241
fi shes may be harmful to the indigenous icthyodenizens. In contrast to the above, the minnow
population introduced in certain lentic bodies in
Europe and America during 1985 declined much
more rapidly than could be accounted for by
direct predation by known numbers of largemouth
bass in the waterbody (Kar 2007a , b ). Introduction
of Clarias gariepinus in some wetlands in
Northeast India during the late 1990s had accelerated the total predation rate and had changed the
composition of the prey community. Predation
rate on many indigenous fi sh species had increased
dramatically because Clarias gariepinus is more
benthic and inhabits waters in the bottom strata
mostly. This increase in predation rate on the
indigenous fi shes had cascading effects on the
zooplankton community in the wetlands.
The fi sh community in Assam is diverse and
consists mainly of carnivores and omnivores (Kar
2007a , b ). The principal fi shes include the following: featherbacks, shads, carps and minnows,
loaches, catfi shes, murrels, killifi shes, glassfi shes, gobies and globefi shes. These fi shes vary
in size from 1 cm to 4.3 m in total length. Such
assemblages, which generally have large piscivores, are common in wetlands of Northeast
India, Myanmar, Thailand, China, Lao PDR,
Kampuchea, Philippines, etc. In these wetlands,
the zooplankton community is generally dominated by rotifers ( Brachionus spp., Lecane spp.,
Filinia spp., etc.), cladocerans ( Ceriodaphnia
spp., Simocephalus spp., Moina spp., Bosmina
spp., Alona spp., Pleuroxus spp., etc.) and copepods ( Cyclops spp., Diaptomus spp., etc.).
The infl ow and outfl ow channels sometimes
reveal rich diversity and high yield of fi shes. This
is refl ected by high catch of fi shes in the channels.
Sometimes, rapid increases in catch ratio in outfl ow channel to the wetland demonstrated high
rate of emigration as the main cause of reduction
fi sh biomass. Further, behavioural responses of
prey fi sh to predation may depend on the density
of prey fi sh in the wetland. Habitat heterogeneity
and habitat selection may also infl uence predator–
prey interaction. The small wetlands may provide
only a modest amount of refuge where prey fi shes
could escape the piscivores. Lack of proper refuge
may lead to total disappearance of a species from
the wetland. Further, cannibalism may also be
another distinct feature of piscivore-dominated
system. Cannibalism plays an important role and
is a key factor in determining density and size
structure of certain fi sh species in the wetlands.
In addition to the above, many fi shes may
progress through a trophic ontogeny, in which
diets undergo changes as habitat shifts occur and
the relative sizes of predator and prey change
with growth dynamics. The adult piscivorous
fi shes may alter their diets and may eat a wide
range of other preys when small fi shes are scarce.
This shift of diets between fi sh and non-fi sh
preys may directly affect growth rates of larger
fi shes. This may also regulate the survivorship of
many juveniles which may eventually become
piscivorous.
Over time, availability of quite large population
of minnows might have eventually yielded a planktivore assemblage. As a result, fi sh communities in
the wetlands, which are subject to regular exploitation, will be generally dominated by planktivores
which are less effi cient predators than the competitively superior catfi shes. The zooplankton community in this type of system may include a diversity of
forms, e.g. Bosmina, Chydorus, Daphnia and
Cyclops . The lentic systems and their consequent
epilimnetic trophic system, as described above, is
amongst those most commonly observed in the lentic bodies of the temperate zone.
The linkage between fi sh and other food web
components generally determines the food web
dynamics in the wetland ecosystem. The strength
of these linkages is not only determined by the
abundances and size structure of fi sh population
but may also depend on the vulnerability and relative abundance of prey populations. Sometimes,
high piscivory occurs when prey fi sh are temporarily abundant. Some of the generalisations are
as follows: (a) predation rate varies with time; (b)
amongst all prey items, predation rates are generally more in benthos; and (c) trophic ontogeny of
a strong cohort may play a major role in regulating long-term responses in food webs and the
variability of trophic cascade.
