Crown-of-thorns starfish on the Great Barrier Reef
CURRENT STATE OF KNOWLEDGE November 2003 (revised edition)
Outbreaks of crown-of-thorns starfish Acanthaster planci operators, and the Queensland and Commonwealth have been a major issue on the Great Barrier Reef and Governments about $3 million a year for control measures. other Indo-Pacific reefs for nearly 40 years. The outbreaks have generated great concern among the community Despite more than 30 years of research on this very and considerable debate among scientists. complex problem, and the commitment of millions of dollars on research over the last 17 years, the ultimate Outbreaks generally occur at regular intervals with coral causes of the outbreaks are still not clear. However, cover returning to pre-outbreak levels in the intervening there is now a much greater understanding of the years. In 2003, there is an outbreak on reefs between biology of the starfish, the nature of the outbreaks, Cairns and the Whitsundays which is costing tourism and the pattern of recovery of the reefs. Photo by GBRMPA a verylargenumberofoffspring. crown-of-thorns starfishcouldpotentiallyproduce to Crown-of-thorns starfishspawnfromDecember Br up to20minanhour long venomousspines the entireuppersurfaceofitsbodycoveredin any invertebr of-thorns starfisharethehighestmeasuredfor Indeed, increases starfish eggs duringasinglespawningseason. w T 1 mindiameter large starfishandcangrowtomorethan T thorns starfish Biology ofcrown-of- years it is2-3yearsoldandbreedsforfivetoseven A crown-of-thornsstarfishfirstbreedswhen invisible untiltheyareaboutsixmonthsold. rocks andrubbleonthereefarealmost they areabout1-2mmacross juveniles settle(orrecruit)ontothereefwhen drifting asplanktoninoceancurrents into larv disc. he starfishreleaseeggsandspermintothe he crown-of-thornsstarfishisanunusually ater throughporesonthetopoftheircentr April whenw eeding When theeggsarefertilised, . Each femalecanproduceupto60million tend togathertogetherspawnwhich fertilisation r ae whichspendfromtwotofourweeks
Photo by GBRMPA the chanceoffertilisingeggs Y feed oncoralline algae. oung cr ate . ater temper . It hasupto21arms T herefore own-of-thorns star ates inthefieldforcrown- . . T he starfishcanmove , ature isabout28 a smallpopulationof . T hey liveamong they develop , . with T T he fish he . o al C . scale surveysindicatethatcor more torecoveritscor less than1%. from theusual25-40%ofreefsurfaceto as of cor is intensecompetitionforfoodandmosttypes But whenstarfishareinlargenumbers (10,000 squaremetres). 20-30 crown-of-thornsstarfishperhectare even pilingontopofeachother many crown-of-thornsstarfishpersquaremetre starfish. about 40-50%cor Scientists estimatethatahealthycor in thatpartofthereef much thattheycankillmostofthelivingcor As aresult, only eataportionoftheentirecor years populations ofcrown-of-thornsstarfishformany starfish. from lowlevelsofpredationbycrown-of-thorns species particularly thorns starfishfeedmainlyontabularcor than 20cm)usuallyfeedatnight. feed duringthedaywhilesmallerstarfish(less day to hideinthereefandundercor of-thorns starfish, their timefeeding. Crown-of-thorns starfishspendabouthalf 25 cmindiameter the starfishgrowfromabout1cmto to growmorer months ofage F rocks andrubbleonthereef (cor Y oung crown-of-thornsstarfisheatencrusting eeding andgr Porites . alline) algaewhicharecommonamong Larger starfish(morethan40cm)usually , als willbeeaten, with onlyasmallreductionincor During asevereoutbreak, Some reefsseemtosupportsmall starfish tosurviveandgrow than 10%isneededforjuvenile spp the reefcanrecoverquiter , Such areefcantak . apidly they starttoeatcor that arenotusuallyeatenbythe . they areverycrypticandtend al covercansupportabout When therearefewcrown- owth Acr . During thenexttwoyears , al cover including speciessuch reducing hardcor opora . At aboutsix al coverofmore . spp Results fromfine- . als duringthe T e 10yearsor hey caneatso . al colony Crown-of- there canbe al andbegin and may al reefwith . , apidly al cover there al cover al . al . , , the affectedreefs subsequent goodrecoveryofcor the GreatBarrierReefendedabout 1975, the perimeterwereaffected. be affected. outbreaks of crown-of-thornsstarfish. few reefsineachregioncarriedlargepopulations southw the northofGreatBarrierReefandmoved