Research articles NAT. NAT. HIST. BUL L. SIAM Soc. 49: 161-176 ， 2001

THE ECOLOGY AND CONSERVATION OF THE SMALLSCALE CROAKER BOESEMANIA MICROLEPIS (BLEEKER 185 8- 59) IN THE MAINSTREAM RIVER ，SOUTHERN

lan lan G. Baird ，Bounpheng Phylavanh ，Bounthong Vongsenesouk ， and Khamsouk Xaiyamanivoni

ABSTRACT

百 le smal1scale croaker Boesemania microlepis is a large species in the perciform fami1y Sciaenidae reaching over a metre in length and at least 18 kg in weigh t. Although drums and croakers croakers from 白is family 訂 e marine main1y ，this freshwater species is found in th 巴 Mekong ， Chao Phraya ，Nan ，M 巴k10ng ，Tachin ， and Bang Pakong River basins. It occurs in the Mekong

River River basin in Tb ailand ，Laos ， and ， and parts of peninsul 釘 and Westem . In the Mekong River ，the species is known at least as far north as Louang Phrabang Phrabang in northem Laos. This paper reviews the status of mainstream Mekong River populations populations of 出 e smal1sca1e croaker in southem Laos. We have identified seven known spawning spawning grounds in Kh ong Dis 町ict ， Champasak Pr ovince ， southem Laos based on 出e dry- season season occurrence of croaking individua1s in spawning condition in these areas. We discuss habitat habitat usage and 出 e acoustic sounds made by smallscale croakers. Conservation Zo nes

have have a1r eady been estab1ished to benefit the species ，加d 由is may be an excellent strategy for protecting protecting populations. Boesemania microlepis be considered should for special IUCN conser-

vation vation listing ， based on widespread reports of severely declining of stocks 出is popul 訂 and expensive expensive food fish in southem Laos and northeast Cambodia

Key words: Laos; Mekong River; capture fisheries; biological conservation; fish acoustics

INTRODUCTION

百le smallscale croaker ， Boesemania microlepis (BLEE 阻 R ，1858-59) ， is a benthopelagic fish fish species found in the Mekong ，Chao Phraya ，Nan ，Meklong ，Tachin and Bang Pakong Rivers and some connecting canals in (WONGRATANA ，1985). 泊施 speci is found in in parts of the Mekong River basin in Laos ，Cambodia and Vietnam (WONGRATANA ， 1985; BAIRD ET AL. ，1999) ，and has also been reported from Indonesia and West Malaysia (WONGRATANA ， 1985; Ko 廿 ELAT ET AL. ， 1993; M. Kottelat ，personal communication ，May 1999). 1999). In the Mekong River basin in the Lao People's Democratic Republic (Lao PDR or Laos) ， this large drum or croaker (Order : Sciaenidae) is res 仕icted to the mainstream Mekong River and large lowland 位ibutaries close to the Mekong. Al though most sciaenid croakers 訂 e marine (DAVIDSON ， 1975; GOULD 町 G ， 1980; WONGRATANA ， 1985; 1985; SASAKI ，1997) ， B. microlepis is one of the very few 出at is a freshwater species. In

町(伺刷l a剖all 制aut 白ho 併rs 吟) P.O. Box 86ω0 ，Pakse 弘， Lao PDR. E-mai1: ian 初nba 剖ir 吋d@lao ∞ne t. net Received Received 20 Ma 訂rch 2001; accept 飽ed 27 Ju1y 200 1.

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Fig ure. 1. Lo w 巴r Mekong Basin ECOLOGY AND CONSERVATION OF SMALLSCALE CROAKER 163 the the Mekong River B. microlepis occurs at least as far upriver as Louang Phrabang in northern northern Laos (DAVIDSON ，1975). 百le smallsc a1 e croaker is one of the most desirable and highly highly priced 1釘 'ge food in the Me ¥k ong region (BAIRD ， 1994; BAIRD ET AL. ， 1999). There There has been considerable confusion in the past regarding the taxonomic status of Boesemania Boesemania microlepis. CHEVEY (1 934) ，SERENE (1 951) ，BARDACH (1 959) ，KAWAMOTO ET AL. (1972) ，TAKI (1 974) ，DAVIDSON (1 975) ，YEN ET AL. (1992) ，and others have reported the the species under the names Sciaena soldado ， Nibea soldado ，Otolithoides aenecorpus ， lohnius lohnius soldado and Pseudosciaena soldado. SMITH (1 945) did not list the species as occurring occurring in 百四land. WONGRATANA (1985) finally clarified its true taxonomic identity. RA 町 BOTH (1996) listed the maximum length of B. microlepis as not exceeding 28 cm (SL) ，YEN ET AL. (1992) listed its maximum length as 355 mm ，t北 i (1974) and DAVIDSON (1975) (1975) bo 血 listed maximum length as 45 cm ，and Ko 甘'EL AT ET AL. (1993) listed it as 550 mm. WONGRATANA (1 985) got closer to the truth ，listing maximum weight as at least 6 kg ， with reports from the Menam Noi in Ayutthaya Pr ovince ，τbailand ， of individuals attaining attaining 10 kg. We have seen a number of specimens of 出is species over a me 位e in length ，and in 1993 ， the first author observed at least one that weighed 18 kg from the Mekong River at Ban Hang Kh one ，Kh ong District ，Champasak Province ， southern Laos. SASAKI (1997) also records maximum length for the species as being 1m. DAVIDSON (1975: 95) provides the following description of B. microlepis: It “It is a light brown fish ， with a greenish or grayish tint ， with obscure ripple ripple markings over the upper side of the body 叩 da light underside.τbe fins fins are pa1 e brown to yellow ， but the tail fin is blackish and the spiny dorsal fish fish has a dark edge."

