Perspectives on the Biological Baseline of Fisheries and the Functional Role of Estuaries in India

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Introduction

The estuaries by all ecological standards qualify as renewable natural resource, since they lend themselves to be exploited within sustainable limits. They are the most productive ecosystem in terms of primary productivity, which is transferred to higher trophic levels and result in commensurately high fish production (Miller and Dunn 1980; Peterson, 2003).

The functioning of estuaries is fundamentally governed by the characteristic natural dynamics of riverine and marine influences (Ross, 2000). Most of these influences are defined by the complex interaction of biotic an abiotic factors which decisively act and shape up the pattern of distribution and abundance of fishes in the estuaries( Deegan 1993).

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The freshwater flow including riverine material inputs into the estuary is a major driving force that determine the estuarine hydrology, geomorphology, mouth status, tidal prism, flood pulses, habitat diversity and openness to adjacent ecosystems etc. on the one hand and the temperature, pH, turbidity, sediment load, dissolved oxygen, organic matter and nutrient concentrations and primary and secondary productivity on the other.

The synergetic influence of these multiple factors impinges on the status of estuarine fisheries which heavily relies on a conduciive environment for recruitment, cues for migration, food availability and competition on a continuous long term basis (Thiel et al. 1995; Ziliukas, 2003).

Understanding of the aquatic ecosystem and the fish population behaviour in an estuary therefore, requires critical interpretation and interlinking of watershed parameters, water quality variations and the food web mechanism in a wholistic manner.

The watershed data may define changes of nonpoint sources of nutrients and flows into the estuary which are easily incorporated as a checklist to compare with the water quality data for inferring on the impact of land use changes on quantitative aspects of nutrient loads, organic and inorganic sediment load, dissolved oxygen, salinity fluctuations and algal biomass etc. to serve as input for the estimating the food web structure and the ecosystem response to fish population variations.

Estuaries and the Changing Scenario

The importance of the estuary lies in free connection with open sea in which sea water is measurably diluted with freshwater from land drainage. The salinity consequently varies with position in the estuary. The mixing process is seasonally variable and also influenced by magnitude of discharge, chemical composition of inflow and intensity of biological activity.

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Highly seasonal and variable rainfall pattern often result in perturbations contributing a wide range of spatiotemporal variability in the physic-chemical status and the biotic processes in estuarine systems. Most of the notable cyclic events in an estuary are driven by seasons, tides, day and night cycle, storms, wind speed and flow velocity caused by tides.

The mixing of water, with complex biological, industrial, hydrochemical and sediment characteristics occurs in estuaries most rapidly. Thus each estuary assumes a distinctive character depending on the size, depth, bottom substrate, tidal range, degree of pollution, hydro-chemical mixing pattern and the biological activity.

The estuary is generally shaped by the volume of freshwater discharge and the quantity of sediment input from the river. Open channels are usually formed when the sediment yield is small and tidal energy is high. Low tidal wave, large sediment yield and declining sea level lead to delta formation.  The resident time of freshwater in an estuary is an important characteristic which is calculated as the ratio of rate of freshwater input to the estuarine volume. The water residence time has important significant to the prevailing phytoplankton diversity in estuaries. Longer resident time allows algal species to grow faster than they are flashed out and therefore helps in a higher species composition.

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Depending on the season and the position of salinity gradient in the estuary, nitrogen or phosphate may become limiting to primary production. Usually the freshwater end may encounter phosphate limitation while nitrogen may be limiting in salt water.

The mixing of salt water and freshwater may lead to different density gradients or stratification, according to the tidal force, areal size of the estuary and the water depth. Thus the estuaries may be partially or fully mixed or a salt wedge type.

Generally oligohaline conditions would prevail at the head of the estuary and euhaline conditions in the mouth region. During major floods, however, almost the estuary may turn freshwater dominated. The nature of mixing of dissolved or particulate matter is termed as conservative when their concentration changes proportionately with the salinity level.  But due to addition or removal of substances from the water phase governed by a number of estuarine biological and hydrochemical processes such as adsorption, desorption, precipitation, settling, transformations etc., the same substance may behave conservatively or non-conservatively in different estuaries.

In transitional ecosystems with relatively higher sediment surface to –to-water volume, the water quality is strongly influenced by the sediments and their resident benthic communities. Many solids are inherently thermo-chemically unstable, (eg. organic matter), some are unstable in sediment phase (eg, metal oxides, carbonate minerals), while new minerals could be formed in sediments (eg. pyrites).

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Thus the sediments apparently exercise important regulatory function for the whole ecosystem by their storage and renewal capacity of organic mater and nutrients.  The estuaries are temporally variable systems in many respects. But the chemical processes occurring in estuaries are same with a degree of difference in their intensity and effect. The lowering of flow velocity as the estuary widens toward the mouth cause settlement of sediment matter.

Flocculation of clay minerals leading to rise in turbidity is also common as the salinity increases.  The heavy metals like Cu, Cr, Hg and Pb, normally sourced from riverine input, pass the estuary bound to clay surfaces and transported to inshore waters.

