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The IUCN/SSC Shark Specialist Group

Shark News 3: March 1995

Freshwater elasmobranchs; a questionable future
Leonard I.V.Compagno, Shark Research Center, South African Museum,
and Sid F. Cook, Argus-Mariner Consulting Scientists
Living cartilaginous fishes include approximately 923-1117 species in 171 genera and 55 families (estimate from 22 January 1995). Of these about 43 species of elasmobranchs (mostly rays but with a few sharks) in four families and ten genera are found in freshwater far beyond tidal influences. Some chimaeras occur inshore in enclosed marine bays but do not tolerate fresh water.


Freshwater elasmobranchs (excluding marginal species)

BRACKISH MARGINAL SPECIES:
Whiptail stingrays, Family Dasyatidae: Himantura (1 species).

EURYHALINE SPECIES:
Requiem sharks, Family Carcharhinidae: Carcharhinus (1 species), Glyphis {2 species?).
Sawfishes, Family Pristidae: Anoxypristis (1 species?), Pristis (5 species).
Whiptailed stingrays, Family Dasyatidae: Dasyatis (2 species), Himantura (2 species?), Hypolophus (= Pastinachus, 1 species).

OBLIGATE FRESHWATER SPECIES:
Requiem sharks, Family Carcharhinidae?: Glyphis (1 species?).
South American river stingrays, Family Potamotrygonidae: Paratrygon (1 species), Plesiotrygon (1 species), Potamotrygon (18 species).
Whiptailed stingrays, Family Dasyatidae: Dasyatis (4 species), Himantura (4 species).


Geographic distribution
Freshwater elasmobranchs are found in tropical and warm-temperate rivers and lakes and inshore marine waters (euryhaline species) or are confined to brackish waters (brackish-marginal species) or fresh waters (obligate freshwater species). At least 25 additional species of sharks and rays (marginal species) penetrate fresh water in estuaries or river mouths but are not found far from the sea. Some freshwater elasmobranchs occur or occurred in warm-temperate rivers such as the Mississippi River in the USA or the rivers of Natal in South Africa, but most occur in the tropics of both hemispheres.

The greatest diversity and endemism of freshwater elasmobranchs occurs in the Atlantic drainages of South America with its radiation of river stingrays (Family Potamotrygonidae), but pockets of endemism and diversity also occur in West Africa and in Asia (the Indian subcontinent eastward through Southeast Asia, southern China, Indonesia, New Guinea, the Philippines, and Australia. Freshwater elasmobranchs also occur in the Tigris River system of southern Iraq, from several other rivers in Africa, North America, and from southern Europe (Portugal) and rivers draining into the Mediterranean Sea.

The tropical rivers and lakes where most freshwater elasmobranchs occur are mostly in developing countries with enormous, expanding human populations. Increasing levels of direct exploitation and modification or destruction of riverine and lacustrine ecosystems, especially where uncontrolled human population growth isoccurring, threaten many freshwater elasmobranch stocks and obligate freshwater soecies with extinction.

The plight of freshwater elasmobranchs
Unfortunately freshwater elasmobranchs are not well-know biologically, and have been little studied in terms of fishery management or conservation. Although freshwater elasmobranchs were known for the past few centuries, their dire plight has only been recognised in the past three decades. Only a handful of researchers (most notably Prof. T. Thorson), have paid much attention to their problems.

Freshwater elasmobranchs have all the biological constraints of marine elasmobranchs, including low fecundity, late sexual maturation, long life, and intermittent breeding. In addition, they are limited by habitat limitations that usually do not affect marine elasmobranch populations. They inhabit physically restricted environments (rivers, streams, bayous, estuaries, and lakes) which greatly limit escape from pollutants, habitat modification and destruction, or directed and incidental capture in fisheries.

Due to habitat constraints, freshwater elasmobranchs are probably less capable of withstanding sustained human impact than more fecund freshwater bony fishes or marine elasmobranchs. Also, human impact may be more severe because of the protected nature of freshwater ecosystems, which allow use of simple forms of fishing gear, vessels and impoundments of little use in marine waters. Lakeside and riverside sites have been favoured habitats of Homo sapiens for millennia, because they provide easy access to supplies of water, food, and avenues of transport for commerce.