Planktivorous fi shes may also exhibit much of
the same type of trophic ontogeny which is found
in the early life history of piscivores. They may
15.5 Wetland Fishes
fi shes may be harmful to the indigenous icthyodenizens. In contrast to the above, the minnow
population introduced in certain lentic bodies in
Europe and America during 1985 declined much
more rapidly than could be accounted for by
direct predation by known numbers of largemouth
bass in the waterbody (Kar 2007a , b ). Introduction
of Clarias gariepinus in some wetlands in
Northeast India during the late 1990s had accelerated the total predation rate and had changed the
composition of the prey community. Predation
rate on many indigenous fi sh species had increased
dramatically because Clarias gariepinus is more
benthic and inhabits waters in the bottom strata
mostly. This increase in predation rate on the
indigenous fi shes had cascading effects on the
zooplankton community in the wetlands.
The fi sh community in Assam is diverse and
consists mainly of carnivores and omnivores (Kar
2007a , b ). The principal fi shes include the following: featherbacks, shads, carps and minnows,
loaches, catfi shes, murrels, killifi shes, glassfi shes, gobies and globefi shes. These fi shes vary
in size from 1 cm to 4.3 m in total length. Such
assemblages, which generally have large piscivores, are common in wetlands of Northeast
India, Myanmar, Thailand, China, Lao PDR,
Kampuchea, Philippines, etc. In these wetlands,
the zooplankton community is generally dominated by rotifers ( Brachionus spp., Lecane spp.,
Filinia spp., etc.), cladocerans ( Ceriodaphnia
spp., Simocephalus spp., Moina spp., Bosmina
spp., Alona spp., Pleuroxus spp., etc.) and copepods ( Cyclops spp., Diaptomus spp., etc.).
The infl ow and outfl ow channels sometimes
reveal rich diversity and high yield of fi shes. This
is refl ected by high catch of fi shes in the channels.
Sometimes, rapid increases in catch ratio in outfl ow channel to the wetland demonstrated high
rate of emigration as the main cause of reduction
fi sh biomass. Further, behavioural responses of
prey fi sh to predation may depend on the density
of prey fi sh in the wetland. Habitat heterogeneity
and habitat selection may also infl uence predator–
prey interaction. The small wetlands may provide
only a modest amount of refuge where prey fi shes
could escape the piscivores. Lack of proper refuge
may lead to total disappearance of a species from
the wetland. Further, cannibalism may also be
another distinct feature of piscivore-dominated
system. Cannibalism plays an important role and
is a key factor in determining density and size
structure of certain fi sh species in the wetlands.
In addition to the above, many fi shes may
progress through a trophic ontogeny, in which
diets undergo changes as habitat shifts occur and
the relative sizes of predator and prey change
with growth dynamics. The adult piscivorous
fi shes may alter their diets and may eat a wide
range of other preys when small fi shes are scarce.
This shift of diets between fi sh and non-fi sh
preys may directly affect growth rates of larger
fi shes. This may also regulate the survivorship of
many juveniles which may eventually become
piscivorous.
Over time, availability of quite large population
of minnows might have eventually yielded a planktivore assemblage. As a result, fi sh communities in
the wetlands, which are subject to regular exploitation, will be generally dominated by planktivores
which are less effi cient predators than the competitively superior catfi shes. The zooplankton community in this type of system may include a diversity of
forms, e.g. Bosmina, Chydorus, Daphnia and
Cyclops . The lentic systems and their consequent
epilimnetic trophic system, as described above, is
amongst those most commonly observed in the lentic bodies of the temperate zone.
The linkage between fi sh and other food web
components generally determines the food web
dynamics in the wetland ecosystem. The strength
of these linkages is not only determined by the
abundances and size structure of fi sh population
but may also depend on the vulnerability and relative abundance of prey populations. Sometimes,
high piscivory occurs when prey fi sh are temporarily abundant. Some of the generalisations are
as follows: (a) predation rate varies with time; (b)
amongst all prey items, predation rates are generally more in benthos; and (c) trophic ontogeny of
a strong cohort may play a major role in regulating long-term responses in food webs and the
variability of trophic cascade.
Planktivorous fi shes may also exhibit much of
the same type of trophic ontogeny which is found
in the early life history of piscivores. They may
15.5 Wetland Fishes