The increasing urbanization and the consequent rising demand for abstraction of freshwater from the river catchments heavily interfere with environmental flow requirements for estuarine ecology and sustenance of productivity (Elliot and Quintino, 2007).  A large scale change in flow by river regulation diminishes estuarine habitat in quality and diversity resulting from shifts in ecosystem properties.

Regions where the freshwater pulses are severely restricted from entering the estuary, the natural sequence of estuarine biota, especially the planktonic succession, are adversely affected. The river flow directly impinges on both pelagic productivity and range and extent of the food chain in the estuary. The changes in habitat quality are most pronounced in shallow water areas and adversely affect the life history strategies of estuarine nekton.

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A major off-shoot of human activity like deforestation, advance in agriculture, construction of dams and barrages across rivers etc. is manifest in the critical variation in sediment load which has manifested in altering the natural state of the estuary, erosion, siltation of channels and growth of deltas and imbalance in the hydrochemistry.

Encroachments on the intertidal marshes for agriculture and other activities cause substantial loss of floodplains adjacent to rivers and estuaries as a result of which tidal ingress is intensified and pushed farther upstream.

Increased nutrient loads, especially a steep increase in phosphate and nitrate concentrations, into some estuaries as the potential cause of eutrophication is also a cause for worry.  Concentration of industries around any rivers also pollutes the estuary and the costal seas with heavy metals, chlorinated hydrocarbons, heat –rise.

Exposure to dissolved or sediment bound chemical toxicants badly affects overall productivity, biodiversity and future yield pattern of estuaries. Industrial, agricultural and municipal wastes input heavily add to the load of organic matter into the estuaries fuelling excessive respiration and alarming depression in dissolved oxygen concentration. This has forced many of the estuaries in recent times to shed natural successions and rolling into artificial cycles.  The flora and fauna including fish therefore tend to respond to these changes in a variety of ways.

Biological Basis of Fish Production

The wide faunal diversity seen in estuaries largely owes to high biological productivity in a highly supportive dual freshwater and marine environment. (Potter et al., 1986; Castel, 1993).  The ecological functions of the estuary provide for the specific needs in the life cycle of a variety of different fish species.

Notably the estuary contributes to recruitment success and serves as a corridor by amphihaline species, plays important role in trophic aspects of most seasonal and opportunistic migrants, acts as nursery ground and provides protection for the larval and juvenile stages against predation (Maes et al 1998; Elie et al. 1990).

The sustained high level of primary production in tropical estuaries, resulting from active material cycling and nutrient availability is largely transferred to higher trophic levels and top predators by food web interactions. The phytoplanktonic and benthic microalgae are the two key primary producer communities that stimulate secondary production and contribute as the major energy source to drive the estuarine food web (Woodwell et al., 1973; Nixon et al., 1986).

Increased phytoplankton productivity, depending on higher nutrient and organic matter availability through increased river flow, naturally triggers higher zooplankton biomass to support increased fish stock.  The consumers that serve as a major link between plankton and higher trophic levels include zooplankton, post larval fish and invertebrates, microbes, meiofauna and eventually the higher animals such as shrimps, crabs and fish. The large food resource of the estuarine ecosystem from high pelagic productivity fundamentally fuels a sizably high level of fish production (Blabber, 1997).

A number of studies have shown that salinity is a prime factor in fish abundance, including piscivores, zooplanktivores, zoobenthivores and detritivores in estuaries.  The salinity tolerance and preference are very specific in different species. It may also vary for a species at different life cycle stages.  While the early juveniles of many marine species are known to be attracted to low salinity, the bulk of older juveniles and adults of the same species are found in mesohaline or polyhaline zones of the estuary.

Majority of coastal marine fishes are strong osmoregulators and generally show tolerance to low salinity rather than extremely high salinity showing high adaptability to estuarine conditions. In general the estuarine spawners show a prolonged tolerance for oligohaline conditions or even freshwater.

Very drastic changes in salinity profile of the estuary may have disturbing effect on the distribution and abundance of fish and fish food resources. Whitfield (1980) concluded from a study that about 5 % of all the coastal marine fishes use estuaries to a large extent. High fish production from the estuaries associated with perennial rivers could possibly owe also to land based cues transmitted along with run off from the catchments to potential marine fish recruits (Whitfield et al., 1994).

The value of mangroves in terms of estuarine productivity and fish yield is also considerably high. It virtually caters to the species specific patterns of habitat use within the estuarine fish community (Allen et al. 1995). The mangrove ecosystem is dominated by lower trophic level species which serve as an important food source for several fish and prawn species.

They naturally combine a range of unique benefits to the estuary, such as foraging profitability, refuge from predations, spawning sites and nursery grounds for new recruits, juveniles and sub-adult fish population. The mangrove vegetation cover and the complex trophic dynamics hold the key to the fish yield in estuaries.

It is recognised that the estuarine fish production level is critical based on the complex trophic dynamics. The quantitative establishment of this relationship is, however, difficult to achieve until the role of many contributing factors, particularly the cost of detritus and forage fish production are mathematically modelled or close to being  measured (Peters and Schaaf, 1991). Iverson (1990) working on a predictive model has shown that total primary production is related to fish production in estuaries and other coastal ecosystems and proposed the following equation which stimulated many more studies.