Restricted habitats
Rivers and lakes are more limited in volume, and very probably in range of habitats that are exploitable by elasmobranchs, than the sea. Freshwater habitats tend to be far less stable than marine equivalents. Shortand long-term fluctuations in temperature, oxygen level, mineral content, turbidity, water flow, rainfall, and major changes in river and lake beds can readily exceed the tolerance of elasmobranchs. Added to natural problems are escalating human-induced problems such as dam-building and other modifications of water courses, fisheries, use of water for irrigation, and an ever-increasing variety and volume of pollutants.

Fresh water may be a marginal habitat for elasmobranchs, as suggested by their low taxonomic, ecological, and morphological diversity compared to freshwater bony fishes and marine cartilaginous fishes. Freshwater elasmobranchs are collectively large animals compared to most freshwater bony fishes, which correlates with their low diversity and habitat specialisation. Elasmobranchs apparently are not competitive in microniches open to small-sized (less than 150 mm total length) fish-like vertebrates at present, and teleosts utterly dominate these niches in fresh water.

Freshwater elasmobranchs are apparently restricted to mostly permanent and relatively large, placid lakes, rivers and large streams with egress to the sea, and are notably absent from more extreme freshwater habitats successfully colonised by bony fishes and by many other aquatic vertebrates. Freshwater elasmobranchs are obligate-aquatic gill-breathing animals that are restricted to well- aerated permanent water and have no ability to breath air directly, to transport themselves out of water, to penetrate major rapids and waterfalls, to aestivate in burrows, orto survive as fertilised eggs when bodies of water become anoxic or dry up. Sea access is vital to certain euryhaline elasmobranchs that range widely in fresh water but cannot reproduce there.

No euryhaline elasmobranchs that reproduce in fresh water and no obligate freshwater elasmobranchs are confined to naturally landlocked bodies of water so far as is known. Perhaps conditions in landlocked rivers and lakes can become more extreme than unmodified sea-run rivers and lakes. These conditions could exceed the tolerance of freshwater elasmobranchs that are trapped in land-locked waters by geological or human-induced events, and cause their extirpation.

Fisheries and other impacts
Although freshwater elasmobranchs were recorded from catches since the early 19th century, very little is known to date of the nature of these fisheries. From the 17th to the 19th century the human impact on freshwater elasmobranchs was probably very low, due to a much smaller world population and small and scattered human populations in most of the tropics, as well as slow spread of the Industrial Revolution from its birthplace in Europe to the rest of the world. The impact of humans upon freshwater elasmobranchs 300 years ago was probably small, limited almost exclusively to small artisanal fisheries for food and other minor products. This changed substantially during the twentieth century with human population tripling, the development of very high human population growth rates in the tropics, and a massive push for resource exploitation and industrialisation in tropical countries.

Now the impact is massive, multifaceted, and includes overfishing of elasmobranchs, marked increases in habitat modification, degradation or destruction, introduction of exceptionally toxic substances from industrial and agricultural activities as well as large volumes of raw sewage and other human wastes into rivers and lakes.

Deforestation proceeds on a massive scale in tropical countries, increasing microclimate modification, damage to soil, destruction of forest ecosystems, lowering of water tables, land erosion, water siltation, and massive flooding. Dams are thrown up helter-skelter for hydroelectric power and water impoundment on the great tropical rivers of the world, with dire implications for those freshwater elasmobranchs that need sea access or which cannot survive extreme conditions in reservoirs and stretches of rivers landlocked above dams.

Mining operations require water for refining, and dump water loaded with toxic heavy metals such as lead, copper and mercury into the rivers. Additionally uranium mining can add a variety of radioactive isotopes to the watershed. Heavy metals and radioactive isotopes are readily passed and concentrated along the food web in freshwater ecosystems, and if not immediately deleterious may later reach damaging concentrations especially in large aquatic predators such as elasmobranchs. Even illegal drug manufacturing contributes toxic organic chemicals to the watershed in South America.

Wars in Central and South America, the Middle East, and Southeast Asia have caused difficulties by increasing wanton exploitation of freshwater elasmobranchs and by creating massive pollution and other environmental damage through destruction of petrochemical complexes and other industrial sites, through extensive use of toxic herbicides to deny cover to guerrillas, and by blasting and mining the countryside with explosives.

