An Evaluation of the Role and Impacts of Alien Finfish In Asian Inland Aquaculture

Page 1

An evaluation of the role and impacts of alien finfish in

Asian inland aquaculture

Sena S De Silva1,Thuy T T Nguyen2, NigelWAbery1�, Upali S Amarasinghe3

1School of Ecology and Environment, Deakin University,Warrnambool,Vic., Australia2Network of Aquaculture Centres in Asia-Paci¢c, Bangkok,Thailand3Department of Zoology, University of Kelaniya, Kelaniya, Sri Lanka

Correspondence: S S De Silva, School of Ecology and Environment, Deakin University, PO Box 423,Warrnambool,Vic. 3280, Australia.

e-mail: [email protected]�Present address: PIRVic, Department of Primary Industries, Snobs Creek, Private Bag 20, Alexandra,Vic.3714, Australia.

Abstract

Asia dominates global aquaculture production ac-counting for over 80% of the total and the mainstayin Asian aquaculture is ¢n¢sh. Over the years, Asiahas experienced a number of inter-continental andintra-continental transfers/introductions/transloca-tion of ¢n¢sh species, between nations and water-sheds, beyond their natural range of distribution,primarily for aquaculture development. In this arti-cle all such species are referred to as alien species.An attempt is made to evaluate the importance ofthe production of alien species in selected Asian na-tions, using statistics of the Food and Agriculture Or-ganization. Also, negative e¡ects, if any, based onliterature surveys, of alien species in relation to dis-placement of indigenous species, and on biodiversityand/or genetic diversity together with associatedpathogen transfers are evaluated. The major alienspecies, based on their signi¢cance to Asian inlandaquaculture considered, are the tilapias, cat¢sh, Chi-nese and Indian major carps and common carp. It isestimated that currently alien species account fornearly 12% of the cultured ¢n¢sh production (2.6million tonnes) in Asia, valued at US$ 2.59 billion,and the contribution exceeds 40%whenAsian coun-tries excluding China are taken into consideration.Inland ¢n¢sh aquaculture in some Asian nations,such as Indonesia and the Philippines, is predomi-nated by alien species, and in some others, e.g. Ban-gladesh and India, the contribution from alienspecies has been increasing steadily. It is suggestedthat overall alien ¢n¢sh species have done little eco-logical harm to native £ora and fauna. However, in

the wake of increasing anthropogenic developmenttaking place in watersheds the resulting environ-ments are often made unconducive to indigenousspecies but not to some alien species, thereby poten-tially and indirectly making the latter invasive.

Keywords: Asian aquaculture, alien species, pro-duction, major carps, tilapias, biodiversity, geneticdiversity, invasive

Introduction

Aquaculture is the fastest growing primary produc-tion sector in the world at present, having recordedan annual growth rate of 11% year�1 over the lastdecade (FAO 2002). Of all aquatic products consumedcurrently, aquaculture accounts for 33%, and theglobal aquaculture production in 2002 was 51.4 mil-lion tonnes, valued at US$ 59.9 billion. Asia domi-nates aquaculture production of the world, andcurrently contributes 87% to the global cultured ¢n-¢sh production of 25.7 million tonnes.However, over the last decade or so, the sector has

had to face criticisms with regard to lack of attentionto negative environmental in£uences resulting fromits practices, in particular, its increasing dependenceon ¢shmeal (Naylor, Goldburg, Mooney, Beveridge,Clay, Folke, Kautsky, Lubchenco, Primavera & Wil-liams 1998; Naylor, Goldburg, Primavera, Kautsky,Beveridge, Clay, Folke, Lubchenco, Mooney & Troell2000), discharge of nutrients, chemicals, antibioticsand other therapeutants to the environment and alsoon in£uences on biodiversity. However, with regard

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to the issue on increasing dependence of aquacultureon ¢shmeal, the observations made by Naylor et al.(1998, 2000) have been refuted (Pike 2000; Hardy2001). Impacts of aquaculture on biodiversity in gen-eral terms have been considered a few times pre-viously (Beveridge, Ross & Kelly 1994; Beardmore,Mair & Lewis1997). In general, alien species are con-sidered to pose a major threat to biodiversity, inrespect of all habitats (IUCN 2000), and aquatichabitats are no exception in this regard (Moyle & Lei-dy1992). Often impacts on biodiversity fromaquacul-ture are purported to result from thegreater degree ofdependence of the latter on alien species, in mostparts of the world (Moyle & Leidy 1992; Rhymer &Simberlo¡ 1996; Collares-Pereira & Cowx 2004among others). Bartley and Casal (1998) consideredthe impacts of introductions on conservation andsustainable use of aquatic biodiversity. Apart fromthe above, there has been no attempt to evaluate therole of alien species in aquaculture and its conse-quent impacts on the £ora and faunaand other issuesrelated to biodiversity.Fish introductions are known to have occurred for

centuries and in respect of Asia one of the ¢rst re-corded introductions was that of the spread of cypri-nids, in particular common carp and crucian carp,associated with the migration of Chinese people(Termvidchakorn, Vidthayanon, Getpetch, Sorrak &Paradonpanichakul 2003). Many of the global intro-ductions were documented byWelcomme (1988), andare now also available in the Database on Introduc-tions of Aquatic Species (DIAS). (http://www.fao.org/¢gis/servlet/static?dom=root&xml=Introsp/introsp_s.xml) and those forAsiawere documented by De Silva(1989) andWelcomme and Vidthayanon (2003). Asiahas experienced many introductions, between na-tions, between watersheds and inter-continentally,and the great majority of these have been for aqua-culture and stock enhancement purposes. It haseven been suggested that aquaculture was a gatewayfor species introductions with consequent negativein£uences on biodiversity (Naylor,Williams & Strong2001).Pillay (1977) believed that the bulk of introductions

for purposes of food ¢sh productionare a post-secondWorld War phenomenon, and indicated that hapha-zard introductions/translocations were a thing ofthe past. However, recently it has been pointed outthat this prophecy has not yet come true in entirety,and many translocations, often conducted for short-term gains, are still a common occurrence (De Silva2001; Nguyen & Na-Nakorn 2004). International

codes of practices are available for e¡ecting aquatictranslocations (Bartley & Minchin1996). Regrettably,however, there appears to be very little adherence toavailable codes of practices in a¡ecting transloca-tions even though most nations are signatories tosuch codes. Often when attention is paid it is re-stricted to inter-continental introductions but noton intra-continental and/or those between water-sheds. In nations where legislation exists to prevent/minimize the spread of alien species, e¡ective imple-mentation of such laws can often be hampered byother factors (Naylor et al. 2001).It is often suggested that the introduction of exotic

