The surveys for freshwater elasmobranchs consisted of three stages: preliminary and full investigations, and preparation for publication.
The purpose of the preliminary investigation was to collect detailed information on the distribution of freshwater elasmobranchs; that is, to find out where and when to stay, and to look for counterparts familiar with the geography, catching methods, transportation, and facilities in the remote areas. Usually a pair of scientists visited many places where elasmobranchs have been reported in freshwater or brackish water. For example, the author visited 10 countries from Canada to Panama during the preliminary investigation for North America.
The next stage was a full investigation at sites selected by the preliminary visit. Usually five or six scientists from
Figure 1. Sampling sites of freshwater elasmobranchs. 1. Lake Naujan, Philippines; 2. Perak River, West Malaysia and Indragiri River, Sumatra, Indonesia; 3. Magdalena River, Colombia; 4. Amazon River Basin; 5. Parana River, Argentina; 6. Sanaga River, Cameroon; 7. Congo River, Democratic Republic of Congo; 8. Gilbert River, Australia;
9. Adelaide River, Australia; 10. Sepik River, Papua New Guinea; 11. Usumacinta River, Mexico; 12. Lake Nicaragua and San Juan River System, Nicaragua and Costa Rica.
differing fields took part, for example, taxonomy, ecology, environmental biology, physiology, biochemistry and genetics. We tried to collect fresh specimens, and for each to examine their physiology and biochemistry. The results of the first two stages were submitted to the Monbusho.
Subsequently, papers were also submitted to refereed journals, including those by Teshima et al. 1987, Taniuchi 1979b, Ogawa and Hirano 1982, Teshima and Hara 1983, Taniuchi and Ishihara 1990, Taniuchi 1991, Kan and Taniuchi l99l,Teshima and Tekashita l992 and Kitamura et al. 1996.
Survey areas are shown in Figure 1. In South Eest Asia, surveys were conducted in Lake Naujan, the Philippines, Perak River, Malaysia, and Indragiri River.
Initially, only sharks were covered targeted by the survey, as noted by the title of the first internal report. The preliminary investigation had indicated the likelihood that live sharks could be caught by longlines or gillnets, particularly in Lake Naujan. Failure to catch sharks during the course of field surveys led to the inclusion of rays, including sawfish and stingrays. Stingrays and sawfish were collected in the Perak River and Indragiri River, both of which flow into the Maraca Straits.
In South America, the Magdalena River basin near San Cristabal, the Amazon River system near Manaus, Brazil, and Iquitos, Peru, and the Parana River near Santa Fe, Argentina were surveyed. Many specimens of freshwater stingrays were caught in the Magdalena and Amazon River basins, but none from the Parana River, probably due to a cold season when the stingrays may migrate upstream or inhabit deeper waters in the river.
In West and Central Africa, field surveys were undertaken in the Sanaga River, Cameroon, and Congo
River, Democratic Republic of Congo. Stingrays were collected from the Sanaga basin near Edea, in freshwater 15km from its river mouth. Although we failed to catch stingray in the Congo River near Boma, an angler provided us with several photos of a stingray, characteristic of Dasyatis ukpam.
In Oceania, field surveys were conducted in the freshwater section of Mitchell and Gilbert River, Queensland, and the Daly and Adelaide River, Northern Territory, Australia. A river shark provided by the staff of Fisheries Research, Primary Industry and Fisheries, Northern Territory, was found to be an undescribed species after dissection, unfortunately too late to reconstruct its original state.
In Mexico, several sharks were collected from the Usumacinta River basin and the west coast of Mexico near Mazatlan. In Lake Nicaragua and San Juan River system, we carried out extensive surveys for freshwater elasmobranchs because the bull shark Carcharhinus leucas and sawfish were believed to be indigenous to the river system, but we fear that both species may be near extinction in the lake. Sharks were collected from the mouth of the Colorado River, a tributary of the San Juan River, and from the coastal area of San Juan de Norte.
Field surveys in Thailand, Laos, India, and Bangladesh took place in 1997. This included the collection of a number of specimens of stingrays from the Chaophraya and Mekong River. We also confirmed the occurrence of several species of stingrays by examining specimens collected from the Ganges River near Bahgarpur above the Farraca Barrage. The preliminary results were submitted to the Ministry of Education in October
1998.
