4
II. REVIEW OF LITERATURE
Estuaries are considered as nutrient sinks or traps, where nutrient dynamics are
unpredictable and significant, as it make estuary a highly productive ecosystem (Odum,
1971).
Among the various brackish water systems, the Hoogly-Matlah estuarine complex
in West Bengal, Chilka lake in Orissa, Pulicate lake and Vellar estuary in Tamil Nadu
along the east coast, the Vembanad lake and its connected backwaters of Kerala, GurpurNethravati and Mulki-Pavanje, Haladi-Chakra, and Kali estuary in Karnataka and
Mandovi-Zuari estuarine complex of Goa along the west coast of India are prominent.
Estuaries of east and west coasts of India differ to a large extent from each other by the
geographic condition, monsoonal regime, the river water influx and tidal range.
2.1 Hydrography- temperature, pH, salinity, dissolved oxygen and nutrients
Seasonal variations in the ecological parameters are studied in estuaries along the
west coast of India. Seasonal variations in hydrographic parameters in Kali estuary
(Karnataka) were studied by Harkantra, (1975a) and Bhat and Neelakantan (1988).
Detailed studies of hydrographic parameters were conducted in Mandovi-Zuari estuary in
Goa (Alagarswamy, 1991; Parulekar et al. 1973; Qasim 1979; Qasim and Sengupta, 1981).
Major work in this aspects carried out in Kerala were that of Qasim and Gopinath (1959),
Josanto (1971) in Cochin back waters and in Ashtamudi estuary by Nair et al. (1983) and
Nair et al. (1984). Along Malabar coast seasonal ecological variations were studied in
Korapuzha Estuary (George and Kartha, 1963) and Beypore Estuary (Premchand et al.
1987).
5
Kennedy (1990) reported that tidal estuary undergoes dynamic interaction of salt
and freshwater, which is largely influenced by tidal exchange and land drainage among
several factors. Hydrographic features such as temperature, salinity, dissolved oxygen and
nutrients govern the distribution of flora and fauna in estuaries (Gouda and Panigrahy,
1995). Thus, the studies on estuaries are more important as these are highly productive and
play an important role as nursery grounds for many commercially important fishes, and
shrimps (Devi et al. 1981).
The hydrography of the Mandovi and Zuari estuarine systems have been studied by
many
scientists
during
different
seasons
(Dehadrai,
1970;
Das
et
al.1972;
Sankaranarayanan and Jayaraman, 1972; Dwivedi et al.1973; Singbal, 1973 and 1976;
Antony et al.1974; Dwivedi et al.1974; Goswami and Singbal, 1974; Thomas Cherian et
al.1975; Varma et al.1975; De Souza, 1977 and 1999; Qasim and Sengupta, 1981; George
et al.1984; De Souza and Sengupta, 1988 and Nayak and Chandramohan, 1989). The
estuarine hydrography of Cochin backwaters were studied by many scientists (Anirudhan
et al.1987; Devi et al.1981; Lakshmanan et al.1982; Manikoth Salih,1974; Menon et
al.2000;.Ramamirtham et al.1986; Pillai, 1993; Shynamma and Balakrishnan, 1973).
The estuaries in Karnataka viz., Nethravati-Gurpur estuary, Mulki-Pavanje estuary,
Sita-Swarna estuary and Haladi-Chakra estuary have gained greater importance because of
their high productivity and potentialities. Menon et al. (1977) opined that the NetravatiGurpur estuary experienced a horohelinicum condition during early part of monsoon
season. A detailed investigation on the hydrography of Nethravati-Gurpur estuary had been
carried out by Bhat (1979). Sahu (1981) studied the seasonal variations of hydrographic
parameters while working on the circulation in the Nethravati-Gurpur estuary. Reddy
(1982) observed seasonal and spatial variations of selected hydrographical parameters of
6
the Mulki estuary, Dakshina Kannada. Nagarajaiah and Gupta (1983) documented seasonal
fluctuation of hydrographic parameters of brackish water ponds of Nethravati estuary. Patil
(1987) and Suresh (1987) have discussed seasonal and spatial variations of hydrographical
parameters of Nethravati-Gurpur estuary.