species is an important factor a¡ecting biodiversity,including ¢n¢sh species (Barel, Dorit, Greenwood,Fryer, Hughes, Jackson, Kawanabe, Lowe-McConnell,Nagoshi, Ribbink, Trewavas, Witte & Yamoka 1985;Moyle & Leidy1992; Rhymer & Simberlo¡ 1996; Nay-lor et al. 2001; Collares-Pereira & Cowx 2004). Suche¡ects on biodiversity have mostly resulted fromcompetition for food and space with indigenous spe-cies (Barel et al. 1985; Moyle & Leidy1992), alterationof habitats (Kottelat &Whitten1996; Collares-Pereira& Cowx 2004), the transmissionof pathogenic organ-isms (Dobson &May1986), as well as through geneticinteractions such as hybridization and introgression(Dowling & Childs 1992; Leary, Allendorf & Forbes1993; Rhymer & Simberlo¡ 1996; Allendorf & Leary1998; Araguas, Sanz, Pla & Garcia-Martin 2004),and other indirect genetic e¡ects (Waples1991).However, in Asia, the role and/or impacts of alien

species in aquaculture and/or in stock enhancementpractices and their consequent e¡ects on biodiver-sity-related issues have been rarely evaluated (Hos-sain, Ehshan & Mazid 1999; De Silva, Phillips &Nguyen 2005). The reasons for this may be manifold.Most countries in the region are still in a growthphase of development of aquaculture, and generallythe emphasis has been on increasing productionand the development of associated technologies.Moreover, the overall success of alien species inaqua-culture production in the region has led to therelative neglect of consideration of possible andpotential negative impacts brought through the in-troduction of such species, until relatively recently. Itis also important to note that the terminology used inrespect of introduced species has changed over theyears. The term alien species now often refers to in-troduced, translocated and/or exotic, especially byinternational organizations, and particularly so afterthe adoption of the term by the Convention on Biolo-gical Diversity (CBD 2003).

Alien ¢n¢sh in Asian inland aquaculture S S De Silva et al. Aquaculture Research, 2006, 37, 1^17

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The aquaculture industry in Asia is still in agrowth phase, but at a reduced rate (De Silva 2001),being only 6.5% from 2001 to 2002 (FAO 2002), andcontinues to provide a signi¢cant source of incomeand employment, particularly in the rural areas,and contribute to poverty alleviation in the develop-ingworld. It is opportune and relevant to evaluate therole of alien species in Asian aquaculture, especiallyin viewof the increasing public concerns with regardto environmental and biodiversity e¡ects of aquacul-ture (Anonymous 2000; De Silva 2001). Accordingly,the aim of this presentation is to assess, as objectivelyas possible, based on available information, the im-pacts and signi¢cance of alien species in Asian aqua-culture. In the context of this paper the analysis waslimited to nations of East, and South and SoutheastAsia in view of the greater inland water resourcesand ¢n¢sh diversity compared with North andWestand Central Asia (IUCN classi¢cation). In addition,in the analysis, for clarity,5-year averages in produc-tion from 1981 to 2000 are presented together withthose for 2001 and 2002. In the case of Vietnam,however, the aquaculture statistics are not providedat the species level, even thoughVietnammakes a sig-ni¢cant contribution to cultured, inland ¢n¢sh pro-duction, which also includes alien species. As such,in the detailed analysis alien ¢n¢sh production forVietnam could not be included.Furthermore, in evaluating the success of a cul-

tured species/species group, particularly in thecurrent context of sustainable development andenvironmental integrity, consideration of productionlevels per se is inadequate. Kutty (1997) suggested thatthe future of aquaculture would not depend on theeconomic viability per se, but that its long-term sus-tainability will only be assured by environmental via-bility. High production levels achieved at the expenseof social and environmental damage/perturbations,as well as at the expense of a net loss of natural ¢sh-ery resources are no longer acceptable to the commu-nity.The public are becoming increasinglyconcernedof and alert to such environmentally detrimentalpractices. As such in assessing the success of analienspecies a more holistic approach is needed, and an at-tempt is made in this paper to achieve this objective.

Alien species

Analien species is considered to be a species that hasbeen moved beyond its natural range of distributiondeliberately and/or accidentally, because of human

activities. However, in the present analysis move-ment of species across watersheds, particularly with-in large nations such as China (mainland) and India,from the previous watershed to the watershed thatthe species has been translocated to is not consideredas alien. For example, the Chinese major carps suchas bighead carp, Aristichthys nobilis Richardson, sil-vercarp Hypophthlamichthys molitrix (Cuv. et Val.),mud carp Cirrhinus molitorella (Val.) and black carpMylopharyngodon piceus (Richardson) are indigenousto theWest (Xi Jiang),Yantze (Chang Jiang) and AmurRiver basins, and some of the Indian major carpssuch as catla Catla catla (Hamilton), rohu Labeo rohita(Hamilton) and mrigal, C. mrigala Hamilton are notindigenous to the southernwatersheds in India. Spe-cies, translocated for example, within each of thenations, although involves movement from theirnatural range of distribution, are not considered asalien to the new areas. This stance was unavoidableas production statistics are available not for eachwa-tershed but for each nation and in the case of Chinaits administrative regions of Macao and Hong Kong(FAO 2002).Some of the reasons for the successful culture of

the more popular alien species, inter- and/or intra-continental, in Asia lie in their biological characteris-tics and suitability for aquaculture. In general, mostof the popular alien species cultured in Asia feed lowin the food chain, and this is particularly relevant asthe great bulk of Asian inland aquaculture is ruraland extensive and/or semi-intensive (De Silva 2001).The developments in the techniques of arti¢cial pro-pagation of commonly cultured alien species in Asia,which have become routine, have also contributedsigni¢cantly to the popularization of these species. Itis not uncommon to see small hatcheries, popularlyreferred to as ‘backyard hatcheries’, in most Asiancountries that have become the backbone for fry and¢ngerling supplies for grow-out operations of suchalien species.

Status of cultured production of alien ¢n¢shspecies

In Asiamore than 50 ¢sh species that are alien to onenationor another are cultured. However, onlya smallnumber of these species make a signi¢cant impact onaquaculture production in the region, and as such, inthe present analysis, species that were produced inexcess of 20000 tonnes year�1 within the last dec-ade are considered. Accordingly, species such as rain-bow trout that has been commonly cultured in Japan

Aquaculture Research, 2006, 37, 1^17 Alien ¢n¢sh in Asian inland aquaculture S S De Silva et al.

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in the past but has accounted for less than20000 tonnes year�1noware not considered.

Alien species of inter-continental origin

The most aquaculturally important alien speciesresulting from inter-continental translocation intoAsia are the tilapias, in particular Mossambique tila-pia (Oreochromis mossambicus Peters) and Nile tilapia(O. niloticus L.). In addition to these two species otherTilapiine species have been introduced into Asia(Pullin 1988; DIAS 2004), but these species have notmade any signi¢cant impact on aquaculture produc-tion in the region.The former two species are distrib-uted throughout Asia, and O. niloticus in particularcurrently plays a signi¢cant role in Asian aquacul-ture, whereas O. mossambicus continues to contri-bute to inland ¢sh production in reservoirs andlakes, to varying degrees, in some nations, such asfor example Sri Lanka and Indonesia (De Silva &Funge-Smith 2005).