Starfish outbreaksgener diving. increased tourismandthepopularityofSCUBA to benoticednowthaninthepastbecauseof is possiblethatstarfishoutbreaksaremorelik Reef w of crown-of-thornsstarfishontheGreatBarrier T starfish outbreaksareanewphenomenonornot. difficult todateaccur become mixedovertimeandthespiculesare During thefirstoutbreakin1960s subsequent eventsintheGreatBarrierReef outbreaks hasbeenaconsistentpatternin further south. flowing oceancurrentsinthearea. the starfishlarv 7,000 years Great BarrierReefduringthepast3,000to suggests thatstarfishhaveoccurredonthe have beenfoundinoldreefsedimentswhich Spicules fromcrown-of-thornsstarfishsk thorns starfishoutbreaks History ofcrown-of- north ofthe by themid-1970s reefs off star Cr he firstdocumentedcaseoflargenumbers own-of-thorns star fish thateatlivingcoral. as atGreenIslandoffCairnsin1962. ard. T , ownsville wereaffectedin1970and mid-shelf reefswerethemostlik T . On somereefs Whitsundays his patternisbelievedtoresultfrom However T ae beingcarriedwiththesoutherly his southw . , the starfishhadreachedreefs ately , fish ar because thesediments ally havestartedin , , ard progressionof more than300km , it isunclearwhether As withthefollowing e oneofthefew only sectionsaround T his outbreakon als onmostof F or example , only a eletons with . ely to It ely ,
Photo by Vicki Harriott A new outbreak was recorded in 1979, again Percentage of reefs with crown-of-thorns starfish outbreaks Porites starting in the Green Island region. Large Number of Reefs Surveyed corals that were metres across and hundreds of 5 0 9 3 8 8 2 8 2 7 4 4 33 6 22 9 9 9 91 8 8 7 11 109 1 1 1 1 100 10 110 10 years old, were eaten by the starfish, even though 246 this type of coral is not usually preferred as food. Once again, this outbreak took about a decade 15 to move south to the Swains Reefs, but largely Active Outbreaks missing the Whitsunday and Pompey regions. 10 Total Outbreaks About 17% of reefs were affected by crown-of- eaking Reefs thorns starfish during this outbreak. 5 Information from Long Term Monitoring % Outbr In 1994, about 15 years after the start of the 0 Program, AIMS. 6 9 7 2 3 7 3 4 5 6 8 2 8 8 8 88 000 001 00 previous outbreak, large numbers of crown-of- 00 19 2 2 1991 19 19 2 1990 199 199 19 2 199 199 199 199 1999 199 thorns starfish were found in the Cooktown Survey Year region. As expected, this was followed by reports of large populations further south along the Great Recovery of reefs from Many coral species spawn only once a year Barrier Reef. By 2003, the highest densities of in a mass spawning in spring or early summer starfish were recorded in the area northeast crown-of-thorns starfish (October to December). Eggs and sperm are of Townsville. outbreaks released into the water where they combine and eggs are fertilised. Coral larvae can be carried There have been persistent outbreaks on reefs Coral reefs recover from outbreaks of crown-of- long distances from the parent colonies by in the Swains area. These starfish populations thorns starfish in a similar way to their recovery currents and tides. Recent research has shown are genetically similar to northern populations from other impacts that reduce coral cover, such that some reefs are more likely than others to but do not follow the pattern as cyclones or coral bleaching. Even on the most receive large numbers of coral larvae from of southward spread of crown- heavily damaged reefs, some corals remain alive. distant reefs in ocean currents. The reefs which of-thorns starfish outbreaks. As these corals grow, they begin the process of receive large numbers of larvae can have many In 2002-03, four of the reef recovery. The rate at which a reef recovers young corals growing on them within a few years seven reefs surveyed in from predation by crown-of-thorns starfish after an outbreak. As a result, coral cover can the Swains sector by the depends on many factors including the rate of return to the normal level within about 10 years. Long-Term Monitoring recruitment of corals to the reef, and the impacts Other reefs that do not receive a plentiful supply Program had active of cyclones and run-off from the land. of coral larvae will take considerably longer to outbreaks. recover, perhaps 15 years or more.