The smallsc a1 e croaker is named "pa kouang" in Lao ， "pla ma" 叩 d “pla hang kew" in in Thai ，“ trey promah" or “trey pama" in Khmer ，and “Ca suu" in Vietnamese (WONGRATANA ， 1985; YEN ET A L.， 1992; RA 凹 BOTH ， 1996; BAIRD ET AL. ， 1999). In In recent years ， there have been persistent reports from fishers in southern Laos and northeastern northeastern Cambodia 白紙 smallscale croaker populations have declined. In some 訂 eas where the species was apparently once common ，it is now either very rare or virtually non- existent existent (ROBERTS ， 1993; BAIRD ， 1994; ROBERTS & BAIRD ， 1995; BAIRD ET AL. ， 1999). B. microlepis microlepis is possibly very sensitive to polluted water ， like other euryhaline fishes (WONGRATANA ， 1985). For 出is reason ，WONG 貼 TANA (1 985) reported 伽 t the species is expected expected to be extirpated from many freshwater habitats in Thailand ，especially the lower courses courses of rivers. Th is expectation was based on 白e gradual disappearance of the species from the tidal influent area of the Tachin River since 1980 (WONGRATANA ， 1985). Unlike Unlike most other species of fish in the Mekong River in northeastern Cambodia and southern southern Laos ， the dry season habitat and presumed spawning grounds of the sm a1 1scale croaker croaker c叩 be easily identified. Medium to large sized B. microlepis make loud croaking voc a1 isations in these deep-water areas (BAIRD ET AL. ，1999). 百lI s deep croaking sound ， described described as “oot-oot-oo(" by the Lao ，is often audible even above the surface of the water ， and it makes the croakers vulnerable to directed gillnet fishing during their spaw 凶ng season. season. Th is species may significantly benefit 合om deep-water Fish Conservation Zones (FCZs) (FCZs) in the mainstream Mekong. In creases in populations of this species in and ぽ ound particular particular FCZs in Kh ong District ，Champasak Province ， southern Lao PDR have been reported. reported. Th ese apparent successes may be a result of villagers having protected B. microlepis 164 164 LAN G. BAIRD ET AL. d回:p -water spawning habitat within the boundaries of certain FCZs (BAIRD ， 1996; HOGAN ， 1997; 1997; BAIRD EI AL. ， 1999). 百lis paper reviews the known ecology and conservation status of smallscale croakers 初出eS 増handone We t1 and area in southem Laos. Seven d関 p-water spawning grounds in Kh ongDis 凶 ct are identified ， based on 血eoccu 町'ence of B. microlepis vocalisations d町 ing the the species' spaw 凶ng season. 百le use of villager-m 阻 aged FCZs as a tool for m 釦 aging 佃 d conse 円泊g smallscale croakers is discussed.

BACKGROUND TO STUDY AREA

百 e Siphandone We t1 and 紅 ea (which literally means 4000 islands) is 1紅 gely siωated in in Kh ong Dis 住ict ，Champasak Province. It is a complex of channels ，rapids ，waterfalls ， islands islands of various sizes ，seasonally inundated forests ， and deep-water pools included as p釘 t of the mainstream Mekong River in 出e ex 佐eme south of Laos (see figure 1 map of Kh ong 泊 regional setting) (CLARIDGE ， 1996; CES 羽， 1998; ALTOBELLIEI AL. ，1998). 官 lere are are approximately 65 ，000 people 泊Kh ong ， the v出 tmajority of whom are ethnic lowland Lao rural peasant semi-subsistence paddy rice farmers with a long history of inhabitation of of the 紅白. Approximately 94 percent of families in 由.e dis 凶 ct subsist on fishing and many sell fish to generate income. 百le estimated total fish catch for the dis 凶.ct in 1996/ 1997 1997 w 部 4000 metric tonnes ， and US$ 1，000 ，000 worth of fish was reportedly exported from the district during the same year (BAIRD ET AL. ，1998a). 百le wild capture fisheries of of Kh ong may be more important 出m 泊飢y other dis 凶ct in Laos. Of 白e 136 villages 泊Kh ong ，86 are situated on islands ， and most of the rest are located along the e槌 tembank of of the Mekong River (BAIRD EI AL. ， 1998a). Th e aquatic environment of the area is characterised characterised by high diversity and productivity. Over 200 fish species have been found in fish fish catches throughout t1t e dis 凶ct since 1993 (BAIRD EI AL. ， 1999).

島ffiTHODS

百lis paper draws upon a literature review and compilation of information about t1t e smallscale smallscale croaker Boesemania microlepis collected during field studies 泊Kh ong Dis 町ict ， Champasak Pr ovince ，southem Lao PDR and other p釘 ts of Laos and northeastem Cambodia between between January 1993 and May 1999. During much of 白紙 period ，t1t e au t1t ors were based 泊 the small rural fishing and fi 紅 m 泊g community of Ban Hang Kh one ，which is situated below below the Kh one Falls in the extreme south of Kh ong Dis 凶ct (s 関 ROBERTS ， 1993; ROBERTS & BAIRD ，1995). At Ban Hang Kh one a wide r佃 geof 紅白泊 al fisheries have been monitored and studied ，some of which catch smallscale croakers (BAIRD ， 1998a). Stomach contents for for B. microlepis and other species have also been investiga 旬d (BAIRD ，1998b). 官 le authors authors have relied heavily on “local wisdom" or “local ecological knowledge" (LEK) as a foundation for studying 血巴 complex biological ，ecological and sociological processes that that effect fish and fisheries in 血e Siphandone We t1 and area. Over the years ，large numbers of of fishers 泊 villages throughout Kh ong and other p紅白 of sou t1t em Laos and northeastem Cambodia have con 凶buted to 0 町 pool of knowledge by providing information about B. microlepis microlepis and other species offish. We have also observed the seasonal cro 北泊gbehaviour ECOLOGY AND CONSERVATION OF SMALLSCALE CROAKER 165