Tilapiine aquaculture production in Asia. The globalTilapiine aquaculture production has increased stea-dily over the last two decades from 87555 tonnes in1980 to 1311372 tonnes in 2002 (FAO 2002). Thebulk of tilapia production occurs in Asia, and has ac-counted for 86% and 60% of the global productionand in value (data not shown), respectively, in 2002and has £uctuated ( � 3%) around these levels overthe past two decades (Fig. 1). Tilapias make an in-creasingly important contribution to Asian inlandaquaculture production and have contributed 45%to the total inland aquaculture production, andabout 4% in value in 2002 (Fig. 2). Nile tilapia is thesixth most produced species in Asian freshwater cul-ture in 2002 andmust be considered themost impor-tant inter-continentally introduced aquaculturespecies intoAsia.Gupta and Acosta (2004a) summarized the cur-

rent tilapia culture methods in Asia, and emphasizedthe diverse array of culture methods used. The resili-ence of tilapias is most apparent from the fact that ithas been used e¡ectively in sewage-¢sh culture sys-tems in India, successfully for over four decades, andthere has never been any reports of disease transmis-sionamongconsumers ormassmortalities of the cul-tured stocks (Nandeesha 2002). The reasons for thesuccess of tilapias in Asian aquaculture were alsodealt with in detail by De Silva, Subasinghe, Bartleyand Lowther (2004), and these authors concludedthat there is no explicit evidence to show that bio-

diversity of the region has been a¡ected by this intro-duction.Within Asia the leading producer of cultured tila-

pias is China, and in fact this is the only alien speciesthat is currently aquaculturally important to this na-tion. However, the contribution of tilapias to the totalinland production in China is small while its propor-tionate contribution in the Philippines and Sri Lankais considerably higher, accounting for � 90% or

0.8

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Pro

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ion

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ion

t)

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89

% A

sia

Rest of world Asia % Asia

Figure 1 Mean yearly production of cultured tilapias inAsian nations (Asia) and in the rest of the world, and thepercent contribution of Asian cultured production to glo-bal tilapia production (% Asia) (data source: FAO statistics:http://www.fao.org/¢/statist/FISOFT/FISHPLUS.asp).

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ion

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% T

ilapi

as

tilapias Inland % tilapias(a)

(b)

Figure 2 Meanyearly production (a) and value (b) of cul-tured tilapias in Asian nations (tilapias) and the produc-tion of other inland species in Asia (Inland), and thepercent contribution of cultured tilapias to the total Asianinland aquaculture production (% tilapias) (data source:FAO statistics: http://www.fao.org/¢/statist/FISOFT/FISHPLUS.asp).

Alien ¢n¢sh in Asian inland aquaculture S S De Silva et al. Aquaculture Research, 2006, 37, 1^17

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greater freshwater ¢n¢sh production and greaterthan 75% of the value of production in 2002 (datanot shown). It should be noted, however, that in mostAsian countries, the contribution of tilapia to thefreshwater ¢n¢sh production quantity and value hasremained static or decreased since 1992. Tilapiineproduction in Philippines is composed of approxi-mately 401% cage and 401% pond culture, withsmall amounts of pen culture, and has contributedabout10% to total domestic ¢sh supplies (FAO 2001).Taiwan’s productionof tilapia is export oriented, withmost exported to the USA (Taiwan Tilapia Alliance2003).The general importance of tilapias to theAsian re-

gion is evident from the fact that the ¢rst ever, multi-nation, selective breeding programme on a cultured¢n¢sh species in Asia was on O. niloticus. This multi-nation regional e¡ort culminated in the productionofthe ‘GIFT Strain’ (Genetically Improved Farmed Tila-pias), which has shown an increased growth perfor-mance (body weight gain) of 18% and 58% over theexisting, cultured strains in China and Bangladeshrespectively (Gupta & Acosta 2004b), and a resultingbreak-even price above variable cost by 7^36%(Dey, Bimbao, Young, Regaspi, Kohinoor, Pongthana& Paraguas 2000).

Cat¢sh. As far as we are aware the onlycat¢sh spe-cies translocated inter-continentally and cultured inAsia is channel cat¢sh Ictalurus punctatus (Ra¢n-esque) in the Republic of Korea, of which the Foodand Agriculture Organization lists production onlyfor1996. However, the African cat¢sh Clarias gariepi-nus (Burchell) was introduced intoThailand andViet-nam and is used for hybridizing with the indigenous,walking cat¢sh C. macrocephalus (Gˇnther) in Thai-land. The hybrid is cultured extensively and ispreferred by farmers in Thailand, because of itsconsiderably higher growth rate than that of theindigenous species and the desirable £esh quality(Na-Nakorn 2004).The production of hybrid cat¢sh, based on avail-

able FAO statistics (FAO 2002), occurs exclusively inThailand and in recent years has accounted forgreater than 17% of the world’s cat¢sh production(FAO 2002). However, this would undoubtedly be anoverestimation of its contribution as the production¢gures of cultured cat¢sh (as a separate entity)in Vietnam, a well-established cultured cat¢sh pro-ducer, including that of Pangasius hypophthalmus(Sauvage) (Narog 2003), are unavailable. Althoughthe contribution of hybrid cat¢sh production to the

world’s cat¢sh production is large, the contributionof hybrid cat¢sh to the total inland aquaculture pro-duction of Asia is relatively small (o0.4%).

Alien species of Asian origin

In Asian aquaculture there are a number of speciesthat have beenmoved across nations andwatersheds,within the continent. In the present analysis, basedon indirect evidence, a number of assumptions weremade in respect of cultured cyprinid species produc-tion. All these assumptions were related to the FAOcategories of production, in particular to the FAO ca-tegory nei (not elsewhere included) for cyprinids inDPR Korea, Nepal, Pakistan, Philippines and Sri Lan-ka, whichwe assumed, based on personal experienceand available records, to be Chinese major carps, In-dian major carps, Indian major carps, Chinese majorcarps (bighead carp; Guerrero 2000) and Indian ma-jor carps respectively.

Common carp. There is con£icting evidence withregard to the ancestral wild common carp; for exam-ple Balon (2004) suggests they originated in the Da-nube River. In view of the relatively uncertain statusof the common carp we have, based on Welcomme(1988), considered it to be indigenous to the Chinesemainland but alien to the rest of Asia. Common carpproduction in Asia was 2.9 million tonnes in 2002,with the bulk being in China. Production of commoncarp in its natural range of distribution as well as inareas it has been translocated to has increased ra-pidly over the past two decades (1981^2002), but therate of increase in the latter areas was much greater(41% versus 9% year�1). Also, the production ofalien common carp is greater than that of the othercyprinids and second only to the tilapias.Production of alien common carp in Asia is cur-

rently 705941tonnes and accounts for 24% of the to-tal common carp production in Asia (Table 1). InAsian nations, to which common carp is alien, thegreatest production occurs in India, Indonesia andMyanmar, and the percentage contribution of com-mon carp to the total freshwater ¢n¢sh productionhas in most cases increased except in the Republic ofKorea. In the latter the contribution of common carpto inland aquaculture decreased from about 68% to12% in the period1992^2002 (data not shown).

