Estuaries are known for high productivity sustained by high input of nutrients and freshwater inflow. Increased nutrient loading stimulates phytoplankton production, enriches supply of energy and organic carbon to open waters, and facilitates rapid development into zooplankton-rich ecosystem.
The resultant food web dynamics paves the way to increased fish yield. The abundance, distribution and diversity of finfish and shell fish are, therefore, significantly high raising the value of fisheries resources of estuaries. A number of environmental attributes contribute to the population response in the estuary.
The response mechanisms are strongly influenced by physical variability like changes in hydrodynamic environment (flow pattern, flooding of fringe areas, tide, salinity regime, flushing time, turbidity, nutrient level etc.) as well as the chemical and biological conditions of the estuary.
In addition to providing feeding ground and general living space to for many fish species, the estuaries play the most vital role in nursery function and recruitment and therefore considered as critical habitats for fish productivity. From the ecological perspective, abundant supply of forage fishes is a significant component of estuarine function correlated to important habitat variables favouring prey-predatory distributions in enhancing productivity through the food web.
The knowledge of juvenile habitats and the environmental requirements of different fishes at various life history stages met from a combination of estuarine habitats are essential to fisheries production management.
The fishes found in estuaries may comprise a range of species of commercial importance, while there are vastly large number of ecologically prominent species in terms of biomass and abundance.
The species specific habitat information is grossly inadequate for most of them.
Across the zoogeographic regions fishes found in estuaries belong to some characteristic families categorized as marine migrants or estuarine residents such as Sciaenidae, Ariidae, Carangidae, Centropomidae, Eleotridae Gobiidae Clupeidae, Elopidae, Pomadasydae, Gerreidae, Haemulidae, Mugilidae, Polynemidae, Scombridae and Trichiuridae. With the saline water penetrating a long distance landwards from sea face Estuarine conditions are increasingly prominent towards estuarine mouth.
Fish and pelagic and benthic invertebrates form distinct species assemblages along the longitudinal salinity gradients of estuaries reflecting different salinity tolerances. A pronounced zonation of species distribution observed with respect to salinity regime, although the zones move up and down longitudinal extent of the estuary as the interface between saline and freshwater varies with the state of the tide.
Classification of Fishes
In general the fishes may be broadly classified into three groups:
(i) Freshwater stenohaline species which enter the zone during the flood and retreat upstream at low water according to the penetration of the saline tongue.
(ii) Marine stenohaline species which follow the influx of marine waters into the river for feeding.
(iii) Euryhaline species which move little but which adapt to the changing salinities of the water. These may be of freshwater origin (e.g., members of Cichlidae, Cyprinodontidae and some Siluroidae) or of marine origin (e.g., members of Clupeidae, Atherinidae, Mugilidae, Lutjanidae.
The interactive or additive effects of primary production, nutrient cycling and food webs, nursery ground function, overall estuarine health, habitats and habitat availability become obvious on the distribution of estuarine fishes.
The quality of and connectivity between reproductive, trophic, and refuge habitats have a greater significance in this respect. Diverse assemblage of available habitats and seasonally abundant food resources are sometimes attributed to variety of movement patterns observed in fishes. Fishes tend to move in and out of the estuary or from one area to another within the estuary, generally driven by the need to migrate for reproduction and feeding.
Habitat and environment are intricately linked from the productivity perspective. They have an important functional value in providing a) physiologically essential physicochemical conditions b) sufficient living space c) abundant live food resources (plankton, benthos and prey items) for fish.
Aquatic habitat implies the environment of the community of organisms adapted to a defined or distinct portion of the aquascape, Environment means the total physical, chemical and biological surroundings of the organisms. The habitat structure denotes the arrangement of objects or features of the environment, where a synergy exists between the abiotic and biotic components. The habitat structure holds out the cues and attraction for assemblage of fish and nekton communities.
The relationship between fish stock and habitats are generally concluded from basic information like presence or absence of species, habitat related densities, habitat related reproduction, growth and survival and habitat related production. The essential fish habitats are those that are absolutely necessary or indispensable.