Vedamurthy (1992) documented the distribution of particulate matter in the
Nethravati-Gurpur estuary. Shanthanagouda (2001) documented spatial and seasonal
variations of hydrographic parameters of Nethravati-Gurpur estuary. Tripathi (2002) while
working on the distribution of phytoplankton observed selected hydrographic parameters
of Nethravati-Gurpur estuary. Vijaykumar et al. (2004), while describing the distribution
of crustacean larvae in Nethravati-Gurpur estuary documented selected hydrographical
parameters. Shivakumar (2005) studied the variations of selected hydrographical
parameters of Mulki-Pavanje estuarine complex, Dakshina Kannada.
Woods (2006) studied the hydrography, suspended sediments, chlorophyll a,
chromophoric dissolved organic matter and optical characteristics of the Pearl River
(Zhujiang) estuary. The study indicated that spatial patterns of modelled photo
synthetically available radiation (PAR) during July suggested light limitation of primary
production in low salinity turbid western waters due to high light attenuation. Capo et al.
(2006) studied the morphology, hydrography and sediment dynamics in a mangroove
estuary, the Konkoure Estuary, Guinea. The study indicated that the estuary became
stratified during high river flows and spring tides whereas a salt wedge appears during
neap tides. The suspended matter is transported by the tidal effect within the middle
estuary and is therefore trapped
in the turbidity maximum zone (TMZ). The location of
the TMZ is river-controlled and is correlated with residual currents but not with salinity
front. Buzzelli et al. (2007) studied the hydrographic characterization of two tidal creeks
7
with implications for watershed land use, flushing times, and benthic production and they
revealed that the dissolved oxygen and salinity contributed to an estimated 8-10 times
more phytoplankton-based carbon.
2.2 Sediments – texture and organic matter.
Estuaries are regions of fundamental importance with respect to chemical
processes occurring on the global scale, for they represent major route whereby
lithospheric material is transported to the oceanic mentary domain (Morris, 1978). The
studies of marine processes have paved the way for predicting the geochemical behaviour
of each individual element.
Comprehensive studies on the physico-chemical, biological and ecological aspects
have been made in the sediments of different Indian estuaries. Along the west coast the
important contributions were made by Alagarsamy (1991) and Nasnolkar et al. (1996) in
the Mandovi estuary; Sankaranarayanan and Panampunnayil (1979), Nair et al. (1993) and
Seralathan et al. (1993) in the Cochin backwaters; Reghunadh et al. (1995) in the
Tellicherry mangrove sediments; Nandan and Abdul Azis (1996) in the Kadinamkulam
estuary; Prabhu et al. (1997) in the near shore sediments off Honnavar; and Anilkumar et
al. (1999) in the Baypore estuary. Rao (1971) studied the sedimentological characteristics
of the bottom sediments of Pulicat Lake.
Chemical and textural characteristics of
sediments in the upper reaches of Cauvery estuary were discussed by Ramanathan et al
(1988).
Tropical estuaries are highly dynamic owing to variation in the tidal forces, energy
of rivers at the confluence, which varies measurably, and interplay of these forces result in
a complicated sedimentary environment. In addition to these, the source of hydrodynamic
conditions of the transport media all mineral composition, influence the variation in texture
8
of the sediment. The morphological features play an important role in sediment
distribution. The urbanization and anthropogenic processes are influencing sedimentation
pattern and hence siltation is found to be one of the common problems in tropical estuaries.
For efficient management of estuaries, knowledge of sedimentation and textural variation
is of vital importance. The sediment acts as the reservoir of nutrient replenishment of these
nutrients in times of need and their consequent renewal greatly helps in the biological cycle
of the system. Such an exchange of nutrients depends upon the characteristics of the
sediment and hydrographic features of the estuary. The distribution and abundance of
benthic organisms are primarily influenced by the sediment texture characteristics.
Sanders(1969) and Wharfe (1976, 1977) have done the pioneering works to establish the
interrelationship between the benthos and textural characteristics of sediment in the
Buzzards Bay and the lower Medway estuary, Kent, United Kingdom respectively.
Along the east coast of India, Venkatarathnam (1968) studied the sediments of
Vishakapatnam, Pudimadaka continental shelf and Pulicat Lake-Pennar river complex.