of of the B. microlepis in seven deep-water areas in Kh ong. Much of the basic biological information information we have collected on the species has recen t1 y been published in Lao language (BAIRD ET AL. ，1999) ， but most of the details 釘 'e presented here 泊 English for the frrst time. Although Although we have been using basic hydrophones to record the sounds of B. microlepis microlepis for many years ，J. F. Borsani from 出e Tethys Research In stitute in Mil 佃， Ita1 y， conducted conducted the first systematic acoustic survey of B. microlepis vocalizations in 血.e Ban Hang Kh one area 泊 early April 1998. A basic overview of his research is presented based on BORSANI (1 999).

RESULTS AND DISCUSSION

Boesemania microlepis Spawning Grounds

Field Field investigations throughout Kh ong Dis 凶ct between 1993 and 1999 have determined that that there 釘 'e presently seven confrrmed spawning grounds for B. microlepis in the Mekong River River in 血e Dis 凶ct.百 le vi1l ages adjacent to areas these 訂 e shown on Figure 2. From south south to north ，血 e areas 紅 e described below. 1. 1. Boung Pa Kouang/Boung Pa Chouk (spelled Boong Pba Gooawng and Boong Pba Jook in ROBERTS & BAIRD ，1995). 百lese areas are adjacent 25 to 30 m deep-water pools with with 蜘 ep rock sides ， hard rock and sand substrate ， slow counter-current eddies in 批命y- season ，and m 組 yl 紅 ge boulders at the bottom. Both 紅 eas 釘 'e below the Kh one Falls and adjacent adjacent to the Cambodian border ， south of the communities of Ban Hang Kh one and Ban Hang Sadam. Th e northern channel adjacent to the main pools has been protected by Ban Hang Kh one vi1l age as a dry season FCZ since 1995 ， but the main southern channel where sm a1 1sc a1 e croakers and Irra waddy dolphins Orcaella brevirostris 紅 'e most abundant remains unprotected ，and there is extensive gillnet fishing by Laotians 釦 d Cambodians in 出e area. Cambodians commonly use explosives for fishing during the dry season (BAIRD & MOUNSOUPHOM ， 1997). In 1993 ，Ban Hang Kh one transformed the main spawn 泊g 訂 'ea for sm a1 1sc a1 e croakers ，Boung Pa Kouang (appropriately named after B. microlepis) ， into a vi1l age FCZ ， but Cambodians 仕om the other side of the border refused to respect the protected protected status of the area ， leading to its dissolution 泊 1995 ， and the establishment of a replacement replacement FCZ in the 20m deep inner channel of Boung Pa Chouk ，which is closer to the the vi1l age and outside of 釘 'eas where Cambodians sometimes fish. 百lis area is an important spawning ground for vocalising Hypsibarbus malcolmi (p a pak kom or pa pak nouat) 泊 November-December (ROBERTS & BAIRD ， 1995; BAIRD & PHYLAVANH ， 1999).

2. 2. Veun Tholathi. 百lis is a 25 ・m deep pool with slow counter-current eddies 泊出e 命y season ，a hard rock and sand substrate ，steep sides ， and many large logs on the bottom. 百le 訂 'ea is direc t1 y in front of Ban Don Th olathi ，a small island community situated just no 吋1 of of an extensive we t1 and forest 釘 'ea above the western side of 白e Kh one Falls.τ 'h e approximately approximately 400 x 300 ・m area has been protected as av i1l age FCZ since 1994 ，組d it is one one of the best examples in Kh ong Dis 位ict of how FCZs Can benefit spawn 泊g populations of of B. microlepis. 166 L AN G. BAIRD ET AL.

MounJapamo k Olab1c:t

Fi gure. 2. Location of Boesemania microlepis spawning sites. ECOLOGY AND CONSERVATION OF SMALLSCALE CROAKER 167

Villagers Villagers from Ban Don Th olathi believe that B. microlepis dry-season croaking has

increased increased in Veun Tholathi every ye 訂 since the establishment of 出e FCZ ，and they have reported reported increases in catches of juveniles since establishing the FCZ. In 1999 ，villagers heard heard B. microlepis croaking not only in the FCZ ， but also ， for the first time in many ye ぽ s， in in adjacent 釘 eas ，including the Tha Vat and Hang Don Kouay 紅 eas. Further research and monitoring monitoring is required regard 泊g changes in B. microlepis populations and habitat use ne 釘 Ban Don Th olathi.

3. 3. Khoum Pa Tong/Peo Ta. τ'h ese are two adjacent 21-m deep pools in front of B 叩 Loppadi Loppadi Kh onenoy on the west side of Loppadi Island. Th ey are both 200 x 100 m in size and can be characterised as having sand and small pebble substrates ， steep rock sides ， moderate moderate counter-current eddies in the dry season ， and large boulders in them. Both areas were were established as vi1l age FCZs in 1995. However ，despite efforts by villagers to protect them ，Cambodians from across the river still regularly fish the areas in 出e 合y season using explosives. explosives. As a result ， the FCZs have so far not been effective in rejuvenating populations of of B. microlepis or other fish species. However ， the species has been heard vocalising in the the area ， and if Cambodian fishers cease their destructive fishing practices in the future ， there there is potential for B. microlepis to benefit from the FCZs.