Critical fish habitats refer to habitat quality parameters which are important with regard to consequences. The ‘essential’ and ‘critical’ habitats represent a gradient in the consequence of habitat use. This gradient serves an important purpose as the fish require to migrate between and within habitats after the nursery phase to complete the cycle of recruitment to the fishery. Aquatic boundaries are soft & penetrable with respect to fish usage in which light, salinity, water current and food availability are the dominant factors.
A significant alteration, degradation or loss of habitats occur due to adverse hydrologic, climatological effects and human influences. This often leads to a number of habitats forming a gradient of quality between two extremes of critical habitats (optimal in all respects) and poor habitats (suboptimal conditions). An unfavourable shift in balance between essential and poor habitats would have detrimental effect on fish irrespective of species and life history stages.
Physical habitat classification in aquatic ecosystems are defined by depth, bottom substrate composition, vegetation cover, salinity trending, temperature regime, turbidity and dissolved oxygen. Ecological classification is based on grouping of estuarine fishes according to environmental tolerance.
The estuarine habitat structure comprises tributaries (freshwater streams and tidal brackishwater creeks), inlets, deep water, mid-depth water, subtidal shallows with or without vegetative cover (sea grass, salt marshes, mangrove), intertidal sandbars and mudflats, intertidal wetlands and estuary mouth. This complex set of habitats provide significant habitat value for many diadromous and estuarine fish species. By analyzing estuarine habitats the fish species assemblages can generally be identified. Use of structural habitats depends on the species and life history stage. A majority of commercially important estuarine fish species depend on tidal wetlands for a portion of their life cycle. Many pelagic nekton species comprising a significant biomass component of estuaries, however do not appear to require any specific habitat structure.
Effect of Habitat Parameters on Fish
Identification of apparent association between fish and habitat conditions is recognized as the first step towards analysis of fish abundance and distribution. The biological relationship to estuary fish production is basically traced to competition for food, space and sufficiency of stock for recruitment.
Freshwater flow has been shown to influence catch rate of several estuarine species. Abundance of marine species may be negatively affected by freshwater flow mainly as a distributional response, due to seaward displacement of their habitats.
The hydrodynamic and sediment transport regime associated with freshwater flow plays a characteristic role in distribution of sediment environments shaping them as most productive areas in parts of the estuary. An increasing abundance of benthic and demersal species are found in these areas with finer sediments.
Sediment deposition is associated with stream configuration, topography, flow dynamics, water depth and stream bank location. This characteristics seem to influence the population status of several fish groups like centropomids, gerreids and gobioids which prefer shallow marginal sediment deposits.
Estuarine seagrass habitats and hard bottom conditions show a positive relation to the marine mesolhaline/polyhaline fishes like lutjanids, serranids, gobiids, blenniids etc
Near the estuary mouth, majority of fish species (with the dominance of marine adventitious species thriving on a wide range of food items available) are found to be abundant at above average temperature, where salinity and depth are also high.
Salinity is the most important abiotic factor affecting fish biomass. The population of stenohaline fishes is most sensitive to salinity regime at all life stages (larvae, juveniles and adults) and for reproduction growth and survival. . Some of the typical resident species, however, are found throughout the estuary irrespective of salinity variations. Euryhaline fish population are associated with oligohaline habitats and thrive well with increased freshwater flow conditions. Seasonal migration of fishes in and out of the estuary for often obscures the relative influence of turbidity, depth, dissolved oxygen, pH as predictive habitat parameters.
Natural soft sediments and mud support healthy estuarine fish communities like gerreids, mugilids and engraulids which use benthic animal communities as food. Solft sediments provide optimal habitat conditions for Goboid fishes. Excessively fine sediment like muck are often anoxic and fail to support bottom biota and fish communities.
Estuarine habitats are very complex. The combined occurrence of a mosaic of habitat structures including subtidal and intertidal vegetation cover and optimal environmental conditions are real drivers of energy flow, trophic ecology and fish assemblage.
Habitat fragmentation and changes in the spatial arrangement of vital habitats along the estuarine axis for several reasons often causes a shift in the source/sink balance in habitat distribution and detrimental effect to estuarine fish production and sustenance. A greater clarity in understanding of fish species based habitat requirements, collective spatial characteristics of estuarine habitats such as size and structural connectivity etc. in estuary would come in hand for estuarine fisheries management.