Sasmal et al. (1986) documented the texture and composition of sediment of Hoogly
estuary and near shore environment. Studies on texture, mineralogy, carbon, nitrogen and
phosphorus(CNP) of the sediments of Vishakapatnam shelf were carried out by
Satyanarayana et al. (1993). Jayaraju et al. (1998) worked on the sediment texture, organic
matter and salinity while studying on the benthic foraminifera of Pulicat Lake. Mohan
(2000) analysed the sediment texture in Vellar estuary and concluded that the variation in
textural characteristics was attributed to tidal activity. Islam et al. (2004) while studying
water quality, nutrient dynamics in shrimp farms of the Sundarbans mangrove forest,
Bangladesh, documented sediment profile of the farms.
9
Along the west coast of India many scientists have investigated textural
characteristics of the sediment of Cochin backwaters and adjoining coastal waters. Dora et
al. (1968) documented the textural characteristics of the sediment of Narrakkal mud banks.
Josanto (1971) documented the size distribution of the sediment in Cochin backwaters.
Jacob and Qasim (1974) examined the organic carbon, size fraction, and calorific values of
sediment from the mud banks of Kerala. Sankaranarayanan, and Panampunnayil, (1979)
studied the organic carbon, nitrogen and phosphorus in the sediment of Cochin backwaters.
In the same area, Paropakari (1979) examined organic carbon, nitrogen ratio and nature of
sediments in relation to pollution. Varshney et al. (1981) recorded the physico-chemical
characters of the sediment and overlying water of Narmada estuary. Ramachandra (1981)
reported that sand contributes to 80 % of the sediment composition in Mulki estuary..
Sediments of Cochin estuary were studied in relation to changing hydrographic conditions
by Nair et al. (1993). Sediment characteristics in the estuarine mouth along the Ernakulam
and Mattancherry channel were studied by Seralathan et al. (1993). Prabhu et al.(1993)
carried out studies on the organic carbon and composition of the near shore sediment off
Gangolli. Nandan and Abdul Azis (1996) documented the organic matter of sediments
from the retting and non-retting areas of Kadinamkulam estuary. Physical properties of
sediment in the mud bank area of Cochin were studied by Kumar et al. (1998). Badarudeen
et al. (1998) observed the average sand percentage of 68 % while studying the sediment
grain size in Kunnur region, Karnataka. Devi et al. (1999) recorded five types of
substratum along the southwest coast clayey silt, clayey sand, silty clay, sandy silt and
sand), but in southeast coast, sand percentage was predominant. Shanthanagouda (2001)
studied textural characteristics and organic carbon of the sediment in Nethravati- Gurpur
estuary. Rajesh et al.(2002) documented the sediment composition, organic carbon and
10
total phosphorus and total nitrogen in relation to shrimp farming. Ingole et al. (2002)
observed that the sediment comprises of mainly silt and clay with less of sand in Dobhol
waters. Rajesh et al. (2004) carried out the work on organic carbon, nitrogen, phosphorus,
and textural analysis of the sediment in brackishwater impoundments along Nethravati
estuary, Mangalore.
Algarasamy (1991) reported the organic carbon in the sediment of Mandovi estuary
Goa, which ranged from 0.1 to 3.0 %. Nasnolkar et al. (1996) have carried out a study on
organic carbon, nitrogen and phosphorus in the sediments of Mandovi estuary, Goa.
Varsheny et al. (1999) observed the clayey silt sediment with higher percentage of organic
matter in Thana creek, Mumbai. An investigation on sedimentation of particulate matter in
the Dona Paula Bay, west coast of India was carried out by Bhaskar et al. (2000). Kumary
et al. (2001) studied the sediment characteristics of Poonthura estuary and recorded high
organic carbon content of 2.4 to 83.3 mg/g. Ingole et al. (2002) observed the high mean
value of (1.42 %) organic carbon content in sediments of Dabhol. Ayyankumar(2007)
while working on the organic content in the sediments of Gurpur estuary reported that the
sediments ranged between 0.35 (June) and 3.68% (May).
2.3 Macrobenthos - Distribution and Abundance
A detailed knowledge of the bottom fauna is essential for the determination of the
fishery potential of an estuary. The structure of the shallow water benthic fauna and their
role as the principal food source for the demersal fishes and other predators, and as the
processors of organic productivity of the superficial waters through nutrient regeneration
are well documented by many authors (Rainer, 1982; Miron and Desrosiers, 1990;
Soemodinoto et al.1995; Platell and Potter, 1996.)
11
Chakraborty and Choudhury (1997) have recorded the maximum species richness
of polychaetes during the monsoon season and the minimum during the pre-monsoon
season in Hoogly estuary, and further the authors have attempted to correlate polychaete
species with hydrographical parameters like salinity, temperature, dissolved oxygen etc.
