CULTURE OF SEA CUCUMBER

Holothurian, commonly called as sea cucumbers, is a group of economically important echinoderms.  Which are consumed either fresh or boiled or prepared as beache- de-mer (trepang) by people of Japan, Korea, China etc. Body wall, ovaries, intestine and respiratory trees are considered delicious food for Japanese. Holothurians exhibit a world wide distribution.  About 1200 species are recorded as belonging to the class Holothuridea. There are around 40 edible species.   Along the Indian coast  Holothuria scabra commonly called ‘sand fish’, is the predominant species. Whereas in Japan and Korea Stichopus japonicus  predominates.  Some species live on hard substrates like rocks, coral reef etc and vary in their habitats from foreshore to deep water zones. The other Holothurians of commercial value are  Holothuria nobilis  (black teat fish),  H. Fusogilia (white teat fish),  Actinopyga  echinites  (deep water red fish),   A.milaris  (black fish),  Thelonata ananas (prickly red fish),  Holothuria ata  (lolly fish),  H. Fuscopuncata  (elephants trunk fish) etc.

Both sandfish ( Holothuria scabra) and golden sandfish (H. lessoni) are considered to have the best potential for aquaculture because they have many attributes that make them suitable for hatchery production

BIOLOGY

The internal organs of the cucumber lay within the tube-like body chamber surrounded by the skin and a layer of longitudinal muscle bands. It is the skin and muscle bands that are the edible part of the animal. The internal organs consist of the digestive system for food processing, the gonad for reproduction, the respiratory trees for the removal of oxygen from the water, and a nerve ring that directs the operation of the muscles and tentacles.  The cucumber is capable of eviscerating (casting off) its internal body organs during times of stress which can later be regenerated.

Evisceration

The sea cucumber is a non-selective suspension feeder, taking its food indiscriminately from the surrounding water. The small bits of detritus and microscopic organisms that are floating just above the bottom are trapped by the cucumber's tentacles.There are ten tentacles that are covered with a sticky mucous and are extended in the water until they are filled with food particles. The sexes are separate, but microscopic examination of the gonad is the only reliable way to distinguish males from females. The gonad is located along one side of the body cavity and begins to produce eggs or sperm in the fall for the next spring's spawning season.  Spawning occurs from about late March to mid-April and coincides with the spring plankton bloom.

At this time the eggs and sperm are released into the water column where the eggs are fertilized and develop into a brief larval stage, at which time the animals are bright red, planktonic and called a "pentacula". By the end of May the larvae have evolved into juveniles and settled to the bottom.

LIFE HISTORY

Holothurians usually spawn in the late afternoon or evening or during night. During spawning, the males release the spermatozoa first and the females release the eggs.The males first lift the anterior end and perform swaying movements for some time after which they start releasing sperm and it continues for 1-2 hours. Ripe females, if any are present nearby, exhibit responsive behaviour. The anterior region of the female gets bulged and eggs are released through the gonopore forcefully in a continuous jet.

The mass of eggs are released appear light yellow and mucus like. The fertilization is external. Taking place in water. One adult female release about 1 million eggs. Eggs are spherical, about 180-200 microns in size. The auricularia larva hatches out after 48 hours. The auricularia larva is transparent, pelagic, it is slipper shaped and performs locomotion by the movement of flagella of the ectodermal cells that forms ridges or bands. The larva has a digestive tract consisting of mouth, pharings, stomach, and anus. There are three coelomic sacs- hydrocoel, and right and left somatocoels.

REPRODUCTION

Holothuria  scabra   attains a length of  400 mm and weight of  500g  and lives on sandy muddy bottom and become sexually mature at 18 months. The size at first maturity is 210 mm. the sexes are separate. There is no distinct sexual dimorphism. The ovary in females and testes in male are in the form of a tuft of tubules attached to the dorsal mesentery, through which the gonoduct passes terminating in gonopore situated on the dorsal side near the oral region.

The gonadal development is distinguishable into five stages such as Immature, resting, growing, mature, and post spawning phases. During the immature and resting stage, the ovarian tubules are transparent, short and thin distal end of tubules are club shaped. During grawing phase the tubules are having opaque spherical oocytes 20-120 microns in diameter. During the mature phase, the tubules are swollen containing ripe oocytes of 150-200 . It has been reported to breed twice in a year, first spawning season in from March to May and second during October to December in Gulf of Mannar.

Culture of sea cucumber

Technique for pond culture.

