overall, compared with other gears large-mesh cast nets wereless taxonomically selective, produced equivalent or better repre-sentations of the fauna (richness, abundance and size structure),provided many discrete replicate samples per unit effort, werelogistically simpler, produced more data per unit time, wereusable across the full range of habitats available (except for thoseheavily vegetated with water plants or among fallen timber – noother gear was efficient in these), and eliminated collateral mor-tality common with other gears. Additionally, no other gearsproduced species not captured in the 18 mm cast nets, so addedno additional information on species richness. Consequently,large-mesh cast nets were the sole gear used on subsequent tripsbecause it was considered more profitable to allocate extra effortto increased replication with a single gear type. The use of large-mesh cast nets represents a ‘many-but-small’ sampling strategy(McCune and Lesica 1992). Compared with a ‘few-but-large’approach, such a strategy represents a trade-off that producesrelatively accurate abundance estimates for common speciesbut a relatively poor representation of rare species (and there-fore a relatively incomplete species list) (McCune and Lesica1992). However, although total species richness per site is likelyto be underestimated, estimates of average species per sample


are likely to be precise. These features are reflected in samplesfrom the most species rich location × trip combination (12 Milebrackish, May 2005). One hundred random permutations of totalabundance showed 20 nets produced an estimate of mean totalfish abundance with a standard error 4.9% of the mean. Forspecies richness, on average 20 cast nets returned an estimated9.7, and 40 nets 13.3, of the 16 total species captured (basedon 9999 random permutations (Species Accumulation routine,PRIMER 6)).

Cast net sampling methods and protocolsAt least 50 cast net samples (18 mm mesh × 4.29 m diameter)were collected from each site whenever there was sufficient areafor 50 independent samples. In some cases the area availablebecame limited as the pool dried, in other cases the presenceof water lilies, Nymphaea caerulea (Nymphaeaceae), or theinvasive aquatic weed, Cabomba caroliniana (Cabombaceae),reduced the area that could be sampled. Although data from siteswith low replication extends the range of pools investigated, thelow number of replicates means the data should be treated withcaution: abundant species are likely to be represented reasonablywell, but less common species and species richness are likelyto be underrepresented. In each pool, sampling effort was dis-tributed as evenly as possible over the whole area, with samplescollected over the full range of depths from edges to pool centres.

Cast netting was conducted from a small boat fitted with anelectric motor to minimise site disturbance. Cast net sampleswere not used if: (1) a net throw did not sample greater thanan estimated (by eye) 85% of the actual maximum sample area;(2) the net became snagged on any structure; (3) it appearedthat a site was disturbed during boat positioning; or (4) if thenet operator’s shadow encroached onto the proposed samplingarea. Any variability in sampling volume (i.e. differences in thesurface area of net throws), was randomly distributed throughoutthe study, therefore any biases added variability to the data in arandom manner. In theory the maximum sample area for thecast nets was 14.45 m2 for the 18 mm mesh (4.29 m diameter)net, but measurements from net throws on land indicated thatthe actual sampling area was considerably below the theoreticalmaximum for each net but was not highly variable (net meanarea = 7.35 m2 (s.e. = 0.40, n = 50)).

Data and statistical analysesMost fish captured were released unharmed, except for thoseretained for dietary studies and to develop length-weight rela-tionships. The fish that were retained (Ethics approval number:A852_03) were euthanased in an ice-water slurry and returnedto the laboratory for dissection. Raw abundances of fish per netwere averaged to produce the basic data for multivariate analy-sis: mean numbers of each species of fish per 100 cast nets, foreach wetland pool, on each sampling trip.

The number of cast nets that could be collected varied greatlybetween sites and over time. To correct for this, species richnesswas standardised to that expected in 20 nets using mean estimatesof species accumulation curves (Ugland et al. 2003) estimatedusing random permutations (PRIMER-E v6). Only site × tripcombinations with 20 or more nets were considered in rich-ness comparisons. Estimated species richness was then analysed