Distribution of macrobenthos in three different zones of Vellar estuary was carried
out by Chandran et al. (1982). Murugan and Ayyakkannu (1991), while investigating on
the ecology of benthic macrofauna in Cuddlore-Uppanar backwaters recorded greater
abundance of polychaetes followed by molluscs, crustaceans and others. While studying
the occurrence and abundance of benthic fauna in marine zone of Coleroon estuary,
Jagadeesan and Ayyakkannu (1992) observed the dominance of polychaetes followed by
crustaceans in a marine zone of Coleroon estuary. Devi et al. (1999) observed the low
population density of macrobenthos in south-east coast compared to south-west coast and
found dominance of polychaetes along south-west coast and the dominance of molluscs
along south-east coast. Kathiresan (2000) reviewed the works carried out on the plankton,
nekton, benthos and flora of Pichavaram mangrooves. Kumar and Sivakumar (2001)
investigated the influence of estuarine environment on benthic foraminifera in Uppanar
river estuary. Kailasam and Sivakami (2004) investigated the effect of thermal effluent on
benthic fauna of Tuticorin bay.
Along the west coast of India investigations on benthos in Cochin back waters and
other estuarine complexes from Kerala, Netravathi-Gurpur, Mulki-Pavanje, Udyavar and
Kollur estuaries from Dakshina Kannada, Mondovi-Zuari from Goa, Narmada-Tapti from
Gujrat are of great importance. Kurian (1972) observed the hydrographical features playing
an important role in sedimentation and distribution of fauna in Cochin backwaters. The
influence of salinity on macrobenthos was clearly described by Ansari (1974) while
12
working on the macrobenthic production in Vembanad lake. Kurian et al. (1975) observed
the bottom fauna of the Vembanad lake, which chiefly composed of molluscs, polychaetes
and amphipods and they documented the variation of fauna in relation to sediment
characters and salinity. Nair et al. (1984) while working on ecology of Indian estuaries
observed quantitative distribution of benthic fauna in relation to salinity, dissolved oxygen,
and nature of sediment. Sunilkumar (1995) documented biomass, horizontal zonation and
vertical stratification of polychaete fauna in the littoral sediments of Cochin estuarine
mangrove habitat. In the soils of Cochin backwaters, Sunilkumar (2002) studied the
distribution and dominance of macroinvertebrates in shrimp culture farms.
Harkantra (1975a) carried out a detailed study on the seasonal distribution in the
benthic production of Kali estuary (Uttara Kannada). The author also reported higher
benthic production during pre-monsoon and post-monsoon seasons and further observed
the dominance of bivalves followed by polychaetes. The spatial and seasonal distribution
of macrobenthos of Netravati- Gurpur estuary was documented by Bhat and Gupta (1980).
The authors have observed greater dominance of molluscs followed by polychaetes and
crustaceans. Ramachandra (1981) made a detailed study on the macrobenthos of Mulki
estuary, Dakshina Kannada. His study revealed details of macrobenthos in Mulki estuary.
An investigation on macrobenthos with sediment characteristics of Mulki estuary was
carried out by Ramachandra et al. (1984). Bhat and Neelakantan (1988) investigated on the
influence of environment on the distribution of macrobenthos in Kali estuary. Narasimham
(1991) reported that the river Mulki supports rich clam fisheries contributing 2392
tonnes/year. Prabhu et al. (1993) while working on macrobenthic fauna at the barmouth of
Gangolli river observed the dominance of polychaetes followed by echiuroides and
13
molluscs. Shanthanagouda (2001) documented the species diversity of macrobenthic fauna
in Nethravati- Gurupur estuarine complex.
The Mondovi-Zuari estuarine system in Goa had received considerable attention.
Dwivedi et al. (1974) studied the ecology of macrobenthos in the Mondovi-Zuari and
Camburja estuarine complex. Further the authors have revealed that the sandy substratum
along with euryhaline condition favoured higher macrobenthic biomass. Parulekar and
Dwivedi (1974) investigated the benthic faunal composition of Mondovi estuary in relation
to bottom salinity and sediment characteristics. An annual cycle of macrobenthic fauna,
their distribution, production and trophic relations was studied in Goan estuaries by
Parulekar et al. (1980). The authors further observed dominance of polychaetes and
bivalves in the total macrobenthic population.
