Mangrove Ecosystem And Its Importance In Fisheries

by N. Rajendra Naik¹, V. Kamalendra¹ and Jaya Naik²

¹ Postgraduate Students, ²Research Scholar(UGC)

Karnataka Veterinary, Animal and Fisheries Sciences University

College of Fisheries, Mangalore, Karnataka.

India has approximately 700,000 ha of area covered by mangroves along the estuaries and major deltas. Mangroves are salt-tolerant plants of tropical and subtropical inter-tidal regions of the world. Besides ecological importance in trapping and accreting sediment material to reduce the erosion, mangrove ecosystem plays a significant role in Aquaculture. The fishery potential of these areas is tremendous and provide livelihood to the coastal population as well.

What is a mangrove?

Mangrove is a specialized marine ecosystem consisting of a group of plants growing in muddy, loose and wet soils in tropical and sub-tropical areas, comprising of shallow, coastal waters, deltas and estuaries or lagoons. The specific regions where these plants occur are termed as 'mangrove ecosystem'. These are highly productive but extremely sensitive and fragile.

Which mangrove species to grow?

Out of 45 mangrove species occurring in India, some are true mangrove while others are considered as 'associated' flora. The most dominant mangrove species found along the east and west coast of India are; Rhizophora mucronata, R. apiculata, Bruguiera gymnorrhiza, B. parviflora, Sonneratia alba,    S. caseolaris, Cariops tagal, Heretiera littoralis, Xylocarpus granatum , X. molluscensis, Excoecaria agallocha, Lumnitzera racemosa,  Avicennia officinalis, A. marina.

Common mangrove species in the Florida Keys:

Black mangroves (Avicenia germinans) occurs shoreward to the red mangrove and is characterized by the presence of small pencil-like vertical root shots call pneumatophores. These root shots stand in dense arrays near the high-tide line, enabling the mangrove to obtain oxygen directly from the air.

White mangroves (Laguncularia racemosa) grow on elevated grounds above the high-tide mark and behind the red and black mangroves. The leaves are thick and succulent, rounded at both ends, and the same color on both sides. The root system resembles that of most terrestrial trees and seldom show breathing roots.

Fishes, shellfishes and crustaceans:

Major constituents of this group in the mangrove environment of India are 105 species of fishes, 20 species of shellfishes and more than 225 species of crustaceans. Among these, commercially important are Meretrixsp., Crassostrea sp., Penaeus sp., Scylla serrata and Mugil cephalis. Scylla serrata,the large edible swimming crab, inhabits the muddy bottom of mangrove estuaries, as well as coastal brackish water. Thalassina anomala, the mud lobster is also found along estuaries and tidal rivers. Mud skippers are one of the fish which live on the mud flats associated with mangroves shores.

Birds are a prominent part of most mangrove forests and they are often present in large numbers. The Royal Bengal Tiger is one of the unique resident species of mangroves of the Sunderbans. Reptiles are also common in mangroves, which include snakes, turtles, crocodiles and alligators. The saltwater crocodile, monitor lizard (Varanus sp.), estuarine crocodile, various species of monkeys, otters, deer, fishing cats and wild pigs are some of the most common species of mangrove forests of India.

Flora and Vegetation:

The Indian mangroves comprise approximately 59 species in 41 genera and 29 families. Of these, 34 species belonging to 25 genera and 21 families are present along west coast. There are about 25 mangrove species which have restricted distribution along the east coast and are not found on the west coast. Similarly, there are eight species of mangroves like Sonneratia caseolaris, Suaeda fruticosa, Urochondra setulosa etc. which have been reported only from the west coast. There are approximately 16 mangrove species reported from the Gujarat coast, while Maharashtra has about 20 species, Goa 14 species and Karnataka 10 species. There are hardly three to four species of mangrove which are rarely found along the Kerala coast. The associated mangrove flora is quite common to both the coasts, with minor variations in distribution.

Fauna

There are different types of faunal communities in mangrove waters which are dependent on the water component in one way or the other. The planktonic and benthic animal communities also play a very important role in the mangrove ecosystem just like the terrestrial animals. There are different species of crustaceans like Penaeus indicus, P. merguiensis and P. monodon, while the crabs are represented by Uca sp., Scylla serrata, Thalassina, etc. The fishes are represented by several species like the mud skippers, carangids, clupeids, serranids, mullets, hilsa, seabass, milkfish etc.