using univariate classification and regression trees (Breimanet al. 1984; De’ath and Fabriscius 2000; De’ath 2002) (CART),with location, trip, salinity, turbidity, water temperature, pH, poolsurface area and maximum pool depth as explanatory variables,using TreesPlus software. CARTs recursively partition data intoincreasingly homogeneous subsets. Splits close to the root of thetree are typically more important (yield greater improvement inthe fit of the model) than those that are closer to the bottom of thetree. In the current situation, with a numeric dependent variable,split selection was based on minimising within-group sum-of-squared residual deviation of the resultant groups. Selection ofthe final tree models was conducted using 10-fold cross valida-tion, with the 1-s.e. tree (the smallest tree with cross validationerror within 1 s.e. of that of the tree with the minimum cross val-idation error) selected as the final tree model. This procedure isgenerally considered to produce valid, biologically interpretabletrees (Breiman et al. 1984; De’ath and Fabriscius 2000; De’ath2002; Urban 2002).

Cast net density data were log(x + 1) transformed to down-weight the influence of very common species (allowing lesscommon species some influence on analytical outcomes), andanalysed using non-metric multidimensional scaling (nMDS)based on Bray–Curtis dissimilarities. Preceding analysis, theoriginal data matrix was reduced to include only species occur-ring in more than 10% of samples, to remove any undue influenceof rare species on the analysis (Gauch 1982). Differences in den-sity owing to pool identity and sampling trip were analysed usingANOSIM (PRIMER-E v6). Coherent groups in the multivariatespace were identified using the SIMPROF routine (PRIMER-Ev6), and rank correlation between the multivariate densitydata and salinity, water temperature, turbidity, pH, maximumdepth and pool surface area evaluated using the BEST routine(PRIMER-E v6). Information on the strength and nature of thecorrelation of species with the nMDS space was added to theordination biplot as vectors indicating the direction of greatestincrease in density of species most highly correlated with thespace. The directions of these vectors was determined by regres-sion of each species on the nMDS space, with the length ofvectors reflecting the R2 value for each regression, thus indi-cating the strength of correlation. For more detailed temporalcomparison within a single system, a second nMDS ordinationwas conducted concentrating on just the three 12 Mile Creeksites.

Length–weight relationships were used to convert abun-dances into biomass estimates. Length–weight relationships didnot vary over space and time for any species, consequently, datafor sites and trips were pooled to produce an overall length–weight relationship for each species. Biomass estimates werebiased in respect to smaller individuals that are not well repre-sented in the 18 mm cast nets samples, however, initial samplingusing a 6 mm cast net suggested that smaller fish were not in highdensities at any site so probably would not contribute greatlyto biomass. Overall biomasses for trophic groups included allspecies. Percentage contribution to trophic groups was calcu-lated for all species with average biomass >5 g at any site onany trip.

Trophic groups were defined as follows based on gut con-tent analysis of 200 individuals per species, or as many aspossible if less were available: Benthivores: fed on sedentary


Tab

le3.

Sum

mar

yof

fish

capt

ured

at10

wet

land

pool

site

sT

roph

icgr

oups

,as

defi

ned

inth

ete

xt,a

rein

dica

ted

inbr

acke

tsaf

ter

spec

ies

nam

es(B

:ben

thiv

ore,

D:d

etri

tivor

e,E

:epi

bent

hivo

re,H

:her

bivo

re,O

:om

nivo

re,P

l:pl

ankt

ivor

e,P

:pre

dato

r)

Taxo

n12

Mil

e12

Mil

e12

Mil

eFr

ogm

ore

Gon

ong

Lit

tle

Ger

man

Mun

dura

nM

undu

ran

Mun

dura

nW

oolw

ash

brac

kish

dow

nstr

eam

upst

ream

Lag

oon

mar

ine

Jack

’sm

arin

edo

wns

trea

mup

stre

amL

agoo

n

Aca

ntho

pagr

usau

stra

lis

(O)

14

10A

rius

grae

ffei

(O)

2819

Am

bass

issp

p.(E

)5

1114

79

107

6A

mni

atab

ape

rcoi

des

(O)

8169

Arr

ham

phus

scle

role

pis

(H)

133

Cha

nos

chan

os(D

)2

Cra

tero

ceph

alus

ster

cusm

usca

rum

(E)

730

105

2A

ngui

lla

rein

hard

tii

(O)

31

Elo

psha

wai

ensi

s(P

)18

1E

leot

rid

sp.A

(E)