Major carps. The native ranges of Chinese majorcarps, including the bighead carp, grass carp, silvercarp, mud carp and black carp (Billard 1999), within

Aquaculture Research, 2006, 37, 1^17 Alien ¢n¢sh in Asian inland aquaculture S S De Silva et al.

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Asia are considered to be China, excluding the ad-ministrative regions of Hong Kong and Macao. Thenative range of Indian major carps are considered tobe India, Bangladesh, Myanmar, Pakistan and Nepal(Jhingran & Pullin 1988; McConnell, Leamon, Ski-binski & Mair 2001). Indian major carps are consid-ered to be catla, mrigal and rohu.Major carps account for the greatest freshwater

aquaculture production in the world and in Asia(16.1million tonnes in 2002). In order of their produc-tion volume grass carp ranks ¢rst, followed by silver,common, bighead and crucian carps. However, mostof cyprinid culture in Asia occurs within the naturaldistribution range of each of the species. The produc-tion of cyprinids (including common carp) in areasbeyond their natural range of distribution currentlyaccounts for1.4 million tonnes valued at US$1.21bil-lionor 8.6% and 8.8% of the total cyprinid productionand value respectively (Fig. 3), and these areas havewitnessed a steady increase in production since the1991^1995 period.The production of Indian major carps in areas be-

yond their natural range of distribution is lowand theproportion of this production has decreased over thelast two decades, while the value of production has£uctuated, dipping in the 1996^2000 period, andremaining consistently under 1% of the total pro-duction value (Fig. 4). Of the Indian major carps,production of alien species of rohu is the highest inAsia followed by mrigal and catla (Table 1). Produc-tion of alien species of rohu and mrigal occurs onlyinThailand and Lao PDR, while that of catla occurredexclusively in Lao PDR (FAO 2002). It should be notedagain that there is productionof alien species of majorcarps (both Chinese and Indian) inVietnam, but theTa

ble

1Meanyearlyindigenou

s(Ind.)a

ndalien(Al.)prod

uction(t)

ofChinesean

dIndian

major

carp

speciesan

dcommon

carp

andthepercen

tage

ofprod

uctionthat

isalien(%

Al.)

Species

1981^1985

1986^199

01991^1995

1996^2

000

2001

2002

Al.

Ind.

%Al.

Al.

Ind.

%Al.

Al.

Ind.

%Al.

Al.

Ind.

%Al.

Al.

Ind.

%Al.

Al.

Ind.

%Al.

Big

head

carp

17

017

313

560

5.1

19

015

641

168

2.9

16

577

928

627

1.8

15

574

1541

198

1.8

29

729

1637

943

1.0

29

350

1700

965

1.7

Bla

ck

carp

038

740

0.0

176

71

763

0.2

1117

71

708

1.5

1178

150

370

0.5

939

189

768

0.8

816

224

529

0.4

Catla

312

118

333

0.3

900

202

001

0.4

1408

345

561

0.4

774

577

759

0.2

814

483

877

0.1

972

468

098

0.2

Com

mon

carp

78

831

127

580

38.2

119

529

488

860

19.6

168

293

943

687

15.1

262

777

1890

258

22.2

624

112

2193

223

12.2

705

941

2235

634

24.0

Gra

ss

carp

11

963

235

200

4.8

13

187

701

527

1.8

20

536

1520

666

1.3

102

785

2814

556

2.1

71

961

3310

913

3.5

71

047

3419

593

2.0

Mrigal

329

74

064

0.4

1080

137

321

0.8

1776

315

250

0.6

2423

527

668

0.7

3204

442

577

0.5

3955

420

087

0.9

Mud

carp

134

31

340

0.4

264

72

500

0.4

96

92

200

0.1

134

164

000

0.1

118

220

000

0.1

88

0100.0

Rohu

823

123

355

0.7

1922

210

077

0.9

3236

395

332

0.8

3377

688

553

0.8

4762

597

545

0.5

4330

574

103

0.7

Silv

er

carp

2033

680

440

0.3

3845

1344

066

0.3

15

430

1864

203

0.8

106

709

3082

396

14.4

553005

3275885

3.3

573

646

3401

930

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Pakistan

,Nepal,D

PRKorea

andSriLa

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uctionof

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sition

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onhttp://w

ww.fa

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02468

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81-85 86-90 91-95 96-00 2001 2002Years

Pro

duct

ion

(mill

ion

t)

0123456789

% A

lien

Alien cyprinid Indigenous cyprinid % Alien

Figure 3 Mean yearly cultured production of cyprinidsin Asian nations to which they have been translocated(alien cyprinid) and in nationswhere theyoccur naturally(indigenous cyprinid), and the percent contribution of theformer to total cultured productionof cyprinids in Asia (%alien) (data source: FAO statistics: http://www.fao.org/¢/statist/FISOFT/FISHPLUS.asp).

Alien ¢n¢sh in Asian inland aquaculture S S De Silva et al. Aquaculture Research, 2006, 37, 1^17

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statistics are not available specieswise, and as suchwere not taken into account in the computations.Chinese major carps production beyond their nat-

ural range of distributionaccounted foramuch largerpercentage of the Asian Chinese major carp produc-tion especially in recent years (�7% for both produc-tion and in value; Fig. 5). The production of alienspecies of Chinese major carps has proportionally in-creased, contributing 43% to inland aquacultureproduction and �2.5% in value in Asia in 2001and2002 and are, as a group, now of similar importanceas the common carp.In general, the overall production of cyprinid spe-

cies, in areas beyond their natural range of distri-bution, has increased in Asia, and the relativecontribution of Chinese and Indian major carps, andcommon carp to inland aquaculture production hasalso increased since 1996. The Asian nations withgreatest alien Chinese major carp production are In-dia, Bangladesh and the Philippines, and the contri-bution of these species has increased dramaticallyfor India and Bangladesh, while only a modest in-crease was seen in the Philippines (data not shown).

Overall status of alien species in Asian Inlandaquaculture

The overall contribution of alien ¢n¢sh species toAsian inland ¢n¢sh aquaculture has increased in re-cent years and has reached 2.6million tonnes or12%in 2002 of cultured inland ¢n¢sh production (Fig.6).However, when China is excluded from the analysis

the contribution of alien ¢n¢sh production is greaterthan 42% (Fig.6). It is pertinent to note that the valueof inland cultured alien ¢n¢sh in Asia (whenChina isincluded) is approximately 11% of the total and issimilar to that of quantity produced. However, thevalue when China is excluded is only 21.5% (Fig.7).