The ponds should be located in a area with free tidal current, The size of one pond is between 30-300mu (2-20 hectares) ，water depth is above 1.8-2 m, salinity is above 26 ppt year-round.  Water circulation is very important factor.  Rocks and concrete are to be applied to strengthen the inner layer of pond wall. The best substrate of pond bottom would be solid mud-sand or sand- mud; the substrate will affect the growth rate and survival tremendously. The sea cucumber culture purposes including: mud removal, disinfection and reef building. No feeding required in most of ponds. Sea cucumbers mainly depend on natural food resources. Routine monitoring of salinity, temperature, pH, alkalinity, water colour, transparency and adjust if required.The hatchery should have an algal culture unit to provide sufficient quantity of desired species of algae such as Isochrysis galbana species and Dunaliella species.

 In addition mixed culture predominated by Chaetoceros also shall be required.

Selection and construction of the culture ponds: 

 The farm sites should provide suitable conditions for the growth of sea cucumbers.  A supply of clean   and unpolluted seawater should be easily accessible. Salinity levels should range between 25 and 35, with optimal values around 27-32.  Ponds with muddy and sandy bottoms and of 2-3 hectares in size are preferred; however some operators use ponds as large as 7 hectares.  If necessary, stones or other artificial materials are placed in the pond to provide an adequate substrate for sea cucumbers to aestivate and live through cold winters.  Hard substrates should cover 50-70 % of the bottom.  The depth of water should be between 1.5 to 2 m and the seawater temperature maintained between 0-30 °C.

Dam Pond Culture

Dam pond can be build by concrete based on the local geomorphological characters in a shallow intertidal area, or a bay with rocky substrate, or by small reef islands area, where there is natural distribution of sea cucumber.  The artificial reefs are built by rocks inside the pond. The height of the dam is based on the high tide line, usually is higher than 2.0m above the ground. There are water intake and release holes underneath the dam, so the water exchange can be driven by tidal action. Seeding density:30 seeds/m2 for 1- 2cm seed, 10 seeds/m2 For 5cm seed.

Longline culture in open sea

 In an area there is less wave action, free tidal current, the existing long line system can be used for sea cucumber farming. The mesh size of the lantern nets is around 1.0cm, open and close by zipper, so it is easy to do the routine feeding operation. Scallop net, abalone net and modified plastic bucket can all be used for sea cucumber culture, and most facilities can do the poly-culture with abalone.  Seeding density is around 200- 300seeds/m2 for 5 cm seed, placed 5-8m below the water surface, The density has to be reduced as the sea cucumbers grow. This type of technique can be used as poly-culture with kelp as well.

Seabed cage culture

In a bay area, round or rectangle cages (2ｍ ×1.5ｍ×1ｍ) were built with steel bars, covered with 1.0cm polyethylene mesh netting outside the cage and hold rocks inside the cage. A 5cm space is needed to keep the rock from the frame of the cage to prevent netting damage due to friction. Seeding density: 3-5cm seed at 200-300 seeds/m2.  Routine management includes monitoring for mesh damage and reducing the sea cucumber’s density through the grow-out period.

Artificial breeding techniques

Holothurians hatchery should have a Brood stock maintenance unit, Spawning unit, Larval rearing unit and an Algal culture unit.

Brood stock maintenance unit

The Brood stock is usually collected from wild as well as from commercial catches and induced to spawn immediately or held in land based tanks and conditioned in captivity. 

Sand fish live in high nutrient environment at densities of 100/ha. Tropical sea cucumber can be difficult to hold in captivity and reduced feeding, weight loss, and poor gonad development. The large and healthy specimens which are not injured or eviscerated during capture are chosen for breeding. Collection of breeders is done during the breeding season i.e., March to May or October to December.

Preparation of brood stock tank: 

FRP tanks of 1 ton capacity provided with 6 cm thick sand at the bottom are used for keeping the breeders brought from natural ground. They are stocked at the rate of  20-30 adults in one tank.

The sand is also brought from the natural beds. These animals usually live buried in the sand and hence the sandy bottom is recommended. The tank is filled with filtered, clear sea water of about 32-35ppt. 

The water has to be changed every day, and sand is changed once in a fortnight. Feeding is done with fresh algae brought from the sea and ground to a fine paste which is given in the tank once in a week. Excess food may cause water fouling. In case of any water fouling, the sea cucumbers eviscerate and become useless for breeding. As algal paste settles to the bottom, the sea cucumber ingests the same with sand. If the feeding is not proper the animal gets shrunken and are the specimen not fit for spawning purpose. The Brood stock rearing tanks are kept in an air-conditioned room to maintain a low temperature of 18-20 degree c.