11

6G

loss

amia

apri

on(P

)27

32

Ger

res

eryt

hrou

rus

(B)

113

Ger

res

fila

men

tosu

s(B

)14

8812

51

Gob

idsp

.A(E

)1

Car

assi

usau

ratu

s(D

)2

Her

klot

sich

thys

cast

elna

ui(P

l)13

16H

ypse

leot

ris

sp.(

E)

12

33

Lat

esca

lcar

ifer

(P)

171

32

Lei

ogna

thus

deco

rus

(B)

231

14L

eiog

nath

useq

uulu

s(B

)1

1123

Lut

janu

sar

gent

imac

ulat

us(P

)1

3L

utja

nus

russ

elli

i(P

)2

Liz

asu

bvir

idis

(D)

6921

414

273

7L

eiop

othe

rapo

nun

icol

or(O

)1

1112

Meg

alop

scy

prin

oide

s(P

)2

26

12

Mel

anot

aeni

asp

lend

ida

(E)

413

5713

1M

onod

acty

lus

arge

nteu

s(E

)1

11

1M

ugil

ceph

alus

(D)

384

54

4230

86

Nem

atal

osa

com

e(D

)3

10N

emat

alos

aer

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benthic prey (mainly infaunal invertebrates), Epibenthivores:fed on mobile prey living on or near the bottom (mainly epi-faunal invertebrates), Predators: fed on large mobile prey (fishand mobile macro-invertebrates), Herbivores: fed principally onliving plant material, Detritivores: fed principally of decom-posing organic material, Omnivores: consumed prey from abroad range of trophic groups, Planktivores: fed principally onmicro-invertebrates in the water column. Although the biomassdata were investigated using similar approaches to those usedfor the density data, the important patterns were so distinctthat the presentation of complex statistical analyses is notwarranted.

Results

Over the course of the study 6123 fish from 44 species werecaptured in 18 mm mesh cast nets from the 10 wetland pools(Table 3).

Species richnessAll pools had reasonably low numbers of species with a max-imum total richness of 25 species at Munduran marine, andthe highest in any one sampling trip of 17 at Gonong marine.Comparison of standardised species richness (Fig. 2) showed

Salinity � 6.1

SA � 11000 Salinity � 31.6

Salinity � 6.1

SA � 11000 Salinity � 31.6

3.87 6 7.04 9.34

71.9% variance explained

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e (a

ll)W

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undu

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ong

m (

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dura

n m

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l, N

ov 0

4, M

ay 0

5)

Fig. 2. Classification and regression tree for estimated species richnessper 20 nets (from permuted species accumulation) for Fitzroy EstuarineWetland pools. Bar graphs below terminal branches indicate relative dis-tribution of species richness; numbers indicate mean estimated speciesrichness. Site codes 12 M b = 12 Mile brackish, 12 M down = 12 Miledownstream, 12 M up = 12 Mile upstream, Gonong m = Gonong marine,Munduran m = Munduran marine.

that variations in species richness were mainly attributable tosalinity and pool surface area, with water temperature, turbid-ity, pH and maximum depth having low explanatory power.Low salinity sites (<6.1‰) had low species richness. Theseincluded the freshwater lagoons and in-stream freshwater pools,as well as the 12 Mile brackish and Munduran marine poolsin February 2005 when salinities were low following flooding.Among the low salinity sites larger pools tended to have lowerspecies richness than smaller pools. Among the higher salinitysites (>6.1‰) species richness was greatest when salinity washighest (>31.6‰). The overall pattern was reflected at 12 MileCreek brackish, the site with the most variable salinity, withestimated richness varying from 3.5 when salinity was lowestin February 2005, through to 9.7 when salinity was greatest inMay 2005.

