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Tridacnid Clam Stocks on Helen Reef, Palau, Western Carounels Sli~S

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Tridacnid Clam Stocks on Helen Reef, Palau, Western Carounels Sli~S Tridacnid Clam Stocks on Helen Reef, Palau, Western CaroUnels Sli~s WENDY HIRSCHBERGER Introduction lying at approximately lat. 3°N and Figure I.-Helen Island at Helen Reef long. 131°E (Fig. 1, 2), occupy about atoll, in Palau's southwest islands. Overexp]oitation of tridacnid clam 216 km 2 with a small island (Helen populations appears to be a current Island) located at the northern end. problem in many areas of the Indo­ Depths inside the lagoon exceed 60 m; remote area is uninhabited and receives Pacific. One area where some of the outside, the bottom slopes steeply. This only occasional visits from U.S. Trust effects of harvesting have been studied Territory outer island support ships and is Helen Reef, a small atoll in the south foreign fishing vessels. Palau District, Western Caroline Is­ Wendy Hirschberger is with the Northwest and In May 1971, the NOAA research Alaska Fisheries Center, National Marine lands, Trust Territory of the Pacific Is­ Fisheries Service, NOAA, 2725 Montlake vessel Townsend Cromwell, operated lands. The submerged reef and lagoon, Boulevard East, Seattle, WA 98112. by the National Marine Fisheries Ser- ABSTRACT-A survey of Palau Distrids ratios of live individuals to empty shells showed less than 5 percent dead. These Helen Reef was conducted in May 1976 to continued to remain very low. Only Hip­ smaller species may possibly be used as continue monitoring changes in tridacnid popus hippopus showed a substantial de­ natural population indicators at Helen clam abundances. The densities of the four crease in the percentage dead. This is in Reef. Tridacna squamosa is still rare, prob­ largest tridacnid species were slightly contrast to Tridacna maxima which is sub­ ably owing to poor environmental condi­ higher than observed in 1975; however, ject tn relatively no fishing mortality and tions for this species. 8 Marine Fisheries Review vice, NOAA, conducted a survey of the referred to as the"giant clam," as old Tridacna squamosa usually occurs on Trust Territory's marine resources. individuals may attain lengths of up to coral reef surfaces in depths less than 15 Only a stop was made at Helen Reef at 135 cm (about 4V2 feet). They usually m, most often in protected environ­ this time, and the ship returned in occur on sandy areas of the reef or in ments such as reef canyons and March 1972 to survey the tridacnid areas ofcoral rubble and reef degenera­ fissures, sheltered lagoons, and marine clam populations. In their report, Hes­ tion, within intertidal regions to ap­ lakes. These tridacnids are often refer­ ter and Jones (1974) concluded that the proximate depths of20 m. Although T. red to as "fluted clams" because of tridacnid populations at Helen Reef gigas exhibits a small byssus orifice, their characteristic broad, plate-like were large and that Tridacna gigas and the adults remain unattached to any projections. In size, they can range up T. derasa could possibly withstand a substrate. The dorsal margin and ribs to 45 cm (over 17 inches). Although moderate, controlled fishery. exhibit deep undulation. Tridacna de­ they are never imbedded, T. squamosa A resurvey effort was undertaken in rasa, also a species capable ofreaching are often found nestled among coral and April 1975 by a team of biologists sup­ large sizes, reportedly attain lengths of anchored with a weak but abundant ported by the Palau District Marine Re­ 50 cm (20 inches). These clams often byssus. Hippopus hippopus grow to 40 sources Office, a division of the Trust live towards the outer edge of coral cm (close to 16 inches) in size and usu­ Territory Department of Marine Re­ reefs and are usually unattached, al­ ally occur in sandy areas of the reef sources, in response to increasing re­ though very young individuals «10 never more than 6 m in depth. You ng ports of foreign fishing vessels in the cm length) develop weak byssal an­ H. hippopus attach by a weak, sparse Helen Reef area. They found that chors. Tridacna derasa appear to prefer byssus which disappears with age. tridacnid clam populations had appar­ the more shallow reef areas (approxi­ These clams have heavy triangular ently been reduced since 1972 (Bryan mate depth range, 4-10 m). shells and an undulating dorsal margin and McConnell, 1976). There are two slightly smaller tridac­ with very sharp, triangular, interdigitat­ In 1976 the Palau Marine Resources nids of commercial importance. mg processes. Office requested another resurvey in re­ sponse to continued reports of unau­ thorized fishing vessels in the Helen Reef area. There was concern that con­ Figure 2.