Pelagic community:

The mangrove water, usually rich in detritus are highly suitable for fishing. The major fishery resources found in these waters are detritivorous species of fishes, crabs, crustaceans and molluscs. Roughly about 60% of India's coastal marine fish species is dependent on the mangrove estuarine complex. Some of the most common fishes in Indian mangrove waters are Liza, Mugil, Lates, Polynemus, Sciaena, Setipinna, Pangasium, Hilsa, Ilisha and Etroplus. Prawns are represented by the species of Penaeus and Metapenaeus while the crabs are represented mainly by Scylla serrata. The molluscans of mangrove waters are mainly represented by Crassostrea spp., Mytilus and clams. In the upstream regions, giant prawns like Macrobrachium rosenbergii are also found in large quantities.

Commercial exploration:

In India, mangrove trees are used for house building, furniture and certain household items. Mangrove trees have been the source of firewood in India since ancient time. Tannin is extracted from the bark of some mangrove species like Rhizophora mucronata, Bruguiera gymnorrhiza and Ceriops tagal. Indian mangrove trees have 35% tannin in their bark which is higher compared to other countries. Extracts from mangrove bark are used by Indian fishermen to dye their fishing net and enhance its durability. Honey collection from the mangrove forest is a promising business in India. It has been estimated that Sundarbans mangrove alone produce 111 tons of honey annually. Bark and roots of Aegicera corniculatum and Derris heterophylla are used as mild fish poison. Avicennia spp; Phoenix paludosa and Sonneratia caseolaris are used for human consumption and as cattle feed. Nypa fruticans is tapped for an alcoholic drink.

Importance:

Mangroves shed and drop about seven and a half tons of leaf litter per acre per year. The constantly-shed leaves are quickly broken down by bacteria and fungi and released into the water, providing food for sea-life. Mangroves are the nesting grounds for mammals, amphibians, reptiles, countless unique plants, juvenile fish and invertebrates, sponges, barnacles, oysters, mussels, crabs, shrimps, oysters and many water birds such as the great white heron, reddish egrets, roseate spoonbills, etc.. Mangroves are also recharge underground water supplies by collecting rainwater and slowly releasing it.

The fishes lay their eggs in tangled roots of mangrove trees and later hatch and grow with needed nutrients available. Thus mangroves act as natural nursery grounds. Mangroves offer shelter to the juveniles of a wide variety of marine organisms, notable among them being certain species of penaeid shrimps. A linear relationship exists between shrimp production and the size of the mangrove forest area. Mangroves give recreation to hunters, fishermen, bird-watchers, photographers and others who treasure natural areas.

Mangrove swamps and other low-lying areas along the estuaries are generally preferred for brackishwater fish farming. The species cultivated are Liza parsia, L. tade, Mugil cephalus, Chanos chanos, Penaeus monodon and Fenneropenaeus indicus. Mangroves trap debris and silt, stabilizing the near shore environment and clarifying adjacent open water, which facilitates photosynthesis in marine plants. The fringing network on mangrove buffers natural forces such as hurricanes, wave action, tidal change and run-off, preventing soil loss with its firm, flexible barrier. Beyond serving as a refuge for juvenile marine organisms, mangroves filter sediment and buffer coastlines against erosion and storm surge. The major ecological role of mangroves is the stabilization of the shoreline and prevention of shore erosion. The dense network of prop roots, pneumatophores and stilt roots not only give mechanical support to the plant, but also trap the sediments. The important ecological role of the mangroves is the detritus, which help in feeding and provides breeding and nursery grounds for the juveniles of many commercially important shrimps and fishes.

Threats to mangroves:

Large hurricanes: Hurricane Donna, in 1960, damaged an area of 100,000 acres of the Mangrove zone of South Florida. Loss of trees ranged from 25% to 100% from shearing the trunks above ground, complete over wash of islands and prop root damage from marl and fine organic matter coating the roots.

Shoreline development has replaced mangroves with marinas, dredged channels, airports, seawalls and other commercial and residential construction. Other threats are illegal dumping, beach renourishment, and oil spill, agricultural run-off that contains herbicides, pesticides and sugarcane wastes. Globally mangrove forests are disappearing at a rate of 1-2 percent per year, a pace that surpasses the destruction of adjacent ecosystems, coral reefs and tropical rainforests. The U.N. Food and Agriculture Organization estimates that mangroves are critically endangered or approaching extinction in 26 out of the 120 countries in which they are found. Indian mangroves have been deforested and reclaimed to such an extent that the mangroves along the west coast are very much degraded. This has not only affected the coastline but also the fisheries to a large extent.

Conclusions:

Mangroves serve as a critical nursery for young marine life and therefore play an important role in the health of fisheries and the economic well-being of fishermen. The ecosystem is also considered as most productive and biodiversity providing significant functions in the coastal zones as buffer against erosion, storm surge and tsunamis. Afforestation of mangrove areas on a large scale is the most urgent need of today, if the coastal environment is to be brought back again to its earlier pristine glory.