Patterns in fish composition and densityANOSIM indicated clear differences in composition amongpools (Global r = 0.801, P < 0.0001) but no clear differenceamong trips (Global r = 0.087, P = 0.19). nMDS ordination(Fig. 3a) shows clear groupings of samples (SIMPEROF atP = 0.01), mainly related to the pool identities. Samples from theGonong and Munduran marine pools formed a distinct group,featuring high densities of Gerres filamentosus (Gerreidae)and Liza subviridis (Mugilidae). The two Munduran freshwa-ter pools also formed a distinct group with high densities ofMugil cephalus (Mugilidae). Similarly, the two 12 Mile Creekfreshwater pools formed a discrete group, with the exception ofthe May 2005 sample from the downstream pool, which groupedwith the adjacent cluster, and included all of the samples fromthe large isolated freshwater Frogmore and Woolwash Lagoons.These two freshwater groups where characterised by high densi-ties of the freshwater clupeid Nematalosa erebi. Except for theFebruary 2005 sample, 12 Mile Creek brackish pool samplescomprised a discrete group occupying the central part of theordination, reflecting a combination of marine and freshwaterspecies. The two Little German Jack’s samples were outlierscompared with the other clusters reflecting a low diversitymarine faunal composition. The BEST routine indicated thatsalinity (Spearmans rs = 0.528) was the only variable highly cor-related with the pattern of biological similarities (Spearmans rs:temperature −0.051, depth 0.136, pH 0.160, turbidity −0.072,surface area 0.282). Not surprisingly, the ordination separationbetween marine, brackish and freshwater sites is reflected insalinity (Fig. 3b).

nMDS ordination concentrating on just the three sequential12 Mile Creek sites (Fig. 4) shows an interesting spatio-temporalpattern. For most sampling trips the fauna of the two freshwaterpools were similar to each other, with high densities of freshwa-ter species such as Melanotaenia splendida (Melanotaeniidae),but quite different to assemblages in the 12 Mile brackish pool,where marine spawning species such as Lates calcarifer (Lati-dae), L. subviridis, Elops hawaiensis (Elopidae), and Selenotocamultifacsiata (Scatophagidae) dominated (Fig. 4a). Over timethe fauna of the two freshwater pools tended to shift (largegrey arrow Fig. 4b) gradually to lower densities of the dominantM. splendida. In contrast, the composition of the assemblage ofthe brackish pool was relatively stable for most of the sampling


Dimension 1

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ension 2

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Little German Jack’s

Munduran marine

Munduran downstream

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L. subviridis

G. filamentosus

MARINE

12 MILEBRACKISH LARGE

ISOLATEDFRESH

12 MILEFRESHWATER

MUNDURANFRESHWATER

(a)

(b)

Fig. 3. Multi-dimensional scaling (MDS) ordination, using Bray–Curtis dissimilarities on log(1 + x) transformeddata (Stress 1 = 0.11), comparison of fish density per net, from 18-mm mesh cast nets. (a) Ordination of sites foreach trip. Vectors radiating from the intersection of axes relate to species abundances most highly correlated with theordination space. Vectors indicate the direction of greatest increase in abundance, with length proportional to R2 (largestL. subviridis R2 = 0.784). (b) Bubble plot with bubbles proportional to log10 salinity (highest salinity = 54.6‰).

period. However, there was a clear compositional shift in Febru-ary 2005 with an assemblage similar to those in the freshwaterpools (Fig. 4a, b), but the typical assemblage was re-establishedby May 2005. This change was correlated with changes in thepercentage contributions of marine fish over time (Fig. 5a). Thepercentage of marine spawners was consistently low in the twofreshwater pools. In contrast, marine spawners dominated in thebrackish pool for most of the study period, as would be expected

given its more direct connection to the estuary. In February 2005there was a major change to the pattern with the percentage ofmarine spawners similar in all three pools. This change followedextensive flooding in late 2004 and early 2005, and was the jointresult of a major increase in the abundance of the freshwaterspawning N. erebi (Fig. 5b) in the brackish pool (apparentlydisplaced downstream during flooding), and decreases in theabundance of the major marine spawning species (Fig. 5c, d, e, f)


BRACKISH

FRESH

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ension 2

M. cephalus

L. subviridis

S. multifasciata

12 Mile brackish

12 Mile downstream

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L. calcarifer

(b)

Dimension 1

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July 04

Nov 04

Feb 05

May 05May 04

July 04Nov 04

Feb 05

May 05

Feb 04

May 04

July 04

Nov 04

Feb 05

May 05

(a)

Fig. 4. Non-metric multidimensional scaling (nMDS) ordination using Bray–Curtis dissimilarities onlog(1 + x) transformed data comparing density of fish in 18 mm mesh cast nets from 12 Mile Creekpools. Stress 1 = 0.09. (a) Ordination of sites for each trip with dashed lines enclosing overlapping freshsites and distinct brackish sites. Vectors radiating from the intersection of axes indicate the direction ofgreatest increase of density of the most important species. Vector lengths are proportional to R2 (largestLates calcarifer R2 = 0.803). (b) Ordination showing trip designations. Large grey arrow indicates thedirection of the faunal shift over time at the two freshwater sites, the dotted arrows the temporal shift at thebrackish site.