Habitat degradation and exploitation can affect freshwater elasmobranchs directly, but also indirectly by affecting their prey. Freshwater sharks are broad-spectrum predators, but could be affected by overfishing or destruction of teleost populations. Freshwater stingrays feed on bottom invertebrates, which can be adversely affected by habitat modification and by pollutants.

shark news
Figure 1. Representative examples from the four major elasmobranch families found in fresh waters.
Carcharhinidae: a) the euryhaline bull shark Carcharhinus leucas and b) possibly obligate freshwater Ganges shark Glyphis gangeticus.
Pristidae: c) euryhaline largetooth sawfish Pristis perotetti.
Potamotrygonidae: d) obligate freshwater South American stingray Potamotrygon magdalenae.
Dasyatidae: e) marginal or possibly euryhaline whiptail stingray Dasyatis guttata.

Shark graphics courtesy Compagno (1984). Batoid pen-and-inks by S.F. Cook. From L.J.V. Compagno & S.F. Cook. In press. The exploitation and conservation of freshwater elasmobranchs: status of taxa and prospects for the future. Journal of Aquaculture and Aquatic Science.


Economic and political issues
The problem of excessive exploitation and habitat degradation in environments inhabited by freshwater elasmobranchs is compounded by the widespread incidence of poverty and political instability in developing countries that contain them. There is little emphasis on management of aquatic resources, and often civil strife, regional or civil wars, hunger, disease, poverty, corruption, ineffective government, inadequate education, and many economic problems. Emphasis in such countries is on short-term fixes for problems, or on no fixes whatever, without regard to the ultimate destruction of ecosystems or the animals which inhabit them. In extreme cases the public mentality may be largely directed to human survival and little else.

World fisheries agencies, alarmed at stagnation of marine fisheries world-wide, suggest exploitation of new and under-utilised stocks and species to sustain human population growth rate. This bears ominous implications for freshwater elasmobranchs; it also fails to address the ultimate problem of human population growth and development, which tends to readily defeat such short-sighted half- measures.

Developing countries are increasingly subject to promotion of high-income tourist facilities for First World vacationers, which can introduce unrestricted sport angling for sharks and rays and anti- shark measures to remove elasmobranchs that may occasionally attack tourists. Such practices could be devastating to freshwater elasmobranchs in restricted bodies of water such as Lake Nicaragua or Jamoer Lake in New Guinea.

Vulnerable species
We expect obligate freshwater elasmobranchs with limited geographical distributions (such as many dasyatid and potamotrygonid stingrays and possibly the Ganges shark)or euryhaline species that are trapped by man-made barriers that prevent free transit to estuaries and the ocean to stand at greatest risk from human impact. Euryhaline elasmobranchs may be relatively less vulnerable than obligate freshwater species, but such species are generally confined to warm inshore marine environments that are exploited by low-technology, increasingly intensive artisanal and small-scale commercial fisheries as well as tourist sports fisheries, and coastal development/degradation. Certain euryhaline species may need to reproduce in fresh water, and are affected by problems in freshwater breeding areas.

Priorities for research and management
Although the problems of high-technology, highly visible exploitation of marine sharks by offshore commercial fisheries have been increasingly addressed by conservationists in recent years, very little has been mentioned about the conservation and management of their more vulnerable freshwater counterparts. Small-scale, low- technology fisheries, and those in the tropics and in freshwater, receive far less attention than big oceanic fisheries, such as pelagic gillnetting and longlining. Elasmobranch conservationists are largely concentrated in more temperate countries in Europe, North America and Australia, and have given most of their attention to local exploitation and to high-seas fisheries. Sharks also get far more attention than rays or chimaeras. While much work has been done on selected aspects of freshwater elasmobranch biology, they still remain poorly known biologically, and important aspects of their biology including behavioural ecology and human impact (Including fisheries) on them urgently need to be investigated through dedicated, intensive field studies.

In view of the rapidly accelerating effects of human population growth and habitat destruction in the tropics, it is possible that several stocks and possibly whole species of freshwater elasmobranchs may become extinct in the next century. Particularly worrisome are some South American and Asian river stingrays, euryhaline sawfish, and the rare river sharks (genus Glyphis). Biological data is urgently needed for freshwater elasmobranchs to make it possible to attempt management and conservation. At present there is a vacuum of information, and elasmobranchs can easi ly drop to extinction without notice.

Development of a protocol for rational management and conservation of freshwater sharks and rays is critical, based in part on previous marine guidelines but taking into account the special and unique problems facing freshwater elasmobranchs.