Govindan et al. (1983) investigated benthic biomass and faunal composition in
relation to various environmental conditions of four estuaries of south Gujarat. Varshney et
al. (1999) recorded the dominance of foraminifera, polychaeta, crustacea and pelecypoda
in the macrofaunal composition in Thana creek, Mumbai. Macrobenthic communities of
the coastal waters of Dabhol was investigated by Ingole et al. (2002).
Harkantra (1982) recorded dominance of polychaetes followed by crustacea,
mollusca and echinodermata while studying sub-littoral macrobenthic fauna of the lower
Swansea bay, England. Cattrijsse et al. (1992) reported total 46 species of amphipods
belonging to family Gammorideans, Caprellids and Hyperiid along with the 8 species of
isopods in Voor delta and the Westerschelde estuary.
2.4 Phytoplankton and pigments
A great deal of information is available regarding the occurrence and abundance of
phytoplankton in space and time. The seasonal and spatial variation of phytoplankton is
14
governed by various physico-chemical factors and these factors were known to exhibit
greatest variations in marine environment in general and in estuaries in particular. Many
scientists have carried out investigations on the occurrence and distribution of
phytoplankton of various estuaries located along East and West coasts of India. Several
studies have been carried out on the pigment analysis and their relationship with several
other factors in different estuaries of the world. Pioneering work carried on pigment
analysis and their relationship with environmental parameters was carried out in Barrents
sea by Kreps and Verjbinskaja (1930) and Harvey (1934).
A great deal of information is available on the diurnal and seasonal variation of
chlorophyll ‘a’ and other plant pigment in the estuarine and inshore marine environment
along the Indian coasts. The primary productivity studies were conducted were based on
chlorophyll in Godavari river estuary (Rajaylakshmi and Premswarup, 1975).
Diversity of phytoplankton species and pigment concentration in Vellar estuary was
investigated by Vijayalakshmi and Venugopalan (1975). Diurnal variation in the
chlorophyll-‘a’ concentration was reported by Subramanian and Venugopalan (1980) in
Vellar eatuary with no significant difference in Chlorophyll ‘a’ between surface and
bottom water. Seasonal distribution of phytoplankton pigment in the Vellar estuary was
studied by Joseph (1982). Their studies revealed a direct relationship between salinity and
chlorophyll fractions. An investigation of phytoplankton pigment in relation to primary
production and nutrients in inshore water of Tuticorin was carried by Gopinathan et al.
(1994).
Seasonal variations of certain plant pigment in the coastal waters off Mangalore
was investigated by Manjappa (1987). The distribution of plant pigment in relation to
hydrography of Nethravati-Gurupur estuary was documented by Suresh (1987). Gowda et
15
al. (1997) studied the distribution of chlorophyll-a and phaeo-pigments in NethravatiGurupur estuary. Temporal variation of phytoplankton pigment in relation to nutrient in a
tropical coastal lagoon was studied by Nayar and Gowda (1999). Gowda et al.(2001b)
while investigating on variation of primary production documented the Chlorophyll-a and
carotenoids in Netravati estuary, Mangalore. Diurnal variation of phytoplankton pigments
in Netravati estuary, Mangalore was studied by Gowda et al. (2001b). An investigation on
primary productivity and algal pigments in estuaries of Dakshina Kannada and Udupi
districts was carried out by Gupta et al. (2002). Bollens et al. ( 2006) studied plankton
dynamics of the lower Columbia River estuary. The results indicated that microplankton
groups observed were diatoms, dinoflagellates and ciliates; dominant mesozooplankton
taxa included the copepods and also indicated a strong seasonal cycle, with spring blooms
of diatoms and copepods, followed by compositional shifts in summer toward flagellates,
ciliates and other copepods. With respect to freshwater flow, both biomass and
abundance of microplankton were higher high flow than in low flow. The tidal cycle
sampling showed large variation in abundance and composition of plankton with tidal
stage, especially in diatoms, ciliates and copepods
The spatial and temporal variation of phytoplankton pigments in the western part of
Ria Formosa, Portugal, was investigated by Perira et al., (2007). The analysis revealed that
the diatoms and other algal groups with a similar pigment profile dominated the
phytoplankton community throughout the year. Ramdani et al. (2009) studied the
environmental influence on the qualitative and quantitative composition of phytoplankton
and zooplankton in the North African coastal lagoons. They evaluated the hydrographical
and other influences on the structure, composition and space-time development of the
plankton communities.