(possibly as a result of migration to the estuary or into floodedfringe habitats).

Throughout the study trophic group biomass at all sites wasdominated by detritivores (Fig. 6). This pattern varied littlebetween trips, with the largest variation an increase in the pro-portion of detritivores in 12 Mile Creek brackish in February

2005 owing to increased numbers of N. erebi (see above). Theprincipal detritivores varied between pool types, N. erebi infreshwater pools, and M. cephalus and L. subviridis in marinepools. The biomass of predatory fish was relatively low inmost pools. However, predators (principally L. calcarifer andE. hawaiensis) accounted for 17% of the biomass in the 12 Mile


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Fig. 5. (a) Proportional contribution of marine fish to catches in large castnets for three 12 Mile Creek pools, (b–f) change in density over time of themajor species in the brackish pool.

brackish pool, and 11% of the biomass in Frogmore Lagoon(Strongylura krefftii (Belonidae) and L. calcarifer). Similarly,omnivores (principally Arius graeffei) were important in Frog-more and Woolwash Lagoons the two large, isolated freshwaterlagoons, benthivores (principally Acanthopagrus australis, Ger-res erythrourus (Gerridae) and G. filamentosus) in Gonong andMunduran marine pools, and herbivores in 12 Mile brackish(Selenotoca multifasciata).

DiscussionPatterns of species richnessDespite consisting of 10 pools of a variety of types, includ-ing some highly connected with the estuary, the total speciesrichness of fish from the Fitzroy Floodplain Wetland pools waslow, with only 44 species recorded from all the pools over theentire study. This is surprising because tropical and subtropical

estuarine faunas in the same biogeographic region typically havehigh species richness (91–128 species) compared with thosein other parts of the world (Robertson and Blaber 1992). Thislow richness was also reported in a previous study of wetlandpools in the Fitzroy region (InfoFish 2005), and is consistentwith low species richness reported for the Fitzroy River (Longand McKinnon 2002; R. Johnston, unpubl. data). The overalllow species richness of Fitzroy Floodplain Wetland pools wasreflected at the individual pool level, with a maximum total rich-ness of 25 species at Munduran marine, and a maximum for anyone sampling trip of 17 at Gonong marine.

Variability in species richness (corrected for sample size) wasmainly attributable to salinity and pool surface area, whereaswater temperature, turbidity, pH and maximum depth had lowexplanatory power. The effects of salinity are understandable,with brackish and marine pools, accessible to a diverse estuar-ine fauna (Robertson and Blaber 1992), having highest richness.In these pools richness declined when salinities were depressedas a result of flooding, which apparently excluded species lesstolerant to low salinity conditions. Similarly, the consistent lowspecies richness in freshwater sites is probably a reflectionof a smaller available species inventory (Allen et al. 2002).Large freshwater pools tended to have less species than smallerfreshwater pools, the opposite of what would be expectedfrom species-area theory (McArthur and Wilson 1967; He andLegendre 1996).This anomaly can probably be explained by con-nectivity, with the two large lagoons isolated from other systemsfor most of the study, whereas the smaller freshwater pools werecomponents of streams that connected to other freshwater poolsand to the estuary several times during the study.Thus any loss ofspecies (e.g. owing to predation) in the large lagoons could notbe replaced, whereas regular connectivity in the smaller poolswould have allowed repeated exposure to sources of individualsof a range of species.

Community compositionDifferent pools had distinctly different faunas. These differenceswere strongly correlated with salinity, whereas other physi-cal variables (temperature, depth, pH, turbidity, surface area)appeared to have little impact on faunal composition. At thebroadest level these differences in composition were a func-tion of the proportion of the community comprised of marinespawned species compared with freshwater spawned species.