4. 4. Veun Louk. 百lI s 30 ・m deep area is situated adjacent to Ban Don Lekfai on the west side side Don Lekfai Island. Th e 釘 ea is approximately 150 x 150 m ， and has a pebble and sand substrate ， steep rock sides ， large boulders in 出e area ， and slow counter-cu 町 ent eddies in 出edηseason. Th e area is presently not protected as a FCZ ， and stocks of B. microlepis

have have reportedly been greatly reduced due to m 組 y years of intense dry season g i1l netting. We heard B. microlepis vocalisations in the area in 1994.

5. 5. Veun Phou That. 百lis area is approximately 70 m deep ， and is situated adjacent to Ban Senhatgnai (including the sub-village of Ban Na Phou Th at) on the west side of Khong

Island.τ 'h e 紅 ea is approximately 300?100 m ， and has a hard rock substrate ，large boulders and and logs at its bottom ， moderately steep rock sides ， and slow counter-cuηent eddies in the dry-season. dry-season. It is not presently protected as a FCZ ， and stocks of B. microlepis have reportedly reportedly been reduced due to dry-season g i1l netting. B. microlepis croaking was heard in the the area in early 1998.

6. 6. Veun Te/Veun Va. Th is area is an approx 泊lately 32.5-m deep section of the Mekong River River with slow counter-current eddies in the dry-season ，a hard rock and si It substrate ， steep steep rock sides with caves ，and many large boulders and logs. Th is is one of the deepest parts parts of the Mekong River in Kh ong District ， and is situated between the villages of Ban

Kokpadek and Ban Chan. Villagers have protected the 紅 ea as a FCZ since 1996. Th e FCZ is is the largest in Kh ong ， with a combined area of 18 hectares (600 x 300 m) including the two connected FCZs of Ban Kokpadek and Ban Chan ， which are each 300?300 m in size. Li ke Veun Tholathi ， the Veun Te /V eun Va 紅白 is an excellent example of a FCZ 白紙 is is protec 白19 the spawning habitat of spawning smallscale croakers. V i1l agers have repo ロed increases increases in B. microlepis croaking in the FCZ every year since its establishmen t. Increases in in catches of juvenile croakers were reported in Ban Kokpadek in 1998 ， although catches of of juveniles apparently declined over the same 的 season months in 1999. In any case ， villagers villagers in Ban Chan and Ban Kokpadek remain convinced that B. microlepis is one of the the important beneficiaries of their FCZs. 168 168 L AN G. B A IRD ET AL

Figure . 3. Bo es emania mi crol epis

7. 7. Veun Hat Phou. This area refers to two 2~0-m deep-water pools with hard rock and sand sand substrate ， moderately spiraling count 巴r-current eddies ， and steep rock sides . This

triangular - shaped area is approximately 0. 8 hectares in size ， and is situated on th 巴 west side of of Don Khamaonoy Island adjacent to Ban Don Khamaonoy village in northem Kh ong Distric t. The area has been protected as a village FCZ since 1996. The first author heard B. microlepis vocalising in the area in March 1998 ， and villagers confirmed confirmed that the area is a known spawning ground. They have reported an increase in smallscale smallscale croaker vocalising since the establishment of the FCZ ， and also more juveniles in in their area .

Boesemania microlepis Acoustic Studies

J. J. F. Borsani from 出e Tethys R 巴search Institute in Milan ，Italy ，coll 巴cted acoustic recordings recordings of the vocalizations of Bo esemania mi crol e pis in th 巴 Boung Pa Kouang /Boung

Pa Chouk area near Ban Hang Khon 巴 in lat 巴 March and early April 1998. It was the first time time the sounds of B . microlepis had been professionally recorded and analysed ， although other other scienids are known to produce sounds in other parts of 出e World (BORSANI ， 1999). Th 巴 sounds Borsani recorded were produced during the mid- to late aftemoon ， and consist 巴d of of a series of loud fast pulses produced by the swim bladde r. Borsani thought the vocalizations vocalizations sounded “like the hom of a truck". The sounds ranged up to 6 kH z in frequency ， with the main energy located at 0.5 kH z. These s ounds lasted up to 100 ms ， and were repeated in bouts lasting several minutes (BORSANI ，1999). BORSANl (1 999) suggested lrrawaddy lrrawaddy dolphins (Orcaella brevirostris) n巴ar Ban Hang Kh one might be able to use the sounds sounds of the smallscale croaker for locating and capωring the fish 巴s. We have not observed the the dolphin ' s actually eating B . microlepis ， but local fishers report that they do . As part of Borsani Borsani 's acoustic research ， he was able to determine exact locations of vocalizing fish. Hyperbolic Hyperbolic locations of individual B. microlepis could be identified through recording vocalizations vocalizations of individuals using a two-hydrophone aITay (CESVI ， 1998). Locations of

seven seven large B. nu crol e pis were identified in the 加 'ea from recordings (J. F. Borsani ，personal ECOLOGY AND CONSERVATION OF SMALLSCALE CROAKER 169 communication ， April 1998). 百le sounds of fish could be used for estimating abundance of of spawning pa 出阻d hense monitoring the success of FCZs. CRUZ (1 998) reports that fisheries scientists in the USA 釘 'e now beginning to use acoustic acoustic recordings of the “pu 汀 ing" and “ knocking" sounds of reproducing fish in order to to develop spawning maps that fishery managers can use to protect over-fished species and their their habitats. BORSANI (1999) suggested that monitoring the sounds of spawning smallscale croakers croakers could be an invaluable tool for preventing populations from being over-fished. Th e main limitations are the high cost of the recording equipment and the high level of technical technical skill required to use the equipment and analyse the acoustic data. Even without recording equipment ，however ，villagers in Kh ong have a1r eady learned to to monitor changes in the intensity of B. microlepis vocalizations over years in order to provide provide them with an idea of whether population sizes are increasing or decreasing.τbey simply simply listen to the sounds of the croaking ，加 d qualitatively assess whether there is more or or less compared to last ye 低

