Ox-bow
lakes
#Mahesh. V, #Harshavardhan
D. Joshi,*K. K. Ramteke, #M. B. Katare,*Rajesh M,#G.G.
Fadake, #Vijay A. R.
#College of Fisheries, Mangalore, Karnataka Veterinary
Animal and Fisheries Sciences University, Bidar, Karnataka, India 575 002.
*Central Institute of Fisheries Education, Seven Bunglows,
Versova, Andheri (W.), Mumbai, Maharashtra, India 400061.
Introduction
An Oxbow lake is a U-shaped body of
water formed when a wide meander from the main stem of a river is cut off to create a
lake. This landform is called an oxbow lake for the distinctive curved
shape, named after part of a yoke for oxen. In Australia, an oxbow lake is called
a billabong,
derived from an indigenous language. The word "oxbow" can also refer
to a U-shaped bend in a river or stream, whether or not it is cut off from
the main stream.
Morphological characteristics
Oxbow lakes are very
large, elongated, semi-circular or hourse-shoe shaped, shallow and lentic varies
from 2.0 hectare to more than 1000 hectare. The hydro dynamics of these water
bodies is influenced by the river water inclusion, quality of inflowing water
from the catchment area and the amount of precipitation in which lake is
isolated. thus during monsoon months the spread over are of water increases
considerably where as in winter and summer the water level shrinks
considerably. India has vast resource of Oxbow lake is about 211713 hectare of
water spread area.
Ecological status of Ox-bow lakes
Oxbow lakes are known
for their high production potential, however these ecosystems have been
subjected to environmental degradation resulting in Eutrophication. The very
existence of these ecosystem are under threat. Excessive growth of Macrophytes
has altered the normal grazing chain with the result the unwanted biota have
occupied the niche to a large extent.
Abiotic
features
The soil of these lakes
are alkaline (pH 7.7 to 8.5) and few are influenced by acidic nature (pH 4.2 to
6.8) with moderate nutritional status (PO4 28 to 185 ppm, NO3
60.5 to 536 ppm) and very high concentration of dissolved organic matter (0.32
to 9%). The oxbow lake water is more or less alkaline in nature and the
dissolve oxygen level has high fluctuations 2 to 2.5 ppm indicating stress
condition specially in lake receiving effluents. The concentration of DO in the
lakes; which do not receive effluents, however fluctuation between 4 to 8 ppm.
The alkalinity level is on the increase and found in the range of 8.5 to 10
with relatively higher values in lakes receiving effluents. The nutrients PO4
and NO3 are generally low in ambient waters compared to the soil
phase.
Biotic features
The oxbow lakes in this
basin have been subjected to utter neglected in the past and in absence of
proper care the processes of ecological succession is rapid and as such they
are in the advance stage eutrophication. presently the biotic communities, including plankton population of the
lakes, both in terms of abundance
and texture, have been adversely affected with the greater infestation of
Macrophytes. The fluctuations of planktonic population normally follow the
sequence of Bacillariophyceae, Chlorophyceae, Cyanophyceae, Dinophyceae of
phytoplankton and Copepods, Cladocerans, Rotifers,Protozoans of Zooplankton.
However, of late, the members of Cyanophyceae amongest phytoplankton and
Protozoans amongest Zooplankton tend to increase due to enrichment of systems.
The greater abundencecof nanoplnkton with high dominance of baterioplankton (Thallothrix
sp, Thiopedia sp, Cworobium sp, Sarcina sp etc) in these ecosystems is
indicative of stressed ecological regime. It has been estimated that more than
70% of the primary productivity is being contributed by this group alone. The
growth of periphyton is generally high in these systems which are represented
mainly by diatoms (> 90%) leaves and stems of submerged vegetation usually
from excellent substrata for their proliferation. The oxbow lakes are highly
infested with aquatic weeds (macrophytes) of various types such as submerged,
floating emergent and marginal covering 50 to 100% of the area. The biomass has
been reported in the range of 4 to 25Kg/m2
Role of Aquatic
weeds in Ox-bow lakes
á
The ox-bow lakes basin are highly chocked with weeds and the following
apparent characters conspicuous.
á
The lakes are over polluted with predatory and forage fishes.
á
The prized fishes were low in abundance and their population appeared going
down every year.
á
The euplanktonic population was much lower than the desired level due to
low nutrient availability.
á
A strong succession of weeds is evident affecting the ecosystem adversely.
á
The excessive growth of weeds has reduced the nutrients, Phospate in
particular, as most of the lakes were found having this nutrient in traces.
á
The algalweeds created almost an underwater desert by preventing the sun
light to penetrate. They also cause disturbance in fishing and other activites.
Fish and Fisheries
Ox-bow lakes are one of
the prime resources of fish protien in North Bihar. It is a paradox, however,
that inspite of this fact the lakes have been neglected for such a considerable
period. Practically no work, worth the name, has been done to ascertain their
biological behaviour and production processes.