The distinctly different natures of the fish fauna of differentpools was a reflection of the extent of connectivity to freshwa-ter and marine systems. This connectivity determined both thephysical nature of pools (principally the salinity regime) and thesources of faunal supply; pools with more direct connectionsto the marine environment had extensive marine components,pools without direct connections to the marine environment weredominated by freshwater fish. Compared with differences amongpools the faunas of individual pools were stable over time, andwhen changes did occur they again reflected patterns of connec-tivity. Regular connections to the marine environment allowedfree access of marine fish to Gonong and Munduran marinepools resulting in faunal compositions similar to those in themain estuary. In contrast, both large and small freshwater poolsthat lacked direct connection to the marine environment hadmore restricted and less diverse faunas (see above). Thus the


Ben

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Fig. 6. Comparison of proportions of fish biomass in 10 wetland pools contributed by various trophicgroups. Error bars are ±1 s.e.

stability of fauna in these isolated pools appears to be a func-tion of their isolation, with resetting occurring episodically inresponse to connection events. As a consequence, hydrologicalconnectivity drives faunal composition in these dry tropical estu-arine wetland pools, and even small alterations in connectivity(e.g. human intervention or global climate change) are likely tohave profound impacts on the nature of pool fauna and how theychange over time.

Trophic dominanceThe fish fauna of Fitzroy Wetland pools were dominated bydetritivores. This dominance prevailed regardless of pool type,size, season or connectedness. The pervasive impact of detriti-vores provides a clear indication of the basic similarity of thepools’ ecosystems and ecosystem processes. The identity of the

detritivores varied between pool types, with Nematalosa erebidominating in freshwater pools, and Mugil cephalus and Lizasubviridis in marine pools. All of these were relatively largespecies, despite their low trophic position. In contrast, parallelstudies of the invertebrate fauna of the pools (J. Sheaves, unpubl.data) showed relatively low biomasses of all trophic groups.Although by no means unique, this situation is unusual. In manymarine systems basal food resources (plants or detritus) are fedon primarily by small individuals (Robertson et al. 1992), ofteninvertebrates (Robertson 1991), which are in turn the food oflarger prey.

The trophic domination by large detritivores has far reach-ing consequences. The large biomass of detritivorous fish andthe low density of other primary consumers (herbivorous fish,detritivorous or herbivorous invertebrates), suggest that most


of the biological production in the pools is based on detritusrather than terrestrial plants. This reliance on detritus, despitethe type of pool or pool setting (forest, saltpan, pasture, etc.),suggests strongly that there is considerable redundancy, with theecosystems not critically reliant on one type of primary pro-ducer but able to use whatever sources of carbon are available tomaximise productivity.The direct use of a basal food resource byrelatively large animals means that food webs leading to preda-tors (fish and birds) are short and simple. Consequently, energyis transferred efficiently to organisms at the top of the foodweb with a minimum of thermodynamic loss. The conversionof detritus to a large biomass of fish provides the opportunityfor large amounts of productivity to be exported from the pools,either through migration of the detritivores themselves or themovement of the predators that consume them. This situationis quite different to our understanding of many other marinewetland systems in northern Australia, where tight recyclingretains organic carbon in mangrove forests (Robertson 1986;Robertson et al. 1992). High predatory impact on these detriti-vore populations by birds (W. Houston, pers. comm.) means thatcarbon isn’t only exported to marine or upstream systems duringstream flow or marine connection events, but is transported tophysically unconnected pools and terrestrial systems through themovements of birds.

Despite the overall domination of detritivores the pools couldbe organised into groups based on their trophic compositions. Infreshwater pools domination by detritivores was strong. In con-trast, the more diverse marine pools (Gonong and Munduran)had a small but significant component of benthic feedingfish, whereas the 12 Mile brackish pool was distinguished byrelatively high biomasses of predators and herbivores.

AcknowledgementsThis project was supported by a grant from the Cooperative Research Centrefor Coastal Zone, Estuary and Waterway Management. We thank R. Baker,A. Penny, L. Ortiz Guerra, B. Packett and P. Verwey for their valuable fieldassistance. Our thanks also go to the anonymous reviewers who providedextensive comments that greatly improved the manuscript.

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Manuscript received 22 May 2007, accepted 13 September 2007

Documents

Fish fauna of dry tropical and subtropical estuarine floodplain wetlands