Fish Production as g wet wt m-2 yr-1 = (0.083 Po – 3.08).(E2)n. c

(Where Po istotal primary production as g C m-2 yr-1, E2  is nitrogen transfer efficiency taken as 0.28, n is presumed mean trophic level of harvest taken as 2.5 and c is the factor to convert g C m-2 yr-1 to biomass as g wet  wt. m-2 yr-1 taken as 36)

Using the above equation on a data set of ten selected estuaries Houde and Rutherford (1993) demonstrated that the mean cost of producing one unit of fish carbon was 91.8 units of net primary production carbon.

Estuarine Fish Communities

The structure of estuarine fish communities are known to reflect the status of environmental quality. The amount of freshwater flow into the estuary plays a significant role in augmenting pelagic productivity and maintenance of a range of food chain within the estuary.

Elevated level of inflow and a balanced tidal regime virtually expands the size of the river-estuary interface zone to serve as the most productive area. Reduced freshwater supplies would trigger a rapid shift away from plankton dominated system, where phytoplankton will no longer be important to the fish assemblage in the estuary. A characteristic common too many of the estuarine fishes is the ability to adapt to both low and high salinity. The composition and abundance of fishes largely responds to temporal salinity changes.

The fish assemblages in estuaries are quite varied consisting of marine, migratory and estuarine species as they get to fulfil the basic requirements of some stages of their life cycle, viz., breeding, nursery and feeding habitats as well as the physiological preparations for migration.

In terms of the ecological guild the estuarine fishes may be categorised as a) truly estuarine resident b) marine adventitious visitors c) diadromous migrant d) marine seasonal migrant e) marine juvenile migrant and f) freshwater adventitious fish.  It is generally observed that the truly estuarine residents or the fishes completing the entire life cycle in estuaries are proportionately much less compared to migratory amphihaline fish.

In terms of vertical distribution, the pelagic and demersal species usually dominate the fishes that use the estuary. Carnivorous fish constitute a sizeable part of the fish assemblage as compared to zooplanktivorous, zoobenthivorous and detritivorous fish, in so far as the biomass is concerned. In a strict sense, however, the estuary may be considered as a temporary living environment with wide spatio-temporal variations in fish assemblage structure. A species may even change the guild during life stage or growth cycle in the fluctuating environment of an estuary. It is rather a fair estimation   to state that the estuary is a transitory key habitat or migratory route to many of the fishes.

The fish assemblage pattern is expectedly different in different estuaries and not easy to predict, because of difference in the ecological status and functions, depending on the influence of hydrology, salinity, tidal incursion, turbidity, estuarine area, morphology, mouth configuration, substrate quality, habitat diversity and geographical position etc., besides the extent of anthropogenic influence.

The grouping of estuaries in terms of fish species structure and diversity and yield is unlikely to be very precise. But very useful information could be collected on the changes in fish species occurrence and their abundance in an estuary over a specified time scale, owing to the combined effect of several of the above factors which act as critical criteria for fish to use the estuary. The data base on fish communities may be useful to assess and compare the quality or ecological changes in the estuarine environment in one or among many estuaries. This comparative quantification approach could serve for prioritisation of factors to be managed for ensuring sustainability of estuarine fish resource and conservation of its diversity.

It is often observed that the ecological conditions peculiar to estuarine environment extends several square kilometres into inshore coastal zone for considerable part of the year.

Heavy rainfall in coastal belt and intensity and frequency of flood events leading to high  discharge form rivers and streams through estuarine mouth apparently tend to push the turbid brackishwater zone seawards and help create the biological characteristics similar to estuary. This estuarine affinity in the coastal marine strip is known for considerable fisheries resource akin to the nearby estuaries. It underlines the quantitative and qualitative relationships between river flow and estuarine and inshore fish production. This phenomenon also signifies that the conventional estuary dimension has an ‘external’ arm so far as fisheries resources are concerned.

The river-estuary interface zone, with its high overlap of physic-chemical and prey availability gradient, is the most important nursery area for several marine fish species. This zone is regarded as a significant contributor to recruitment success and production. Flood pulses are also regarded as critically important to the abundance of estuarine spawners.

Very high floods however, may cause a temporary decline in both marine and estuarine fish stock, possibly because of physiological stress.   The size of the area depends on the extent of river flow and it plays a vital role for supporting a large number of juvenile fishes. There are evidence that the estuaries with a strong riverine inputs are also capable of attracting adult and sub-adult marine immigrants.

Basic Features and Performance of Some Estuaries

The coastal and estuarine aquatic resources in India are vast and varied.  The country has a coastline of 8118 km with an exclusive economic zone (EEZ) of 2.015 × 106 km2 comprising over 60% of geographical land area. There are 15 major rivers with a catchment size of more than 2000 km2, 45 medium rivers with catchment are between 2000 and 20000 km2 and 120 small rivers whose catchment measures less than 2000 km2 ., besides many ephemeral streams of lesser significance.

For large-scale analyses of water resources, the country is often separated into some 19 major river basins/drainage regions and the cumulative annual runoff of all these rivers is 1645 km3.  The east flowing rivers have a total drainage are of 172000 km2, while the west flowing rivers drain 63500 km2  catchment.