The species name and the number of specimens collected in each survey are shown in Table 1. Twenty-seven species ofelasmobranch were examined during these field surveys.
Of these, two species of sharks, Carcharhinus leucas and Glyphis sp., six species of stingrays (Dasyatidae), eight species of river stingrays (Potamotrygonidae) and two species of sawfishes (Pristidae) were collected from freshwater. Over 200 specimens of freshwater elasmobranchs were collected and examined for various scientific purposes. Whilst freshwater elasmobranchs are generally not very difficult to collect, it is very difficult to
obtain fresh specimens of elasmobranchs in some areas where fisheries and other activities have led to over-exploitation and environmental deterioration, such as in Lake Nicaragua.
Elasmobranch populations are also endangered if they are not free to recruit from the sea. Physical barriers, such as harbour facilities, prevent migration to and from the sea. Shrimp fisheries employing gill nets e n t r a p elasmobranchs in river mouths, hindering upriver migration. In Lake Nicaragua, freshwater fishers believe that gill nets for shrimps set at the river mouth of San Juan
Table 1. Species name and number of specimens captured Locality Species name
Lake Naujan (Philippines) Perak River (Malaysia)
Indragiri River (Sumatra, Indonesia)
Magdalena River (Colombia) Amazon River (Manaus, Brazil)
Amazon River (Iquitos, Peru)
Sanaga River (Cameroon)
Congo River (Dem. Rep. of Congo) Gilbert River (Australia)
Adelaide River (Australia) Daly River (Australia)
Pentecoste River (Australia) Sepik River (Papua New Guinea) Lake Murray (Papua New Guinea) Oriomo River (Papua New Guinea)
Usumacinta River (Mexico) Lake Nicaragua (Nicaragua) Colorado River (Costa Rica)
Carcharhinus melanopterus Chiloscyllium indicum Rhizoprionodon acutus Scoliodon laticaudus Rynchobatus djiddensis Dasyatis bennetti Dasyatis bennetti ? Pastinachus sephen Pristis microdon
Potamotrygon magdalenae Potamotrygon histrix Potamotrygon motoro Potamotrygon signata Potamotrygon circularis Potamotrygon scobina Potamotrygon sp.
Paratryon thayeri Potamotrygon histrix Potamotrygon motoro Potamotrygon laticeps Disceus thayeri Dasyatis garouaensis Dasyatis ukpam Dasyatis margarita Pristis microdon Carcharhinus leucas Glyphis sp.
Carcharhinus leucas Pristis microdon Himantura chaophraya Pristis clavata
Carcharhinus leucas Pristis microdon Pristis microdon Anoxypristis cuspidatas Pristis microdon Himantura uarnak Carcharhinus leucas Pristis perotteti Carcharhinus leucas
in each locality.
No. of
specimens Remarks X2
W6 W1, X6
X1 X1 X2 X1 W2, X3 W44, X74 W1,X1
W2 W1, X1
W2 W1 X1 W3, X1
W2 W3, X1
X1 X2 W14, X21
X1 W4, X3 W2, X3 W1,X1
X1 W1, X1 W2, X2 X1 W5,X5
W4 W8,X4 W12, X11
W1 W1 X1 W3, X2
W1 W1,X1
freshwater, identified by photos collected in fish market
freshwater freshwater
freshwater freshwater
freshwater
freshwater brackish water freshwater freshwater freshwater
salt water freshwater freshwater brackish water
freshwater freshwater freshwater
River prevent elasmobranchs from coming up the river. In contrast, freshwater stingrays were relatively easy to obtain from a fish dealer on the Chaophraya River, Thailand.
Before the survey, we had been told that Himantura chaophraya was an endangered species, but we have already examined several specimens of this stingray. It is important that better data are available for freshwater elasmobranchs, allowing a proper assessment of status.
Acknowledgements
Thanks go to the members of each field survey: K. Mizue, K. Hirayama, M. Shimizu, T. Hirano, T. Muramatsu, S.
Sonoda, K. Teshima, S. Watabe, K. Ogawa, H. Ishihara, H. Kitamura, T. Otake, S. Tanaka, M. Murakami, M.
Sano, O. Baba, T. Kitamura, T. Yamashita, M. Ahmed, P. Last, T. Kang, S. Martinez and O. Sosa-Nishizaki.
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