Biology Biology and Ecology of B. microlepis

Th e most important ecological and biological factors related to smallscale croaker management and conservation are: (1) the species is a persistent dry-season inhabitant of deep-water deep-water pools in the Mekong River in southern Laos and northeastern Cambodia; (2) 白e species appe 訂 s to be rather sedentary in 出e river ， without any known migrations or movements into 凶 butaries; and (3) the species spawns in deep-water sections of the Mekong River in southern Laos and northeastern Cambodia during the height of the 世y- season ， sometime between March and early May (CESVI ，1998). BARDACH (1959) suggested that that the species spawns in May or June in Cambodia. Smallscale Smallscale croakers have not been recorded in catches of strongly migratory fish species species moving up the Khone Falls in the Mekong River (SINGHANOUVONG ET AL. ，1996A ， 1996B; 1996B; BAIRD ，1998b). 百ley have been recorded in small numbers in "dai" bagnet fishery catches catches in the Tonle Sap Ri ver in Cambodia ， a fishery which prim 釘 ily t釘 gets fish migrating 合om the Great Lake to the Mekong River at the beginning of the dry-season ， and in December 1996 ， 199 me 回c tonnes of “trey prama" was estimated to have been landed (DIEP (DIEP ET AL. ，1998). 百1U S it appears 由at the species moves from the Great Lake to the Mekong River at the very beginning of the dry season. Other 合'eshwater croakers in. the family family Sciaenidae ， such as Plagioscion squamosissimus (Heckel) from the Am azon River in in South America ， have not been shown to engage in clear-cut migrations (GOULD 町 G ， 1980). 1980). BARDACH (1 959: 25) stated 伽 t，

[smallscale [smallscale croaker] “ spawning locations 釘 e unknown but the distribution of of samples [in the Great Lake and Tonle Sap River] suggests more circumscribed circumscribed migratory habits than those of the bulk of the “ white fishes"

which which mostly move from inundated 釘 'eas to rivers and back again." He therefore recommended “that research be done on the spawning habit of the fishes" (BARDACH ， 1959: 25). Smallscale Smallscale croakers are known to be predatory (RA 悶 BOTH ， 1996). We examined the stomach stomach contents of 28 仕eshly-caught specimens 15 -4 0 cm in length: 14% had empty stomachs ，14% had remains of fish ，14% had insects ， and 75% had the remains of 170 170 L 釧 G. BAIRD Ef AL.

in in their stomachs. Shrimp were found in 、stomachs of fish caught in the low and high-water seasons. seasons. One fish had bark in its stomach ， another had a few leaves ，and a 血ird had a stomach stomach full of gastropod snails. It is not known what species of shrimp the smallscale croaker croaker consumes ， as 0凶y seven species of crabs and ten species of shrimp have been recorded recorded from the Mekong Basin so f;低官 tis probably accounts for only about 15 percent of of the species 白at actually occ 町 there (KOTIELAT & WHITTEN ， 1996). According According to WONGRATANA (1 985) ，fishers liv 均 along 恥 lower co 町 ses of 恥 Tachin Tachin and Bang Pakong Ri vers catch the species during high tides using long lines in shallow shallow waters of 2-3 m using fiddler crabs (Uca spp.) 叩 d rice crabs (Somanniathelpusa spp.) spp.) as bai t.百 e best time of year to catch them there is June and July ，and the best fishing spots spots are reportedly ne 紅白.e b拙 s oftums 泊 rivers (WONGRATANA ， 1985). B. microlepis may be one of the few fish species in the Mekong Ri ver 白紙 feeds prim 組 ly on . However ，Hemisiluris mekongensis is one of 血em 吋or species caught on shrimp-bait 吋 long long 1泊es ，and is likely to feed heavily of shrimp as well (BAIRD ， 1998b). GOULD 町 G& FE 限 EIRA (1 984) found that Plagiscion squamosissimus ，a freshwater sciaenid sciaenid croaker in the Am azon ， feeds mainly on shr 加 p.τbey discovered 白紙 the species' diet diet changed from mostly shrimp when in white water rivers to mostly fish when in clear water water and black water rivers. Since the Mekong Ri ver could certainly be described as a turbid turbid whitewater river ，especially in the rainy season ，it is not su 甲ris 泊g that B. microlepis appe 紅 s to mainly consume shrimp. Because shrimp productivity 泊 whitewater rivers tends to to be higher 血an for clearwater rivers (GoULD 別 G& 白眼EIRA ，1984) ，it is not surprising 白紙 B. microlepis r; 紅 ely ventures very fi 釘 up Mekong River tributaries ，especially during the the clearer water dry-season. However ，it is likely 白紙血e croakers move into 紅 'eas with inundated inundated vegetation d町 ing 血e high-water season ，since shrimp commonly congregate on f1 00ded 'vegetation and roots in the rainy season (BAIRD ， 1998b). Freshwater sciaenid croakers 仕om the Am azon have been found in f1 00d pla 泊紅白s during the high ・wa 旬r season season (Go ULD 町 G ， 1980). L 紅 ge individuals of B. microlepis tend to prefer deep parts of pools ，and spend little time time in shallow 紅 e拙. Croakers 紅 e only rarely ever seen in shallow 釘 'eas ne 紅 shore 泊出e Am azon (GOULD 町 G， 1980). Howeve l"， in Kh ong District ，fishers often encounter juvenile croakers croakers congr 句 ating along the edges of sand banks in the Mekong Ri ver. Neither Kh ong nor nor Am azon fishers report distinct upstream migrations of croakers. In the Am azon ，fishers visualize visualize their movements as “ hunting schools" 出at 釘 e often found near beach areas (GOULD 町 G， 1980). Th e Lao also believe 白創出e species is a “hunter". B. B. microlepis tend to begin vocal i; ang 泊. deep-water areas during the dry-season at around around 1500 h each day ， and continue their loud continuous croaking until after dark. Vocalizations Vocalizations have not been reported during. the: ra 泊y season. It is presently not known what the exact linkage is between croaking and spawning behaviour ，al 血ough a link is highly highly suspected. Croaking may begin before spawn 加g actually occurs. Hypsibarbus malcolmi malcolmi vocalize for a shorter period of time ， and their croaks may be more directly associated associated with spawning behaviour (BAIRD & PHYLAVANH ， 1999). However ，it is possible that that B. microlepis croaks 紅 e equally associated with spawn 泊g behaviour ， but that 出e spawning spawning se 出 on for the species is more drawn ou t. Villagers associate particularly loud smallscale smallscale croaker vocalizations wi 出 especially hot and sunny days. A similar association has has been noted for spawning and vocalizing H. malcolmi c紅 ps in November (BAIRD & PHYLA V ANH ，1999). 官 le relationship between the intensity of B. microlepis and H. malcolmi ECOLOGY AND CONSERVATION OF SMALLSCALE CROAKER 171