16
2.5 Suspended Particulate Matter (SPM)
A great deal of work has been carried out on various aspects of seston.
Chalapathirao and Satynarayanarao (1973) made observations on the distribution of
particulate organic matter in inshore waters of Bay of Bengal off the coast of Waltair and
reported peak values of particulate organic matter during peak phytoplankton growth in
summer and also revealed an increase in particulate organic carbon and nitrogen in surface
waters during the period of heavy rains. The distribution of suspended particulate matter in
the waters of eastern continental margin of India was studied by Madhusudanarao (1985)
and reported a decrease in total particulate away from the coast and high concentrations in
some river mouths and in near shore regions. The total SPM material in the clambeds in
Mulki estuary present study varied from 13.2 to 134. 00 mg/l. In Cochin backwater,
Saraladevi (1989) reported the values of total suspended solids between 3 and 253 mg/l
Vedamurthy (1992) has reported the values between 7 and 218 mg/l. The values of total
suspended solids in Gangolli estuary varied between 10 to 173 mg/l (Reddy et al. 1992)..
Padyar (1999), while studying the particulate matter in Mulki - Pavanje estuary reported
that the suspended solids varied between 13 to 143 mg/l. The author opined that the higher
values in the monsoon and post monsoon due to terrigenous input and lower value during
pre monsoon due to lesser inputs.
2.6 Clam habitat – Population ecology
The estuarine ecosystem reveals the complexity of the operating forces of both
marine and fresh water origin, induced mainly by tidal incursion, current patterns and the
magnitude of fresh water discharge at different periods and seasons (Cameron and
Pritchard, 1963). The abiotic factors vary from habitat to habitat and are constantly under
fluctuations within a habitat. They interact with each other and also influence greatly the
17
biota. The climatic, physical and chemical factors are the three major types of determining
factors apart from the topography.
Water quality reflects the collective influence of various physic-chemical
characters and the influence of meteorological conditions. This would determine the type
and growth rate and reproduction of organisms in any locality (Vink, 1983). The various
environmental and biological characteristics in a water body have a dynamic equilibrium
with one another, while the physical and chemical parameters in themselves, collectively
determine the water course favourable for aquatic organisms. The nature and distribution
of flora and fauna in an estuary are mainly controlled by fluctuations in the physical and
chemical characteristics of the water, such as temperature, transparency, pH, salinity,
dissolved oxygen and nutrients (Murugan and Ayyakkannu, 1991).
The productivity of the ecosystem differs significantly and the annual productivity
indicates the trophic status and it indicates the health of a system. Influence of
environmental parameters especially temperature and salinity are studied in relation to the
distribution, reproduction and condition of bivalves worldwide. Adaptation of the clam P.
laterisulca in Kalbadevi estuary was studied by Mane and Dhamne (1980). Cywiak et al.
1989, reported that abiotic variables such as temperature and salinity caused mortality of
bivalves Donax trunculus and Donax semistriatus along the Mediterranean coast. It was
also reported that these species were susceptible to environmental stresses. Study on the
influence of temperature and salinity on reproduction of Ostrea edulis showed that oyster
is an opportunistic organism which concentrates its reproductive efforts during a short
period of favourable condition and it is directly dependent on nutritive availability in the
environment (Ruiz et al. 1992).The relative importance of biotic versus abiotic factors in
determining the spatial distribution of intertidal clams was studied by Schoeman and
18
Richardson (2002) and further they observed that a mixture of biotic and abiotic factors
mediates abundance of clams. Marin et al. (2003) have observed that condition index and
total energy content of the clam, Tapes phillipinarum, was influenced by environmental
factors.
The relationships between gametogensis and environmental parameters have also
been described in other bivalve groups (Ruiz et al. 1992; Cano et al. 1997; CeballosVazquez et al. 2000 and Luna-Gonzalez et al. 2000).
In the populations of Donax
denticulatus in Puerto Rico, it was observed that higher population density and bigger
clams were found in water having higher concentration of total organic carbon, particulate
organic carbon and nitrate-nitrogen. This study indicated the relationships between water,
sediment quality and clam density (Sastre, 1984).
