Environmental Engineering Reference
In-Depth Information
Table 2.12
Classi
cation of zooplankton on the basis of
size (see Fig.
2.18
)
Type
shery, there has been an increased demand for
tropical marine aquarium
shes in recent years.
This has opened up new avenues for alternative
livelihood and a lucrative money spinning trade
for marine ornamental
Size range
Nannozooplankton
<20
µ
m
shes.
Coral reefs (Fig.
2.22
) also provide diverse
ecological niches for shelter as well as food and
thus sustain a wide variety of marine ornamental
20
-
200
µ
m
Microzooplankton
Mesozooplankton
200
µ
mto2mm
Macrozooplankton
2
-
20 mm
Megazooplankton
>20 mm
sh species (Fig.
2.23
). About 400 species of
ornamental
Source
Santhanam and Srinivasan (
1998
)
shes belonging to 175 genera and
50 families are reported from Indian waters.
stock and the coastal environmental changes
(Tang and Kawamura
2001
). Modern oceano-
graphic vessels are therefore linked to satellites
via computers allowing scientists to use imme-
diate data to plan their sampling programmes
while at sea.
Fishes are mostly concentrated in upwelling
areas, shallow coastal areas and estuaries. The
surface waters support much greater populations
of
2.2.3 Chondrichthyes
Elasmobranchs
(
chondrichthyes
) constitute a
vital segment of marine and estuarine nekton and
are of great commercial importance all over the
globe, apart from being a major component in
marine food web. About 350 species of sharks and
320 species of rays are known to exist. Nearly all
are marine, although a few species inhabit estu-
aries, and a very few are permanent inhabitants of
fresh water. It has been observed that sharks
usually prefer swimming in open waters, whereas
rays tend to be found on or near the bottom.
Sharks
are thus dominant species of elasmo-
branchs and play a vital role in both ecology and
economics. The various direct and indirect
products obtained from sharks are today used in
food, tourism and pharmaceutical industries.
Because of such multiple uses, the community is
presently under threat due to overexploitation.
Deterioration of water quality due to anthropo-
genic activities has increased the magnitude of
threat. Few species of sharks are so sensitive in
nature that they cannot withstand the alteration of
sh per unit volume of water than the deeper
zones, where food resources are very less in
terms of quality (diversity) and quantity. The
presence of mangroves and other associate
fl
oral
species also regulate the distribution of
shes in
the marine and estuarine compartments.
The marine and estuarine ecosystem is the
dwelling place of a wide spectrum of
ornamental
shes
, which have unique economic linkage.
Marine aquarium trade is rapidly expanding and
there is a growing demand for tropical marine
aquarium
shes in the international market.
Globally, the aquarium industry is valued at $4 to
$15 billion. In USA, 89 million freshwater
shes
are being maintained in 12.1 million tanks, while
5.6 million tropical marine
shes in 2.1 million
tanks. In India, after the scienti
c and technolog-
ical advancements in the sector of ornamental
Table 2.13
Classication of zooplankton on the basis of duration of planktonic life
Type
Description
Holoplankton
This group includes organisms which are planktonic throughout their life cycle, e.g. tintinnids,
cladocerans, copepods, chaetognaths, etc. Figure
2.19
shows a very common holoplankton found in
the marine and estuarine waters of tropics
Meroplankton
This group encompasses those organisms which remain planktonic only for a portion of their life
cycle, e.g. larvae of benthic invertebrates and sh larvae (ichthyoplankton). In the estuaries of
Indian Sundarbans, larvae of prawn (
) are found in plenty (Fig.
2.20
),
which nally become a semi-benthic species in the adult stage (Fig.
2.21
)
Macrobrachium rosenbergii
Source
Santhanam and Srinivasan (
1998
)