Many of these rivers form estuaries on the east or west coast of India spread across 10 states as they meet the sea.  The estuarine ecosystems are extremely diverse attributed to geomorphologic and climatic variations along the coast.  The estuarine habitats include creeks, tidal flats, mudflats, mangroves, marshes, seagras beds, deltaic plains and wetlands.

The mangrove ecosystems occur on both east and west costs spread over 4120 km2.  The important mangroves are Sundarbans, Bhitarkanika, Krishna-Godavari delta, Gulf of Kutchh, Pichavarma a Vedaranyan.  In contrast to the large number of estuaries and coastal ecosystems existing in the country, the detailed studies and investigations on ecology, fisheries and production functions are limited to only a few specific estuaries, Very little or patchy information is available for many of the estuaries. The east-coast estuaries in general are characterised by input of heavy sediment load trapped in the system due to slow and gradual flow of water and consequently lead to extensive sandbars, islands and deltas appearing towards the mouth.

But the west cost estuaries are in sharp contrast for their fast flowing rivers meandering mountainous terrains resulting in almost entire load of sediments flowing into the sea.

Rivers Forming Important Estuaries

River Direction Important Rivers

East flowing rivers (major) – Ganga (Hooghly), Subarnarekha, Brahmani, Mahanadi, Godavari, Krishna, Cauveri, Pennar,

East flowing rivers (medium) – 24 nos with total drainage area of 172000 km. Bairatarani, Rushikulya, Matai, Kortlaiyar, Palar, Ponnaniyar, Vellar, Vaigai, Tambrapani

West flowing rivers (major) –  Sabarmati, Mahi, Narmada, Tapti

West flowing rivers (medium) – 17nos.  Total drainage are of 63500 km. Kalindi, Bedti, Sharavato, Bharathapizha, Periyar, Pamba

Hooghly Estuary

The funnel shaped Hooghly estuarine system of the Gangetic detla is the largest in India and is classified as a positive mixohaline estuary.  The maximum and minimum semi-diurnal tidal range of 5.5 to 1.8 m is recorded at the lower estuary with two high and two low tides in a day.

The salinity gradient is highly variable ranging from 3.6 – 32.77 gl-1 depending on the place and season. No well defined saline water is observed at the bottom.  The tidal influence is felt up to a distance of 285 km from sea mouth. The salt front measuring ~0.5 gl-1 is observed only up to Diamond Harbour. The estuary is ~4.5 km wide with a maximum depth of 37 m a high tide.  The suspended matter load of 1-3 gl-1 is recorded in the estuary accounting for ~11 × 106 kg sediment annually flowing through the estuary.

Based on salinity characteristics and tidal influence the estuary is divided into 3 zone – (i) freshwater tidal zone from Nabdwip to Nabadganj (ii) Brackishwater Transitional Zone from Nababganj to Diamond Harbour and (iii)  Saline zone from Diamond Harbour to sea mouth.

The physicochemical characteristics and the primary production in the estuary are highly variable.  The transparency is generally low ( 18.4 – 31.4 cm) with the lowest value in the gradient zone.  The dissolved oxygen (av. 6.8  mgl-1)and pH ( av 7.9 -8.3)  are generally maintained at a conducive level for estuarine biota  under influence of adequate freshwater discharge. The nitrate  (0.15 – 0.29 mgl-1) and phosphate (0.05 – 0.11 mgl-1)  concentrations are maximum at the freshwater zone.

During pre-monsoon nitrate, phosphate and silicate are apparently consumed at a faster rate by higher biological activity.  During monsoon the system is recharged by nutrients to a significant extent.  The flux of nutrients from Hooghly into inshore is estimated as 366 t nitrate, 67.2 t phosphate and 3690 t silicate annually.  The oscillatory tidal movement may not allow the nutrient to flash out fully and at least 4 -9 % of is retained in the system.

The estuarine region served an important sink for several elements.  Most of the riverine dissolved and particulate matter could be expected to settle to the bottom at the confluence of river and sea.  The heavy metal in Hooghly estuary were mostly in insoluble bound state and very low quantities apparently are liberated into water phase at the soil-water interface.

The primary productivity varied significantly depending on climatic factors, water velocity, turbulence and turbidity level.  Maximum primary productivity was recorded in post-monsoon and summer due to relatively stable conditions in the estuary.

The primary production at the saline zone varied from 15.6 -41.7 mg C m-3 h-1, while the most productive freshwater zone had a productivity level of ~63 mg C m-3 h-1.  The photic depth is considerably low in the transition and saline zones due to heavy suspended matter and turbulence.  The phytoplankton composition is primarily influenced by variability in salinity, transparency and nutrient concentration and had an abundance of of diatoms, followed by green algae, bluegreen algae and flagellates.  Copepods were the major constituent of zooplankton flowed by Cyclops.

Fifty three species of zooplankton were recorded in the estuary with highest abundance in pre-monsoon.  The average density of zooplankton in the estuary is estimated as 1200 – 9550 no /m-2… The macrozoobenthic fauna of Hooghly varied from 25 =125 nos m-2 and the gastropods and polychaetes contributed morethan 55 %  and 14 % respectively.