vocalizations vocalizations and daily changes 泊 water temperature during their spawning seasons ，and the the degree of association between croaking and actual spawning behaviour ，is not known. Vocalisations Vocalisations of B. microlepis differ considerably from those of H. malcolmi ， which emits emits a higher sounding croak in November-December. 百ley also differ marked1y from 血e croaks of the small cyprinid Henicorhynchus lobatus ， which make even higher pitched vocalizations vocalizations in May and June before populations migrate south 旬to Cambodia to spawn (ROBERTS & BAIRD ， 1995; BAIRD ET AL. ， 1998A; BAIRD ET AL. ， 1999). Since all three species species have very different sound 泊g vocalizations 出at are emitted at different times of the ye 紅， there is little chance of confusing their sounds. In In southem Laos B. microlepis are usually caught in large-mesh gil1 nets ， but also in

some smaller hook 佃 d line ，trap and c儲 tnet fisheries. WONGRATANA (1 985) sta 附伽t the species species is mainly caught in gil1 nets ，castnets and sometimes bagnets. Goulding (1 980) reported reported that sciaenid croakers from the Amazon are 台-e quently caught in gil1 nets ，and often often by the mouth. He suggested that croakers pursue 出eir p児 Y with open mouths ，and 白at when 白ey collide with ag il1 net the line enters the mouth and cannot be dislodged. 百lis habit of chasing prey wi 血 open mouths makes at least Am azon croakers more vulnerable to to gil1 nett 泊g than m 阻 y other species ，叩d this may be p紅 t of the reason why gil1 netting has has apparently had such a severe impact on populations of B. microlepis in Laos and other p紅 ts of the Mekong basin. In In the Amazon ，fish-baited hand lines are used to locate the depth at which croakers reside. reside. Fishers lower the bait repeatedly to determine at what depth the 自由 bite. 百le nature of of the fishery suggested to GOULDING (1980) that “croakers are sensitive either to depth or 旬mperature and inhabit vertical distinct zones in the water." He could not ，however ， confrrm confrrm whether these 釘 eas were natural feeding zones. It seems highly likely that B. microlepis microlepis inhabit distinct vertical zones in the Mekong ， and specific habitat preferences may explain why there 紅 eo n1 y seven confirmed spaw 凶ng grounds for the sp 配 ies 血roughout Kh ong Dis 回ct. It is not clear exactly what constitutes suitable dry-season habitat for smallscale smallscale croakers ， but all seven of the presumed spawning areas for smallscale croakers found found in Kh ong 訂 e characterized by being over 20 m deep ， having hard rock or pebble and silt or sand substrate ， having slow to moderate counter-cu 汀-e nt eddies 泊白e 合y- season ，and having steep rock sides descending into the pools. V il1 agers believe that large croakers croakers stay close to the sides of these cliffs ， and in February and March fishers at Ban Hang Kh one have been observed setting 18 ・cm mesh nylon gil1 nets down into deep pools adjacent adjacent to 出e steep cliffs in order to catch 1紅 ge croakers in spaw 凶ng condition.

Boesemania microlepis Conservation Listings and International Trade

Th e smallscale croaker Boesemania microlepis is presently listed in schedule n of Decre 泡 No. 118 /M CC of the Ministry of Agriculture and Forestry ，Lao PDR. However ，it is is listed as Preudosciaena soldado (Lacepede) ，a junior synonym for the species also used by BARDACH (1 959) ，TA 阻(1 974) and DAVIDSON (1975). A 1991 amendment to this decree decree made it iIl egal for 叩 yone to catch schedule n species during their spawning seasons ， or or to sell individuals of 血e species throughout the year. However ，smallscale croakers 釘 e still still widely caught ， sold and 住aded in Laos (BAIRD ，1994). DAVIDSON (1975) reported 白紙 the the main fishing season for the species is between January and April ，which coincides with its its spawning period. 172 172 LAN G. BAIRD EF AL