-Palau District's southwest islands. tinued fishing in this area could lead to severe stock depletions, with small chances ofrecovery. This report gives a description of the 1976 resurvey effort, monitoring changes in the abundance of tridacnid clams on Helen Reef, and provides population densities of the area with comparison to earlier sur­ veys. Natural History Because of its isolation, Helen Reef Yap I.. at one time supported large tridacnid clam populations. In one instance, Motoda (1938) reported observing ap­ J Palau Is. proximately 38-46 T. gigas within 100 2 CAROLINE ISLANDS m . Although populations have been greatly reduced over the years, all six . Sansaral I. species of the Tridacnidae family still Pula Anna I.. 'Merir I occur on Helen Reef: T. gigas (Linne), T. derasa (Roding), T. squamosa Tabi 1,'1 ... HELEN REEF I Lamarck, T. maxima (Roding), T. crocea Lamarck, and Hippopus hip­ popus (Linne). These beautiful and " sometimes colossal bivalves are found ~:"9> ~ .. 4.::""7'~'1_ only in the Indo-Pacific faunal region ... (Fig. 3). ~""'~T The two largest Tridacna species are ,'. .... T. gigas (Fig. 4a) and T. derasa. C'll". ~..... Tridacna gigas is the species usually February J980 9 60° 80° 100° 160° 180° 160° 140° . _. .. A~~II __• H"'~A",,,,,,,, _________0_ '-_._ 4. ~_. 20°t----~I,_-+____::,.L____"t-h4_h_-+,;_____----'-+__'o__;;M=...... ". ''''-1".1'''' 0'i" '/."'" (~1-' 001-- ·--:;---t-----f--~t~~:_l~~.-f~~~~""''"":::_If__-~;_-'--+---,--'1o.::.;.~,,-,-.. f--I\--'~----~-=.~C:.:.'~''~-~I---+---I ',,: ~ ~«",: . ,.\'lOINI" •• I("t"; '. ,-rOKII..a.U IS. 20° COOK ". Tridacna gigas 60° 80° 100° 120° 140° 160° 180° 160° 20° 1-------.::~~--I---_,..L~~-_l--tL_f:.,;:.--7"=t:;==:..",_.;;:_--_+-....:,....,M=••;;:-:I.... H.. la.. .......... .' .~:: ~.t,,'"'''''''11''1.1.. ~. '.-~ . ~OI.NUl .. ...TOKIU.U II. 200 .....::.-----j-------t--·----:?¥- ..·~o~~... C.OOK -----------,.. Tridacna derasa 20° 60° 80° 100° 160° 180° 160° ' .. ~A~";I__• Ii... "':'.... 'A,,.. -------------- --------_..... -. 20°t---H~~ ---~'f"''<;:__----=.~_+---.-',_J \. O°I----..-A~_l'ii_-----1 Tridacna sauamosa Figure 3.-Geographical ranges of tridacnid clams (from Rosewater, J965). 10 Marine Fisheries Review 80° 160° 180° 160° 140° ' .. ~A~"I...... __• 1 :"" """, -------_~_~_~/~ ------------ 20° I---...::.J\--+--,,...L---'_.L-+----.;h~--_I_ ,-----'-I:--+.M;;....'.A..o.:-+---c;;;""--+-----~-~··;;;·~"'b_----+_-__I . .. s..,...... ......... -,.. ."~ M tI"''''' r ,.' ....'~ .~t4~ ',.'.:~ O·l..lqa.· .._ .....1". .... ....'., "'T.~~. 200~,---+----+--_-=::;;;j;#~====~~ Hippopus hippopus 100° 160° 180° 160° ", 'T' 200t--/f-+Hf--±-=::::==f===:::::=-t---7-t:::::==::t:-~--:-_'_jr.::.'~·:.,'T:\·:::;:-~'-"':·:"'':.t_--:-;;,~,:;,'::-.-..;:,~=.tr·"-"-·........_·_·-t-.-__-...-.-__-__-.-'__Ij.-.-..-__-__-.-..-.--1.. ------.;..._.~"-I.;;;.::;:.- - Tridacna maxima 80° 100° 160° 180° 140° ~.': ,.",·,./1 _ "f"~""'I""", -- ------------ -- -- --'.... -.-'''.- ---- ------ - - -- --- 200r---'p-t~,...L-__'l--::t_-i7'lr-~;----'-t_~;;,;;-;:;;:-+---,=--f__-----;:.~·~::r_--_+____1 ·",'I'".. ~.M.Alt'".I.LI" . '~I .... .... ~ - .. .... .... :.. ,.,../"- ~.'iltvt ,~f. '~J'- 0° l----~._-f_---_\~~~'=--e:~~f-,~~~-.",,""'- ~--:_:--'--\;:+---,----.:'!,"'.':c.--:c.-._+....:.._---'- ..,._i_..:~~,~••~..~-=-,--+--1 ~~':.'. " . ","OINI" " '....... :; f((·.•• '<"<"~ ','TOI(Il"Il.\lIS ..oc.o.· ......"" .. '. "'Ton. .~~ , ..."l&. 2001-:. -----l------+--------:;;..,(;-----t~~----'-t-~~---'w Tridacna crocea Figure 3. - Continued. February 1980 IJ Figure 4a.-Tridacna gigas, about 3 feet in length, on the Figure 4b.-Tridacna maxima partially embedded in coral. reef at low tide. The larger Tridacna species are the reef flats and reef edge, remaining for counting two or three assigned clam clams which are most often utilized by firmly anchored by a byssus. species. Palauans and foreign fishermen. The Total population sizes (Nj) Survey Methods for Helen mantle and the adductor muscle tissues Reef atoll were estimated by the equa­ are highly favored food items, the Attempts were made during the May tion (Cochran, 1963): larger clams producing a higher yield 1976 survey to duplicate, as closely as per expended fishing effort. Except for possible, sampling locations and field i=1~ A a small trade as a tourist item, shells are methods used during the 1975 survey N= --YilJ - J [ n W discarded. (Bryan and McConnell, 1976). Tran­ ~ Ii The two smallest species, T. maxima sect lengths were determined from the i=1 (Fig. 4b) and T. crocea, are not usually U.S. Navy Hydrographic Office Chart where Yij is the number of individuals harvested by foreign fishermen and are of Helen Reef, No. 6072. The same of tridacnid clam species j observed minimally used for food by Palauans. transects as reported by Bryan and along transect i, Ii is the length of trans­ Tridacna maxima can grow to 35 cm McConnell (1976) were resurveyed. ect i, w is the transect path width ( = 2 (close to 14 inches). It can often be The
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