Biodiversity of the Hooghly consisted of terrestrial, freshwater and marine communities.  A total of 1498 species of all living organism have been reported from this estuary.  The aquatic fauna of Hooghly estuary comprises ~ 76 % of the total faunal occurrence.

The estimated total annual fish catch in Hooghly estuarine system fluctuated from 62165 – 72098 t during 1999-2003. The average catch figure showed a sharp increase by 36.1 % compared to the previous 5-year period. The lower estuarine zone including the entire Sundarbans accounted for highest production of fish contributing ~ 97% of the total catch, as compared to the gradient zone and the freshwater zone. The maximum average catch of 59.2% occurred in winter months of November, December and January, while the minimum catch of 4.5% was recorded in summer months.

The dominant fish species in order of abundance in Hooghly comparised Harpondon nehereus (16.4%), Tenualosailisha (15.7%),  Pamapama (11.2%), Setipinna spp. (8.3%), Trichurus spp. (7.5%), Prawns (6.0%), Arius jella (5.4%), Sciana biauritus (3.1 %), Coillia spp (2.7%), Papmus argenteus (2.6%), Ilisha megaloptera (1.9%) and Mackere (1.2%).  These species together accounted for 82.0% of the total catch.

Godavari Estuary

The 1300 km long Godavari has a 50 km long  (or 3.76 % of the total length) estuarine part divided into Goutami arm and Vashit arm in Andhra Pradesh. The brackishwater tidal zone stretches to 40 km from mouth. The monsoon turns the estuary riverine dominated marked by recurrent floods and huge influx of organic sediments and nutrients. Post monsoon through summer largely dried up conditions prevail. The freshwater discharge through the Goutami arm of the estuary is comparatively higher.

The estuarine width and water depth had no marked change throughout the year.  The lower estuary of Goutami ( ~2.5 km wide, ~3.5 m deep at Yanam) extends up to 20 km, while it is up to 10 km on the Vashist arm. The monsoon months (July-Sept) accounted for about 95% of mean annual discharge of water estimated at ~ 90 × 109 m3 , which is cited as the cause of extensive bottom erosion and formation of sandbars. The estuarine stretch is dotted with alternation of shallow and deeper areas. Fluvial effects evidently produce large nutrient concentration in the wet season. During Sept. the estuary is well stratified as a result of maximal runoff.  Beyond monsoon, the estuary is deprived of natural flow, regulated by Dowleswaram barrage and gradually turns well mixed with tidal effect dominating the lean season.  The tides are semidiurnal with an amplitude range of 0.6 -2.0 m. The estuarine flushing time is inversely proportional to freshwater discharge.

Temperature, salinity, dissolved oxygen and nutrient concentrations primarily influenced aquatic productivity and faunal distribution including fish in Godavari.  Freshwater conditions (<0.5 ppt) prevailed from Dowleswaram to estuary mouth from July – September. The river discharge controls the salinity pattern.  Salinity level showed increasing trend from October through the summer.

Salinity increased from 0.54 – 24.4 ppt at Antaravedi and from 0.39 – 19.16 ppt at Guthunadeevi from September to December, while salinity values peaked to 33.47 ppt at the mouth in May.  pH values ranged from 7.4 – 8.4 from September to January with small changes till summer.  The temperature varied by an average of 7oC from pos-monsoon to summer (26 – 33oC).  Dissolved oxygen maintained at or above saturation level, with marginal variations (6 – 8 mg l-1) with season and distance from the mouth.

The concentration of dissolved oxygen increased during ebb tide. The total suspended solids dropped drastically from the monsoonal level (av. 768 mg l-1) to post monsoon level (av. 29 mg l-1). In monsoon period 80 -90% of total nitrogen pool was contributed by nitrate-N. Commensurate with decreased inflow, the concentration of total inorganic nitrogen in estuarine waters tended to decline and the major form of nitrogen was organic Nitrate –N concentrations peaked to 0.396 mg l-1 in January from the lowest value of 0.034 mg l-1 in September, and reached a moderate level (0.14 mg l-1) in May.  The mid estuary generally showed higher nitrate – N concentration compared to the freshwater zone or the saline zone.

The Phosphate –P peaked (0.093 – 0.12 mg l-1) in September with little change with the axial distance from the mouth. The concentration gradually decreases through post-monsoon (0.016 – 0.046 mg l-1) in May.

The Gross primary productivity reflected the seasonal variation of nutrient concentration.  The GPP was the lowest in September (av. 73.59 mg C m-3 h-1) ; moderately higher (12.35 mg C n-3h-1) in December and highest (138.39 mg C m-3h-1) in May.

A total of 110 species of fish belonging to 52 families and 15 orders were collected and identified in Godavari estuary apart from 11 prawn and 6 crab species. Bregmaceros mcclellandi, the spotted codlet considered to be rare in the east-coast of India was collected at the estuary mouth at Antaravedi.  Tenualosa ilisha  is the mot dominant fishery in Sept and Nov. with maximum landing at Babbarlanka and Antaravedi.