B. B. microlepis is ag 'O'O d candidate f'O r listing in the IUCN Red Data B 'O'O k， but s 'O far insufficient insufficient inf 'O rmati 'O n 'O n the bi 'O l'O gy ，ec 'O l'O gy ，and c'O nservati 'O n st 泊施。f 出e species has been available. M 'O re studies need t 'O be c'O nducted ，especially related t'O由 e species' m 'O vements in the river and repr 'O duction. B. B. microlepis may als 'O be a candidate for listing 泊 Appendix n 'O f the C 'O nventi 'O n 'O n the the In ternati 'O nal Trade in Endangered Species (CITES). Between N 'O vember 10 and December 29 ，1993 ， during 17 days 'O fm 'O nit 'O ring the fresh fish trade between La 'O PDR and Thailand at at Ban Veun Berk ，Ub'O n Ratchathani Province ，367 kg 'O f iced fresh B. microlepis were 'O bserved being imp 'O rted int 'O Thailand. Th is represented 2.8% of the t 'O tal weight 'O ff 詑sh fish fish imp 'O rted thr 'O ugh 血is maj 'O r fish trad 泊gp 'O int 'O ver the peri 'O d investigated (Baird ， 1994). 百le smallscale croaker was the eighth m 'O st abundant species by weight 仕組ed 由r'O ugh Ban Veun Berk 合om La 'O s during 白紙 peri 'O d (B AIRD ，1994). H 'O wever ， B. microlepis are are mainly caught later in the dry-seas 'O n between January and April (DAVIDSON ，1975) ， and the peri 'O d when fish trade was monit 'O red at Ban Veun Berk was n'O t血e peak seas 'O n f'O r catching B. microlepis. Therefore ，it seems likely that many t'O nnes 'O f smallscale cr 'O akers were imp 'O rted int 'O Thailand as f'O od fish in early 1994. The auth 'O r has als 'O se 四 large large am 'O unts 'O f large B. microlepis imported int 'O La 'O s 合om Stung Treng Province ， n 'O rtheastern Camb 'O dia. In In December 1993 ， the price 'O f B. microlepis at Ban Veun Kh創 n ，'O n the La 'O side 'O f 出e La 'O /Camb 'O dia b'O rder ，was the equivalent 'O f US$ 2.4 0/k g. Th e price 'O f B. microlepis at at Ban Veun Berk ，百聞 land ，was US$ 2.60 /k g (B AIRD ，1994) ，加 d in May 1999 B. microlepis s'O ld f'O r 15 ，000 kip /k g (US$ 2.14 /k g) 血 Pakse ，組 d 20 ，000 kip /k g (US$ 2.86 /k g) at the La 'On'h ai b 'O rder. Th is makes the species 'O ne 'O f the m 'O st highly valued 合'eshwater f'O'O d fishes fishes in the regi 'O n. Th e high ec 'O n'O mic value 'O f the species ，its vulnerability t 'O gillnetting during during its dry seas 'O n spawning seas 'O n， and improved transp 'O rtati 'O nr 'O utes and netw 'O rks f'O r trading fresh fish 'O n ice t 'O distant markets are pr 'O bably all significant fact 'O rs that have lead lead t 'O increased fishing f'O r B. microlepis and resultant declines in p'O pulati 'O ns in many p訂 ts 'O f the Mek 'O ng River.

Boesemania microlepis and Mainstream Mekong River Fish Conservation Zones

P'O pulati 'O ns 'O f Boesemania microlepis are apparently declining 白r'O ugh 'O ut much 'O f the the Mek 'O ng River basin. Wh ile Bardach (1959) listβd smallscale cr 'O akers 酪'O ne 'O f 15 'O r s'O“ principle c'O rnnlercial species" 'O f freshwater fish in Camb 'O dia ，st 'O cks 'O f the species there there have apparently greatly declined since then (T 'O uch Saeng Tana ，pers 'O nal c'O mmunication ， 1996). Furtherm 'O re ， in 1993 villagers from Ban Hang Kh'O ne rep 'O rted that in in recent decades smallscale croaker p 'O pulations have plummeted. At the time ，many believed believed that there were 'O nly as few as 10 or 20 percent as many individuals 'O f B. microlepis microlepis ar 'O und their village c'O mpared t 'O 1970. Declines were believed t 'O be mainly the result result 'O fa large increase in large-mesh gillnet use in deep-water 紅 eas (ROBERTS ， 1993). In In the 1970s ，when gillnet use began increasing in 血e Kh'O ne Falls area ，and s'O uthern La 'O s as as a wh 'O le ，'O ne fisher 合'O m Ban Hang Kh'O ne rep 'O rted catching 700 kg 'O f smallscale cr 'O aker using g出nets each ye 低 Alth 'O ugh small individuals 釘 'e still c'O rnnl 'O nly enc 'O untered in in fish catches (ROBERTS & BAIRD ，1995) ，few large individuals 紅 'e landed. 百le establishment 'O f Fish C 'O nservati 'O nZ 'O nes (FCZs) c'O vering deep-water dry-seas 'O n spawning spawning habitat for B. microlepis may be 'O ne 'O f the best 'O pti 'O ns for reviving apparently ECOLOGY AND CONSERVATION OF SMALLSCALE CROAKER 173 depleted depleted populations in the Mekong River. Veun Tholathi and Veun Te /V eun Va are already two excellent examples of deep-water smallscale croaker spawning grounds that are being successfully successfully protected 台om dry-season gillnetting. 百 1e frrst of the two areas was established as as a village FCZ in 1994 ， while villagers set up the second area in 1996. It おis useful 旬Ot consider consider BARDACαH'S 、s (1959 佼: 22 勾) stat 飽em 巴瓜n1t 孔tし， subsequently subsequently of fry and fingerlings is undisputedly one the efficient tools of fish conservation." conservation." In出 e same vain ， Joseph Luczkovich ， an associate professor of biology at East East Carolina University in the USA ，was quoted as saying ，“ Almost every fisher person worldwide worldwide understands [that] if you over-harvest spawning aggregations ， you're not going to to have a future" (CRUZ ，1998:1). FCZs have been successful in reviving depressed populations of B. microlepis at Ban Don Tholathi ，B 加 Chan and Ban Kokpadek ， the villages adjacent to the above-mentioned two Kh ong Dis 回ct FCZs. In those communities ，fishers have widely reported large increases in 合 y-season croaking of large B. microlepis. 百1ey have observed increased numbers of juvenile juvenile croakers caught in 2.5 cm mesh gillnets at Ban Don Tholathi in November ，and in 4- 5 cm mesh gillnets at Ban Kokpadek and Ban Chan in Janu 紅 y and February. Since the two FCZs are about 40 km apa 託， it is highly unlikely that there is any interaction between 出e B. microlepis populations in these areas. Wh ile FCZs properly situated to protect spawning habitat of B. microlepis have the potential potential to benefit the species greatly ， other biological and ecological factors not yet fully understood understood are also likely to influence whether the species is able to successfully spawn. Al 血ough villagers from Ban Don Th olathi ，Ban Chan and Ban Kokpadek have reported annual annual increases in B. microlepis vocalizing since the establishment of their FCZs ，血 e numbers of juveniles caught in gillnets from year to year apparently varies. Although villagers villagers from Ban Don Tholathi have reported large numbers of juveniles eveηye 紅 since establishing establishing their FCZ ，this has not been the case for Ban Kokpadek and Ban Chan. During the the dry season of 1998 ，villagers from Ban Kokpadek reported catching the 1紅 gest quantities of of juvenile B. microlepis in living memory. However ，there were very few juvenile sciaenid croakers croakers less 白血 one year old caught at B 佃 Kokpadek during the dry season of 1999 ， even even though B. microlepis spawners in the village FCZ were well protected the previous ye 低 is It possible that factors related to hydrology and water temperature reduced spawning in in 1998 ， in con 仕ast with 1997. Although FCZs are capable of protecting smallscale croakers at spawning sites ， ecological ecological factors unrelated to FCZs must also be suitable ， or spawning may still be unsuccessfu l. However ，this should not be taken as justification for neglecting to conserve spawning spawning B. microlepis. In stead ，we must recognise that we are dealing with complicated and stochastic ecological processes that cannot always be controlled through human managemen t. Wh en other factors 紅 'e amenable for successful spawning ，出 e protection of spawners spawners is likely to have very positive results ，especially when populations of the species are are already low. Th e question is how much ， and when and where ， does the establishment of of FCZs most influence the chances of successful spawning and recruitmen t. Again ，more research research is required. 174 174 LAN G. BAIRD ET AL.