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Figure 4 Meanyearlycultured production of Indianma-jor cyprinids in Asian nations to which they have beentranslocated (alien Indian cyprinid) and in nations wherethey occur naturally (Indigenous Indian cyprinid), andthe percent contribution of the former to total cultureproduction of cyprinids in Asia (% alien) (data source:FAO statistics: http://www.fao.org/¢/statist/FISOFT/FISHPLUS.asp).

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Figure 5 Mean yearly cultured production of Chinesecyprinids in Asian nations to which they have been trans-located (alien Chinese cyprinids) and in China, wherethey occur naturally (Indigenous Chinese cyprinids), andthe percent contribution of the former to total Asiancultured cyprinid production (% alien) (data source:FAO statistics: http://www.fao.org/¢/statist/FISOFT/FISHPLUS.asp).

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Figure 6 Mean yearly cultured production of ¢n¢sh in-digenous and alien to (a) all Asian nations and (b) Asiannations excluding China, and the percent contribution ofthe latter to the total Asian freshwater ¢n¢sh culture (%alien) (data source: FAO statistics: http://www.fao.org/¢/statist/FISOFT/FISHPLUS.asp).

Aquaculture Research, 2006, 37, 1^17 Alien ¢n¢sh in Asian inland aquaculture S S De Silva et al.

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Countrywise the greatest production of alien ¢n-¢sh species occurs by rank in India (861168 tonnes)followed by China (706585 tonnes), Indonesia

(278907 tonnes), Thailand (197343 tonnes), the Phi-lippines (132136 tonnes) and Taiwan (82223 tonnes)and then others. India and China outproduce thealien species of the other nations by a large margin,but only since the1980s (Table 2). However, when theland area of each country is taken into account, thehighest density of alien species production occurs inTaiwan (2549 kg km�2), Hong Kong (1212 kg km�2),followed by the Philippines (443 kg km�2),Thailand(386 kg km�2), India (290 kg km�2) and then others(Table 2).

Country perspectives

It is not intended here to provide a detailed synopsisonaquaculture in each of the countries but to consid-er, very brie£y, the key points on the trends of the in-dustry, with a view to put into perspective the dataalready presented on aquaculture production andthe importance of alien species to the sector in eachcountry.

Bangladesh

The freshwater inland aquaculture production inBangladesh is the third highest in the world, andits aquaculture activities are characterized by pond

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Table 2 Meanyearly production (tonnes) of cultured alien ¢n¢sh species in selected Asian nations and regional total, for theperiod1980^2002 (based on http://www.fao.org/¢/statist/FISOFT/FISHPLUS.asp)

Country (landarea in km2) 1981^1985 1986^1990 1991^1995 1996^2000 2001 2002

Bangladesh (133 910) 0 0 3091 20 205 26 781 25 975

Bhutan (47 000) 0 0 0 0 0 0

Brunei (5270) 0 1 8 27 0 84

Cambodia (176 520) 636 2165 3985 6253 6853 7193

China (9 326 410) 15 680 59 769 203 837 519 333 671 666 706 585

Hong Kong (1042) 6110 5114 3502 2495 1631 1263

India (2 973 190) 50 340 14 332 148 018 782 862 861 168

Indonesia (1 826 440) 64 679 111 049 157 968 207 302 264 444 278 907

Japan (374 744) 45 453 42 715 36 723 29 639 24 887 23 439

DPR Korea (120 410) 3372 4200 4780 1863 2200 2200

Republic of Korea (98 190) 641 7699 13 976 12 062 5076 4908

PDR Lao (230 800) 2498 7200 11 268 25 010 47 500 56 730

Malaysia (328 550) 2819 3948.8 9152.6 17 091 18 922 23 139

Myanmar (657 740) 0 28 53 0 30 000 32 100

Nepal (136 800) 599 1661 7325 9047 11 749 12 135

Philippines (298 170) 18 666 58 423 82 197 83 380 116 806 132 136

Singapore (683) 2726 4600 3200 3625 3194 3817

Sri Lanka (64 740) 0 0 0.2 80 64 147

Taiwan (32 260) 74 688 73 726 68 349 55 687 79 524 82 223

Thailand (511 770) 14 257 38 716 91 179 154 646 191 119 197 343

Total 252 873 421 356 714 926 1 295 762 2 285 278 2 451 492

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culture and in seasonal and semi-enclosed waters(70%). The bulk of the aquaculture practices are ex-tensive having small amounts of semi-intensive andintensive culture, 10% and 1% respectively (FAO2001; Islam 2001). Fish consumption contributed tothe bulk of the animal protein consumption of the lo-cal population (FAO 2001) of which freshwater ¢shcontributed more than 50% to the intake. Rural ¢shconsumption was approximately 16% tilapia and28% carps, and accounted for 15% of food expendi-ture and 11% of total expenditure (Dey et al. 2000).Bangladesh has a large freshwater ¢sheryand the re-lative contribution of cultured ¢sh volume is aboutthe same as for the capture ¢sheries in recent years(FAO 2002). The dominant species are silver carp fol-lowed by rohu, catla, mrigal, grass and common carpand other unidenti¢ed ¢sh species. The contributionof cultured alien species is high and its contributionto the total food ¢sh production is increasing (28%of cultured ¢n¢sh in 2002) (Table 2) suggestingthat cultured alien ¢n¢sh currently contribute sig-ni¢cantly to the food security particularly in ruralBangladesh.

India

The freshwater inland aquaculture production of In-dia (2076734 tonnes in 2002) is second only to thatof China. Freshwater aquaculture in the 1980s andearly1990s was almost exclusively based on the pro-duction of indigenous species. However, in the late1990s alien species production increased appreciablyto the current level of �40% in 2002. The alien spe-cies cultured are common carp and Chinese majorcarps, of which silver and grass carps are most signif-icant; common carp and silver carp production isalmost equal with a lesser volume of grass carp.

Indonesia

Indonesian inland culture is dominated by the alienproduction of common carp, which is highly re-garded by the local population. Di¡erent strains ofcommon carp are popular in di¡erent areas. Com-mon carp culture was mostly in rice paddy-¢sh pro-duction in 1986^1995. However, there is a shifttowards more pond and cage culture, while paddy-¢sh culture still remains the higher contributor (Kon-tara & Maswardi 1999). Pond and cage culture aremostly semi-intensive, arti¢cial food, such as ricebran or feed pellets, being used on a regular basis

(Supardan 1993). The production of Nile tilapia hasalso increased to signi¢cant levels in recent years, inparticular when cage culture of Nile tilapias was en-couraged, as an alternative livelihood to displacedpersons, in a number of reservoirs that wereimpounded in the last 40 years or so (Costa-Pierce &Hadikusumah1990).

Lao PDR

Freshwater aquaculture in PDR Lao is dominated bythe production of tilapias and common carp, withmodest amounts of the Chinese and Indian majorcarps. Production is almost entirely ( � 95%) com-posed of alien species. An estimated 31% of all ¢shin the market in Lao are from aquaculture ponds;the species composition of cultured ¢sh available inmarkets was estimated at only 0.4^7.1% indigenousspecies (Bush 2003). Thus, cultured alien speciesmay account for more than a quarter of ¢sh con-sumption in Lao PDR.