Hilsa migrated to Dowleswaram in monsoon but during post-monsoon and winter hilsa fisheries was restricted to Narsapuram (Vashit aram) and Kapileswarpuram (Goutami arm).The contribution in catch by weight from hilsa was ~ 20%, from  E.vacha and Pangsius pangasiu ~18%, while large species of prawns also contributed substantially (~12%).

Trend of Fisheries in Godavari Estuary

Fish Catch (t)

1980s

2006-07

2007-08

Gautami Branch

Shark & Skates

218.00

30.00

31.00

Hilsa

231.00

7.00

8.00

Other Clupieds

210.00

51.00

53.00

Catfish

260.00

3.00

4.00

Mullets

223.00

2.50

3.00

Polynemids (Perches)

13.00

114.00

111.00

Caranx & other Carangids

130.00

11.00

8.00

Ribbon fish

131.00

11.00

39.00

Mackerals & other Scombridge

103.00

39.00

22.00

Pomfrets

103.00

29.00

5.00

Prawns

311.00

133.00

135.00

Crabs

110.00

22.00

25.00

Others

640.00

484.50

462.00

Sub-Total

2795.00

910.00

907.00

Vashistha Branch

Mostly perches and prawns & crabs

1205.00

182.00

193.00

GRAND TOTAL

4000.00

1092.00

1100.00

The Shark & Skates declined in the estuary to the tune of >85% in Godavari estuary as compared to 1980s. Alarming decline in mullet fishery to the tune of >98% has also been estimated compared 1980s level. The perch fishery, especially Lobotes surinamensis, has increased nearly nine fold from 13 t in 1980s to >111 t, currently. The dominant Labeo fimbriatus , which was contributing to the tune of > 17% to the fishery of freshwater stretch in 80s has nearly collapsed (Only two specimens could be collected during the last two years) .There is also a sharp decline in Hilsa fishery, from 231 t (1980s) to merely 07 t (2008).

Mahanadi Estuary

The Mahanadi estuarine system, comprising two important estuarine arms was investigated during the year with additional focus on the Devi estuary for a complete study.  The Mahanadi and Devi divided at Cuttack are subject to major hydrological changes due to barrages and irrigation canal networks  holding up and withdrawing water.

The excessive control and water withdrawal-induced low flow become glaringly critical to the estuarine functions and ecological processes, mostly dependent on effective and functional salinity gradient, nutrient delivery from upstream river, suspended sediments, diversity of physical habitat conditions, tidal events etc.  This is reflected in the estuarine productivity variations and the size and quality of biotic components, especially in the assessment of the distribution of adult and juveniles of migrant and resident fishes including their recruitment.

The observations revealed that hydrodynamic processes associated with high flow and number of flood pulses in the wet season from June to October (Monsoon), with features specific to each year was most crucial to revitalizing the estuarine ecosystem for the rest of the year. Delivery of organic and inorganic material from river and coastal waters into the estuary are fundamental and derived from land runoff and tidal regimen, directly impacted by the rainfall pattern.  In terms of size and depth and the length of the mixing zone, Devi estuary was more prominent and extensive, compared to the Mahanadi main channel estuary even during dry seasons.

The estuarine region experienced near normal rainfall (1450 mm) in 2006 with one high flood and several low flood pulses extending up to the end of October.  The estimated annual discharge in Devi was 65.66 ×109 m3, more than 90 % of which was accounted for by the wet season discharge and it was clearly an outcome of the rainfall received.

The monthly average discharge figures during post-monsoon and pre-monsoon were 0.3965 ×109 m3 and 0.0873 ×109 m3 respectively.  In comparison, the Mahanadi main channel estuary had 44.803 ×109 m3 annual discharge and drastically lower freshwater flow in summer (monthly av. discharge, 0.1884 ×109 m3).  The vast differences in discharge from wet to dry seasons had strongly affected the water exchange in the estuary.  Devi had a large cross section area at the mouth and the mean tidal amplitude was 1.75 m. while it was 0.8 – 2.0 m for Mahanadi main channel.

The combined effect of tidal activity, freshwater flow and turbulence determined the vertical salinity gradient.  The estuary is partially mixed for most of the wet season and winter, while well mixed conditions prevailed for a brief period of low flow in summer.

The marine zone in Devi extended to 7 km from mouth while the salt front indicating the extent of brackishwater tidal zone extended up to 40 km.  The tidal effect (freshwater tidal zone) however, was felt up to 60 km from sea. The mid-estuary was 3.5 km wide and 10- 12 m in depth, which almost doubled at the tip of the lower estuary.

Contrastingly, the Mahanadi main channel was shallow and narrow.  The extended transition zone of Devi is integrated with important physical habitats like mudflats, intertidal marshes that are biologically active beyond monsoon season.  This zone served as the deposit zone for large influx of suspended matter during high monsoon flow, carrying highest total suspended matter (624 mg l-1).  The flood pulses tend to increase the nutrient transport along with the sediment, leading to a variety of biological and biochemical responses in other seasons as well and the process is effectively helped by variability in water residence time.

The physical and chemical parameters indicated strong spatio-temporal variability.  The fresh-saline water transition zone was marked for dynamic changes and processing of dissolved and suspended material entering the estuary.  The freshwater tidal zone was relatively steady.