Year 1996 – 1997 Cooktown Year 1999 – 2000 Year 1992 – 1993 Year 2002 – 2003 Cairns
Cooktown Cooktown
Cairns Cooktown Cairns
Townsville Cairns Townsville Townsville
Townsville Mackay Mackay Crown-of-thorns starfish per tow Mackay Mackay 0 – 0.01 Gladstone 0.01 – 0.1 Gladstone 0.1 – 0.2 Gladstone 0.2 – 1 (Incipient Outbreak) > 1 (Active Outbreak) Changes in density of crown-of-thorns starfish on No Data the Great Barrier Reef over the past decade. Gladstone Maps courtesy of AIMS The giant triton snail was highly prized and heavily collected prior to its protection in 1969. Numbers of triton snails on the Reef remain low. However, the triton shell can eat only about one crown-of-thorns starfish per week so its capacity Photo by AIMS A diver using a manta board is towed behind to prevent starfish outbreaks seems limited. a small boat to survey a reef for crown-of-thorns starfish. Humphead maori wrasse are reputed to be active crown-of-thorns starfish predators. Whilst maori Natural fluctuations wrasse have historically been taken in the Great Although coral cover on an individual reef can Like many marine invertebrates, crown-of- Barrier Reef line fishery – and more recently reach levels that are considered normal on the thorns starfish produce large numbers of eggs - in the live fish fishery – from December 2003, Great Barrier Reef, the same combination of coral potentially 1,000 million eggs in the lifetime of it became illegal under Queensland fisheries species may not be present as existed before the a single female. Fluctuations in the environment legislation to fish for, or keep this species. outbreak. Some species appear quickly after a can change the survival rate of the larvae. If larval reef is damaged and grow rapidly. Other species survival is improved from one larva in 100 million, Predation by other reef fish on juvenile starfish are slower to appear and slower growing. These to one larva in 10 million, there would be a might also limit crown-of-thorns starfish corals may not reach previous levels on the reefs 10-fold increase in the population of crown-of- populations. Juvenile starfish are most likely to for some decades. This change in species diversity thorns starfish within one generation. Once there be eaten by fish about six months after settling is one of the concerns if the interval between is a small increase in the starfish population, the when they start to feed on coral. If numbers of outbreaks becomes shorter than the time required success of subsequent spawning aggregations the predator fish were depleted by fishing for the coral community to completely recover. In would ensure that even more larvae could survive activities this might allow an abnormally large this situation, reefs might lose some biodiversity in successive generations. number of starfish to survive to maturity. There is and be dominated by a small number of those no substantial evidence at this time to show that coral species that can recover rapidly. Natural fluctuations in temperature, salinity or commercially exploited fish eat significant numbers availability of planktonic food could all contribute of juvenile crown-of-thorns starfish. to improving the survival of crown-of-thorns What causes outbreaks starfish larvae. There has been some suggestion Because the numbers of starfish larvae that usually that outbreaks of the crown-of-thorns starfish survive and then settle is unknown and it is of crown-of-thorns are linked to the timing of the El Niño-Southern difficult to estimate feeding rates of predators starfish? Oscillation (ENSO) system which can cause in the field, it is unclear what is the minimum dramatic shifts in local climate around the predation rate that would be needed to prevent While there have been many theories on the Pacific Ocean. outbreaks. causes of outbreaks of the crown-of-thorns starfish, there are three theories that are Removal of predators Human influence on water quality supported by scientists. These theories have neither been proved nor disproved. The theories are: Although crown-of-thorns starfish have few Many reports from around the world have predators, one theory suggests that they play an suggested that there is a relationship between ■ Fluctuations in crown-of-thorns starfish important role in keeping starfish populations in periods of high rainfall and the beginning of population are a natural phenomenon. check. Predators of adult crown-of-thorns starfish crown-of-thorns starfish outbreaks. Periods ■ Removal of natural predators of the crown- include the giant triton snail, the humphead maori of high rainfall after drought or extended dry of-thorns starfish has allowed populations wrasse, starry pufferfish and titan triggerfish. periods cause water with low salinity, high to expand. sediment and high nutrient loads to be washed ■ Human use of the coastal zone has increased into the waters of the Great Barrier Reef. the nutrients flowing to the sea and resulted in an increase in planktonic food for larvae of High nutrient levels can cause an increase in crown-of-thorns starfish. The improved microscopic algae in the water, providing food survival of larvae has led to an increase for the developing crown-of-thorns starfish in the number of adult starfish which larvae. This can increase the number of larvae