Philippines

In the early1980s the bulk of freshwater aquacultureproduction in the Philippines was indigenous species( � 75%), after which there has been a sharp changeto the production of alien species, contributing about90% to the current production.The majority of fresh-water aquaculture production in the Philippines isNile tilapia, with modest amounts of cyprinids nei,most likely Chinese major carp species such as big-head carp (Guerrero 2000), other tilapias and indi-genous species. Tilapia is the most preferred andconsumed ¢sh species, and has by-passed the tradi-tionally preferred milk¢sh. Despite the high level ofconsumer preference, tilapia prices are still relativelylower than other popular species (Dey et al. 2000).

China

China is not only the world’s most populous nation,but also one in which the per capita consumption ofaquatic foods is very high (Ye1999). It is therefore notsurprising that in order to meet the increasing de-mand for aquatic food supplies, in thewake of dwind-ling capture ¢shery supplies, all forms of aquaculturehave developed at a considerable pace over the lasttwo decades in China (Yiangliang 2001; FAO 2004).It has been pointed out that public sector policies

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stimulated the aquaculture developments in Chinaover the last two decades, and continue to do so evento date (FAO 2004).China is the leading aquaculture nation in the

world, accounting for 71.2% of all cultured organ-isms produced globally and 65.8% and 70.8% of allcultured ¢n¢sh and inland ¢n¢sh production respec-tively. In general, China, among Asian nations, is oneof the least dependent onalien ¢n¢sh for aquacultureproduction, themain cultured alien species being thetilapias. China is blessed with a very diverse inland¢sh fauna and among which are some of the fastestgrowing species, in particular the Chinese majorcarps, feeding low in the food chain, and relatively ea-sily propagated arti¢cially, all traits that contribute tothe aquaculture success in China. It is needless topoint out that freshwater ¢n¢sh aquaculture is veryvaried in China, and no attempt will be made here tosummarize the vast array of aquaculture practicesthat vary from extensive to intensive practices ofmono- and polyculture, in ponds, cages, pens, smallreservoirs and lakes, etc. Details on Chinese aquacul-ture were reviewed byYiangliang (2001).

Thailand

The aquaculture developments in Thailand over thelast15 years or so have focussed primarily on shrimpculture and associated issues, making Thailand thecurrent leader in shrimp production (FAO 2002),and literally pushing freshwater ¢n¢sh culturesomewhat backstage. Indeed, the spread of shrimpculture inland, in particular the exotic Penaeus van-namei (Boone), has further reduced the emphasis oninland ¢n¢sh culture (H. Kongkeo, pers. comm.).Freshwater ¢n¢sh aquaculture in Thailand is domi-nated by the production of alien species, in particularNile tilapia followed by hybrid cat¢sh. Moderateamounts of common carp together with smallamounts of rohu, mrigal, silver carps and Mozambi-que tilapia are produced, and in total alien speciesaccount for 66.9% of the total inland ¢n¢sh aquacul-ture production in the country, and marginally lessso in value (61.0%).

Problems associated with cultured alienfinfish in Asia

Problems associated with translocations, thereforealien species, can be manifold. These could bebroadly classi¢ed as problems that impact the indi-

genous £ora and fauna almost immediately, such asthose resulting from the accidental transfer of a viru-lent pathogenwith the ¢sh species, and/or long-terme¡ects that are not easily observed and/or when acause/e¡ect cannot be discerned objectively. The lat-ter e¡ects tend to be ecological and/or genetic. Oftenthe socio-economic impacts resulting from translo-cations tend to be ignored. Bartley and Casal (1998)pointed out that such considerations are of signi¢-cance when impacts of introductions on conserva-tion and biodiversity are assessed.Introduction of new species across nations, geo-

graphical regions, river basins and inter-conti-nentally have been a common practice in Asia,primarily for aquaculture purposes (Welcomme1984, 1988; De Silva 1989; DIAS 2004). Beverton(1992) reviewing the global state of introductionsconcluded that the majority of introduced specieshave proved either non-viable or ecologically neutral;a small proportion has been bene¢cial, and some, no-tablygeneral colonizers and powerful predators, haveseriously harmed the native ¢sh fauna. Of the 1155¢sh introductions for aquaculture (global) only 6.8%and 0.7% were considered ecologically and socio-economically adverse, respectively, but 4.5% and24.5% were bene¢cial (Bartley & Casal1998).

Diseases/pathogens

There are records of introduction of metazoan para-sites associated with intra-continental ¢n¢sh trans-locations in Asia, in the early periods of aquaculturedevelopment in the region. For example, the cestodeworm Bothriocephalus gowkonensis was introducedinto Sri Lanka with the introduction of Chinese ma-jor carps into the island for aquaculture purposes(Fernando & Furtado 1963). Fernando and Hanek(1973) reported the introduction of three copepodspecies, with alien species but further details werenot given. However, none of these parasites, or forthat matter any parasite introduced through alienspecies in Asia, have been reported to reach the scaleassociated with fungal plague that decimated theEuropean cray¢sh stocks that in turn is linked to thetranslocation of the North American cray¢sh(Galuppi, Quaglio, Maxia, Morolli & Tampieri 2002;Edgerton, Henttonent, Jussila, Mannonent, Paaso-nens,Taugb�l, Edsman & Souty-Grosset 2004). Also,in general there had not been an update of metazoanparasite transfers associated with ¢n¢sh introduc-tions in the last decade or so, and this could be aresult of improved quarantine measures that are

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practiced in association with introductions (FAO/NACA 2001).In the recent years the emphasis has been on the

introduction of bacterial and viral pathogens asso-ciated with translocations in aquaculture, and themost recent being the transfer of the ‘Koi herpesvirus’ into Indonesia, purportedly from Hong Kong,that resulted in adverse a¡ects on the carp aquacul-ture industry in Indonesia (Bondad-Reantaso 2004).Arthur (2005) listed the seven trans-boundary dis-eases in cultured ¢n¢sh in the Asia-Paci¢c region, ofwhich ¢ve are listed by the O⁄ce International desEŁ pizooties, and pointed out the enormity of the pro-blem facing the sector in the region (Table 3).