Maske et al (2004) studied salinity tolerance and related biochemical changes in
three clams from Bhatye estuary, Ratnagiri district of Maharashtra. It was observed that K.
opima was highly sensitive while M. meretrix was more resistant than M. casta.
Ringwood and Kepler (2002) studied the relationship between important water
chemistry parameters such as pH, salinity and dissolved oxygen and biotic performance
based on clam, Mercenaria mercenaria growth. Studies indicated that the salinity was
identified as an important determinant of clam growth over wide salinity ranges (1035ppt), pH was also found to be a very important parameter, especially in low-salinity
regimes (< 25ppt). Further his study indicated that when an average pH levels fell below
7.5 or minimum pH levels fell below 7.2, the growth rates were < 50% that of clams
deployed under higher pH conditions. Estuarine systems are generally perceived as being
well-buffered zone so pH is frequently assumed to be unimportant, but the results
suggested that pH levels can decline in estuarine systems to levels that can adversely
19
affect biological responses. Stecher et al. (2003) studied the population characteristics of
abundant bivalves (Mollusca, Vesicomyidae) at a sulphide-rich seafloor site near Lihir
Island, Papua New Guinea. He found that the average biomass value was 13.3 kg/m2, with
maximum values over 29 kg/m2 (wet mass with shells). Whiteley et al. (2007) studied the
ecological implications of intertidal mariculture and observed differences in bivalve
community structure between farm and reference sites. The authors opined that predation
and competition play minor roles in structuring communities in soft-bottomed
environments. Mendoza (2007) studied the community structure and nutrition of deep
methane-seep macrobenthos from the North Pacific (Aleutian) Margin and the Gulf of
Mexico. The studies indicated a characteristic communities or features common to the
three deep-water seeps (>3000 m), but common properties across habitats (mat, clam bed,
pogonophorans), independent of location or water depth. In general, macrofaunal densities
were lower (except at Florida microbial mats), community structure was similar, and
reliance on chemosynthesis was greater than observed in
shallower seeps off California
and Oregon. Flye-Sainte-Marie et al. (2008) studied the effect of sediment grain-size on
development of brown ring disease in the Manila clam Ruditapes philippinarum. The
results indicated that the presence of large sediment grains in natural habitats, which
become lodged in the shell opening will induce the Brown ring disease (BRD).
Nizzoli et al. (2007) studied oxygen and ammonium dynamics during a farming
cycle of a bivalve Tapes philippinarum. They opined that the oxygen consumption and
ammonium production at the high density area were on an average, 3 to 4 and 1.9 to 4.9
folds higher than those measured in the control field respectively; and their growth rates
were positively correlated with clam biomass. The clams have the potential to drive
benthic metabolism in farmed areas and to sustain macroalgal growth through regeneration
20
of limiting nutrient in seawater. While studying the influence of seed size, level of
protection and substratum type on mortality early in the culture period, Onge and Miron
(2007) studied on
erosion and transport of juvenile softshell clams (Mya arenaria).The
results indicated that stream velocity and sediment type interact together on the erosion of
clams from the sediment. Juveniles were eroded in great numbers in sand while mud
retained them more easily. Field results confirmed the bearing of free-stream velocity, shell
length and type of sediment on the erosion rate of clams.
The ecology of clambeds in India had been studied by several authors. Some of
them are on Anadara granosa by Narasimham et al. (1984); Narasimham, (1985); Meretrix
casta by Parulekar et al. (1973); Harkantra, (1975, 1975a); Rao et al. (1980) and
Sreenivasan (1983 a, 1983b; Paphia malabarica by Parulekar et al. (1984) and Villorita
cyprinoid.s Rao et al.(1989). Reports on the habitat preferences, density, biomass and
distribution are available for A. granosa (Radhakrishna and Ganapathi, 1968), Meretrix
ovum (Desai, 1971 and Kurian, 1972), Donax cuneatus (Victor and Subramaniam, 1988),
Donax sp. (Ansell et al. 1972 a, 1973), M. casta (Balasubramanyan and Natarajan, 1987
and Modassir, 1990).
An understanding of the ecology of clam bed is very essential in order to evaluate
the influence of different environmental factors and their interactions. This will help in
future management of fishing and farming of clams in an area.
2.7 Allometric relationships- morphometric measurements
Studying bivalve growth and establishing allometric relationships are essential for
generating useful information for managing resources and understanding changing
environmental conditions (Palmer, 1990).