results in outbreaks. A that survive and lead to larger adult starfish populations. Low salinity also increases the survival of crown-of-thorns starfish larvae. Photo by GBRMP The giant triton shell is one of the few predators of adult crown-of-thorns starfish. important issuesintheGreatBarrierReef by managementresponsesbecausetheyare outbreaks Regardless ofwhethersuchfactorscause as playingaroleincrown-of-thornsoutbreaks and poorw Some humaninfluences expect ahighcoverofhardcor areas thatarevisitedfrequentlybytouristswho and areonlypr However have beendevelopedtocontrolstarfishnumbers of crown-of-thornsstarfish. T numbers (seeSurveyMethodsonnextpage). techniques tomonitorstarfishdistributionand has supportedthedevelopmentofsever Barrier ReefMarineP reef ecosystem. of thestarfishmayalsobeanatur of theGreatBarrierReefecosystem. Crown-of-thorns starfishareanatur starfish outbr Managing cr crown-of-thorns starfishover200years would leadtomorefrequentoutbreaksof increase inlarv mathematical modelsthatmimica10-fold the frequencyorintensityofoutbreaks which couldpossiblycauseoutbreaksorincrease nutrient loadcouldimprovesurviv since Europeansettlement. the adjacentrivershasincreasedsever reaching theGreatBarrierReeflagoonfrom crown-of-thorns starfish. important tounderstandthedistributionof T be appropriate? eliminate crown-of-thornsstarfish, T of outbreaks here arefewoptionstomanageoutbreaks o developthebestcontrolmeasures herefore , , they arelabour , flood eventscouldbeanatur they arealreadybeingaddressed ater quality . However al surviv actical insmallareas own-of-thorns So eaks , even ifitwerepossibleto ark , , al showsthatthis the amountofnutrients , have beensuggested -intensive andexpensive such asoverfishing Authority (GBRMP T herefore T Sever his increased al. al techniques al oflarv al partofthe , would it the Great , al part Outbreaks for example , it is al-fold . . Recent al cause al survey . ae A) . , . ,
Photo by GBRMPA other marinelife may berequiredonadailybasis of aspecificareamustbeanongoingeffortand within afewdays acid) solutionintothestarfish, tr T quickly movefromoneareatoanother areas canbeprotected. densities starfish fromreefswheretheyareinoutbreak It isimpossibletoer researchers andthemarinetourismindustry Reef MarineP thorns starfishinsmallareasbytheGreatBarrier methods tocontrollargenumbersofcrown-of- Consider Other contr major diseasesincrown-of-thornsstarfish. and longspines predators becauseoftheirtoughandto starfish anddiseases with sodiumbisulfate solution. Cr high mortalityofthelarv Natur Natur thorns starfishpopulations Controlling crown-of- During asever he recommendedcontrolmethodinvolves ained diversinjectingsodiumbisulfate(dry own-of-thorns star al controlsonstarfishpopulationsinclude al contr . able efforthasgoneintodeveloping However ark ol methods e outbr . . T ols . Authority (GBRMP here islittleevidenceofany , T with sufficienteffort, his chemicalisnon-to adicate crown-of-thorns . fish ar Adult starfishhavefew eak, cr Because starfishcan ae , e killedbyinjecting predation ofsmall own-of-thorns star which killsthem . A), , CRC Reef xic control small xic to ‘skin’ . fish canbesodensetheypileontopofeachother spend upto$300,000eachperyearincrown- Some tourismoper T Reef ResearchF T the Cairns thorns starfishcontrolprogr $2 milliontoatwo-year Queensland Governmentscommitted of starfish. every dayinanefforttok oper of-thorns starfishcontrol. to 1mindiameter Cr ■ ■ ■ T or causesofcrown-of-thornsstarfishoutbreaks Scientists willcontinuetosearchforthecause Future research his controlmethodisextremelycostly his progr hey willalso: own-of-thorns star to controlcrown-of-thornsstarfish. Investigate morecost-effectivemethods populations Continue tomonitorcrown-of-thorns and crown-of-thornsstarfishoutbreaks Investigate linksbetweenterrestrialrun-off ators mayneedtoinject200500starfish am issupportedbytheGreatBarrier , In 2001, T ownsville and . oundation. . ators intheCairnsregion the Commonwealth fish cangr , industry-run crown-of- During activeoutbreaks eep selected Whitsunday areas am thatservices ow . sites free and . . . . ,
Photo by David Wachenfeld, Photo by AIMS Triggerfish Images Survey methods for crown-of-thorns starfish
The distribution of crown-of-thorns starfish on the Great Barrier Reef has been monitored regularly using two different and complementary methods: broad-scale reef surveys by the Long-Term Monitoring Program supported by the Australian Institute of Marine Science (AIMS) and CRC Reef; and the fine-scale reef surveys carried out by Reefwatch Australia, funded by CRC Reef.