Ecological impacts

Introductions intoAsian nations, inter- and/or intra-continentally, fortunately, thus far, have not causedmajor impacts resulting from predation, such as thatcomparable with the introduction of the Nile perch(Lates niloticus L.) into Lake Victoria, East Africa,which is purported to have resulted in the loss of hap-lochromine species indigenous to the Lake (Barel et al.1985; Ogutu-Ohwayo1990).The concurrent issues re-lated to this introduction are complex and the dialo-gue and debate on such issues are expected tocontinue for a long time, and perhaps a consensuson gains/losses, in relation to ¢sheries, biodiversity,ecological and socio-economic status, resulting fromthis translocation will never be reached (Ogutu-Oh-wayo 1990; Kudhongania,Twongo & Ogutu-Ohwayo1992; Riedmiller 1994). However, there has beenmuch controversy with regard to the introduction ofcichlid species, in particular O. mossambicus and O.niloticus, for aquaculture purposes into some Asiannations and their negative impacts on indigenousstocks, in spite of the fact that Nile tilapia was pur-ported to be the ‘aquatic chicken’ for Asia (Smith &

Pullin 1984), and currently it contributes over a mil-lion tonnes toAsian aquaculture production.Of these the most cited detrimental e¡ects because

of tilapias in the region have been alleged in the caseof the near extinction of the small endemic goby(sinarapan), Mistichthys luzonensis (Smith), in LakeBuhi, in the Philippines (Baluyut 1983; Soliman1994). However, a critical examination of the avail-able evidence suggests that the decline of sinarapanwas probably a result of a number of factors, the leastin£uential of these being the presence of the exoticcichlid (De Silva et al. 2004). It is heartening to notethat with better management of the ¢shery activitiesin Lake Buhi, sinarapan is staging a recovery (DeSilva et al. 2004).De Silva et al. (2004) reviewed the status of alien

tilapias in the Asia-Paci¢c region, and considered indetail the possible negative impacts on indigenousstocks and on biodiversity, in general. These authorsconcluded that tilapia species introduced into Asia,all for aquaculture purposes, have had no apparentnegative impacts on biodiversity, either directly and/or indirectly, and that the purported negative e¡ectsare mostly ill founded. However, these authors cau-tioned that general degradation of habitats in Asiabecause of rapid and often unplanned developmentcould permit a generalized, non-catholic, tolerantspecies such as O. niloticus and O. mossambicus to be-come invasive (De Silva et al. 2004, 2005).In Asia, one of the worst documented negative ef-

fects on ¢sh biodiversity has resulted from withincountry translocations, in Donghu Lake, Wuhan,China, when the introduction of grass carp resultedin the decimation of submerged macrophytes, andthe consequent ecological changes brought aboutan upsurge of bighead and silver carps and also thedisappearance of most of the 60 ¢sh species nativeto the lake (Chen1989).

E¡ects on genetic diversity

Although Asia has experienced a large number oftranslocations of aquatic species since World War I(Welcomme 1988; De Silva 1989; Welcomme &Vidthayanon 2003), there is very little informationavailable on Asian ¢sh species with regard to genetice¡ects arising from hybridization and introgression.Past experiences, particularly from the west, and onsalmonids, emphasize the extent to which cultured/hatchery bred alien species/stocks, either throughescapement and/or as a result of being used for

Table 3 Summary of occurrences of OIE listed trans-boundary diseases in ¢n¢sh in the Asia-Paci¢c region, andthe individual nations a¡ected (based on data from Arthur2005)

Disease Countries affected

Marteilia sydneyi Australia

Mikrocytos roughleyi Australia

Perkinsis olseni/atlanticus Korean Republic, Australia

Haplosporidium nelsoni Japan

Marteilioides chungmuensis Japan

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purposes of stock enhancement, have a¡ected indi-genous stocks. There are many such examples fromNorth America, especially in respect of salmonid spe-cies (Dowling & Childs 1992; Leary et al. 1993; Allen-dorf & Leary1998).To date, there are onlya limited numbers of studies

of genetic e¡ects arising from hybridization and in-trogression conducted in Asia, which are attributedto aquaculture developments (Nguyen & Na-Nakorn2004; Simonsen, Hansen, Sarder & Alan 2004). Sena-nan, Kapuscinski, Na-Nakorn and Miller (2004),based on diagnostic alleles at six allozyme loci andonemicrosatellite locus, demonstrated that introgres-sion of African cat¢sh (C. gariepinus) genes into thenative walking cat¢sh C. macrocephalus has occurredin four wild and two broodstock populations in cen-tralThailand. Introgression in12 natural populationsof C. macrocephalus, nine from Chaophraya river sys-tem, one from Mekong river system, two from thesouth and a hatchery population was also reportedby Na-Nakorn, Kamonrat and Ngamsiri (2004). Allthese observations show that the native gene poolsof C. macrocephalus have been diluted and are threa-tened and if appropriate management strategies arenot undertaken, according to Na-Nakorn et al. (2004)the indigenous walking cat¢sh will be potentiallythreatened as a result of massive backcrossing withhybrid cat¢sh, which is the preferred cat¢sh of Thaicat¢sh farmers (Na-Nakorn 2004). Acomparable pro-blem has occurred in Bangladesh through the use ofhybrid C. batrachus � C. gariepinus for aquaculture(Rahman, Bhadra, Begum, Islam & Hussain1995).Any factors such as predation, competition and

disease transmission from exotic species/populationscan reduce population size of native species andeventually lead to inbreeding, and consequently re-sult in loss of genetic diversity. Although inbreedingcan only happen when the bottleneck is severe andlasts for many generations, it has signi¢cant long-term negative e¡ects. In the short term, signi¢cantreduction in population size can disrupt various de-mographic features of a population and may lead toextinction from severe perturbation in environmen-tal conditions (Waples1991).The presence of alien species mayalso alter the ge-

netic composition of native ones via the change in se-lective pressures, for example predation on certainphenotypes or competition for a certain kind of food.These changes are di⁄cult to demonstrate. However,the potential for deleterious e¡ects on native stocksare likely and need to be considered in the planningstages of any translocation events.

In Asia, there is a general resurgence of culture-based ¢sheries in small water bodies, a practicewhichuses existing water resources, suits rural com-munities and is less resource intensive (De Silva2003), and stock enhancement practices to supple-ment existing artisanal ¢sheries. Most Asian nationsutilize alien species in culture-based ¢sheries (De Sil-va 2003) and in both practices there is a greater prob-ability for the stocked species to spread into naturalhabitats and mingle with indigenous species, thanin conventional aquaculture. In fact there is evidenceaccumulating from Thailand that stock enhance-ment practices using captive bred Java barb, Barbodesgonionotus (Bleeker), although an indigenous species,have led to an alteration in genetic diversity of natur-al populations (Kamonrat1996), a clear warning sig-nal for the need formore planned stockenhancementprogrammes in the region.

E¡ects on production of indigenous species

Although up to now it has been demonstrated thataquaculture of alien species in Asia has had a posi-tive impact on production, there are isolated in-stances that escapee cultured alien species havedisplaced and markedly reduced the production le-vels of supposedly more desirable indigenous species.One such example is reported from the ¢shery of theGobhindasagar reservoir, Himachal Pradesh, India,where escapees from aquaculture (cage) practices ofsilver carp gradually replaced the more desirable in-digenous Indian major carp, catla, within a few years(Sugunan1995). In the same incident it was reportedthat cultured common carp also entered the reser-voir and 3 years later were purposely stocked for cap-ture ¢shery purposes, but despite this common carpcatches have remained at low levels (Sugunan1995).Although cause and e¡ect cannot be discerned, it isprobable that the establishment of silver carp breed-ing populations, in the upper reaches of the reservoir,itself a very rare occurrence, mayhave led to this cur-rent situation. A comparable situation is inwhich es-capees from cage culture operations of alien Niletilapia in Kaptai Lake (man-made), Bangladesh, arepurported to have been responsible for the decline ofcatches of Indian major carps in the reservoir (Hal-dar, Ahmed, Alamgir, Akhter & Rahaman 2002). Inthis instance, however, the available evidence doesnot suggest that Nile tilapia has been responsible forthe decline in the major carp catches, but that distur-bance to the spawning grounds of the latter andcatching of juveniles appear to be.