21
Different environmental parameters and physiological factors are found to influence
shell growth in bivalves
(Gasper et al., 2002).
Environmental factors latitude, depth,
shore level, tidal level, currents water, turbulence, wave exposure type of bottom and
sediment are known to influence shell morphology and relative proportions of many
bivalve species, ( Beukema and Meehan, 1985; Claxton et al. 1988; Franz, 1993; Dame,
1972 ; Fuiman et al., 1999; Hinch and Bailey, 1988; Akester and Martel, 2000; Newell
and Hidu, 1982).
In India, allometric relationships of several bivalves have been studied. The
important studies were those of Durve and Dharmaraja (1965, 1970), Alagarswami
(1966), Parulekar et al., (1973), Alagaraswami and Chellam (1977), Shafee (1978),
Ansari
et al., (1979),
Mohan (1980), Mohan and Damodaran (1981), Sreenivasan
(1983), Mohan et al. (1984) and Rao (1988). Studies on
Paphia malabarica
are
limited to the length- weight relationship by Rao (1988) from Mulki Estuary in
Dakishna Kannada, and dimensional variations by Appukuttan (1993) from Ashtamudi
Estuary in south Kerala.
2.8 Condition index-growth factor
Bivalves are known for delicacy at sea food market. Like any commercial food, it is
necessary that bivalves should possess good quality standards such as amount of meat and
appearance. In bivalves, condition index or fattening index or simply condition, is one of
the most satisfactory evaluation methods for estimating the amount of meat related to shell
cavity (Nishida, 2006). Variation in meat content is observed in most of the bivalves
depending on their physiological condition and also changes in the environmental
condition. Condition of bivalves is recognized as the degree of the fatness or the extent to
22
which the meat fills the shell cavity. The body size undergoes changes and such changes
are often associated with the breeding cycle. This is accomplished by the development of
an increase in size of the reproductive organs followed by a considerable reduction in size
after spawning. The meat weight and spawning activity of the bivalves are the two
important factors which should be taken into account in any judicious exploitation of the
resource. The knowledge of the changes in meat content of the bivalves is therefore
important for the culturist, as these greatly affect the meat yield and financial returns
(Rajapandian and Rajan, 1987).
Reports on the studies on the condition index of the bivalves are available from
abroad and India. Schumacker et al. (1996) studied the condition index of oysters to
monitor the aquatic environment of Willapa Bay in Washington and has observed that
gravimetric and volumetric methods produce linearly correlated indices when performed
on the same oysters and that less time consuming and more precise gravimetric method can
be used as an accurate gauge of oyster. Control of temperature over the condition index
was studied by Fischer et al. (1996), they have observed that temperature controls the
condition index but it has not known whether the changes resulted directly from the
temperature or from the temperature driven reproductive and metabolic cycles. Food
limited growth and condition index were studied by Rheault and Rice (1996) in eastern
oyster, Crassostrea virginica. Seasonal changes in condition index in scallop Pecten
maximas in relation to environmental condition was studied by Pazos et al. (1997).
Okumus and Stirling (1998) studied the meat weight, condition index and biochemical
composition of mussels (Mytilus edulis L.) in suspended cultures.
From Indian waters, seasonal changes in condition index of bivalves were studied
by Venketaraman and Chari (1951), Abraham (1953), Nayar (1955), Durve (1964, 1970),
23
Nagabhushanam and Talikhedkar (1977a, 1977b), Krishnakumari et al. (1977), Mane and
Nagabhushanam, (1979), Joseph (1979), Balasubramaniam et al., (1979), Narasimham
(1984, 1988), Rajapandian and Rajan (1987), Joseph and Madhystha (1987), Thippeswamy
and Joseph (1988) and Rao (1988). The important species include Perna viridis Linnaeus,
Villorita cyprinoides (Gray), Katelysia opima (Gmelin), Crassostrea madrasensis
(Preston), Meretrix meretrix (Linnaeus), M. casta Chemnitz, Donax fabs Gmelin, D.
incarnates Gmelin, D.cuneatus Linnaeus, Placenta placenta (Linnaeus) and Anadara
rhombea (Born). In all the studies the condition index was correlated with reproduction
and it was observed that high condition index was observed
just before the spawning
season. Sujitha Thomas (2004 ) indicated that high condition index P.malabarica in
Dharmadom estuary was observed in September i.e just before spawning.