Low freshwater discharge in premonsoon established a clear salinity gradient of 0.6-18.0 ppt over 65 km stretch from the mouth.  Monsoon turned the estuary into oligohaline (0.3 -5 ppt) but the system switched to mesohaline conditions (0.7 – 10.8 ppt) in winter.  Difference in salinity in surface and bottom water was marked in monsoon and post-monsoon at the mid-estuary indicating vertically mixing was partial.  The position of salt front varied with season; determined by freshwater flow and tidal amplitude.  While temperature reached a maximum of 32.5oC in June and the minimum of 22.0oC occurred in Jan.  The marine zone was relatively cooler compared to outer estuarine sites in all seasons.

The pH varied widely with season and also longitudinally along the estuary. Monsoon was marked by almost a steady pH (7.5-7.6), while postmonsoon pH was generally alkaline (8.0 -8.2) coinciding with biological activity.  A near neutral pH (7.4) at the riverine end, alkaline pH at inner estuary (8.4) and acidic pH (6.2-6.6) at mid-estuary were other hallmarks of pre-monsoon conditions, indicative of decomposition of organic matter favoured by high temperature.

The DO was higher (8.2 – 8.4 mg l-1) throughout the estuary in post-monsoon compared to values ranging 4.8 – 6.4 mg l-1 in premonsoon and 5.6 – 6.8 mg l-1 in monsoon.  A shift towards high DO and alkaline pH in the mid estuary region after monsoon floods showed active nutrient regeneration as well as nutrient uptake by phytoplanktonic organisms.  The organic load during monsoon high flow may be reason for decreased DO concentration.

The external inputs due to land runoff tended to elevate NO3-N concentrations (0.07 – 0.104 mg l-1) at the riverine end in monsoon but the dilution effect caused a decrease (0.032 – 0.068  mg l-1) towards the mouth region.  High values of NO3-N (0.14 – 0.28 mg l-1) in post monsoon and (0.064 – 0.10 mg l-1) in premonsoon at the mid-estuary supported high productivity and also indicated a source for NO3-N through mineralization of organic matter.

This is further supported from consistently higher values of NH4-N (0.04 – 0.1 mg l-1) in post monsoon and (0.042 – 0.17 mg l-1) in pre-monsoon, up from an average level of 0.02-0.04 mg l-1 in monsoon.  PO4-P concentrations marked high values in monsoon (0.078 – 0.122 mg l-1) in the entire estuary and then leading to a gradual decrease (0.016 – 0.076 mg l-1) in post monsoon.  A rise in the PO4-P level located in the middle and upper estuary (0.054- 0.096 mg l-1) could possibly arise from P regeneration from sediments favoured by slow flushing rate and shallow water level and low temperature.

Silicate maintained high concentrations (11.0 – 15.4 mg l-1) in monsoon as expected with high riverine discharge.  As biological productivity shot up with the onset of postmonsoon the distribution of Si concentration decreased in the lower estuarine region (6.60 – 7.60 mg l-1 in winter and 5.2 – 9.5 mg l-1¬ in summer).

The soil pH was generally alkaline during monsoon (7.02 – 7.38) and post monsoon (7.25 – 8.18) but tended towards slightly acidic reaction (6.62 – 6.9) during summer in the marine zone and brackishwater tidal zone.  The available – N showed comparatively high values in monsoon (10.64 – 14.54 mg/100g) and postmonsoon (11.2 – 17.36 mg /100g) , but dropped down to 2.80 -5.6 mg/100g in summer.

Available-P was higher in post monsoon (6.40 – 11.2 mg/100g), but the monsoon values were the lowest (0.463 – 0.716 mg/100g) with a marginal improvement in summer.  The organic C % was remarkably low in the soil all through the year ranging between 0.12  and 0.6 (monsoon), 0.03  and 0.06 (postmonsoon and 0.06 – 0.18 (premonsoon) indicative of better utilization of organic matter in nutrient regeneration.

Subarnarekha Estuary

The Subarnarekha estuary is the least studied, small rainfed river-estuary on the east cost of India connecting Bay of Bengal at Kirtanya (Orissa) and extends 43 km from sea face to the point of feeble tidal impact at Jaleswar.  The estuary is characterized by shallower (0.6 – 1.5 m) water channel in freshwater tidal zone (15 km),  leading to a 20 km – long, wider and deeper trough as the transitional zone  ( 3 -8 m  water depth)  at Dahamunda and again turning shallow ranging 1-3 m water depth towards sea mouth (8 km long saline zone) at Kirtanya.  From post monsoon to summer an extremely lean flow feeds the estuary.

As the estuary volume is less, its hydrological conditions varied with season as a function of change in freshwater input and tidal functions. Subarnarekha has the mean annual runoff (MAR) of 12.4 × 109  m3. The estuary witnessed large scale changes during south-west monsoon.  With reversal of salinity due to high discharge/ floods, the estuary turned freshwater dominated by a unidirectional flow towards sea. Frequent high floods at close intervals during July-Sept resulted in  wide spread inundation, while the dry spell from Jan – Jun led to development of  dried up conditions even at the estuary mouth.

