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.
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.
|
|
|
|
|