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Marketing

Cultured alien species in the past mostly served thedomestic markets. Indeed, even now cultured aliencarp species cater to the domestic markets. However,the most dramatic change has occurred with cul-tured Nile tilapia, which is being increasingly ex-ported to countries such as the USA and Europe. Forexample, tilapias account for nearly 70% (Fig.8) of allimported freshwater ¢sh to the USA, and of this im-ports from Asia account for about 80%. Moreover,the notion that some of the alien species, e.g. tilapias,are ‘poor man’s ¢sh’ is no longer true, and culturedalien species are now often a major ¢sh commoditysold in urban markets. For example, in the Philip-pines, Nile tilapia has displaced the traditional milk-¢sh, Chanos chanos, as the most popularly consumed¢sh species, and in Indonesia two of the mostfavoured freshwater species are alien species ^ Niletilapia and common carp.

Conclusions

Global ¢sh production and consumption patternshave witnessed marked changes since the early1970s. One of the main trends has been an increasein consumption of ¢sh in the developing world, esti-mated to account for approximately 70% of globalproduction (Delgado, Wada, Rosegrant, Meijer &Ahmed 2003). Also, it is reported that the consump-tion of freshwater ¢sh has increased more than10-fold between 1981 and 1997, amounting to about20^25% of the animal protein intake, particularly inrural populations in the developing world (Delgadoet al. 2003). These changes have brought about anincreasing emphasis on the development of inland

¢sheries as a signi¢cant contributor to rural ¢shfood supplies (Welcomme & Bartley 1998; De Silva2003).Overall, cultured alien species play a major role in

the sector and are an important ¢sh food source andan income-generating source in Asia, currently ac-counting for nearly12% of the inland cultured ¢n¢shproduction (2.6 million tonnes), valued at US$ 2.59billion. As most inland ¢n¢sh aquaculture in Asiatends to be predominantly rural and small scale, theimpacts on the socio-economic conditions of the rur-al sector are believed to be signi¢cant, even though itis di⁄cult, if not impossible, to quantify themonetarygains. Moreover, it provides a relatively easilyaccessi-ble fresh and an a¡ordable source of much neededanimal protein to rural communities. However, withincreasing emphasis and public concerns with re-gard to alien species (Naylor et al. 2001) and theirin£uences on biodiversity, it is important thatcontinued vigilance is maintained on their impactsand suitable and urgent action be taken when nega-tive impacts are observed. Although many alien spe-cies have established themselves to becomeintegrated into the local biota, and have not resultedin any noticeable adverse impacts on biodiversity,somewell-intentioned ones have had unexpected ne-gative consequences, if not in Asia but elsewhere.Theunpredictability of the impacts of alien species ledMoyle, Li and Barton (1986) to call the general resultof deliberate introductions as a ‘Frankstein E¡ect’.The uncertainty of negative impacts arising from in-troductions, as well as the possibility that such im-pacts could be manifested after a long lapse of time,makes studies of impact over long periods of time im-perative. This has been seriously lacking in Asia. Infact, to the authors’ knowledge there had not been asingle dedicated study on impacts on any of the alienspecies in Asia. The current assessments are mostlybased on isolated, short-term studies. The need fordedicated impact assessment studies is increasinglywarranted because there is a general deterioration ofenvironmental conditions, because of ever increas-inganthropogenic activities.The latter make the con-ditions less congenial to indigenous species andconsequently favour alien, robust species such astilapias, possibly leading to designating such alienspecies as invasive, possibly unjusti¢ably. It is heart-ening, however, that in a recent general assessmentof the impacts of alien species in the Lower MekongBasin (LMB), this being reputed to have the most di-verse ¢n¢sh fauna in Asia and the source of a largecapture ¢shery (Mattson 2005), it was concluded that

0

10000

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40000

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1995 1996 1997 1998 1999 2000 2001 2002 2003

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Figure 8 Changes in the imports of tilapia (in quantityand value) and the quantity of tilapia imported as a per-cent of the total freshwater ¢sh imports to the US from1995 to 2003. (data source: http://www.st.nmfs.noaa.gov/st1/trade/trade).

Aquaculture Research, 2006, 37, 1^17 Alien ¢n¢sh in Asian inland aquaculture S S De Silva et al.

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the impacts in the LMB have thus far beenveryminor(Welcomme & Vidthayanon 2003).From a production viewpoint alien species play a

major role in Asian aquaculture, and their role hasbeen increasing with the growth of the sector in thelast two decades, and is expected to continue to in-crease in the foreseeable future. Indeed, there are ex-amples from outside Asia, such as in Chile, wherealmost the whole aquaculture industry is based onalien species (Gajardo & Laikre 2003). The relativelyhigh dependence of Asian aquaculture on alien spe-cies may also well be an accident of history, in thatthe initial successes on arti¢cial propagation wereprimarily achieved with Chinese and Indian majorcarp species, which was a determining factor on thepopularization of the sector.On the other hand, in small islands where ¢sh

biodiversity is low as shown by the theory of islandbiogeography (McArthur & Wilson 1967), speciessuitable for aquaculture may be rare and/or unavail-able in the indigenous ¢sh fauna and as such depen-dence on alien ¢sh species in these countries isinevitable, if aquaculture is to be chosen as a meansfor food security. As discussed previously alien Tila-piine ¢sh, which continue to provide a cheap sourceof animal protein for rural poor, as well as supportintensive practices, in someAsian countries, providea good example in this regard.In the above overall context, and as pointed out by

Bartley and Casal (1998), the issue is not to prohibitalien species or to abandon regulations of theirmovement, but rather assess risks and bene¢ts asso-ciated with their use and then, if appropriate, developand implement a plan for their responsible use.Whatis most urgently needed in the Asian context are fo-cussed, scienti¢c studies on the impact of alien spe-cies on biodiversity and their potential invasiveness,and perhaps a concerted e¡ort to encourage theaquaculture of indigenous species, without compro-mising the socio-economic balance. Admittedly,there has been recognition of the latter and an up-surge in e¡orts to develop aquaculture techniquesfor indigenous ¢n¢sh and popularization thereofin the Asian region (Bakos 1997; Sverdrup-Jensen2002; Ingram, Sungan, Gooley, Sih,Tinggi & De Silva2005), and also elsewhere (Ross & Beveridge1995).

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