The
present investigation indicated that the lakes are in very poor health owing to
high degree of eutrophication as reflected by the massive infestation of
aquatic weeds, besides other indiscriminate human interference. Most of the
lakes are chocked with weeds to the tune of 50 to 100% and as a result they are
in the advance stage of swampification. Over growth of macrophytes, in these
ecosystems, has done tremendous harm to fish and fisheries by promoting sps of
less economic values at the cost of prized ones. The changed physical state of
lakes provide the lost opportunity for breeding of carps. The fishermen are
forced to operate such small bar nets owing to very poor availability of bigger
fish. In the process of going for minor carps and smaller fishes the newly
recruited major carps, especially during monsoon, are also fishedout. The
prized carps like Cattla cattla, Labeo rohita, Cirrihinus mrigala and Labeo
calbasu in the fish production has been estimated to a meagre 3 to 5% only. In
those lakes where ingress of carp seed is taking place or some stocking is
done, its percentage in the total catch has been found to be relatively better
(26 to 28% only).
In
oxbow lakes the catch is dominated by catfishes (60 to 80%) in terms of biomass
where forage fishes dominate . The medium sized fishes like Notopteus
notopteres, Mystus carasius, Clarias batrachus, Channa gachua, Mastacembelus
armatus, Mastacembelus pancalus and big fishes like Wallago attu, Channa
marulias and Channa striatus dominate the fishery to the tune of 35%. The
miscellaneous fishes are found to be the main stay for marketing and among them
Nandus nandus, Oxygester sps, Puntis sps, Mystus vittatus are important. Infact
these small fishes account for 50% or of the daily catch, provide the basis for
survival to the fisher man community. Shrimp fishery has been found very common
in these lakes as indicated by the extensive use of a large number of traps
specially during the summer months. The period between March to June was found
to be the best for shrimp fishery. Which at times contribute up to 30% of the
total catch.
The
short term leasing policy of Fisheries department is another important factor
affecting the fishery adversely. The lessees are not very keen to invest money
for the development of fishery in these lakes. On the contrary they try to
catch the entire fish stock irrespective of size and quality, resulting in
further depletion of fish population of these lakes is allowed to build up.
The
operation of gears vary in accordance with the depth of water, nature of fish
to be caught, locally available raw materials like Cotton or Jute are more
practice as compared to Nylon nets. Bamboo reeds and strips for forming
barriers across the width of the lake are fairly common. The primitive types
and of smaller mesh bar which itself indicator that the lakes are dominated by
smaller variety of fish sps. The most disturbing trend is using of Chatti
jal, a
drag net made of mosquito net clothing which are not at all suitable in these
systems. This amounts to converting these fragile ecosystems into fishery
deserts by netting out all the fishes without any discrimination of sex, size
or quality. The small mesh size gill nets (Tiar nets) are extensively used
largely due to the dominance of small forage fishes in the system. Other than
these gears, Cast net, Scoop nets, Bag nets, also used occasionally.
Pen
culture can do easily in the Oxbow lakes. It may be benificial.
Example
The Reelfoot
Lake in west Tennessee is an oxbow lake formed when the Mississippi
Riverchanged course following the New Madrid Earthquake of 1811—1812. There are many oxbow lakes
alongside the Mississippi River and its tributaries. The largest oxbow lake
in North America, Lake
Chicot (located near Lake Village, Arkansas),
was originally part of the Mississippi
River.
The Oxbow (Connecticut River),
a 2.5-mile (4.0 km) bend in the Connecticut
River, is disconnected at one end.
The town of Horseshoe Lake, Arkansas is named after the horseshoe-shaped
oxbow lake on which it is located.
Cuckmere Haven in Sussex, England contains a widely meandering river with many oxbow lakes, often referred
to in physical geography textbooks.
Kanwar Lake Bird Sanctuary, India contains rare and endangered migratory
birds and is one of Asia's largest oxbow lakes.
Carter Lake, Iowa was created after severe flooding in
1877 led to the river shifting approximately 1.25 mi to the southeast.
In India Brahmaputra river has maximum Oxbow lake area, followed by
Ganga river system.
Artificial
Oxbow Lake
Oxbow lakes may be formed when a river
channel is straightened artificially to improve navigation or for flood alleviation.
This occurred notably on the upper Rhine in Germany in the nineteenth century.
An example of an entirely artificial
waterway with oxbows is the Oxford Canal in England. When originally constructed,
it had a very meandering course, following the contours of the land, but
the northern part of the canal was straightened out between 1829 and 1834.
The work reduced its length from 91 to 77 and half miles (approximately)
and left a number of oxbow-shaped sections isolated
from the new course.
References
AYYAPPAN,S;(2006).Oxbow lake
fisheries.Handbook of Fisheries and Aquaculture.ICAR
BOUGHEY, JOSEPH;(1994). Hadfield's British
Canals. Sutton
Publishing. ISBN 0750918403.
CONSTANTINE,
& THOMAS, DUNNE; (2008). "Meander Cut Off And The Controls On The
Production Of Oxbow Lakes", Geology, 36(1)
SHIHA,M, and JHA,
B.C.Ecology and Fisheries of Oxbow lakes of North Bihar.CICFRI
SUGUNAN,V.V, and
SINHA,M.Fisheries Enhancement of small Reservoirs & floodplain lakes in
India
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2000). "The
New Management of Rivers and Wetlands in Central Europe".
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