Alyssa Marshell & Peter MumbyMarine Spatial Ecology Lab

School of Biological Sciences12th ICRS Cairns, July 2012

The ecological role of grazing surgeonfish on Heron Island reef

© Mark Priest

• Main consumers of primary production (50 - 100% consumed daily)

• Reduce algal establishment, growth & competition with corals

• Increase availability of coral recruitment substrate

• Structure benthic communities

• Herbivory = key ecological process- important for reef resilience

Essential to understand species-specific roles

Herbivorous coral reef fishes

• Widespread, common & abundant

• Mostly herbivores (27/41 GBR species)

• Little known of ecological roles of grazer/detritivores- foraging morphology/strategies- interspecific grazing relationships- diet/trophodynamic analyses

Acanthuridae (surgeonfishes, unicornfishes, tangs)

-> investigate roles of key grazing surgeonfish spp.

Acanthurus nigrofuscus - Brown surgeonfish Grazer - Epilithic microalgae (crops)

Ctenochaetus striatus – Lined Bristletooth surgeonfish Detritivore - Detritus & sediment (brushes)

Common Indo-Pacific surgeonfish species

Sparse/Younger

Dense/Older

vs.

Both feed upon epilithic algal matrix Grazing impact on algal turfs?

Objectives:• Individual impact = bite size/volume x # of bites per day• Population impact = mean abundance x individual impact

-> Aquaria experimental trials – bite size/volume & grazing impact-> Field surveys – abundance, biomass, bite rates

Grazer Detritivore

n = 9, χ = 16.2cm n = 8, χ = 19.7 cm

-> Algal turf biomass per bite/bite size-> Grazing impact on algal turfs

BEFORE & AFTER trialMean algal turf biomass (g C/cm-2)

Algal turf biomass per bitemean biomassbefore – mean biomassafter

no. of bites cm-2

Bite size – measure grazing scars

Methods – aquaria experimental trials

Sparse/short

Dense/long

vs.

Methods - survey sites

Heron Island, southern GBR

Six sites - 3 windward, 3 leeward - Deep (12m), Shallow (4m)

Methods – field surveysSix sites

Herbivorous fish surveys• 8 x Deep, 8 x Shallow (30 x 5m)• surgeonfish, parrotfish, rabbitfish• Benthic photo quadrats, rugosity• Morning surveys, at high tide

Key surgeonfish spp. bite observations • 5-min observations• Mean # bites per day

-> Sparse turf preferred

Grazer Detritivore

Number of bites

-> Sparse turf preferred

-> Dense = more turf per bite

Grazer Detritivore

Mean biomass per bite

Detritivore removed more turf than grazer

BEFORE

AFTER

BEFORE

Detritivore

Grazer

Aquaria trials – removed biomass vs.

Sediment removed

Turf removed

*

*

^

^

* p = < 0.001^ p = < 0.004

A. nigrofuscus C. striatus

Ma

ss o

f a

lga

l tur

f re

mov

ed p

er d

ay

(g C

d-1

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4Turf removed (g.C.day-1)

= Bite volume (g) × # of bites/day- sparse turf data

Individual Grazing Impact

0.88 0.07

A. nigrofuscus C. striatus

Ma

ss o

f a

lga

l tur

f re

mov

ed p

er d

ay

(g C

d-1

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4Turf removed (g.C.day-1)

= Bite volume (g) × # of bites/day- sparse turf data

= Bite size (m2) × # of bites/day

Individual Grazing Impact

A. nigrofuscus C. striatus

Sub

stra

te a

rea

graz

ed p

er d

ay (

m2 d

-1)

0.0

0.7

0.8

0.9

1.0Area grazed (m2 day-1 )

(Purcell & Bellwood 1993)

0.9

0.003

0.88 0.07

0.002 ± 0.0003 cm2 0.8 ± 0.03 cm2

177.6° gape112.8° gape

Fish survey data - Herbivorous fish mean biomass (g/m2)

-> Surgeonfish - dominate biomass at shallow depths

Leeward Windward

Deep Deep ShallowShallow

Mea

n ab

und

ance

(in

div

idua

ls/h

a)

-300

-200

-100

0

A. nigrofuscus C. striatus

Ma

ss o

f m

ate

rial r

em

ove

d (

g.C

. h

a-1.d

ay-1

)

0

200

300

Mass removed (g.C.ha-1.day-1)

= Individual impact × mean abundance- Size class 15 - 20cm- Sparse turfs- Shallow reef slope (~4m)

Population Grazing Impact

Abundance (individuals/ha)

239 g2.7 g

v

Summary

Reconsider role of Ctenochaetus spp. in algal turf dynamics

Revaluate exclusion of Ctenochaetus from resilience monitoring

• Most abundant surgeonfish species - detritivorous Ctenochaetus spp.

- previously thought to cause little damage to algal turfs- capacity to remove significant quantities of algal turfs

• Need to quantify Ctenochaetus impact on algal turfs on the reef

Volunteers: Julia Lawson, YY Chang, Chris Doropoulos, George Roff, Mark Priest

Heron Island Research Station Staff

Marine Spatial Ecology Lab

ARC Laureate Fellowship Funding (Peter Mumby)

Photo Credits: Mark Priest, www.hawaiifishes.com, www.guamreeflife.com

Acknowledgements

Marshell, A. Mumby, P.J. (2012) Revisiting the functional roles of the surgeonfish Acanthurus nigrofuscus and Ctenochaetus striatus.

Online First, Coral Reefs

Species

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Most abundant surgeonfishLeeward Deep Leeward Shallow

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