LONG-TERM MONITORING PROGRAM The Long-Term Monitoring Program is designed to monitor broad-scale changes in several parameters that are indicators of reef health across the Great Barrier Reef. The program is undertaken by scientists from the Australian Institute of Marine Science (AIMS) who have been conducting surveys since 1985. Ensuring the future of the Each year, the team uses manta tows to survey the perimeters of about 100 reefs. During a manta tow, a diver using a ‘manta board’ is towed behind a small boat around each reef. The boat stops world’s coral reefs every two minutes so the diver can record evidence of crown-of-thorns starfish (sightings or feeding scars) and estimates of reef-wide coral cover. During the Long-Term Monitoring Program, sites on CRC Reef Research Centre Ltd 48 of these reefs are surveyed more intensively for coral cover and resident fish populations. is a knowledge-based partnership of coral reef researchers, managers and Divers using manta tows can see adult crown-of-thorns starfish that are bigger than about 15 cm industry. Its mission is to plan, fund in diameter. The numbers of starfish seen during two-minute tows are used to estimate the status and manage world-leading science for of the crown-of-thorns starfish populations. An ‘incipient’ outbreak is the density of starfish at which the sustainable use of the Great Barrier coral damage is likely: 0.22 starfish per two-minute tow. During an ‘active’ outbreak, densities reach Reef World Heritage Area. >1.0 starfish per two-minute tow which would certainly damage reefs. It is interesting to note that some reefs have been unaffected by crown-of-thorns starfish since the beginning of the monitoring CRC Reef Research Centre Ltd program in 1985. is a joint venture between the Association of Marine Park Tourism Information about the current status of crown-of-thorns starfish on the Great Barrier Reef from Operators, Australian Institute of the Long-Term Monitoring Program is available on the AIMS website at: Marine Science, Great Barrier Reef www.aims.gov.au/monmap/COTSPage/COTSPage.html. Marine Park Authority, Great Barrier Reef Research Foundation, James Cook FINE-SCALE SURVEYS University, Queensland Department of Since 1994, up to 21 reefs between Cooktown and Townsville have been surveyed each year using Primary Industries, Queensland Seafood the fine-scale survey method. As the name suggests, the fine-scale surveys look in detail at small Industry Association and Sunfish areas of reef and reveal the structure of the crown-of-thorns starfish populations. Queensland Inc.
At each reef that is surveyed, SCUBA divers search for crown-of-thorns starfish in 40 transects (50 m long x 5 m wide) at 20 different places on the reef. The divers count all the starfish in each CRC Reef Research Centre transect, including juveniles (up to 13 cm diameter, estimated to be one year old), sub-adults PO Box 772 Townsville (14-25 cm diameter, estimated to be about two years old) and adults (more than 26 cm diameter, Queensland 4810 Australia estimated to be three years old or more). The smaller starfish cannot be seen during manta tow surveys. Email: [email protected] Website: www.reef.crc.org.au Incipient outbreaks are recorded when sub-adult and adult densities together reach more than 30 per hectare (10,000 square metres). Active outbreaks are defined when there are more than 30 adult crown-of-thorns starfish per hectare (10,000 square metres). Established and supported under the Australian COMPARING LONG-TERM MONITORING AND FINE-SCALE SURVEYS Government’s Cooperative An analysis of the difference between manta tows and fine-scale surveys showed that: Research Centres ■ Manta tow surveys underestimate the numbers (densities) of crown-of-thorns starfish compared Program with fine-scale surveys on the same reef. ■ Both techniques show a similar progression of the outbreak and rate of southerly drift of the This brochure was written by Assoc Prof Vicki Harriott, outbreak. Southern Cross University (formerly of CRC Reef), Therefore, the fine-scale surveys are most useful to calculate densities of crown-of-thorns starfish Dr Louise Goggin, CRC Reef, and Dr Hugh Sweatman, on a small-scale. Manta tows are better for following broad-scale changes in crown-of-thorns Australian Institute of Marine Science. starfish density across the entire Great Barrier Reef. Published by CRC Reef Research Centre Ltd 2003 Printed on recycled paper