CONCLUSIONS

Thi s paper ou t1in es 白e critic a1 ecologic a1 and conservation factors most relevant 加 sm a1 1sc a1 e croakers in 血e mains 佐eam Mekong Ri ver of southem Laos ，a1 though gaps in knowledge remain regarding the species' life-cycle s旬.ges and habitat preferences.τ 'h e establishment establishment of FCZs to revive populations of Boesemania microlepis is no gu 紅釦tee 白紙 croaker croaker populations w i1l I1i crease over as 泊gle year ， but if spawning grounds for the sp 即 ies are protected over a number of years ， as with Veun Tholathi and Veun Te/V eun Va in in Kh ong Di s凶 ct ， there s田 ms to be a strong likelihood 白紙 croaker populations w i1l benefit. benefit. Still ， the situation needs to be monitored over many ye 紅 s in order to learn more about about the impact FCZs can have on sm a1 1sc a1 e croaker populations. V i1l age-based FCZs 紅 e only likely to receive adequate protection when communities largely largely agr 関 with their establishment and are committed to enforcing rules themselves without without paymen t. Forcing unwanted FCZs on vi1l agers is unlikely to achieve 血e desired conservation conservation or soci a1 results. Participatory extension activities designed to increase fisher awareness awareness reg 紅 ding the possible benefits of establishing FCZs ， or initiating other fish management and conservation strategies ，紅e likely ωbe an important factor in promoting vi1l age-based sustainable fisheries.

ACKNOWLEDGEMENTS

We would like to acknowledge Mr. Phongsavath Ki souvann a1 ath ， Mr. Vixay In thaphaisy ， and Mr. Peter Cunningham for their assistance with the research. Mr. J. F. Borsani from 由e Tethys Research In stitute in Mi lan ，Ita1 y collected some acoustic data in B 組 Hang Kh one. Mr. Simon Bush provided some information reg 紅 'ding 白e present market prices for for Boesemania microlepis in Southem Laos. Dr. Maurice Kottelat assisted by providing an important reference and ideas. Thanks a1 so for the useful comments received 針。 m 白e anonymous reviews of the article. Support for the study was provided through the Environment a1 Pro tection and Community Development 泊 Siphandone We t1 and Pr oj 民 t (EPCDSWP) ，which is being implemented by the NGO CESVI Cooperation and Development and funded by the Europe 組 Commission. We would a1 so like to 血ank 出e Agriculture Agriculture and Fores 町 Division of Champasak Province ， the Agric u1 ture and Fores 句 Office Office of Kh ong District ，and 白e Center for Pr otected Ar eas and Watershed M 佃 agement (CPA V¥瓜 1) at 白eDep 紅佃lent of Fores 住y in Vientiane ， for their support in conducting 白is research. research. Th e map of the Lower Mekong Basin was prepared by Mr. Ole Haggen ， Geography Dep 紅佃len t， U 凶versity of Victoria ，C 組 ad a， and 出e map i1l us 佐ating Boesemania microlepis microlepis spawning grounds was done by Mr. Giuseppe Daconto ，CESV I. F泊a1 1y ，we would especi a1 1y like to acknowledge 也econ 凶bution 血atm 阻 y villagers in Kh ong Dis 凶ct have made to the study. Without their assistance and cooperation ，出 is research would not have been impossible. ECOLOGY AND CONSERVATION OF SMALLSCALE CROAKER 175

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