The sufficiently long and wide mid-estuary, however, retains water all through the year. During prolonged flood events, the estuarine processes are apparently bypassed as high flow is rapidly directed seawards. The monsoonal load of nutrients rich sediment and organic matter is considerably trapped in the central deeper part of the estuary and the recovery of chemical and biological processes are apparently triggered primary productivity  in post-monsoon through summer.

The tidal water traversed about 10 km before opening into a wider and deeper transitional zone resulting in dropping of the tidal height and water current observed at the mouth.  Because of weak current and insufficient force the suspended matter in water tends to be deposited in the mid-estuary as it is not completely driven out of the estuary.
The sea water exchange is predominantly by tide which is semi-diurnal in nature. The mean tidal range is about 1.2 m during at sping tide and 0.7 m at neap tide at Rasulpur-Ramnagar (mid estuary).  Partially mixed conditions prevailed in the estuary except in the monsoon season.

The water quality is apparently structured by the natural variations monsoonal cycle and hydrodynamic characteristics of the estuary in the in terms of catchment runoff volume, system area salinity gradient and nutrient distribution.

Abundant rain and floods turned the estuary riverine dominated upstream to downstream during monsoon (salinity <0.5 ppt).  The total suspended matter fell from a high of 677 mg l-1 to a low of 11.00 – 23.00 mg l-1 in the post monsoon. A weak salinity gradient (5.35 – 0.05 ppt in Nov. and 7.0 – 0.39 ppt in Jan) appeared only in post-monsoon over a smaller stretch, which improved considerably (21 – 0.5 ppt ) in summer over whole of the estuary. The average salinity level was recorded as 1.33 ppt in the mid estuary region and 0.10 ppt in the upper estuary in April.

Non-conservative behavior of nutrients due to physical and biogeochemical processes was evident from the non-linear distribution against salinity. The total alkalinity ranged from 90 – 116.0 mg l-1 indicative of good buffering capacity of water. The flood water showed lowest pH (av.6.8) and dissolved oxygen concentration (av. 5.8 mg l-1). Higher and almost stable pH (8.0 – 8.4) and dissolved oxygen values (7.4 – 8.8 mg l-1) were recorded in the mid-estuary in drier seasons.

Generally pH increased and D.O decreased with increasing salinity The DO, pH , turbidity,  salinity  and water flow levels were conducive to good nitrification in the transitional zone. Major contribution of N (82% of the Total N) was through river discharge with only a small fraction (22%) as organic N. The net influx of nutrients increased as the riverine influence increased. During wet season the nutrient input is unassimilated due to short residence time and slow uptake by phytoplankton.

A substantial quantity of sediment bound nutrients is retained in the mid-estuary and released by mineralization in the post monsoon recovery phase.  N03-N values were higher (av. 0.186 mg l-1) in monsoon but gradually declined (0.036- 0.052 mg l-1) during dry season.  Higher values (0.072 -0.078 mg l-1) were again recorded with the progress of dry season.

A closely similar trend was noticed for PO4-P with values at 0.108 -0.84 mg l-1 during August, 0.024 -0.052 mg l-1 in November and 0.05 -0.08 mg l-1 in April. This average concentrations of dissolved inorganic N and P levels indicated good productivity potential. The silicate concentration ranged between 16.6 – 20.0   mgl-1).
The Gross primary productivity was higher (187.5 mg C m-3h-1 ) in the central part of the estuary as compared to the other zones during post monsoon and summer.

The mid estuary and the estuary mouth served as active fishing grounds. Fishing activity was minimal during high flow period and resumed only after floods receded.  The fishing gear in use includes set-barrier, bag net, drift gillnet, cast net and hook & lines.

An effective indigenous fishing method known as Khadan fishing is a common sight in the upper and middle estuary. The dominance of carp fishery in the freshwater zone, and mullets in the transitional zone was observed during the study.  The saline zone however, supported a variety of fishes and decapods crustaceans.

Freshwater prawn M. rosenbergii occurs sizeably in the middle and upper estuary during dry season. Hilsa is the most dominant species at the estuary mouth constituting ~ 25% to the total catch .The estimated fish production in Subarnarekha is 205t yr-1 of which the saline zone (including part of on shore catch) alone contributes  ~195 t.  The av. production in   the mid estuary is ~ 4 –  8 kg  km-1 d-1 and in the freshwater tidal zone is 2 -4 kg km-1 d-1.

Concluding Remarks

The estuarine ecosystems are very fragile and are greatly disturbed and even degraded to a large extent affecting fisheries and biodiversity.  This unique environment and high productivity could be maintained and conserved only by a comprehensive strategic plan of action with due consideration of every important aspect liked with estuarine ecosystem functioning. A beginning however, must be made by addressing the most critical factor of ensuring environmental flow to all estuaries.

References

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Whitfield. A. K.., Paterson, A. W., Bok, A. H. and Kok, H. M., 1994.  A comparison of the ichthyofauna in two permanently open eastern Cape estuaries.  S. Afr. J. Zool. 29 : 175-185.

By

Dr. Bana Behari Satpathy

Email: bbsatpathy-at-yahoo.co.in

 

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