Spawning unit

Spawning is carried out in rectangular FRP tanks of about 100 litre capacity. The provision for an immersion heater with thermostat, thermometer and aerator are provided in the tank for thermal stimulation of spawners. . After introduction the spawners into the tank having filtered, clear and clean sea water, the temperature of the water is raised by 3c-5c by using the immersion heater. This thermal stimulation induces the sea cucumbers to spawn. This is the most widely used and most reliable method to induce the holothurians to spawn.

Apart from the thermal stimulation there are three other ways to do the breeding of holothurians. These are (1) Natural Spawning (2) Stripping and (3) stimulation through drying and powerful jet of water.

1. Natural spawning

       The male and female may release the gametes into the surrounding water without any artificial stimulation.

2. Stripping

This is done mainly on an experimental scale only.  The animals are cut open from cloaca to mouth through the dorsal side. The ovary which is translucent is taken out from the female and the same is slightly dried in a shade. It is then placed in sea water in a petridish and punctured with the scissors to release the eggs into the sea water. In the same way, the testis is taken out and cut into pieces. When the sperm move out in the water it is mixed with the eggs kept in a beaker with sea water. Mild aeration fecilates higher rate of fertilization.

3. Stimulation through Drying and powerful jet of sea water

The breeders conditioned for more than a weak in the hatchery are utilized for this purpose . First of all water in the brood stock tank is removed and specimen are dried in a shade for about half an hour. After this a powerful jet of water is sprayed on the specimens for a few minutes. Then the animals are put back into the tanks with sea water.

After 1-2 hours, the animals move up the tank wall and exhibit swaying movement indicating the release of gametes. First the males release the sperm and after one hour the females release the eggs.

Larval rearing unit

After the spawn and eggs are released, the breeders are removed from the tank carefully. The fertilized eggs are removed to the rearing tanks. The auricularia lavae hatch out after 48 hours. The healthy larva occupies the surface layer of water whereas dead or deformed one settles at the lower layer of water column or at the bottom of the tank. Those settled at the bottom are siphoned out. The healthy larvae are collected in a sieve and counted using a plankton counting chamber. Then the larva are released into the rearing tank containing clean, clean filtered sea water at density of 300 to 700 numbers per liter. The larvae are taken out once in 3 days to clean the tank to avoid infestation of other organisms.The larva is fed on micro algae Isochrysis galbana, two times a day.. This may increased or decreased depending on the stage of larvae. After 4-5 days, the larvae may be fed with mixed culture of phytoplankton mainly having Chaetoceros.

Under the above conditions of rearing, the auricularia develops to Doliolaria larva between 2-3days.  The Doliolaria  larva transforms into the pentacula larva within 10days. The pentacula is the creeping stage.  Late pentacula larvae settle on hard surface provided suitable substratum is provided in the tank. Hence, Artificial settling bases (settlers) are provided for them to settle. 

Two types of settling bases are tried.

(1)  polythene sheets are taken and kept in a tank kept outdoors having enough sunlight. For 4-5 days ,  filtered sea water is circulated continuously. Benthic Diatoms and other algae settle on these sheets. These are kept suspended in the water having late Doliolaria which are about to settle. The larvae settle on them as it gets food and hard surfaces.

 (2) In another type of settlers used, the polytheen sheets are kept in a tank having sea water. In this, filtered algal extract (50 mu filter) is added. Usually species of algae sargassum are used to make the extract. Algal extracts shall stick to the sheets. Fresh extract is put daily and the water is also changed daily. After 4-5 days. when the sheet is covered with algal extract it is given as settling base for the larvae. Juveniles that settle to the hard substrate has very weak motility. Hence, algal extract is again given daily twice morning and evening. Which is filtered through 40 micros sieve, After one month, 80 micron sieve can be used, and large sized juveniles of 15-20 mm size are separated and put in tank with very fine sand. There are also fed on algal extract. Optimum density of larvae should be adjusted to 200-500individuals per square meter.

Nursing of juvenile sea cucumbers: 

As the juveniles grow, the water quality and dissolved oxygen must be maintained at the optimal level.

 Increasing aeration and water exchange rates becomes necessary. The oxygen level has to be maintained above 5 mg/L. It is also important to use formulated feed that can be digested and absorbed easily. Experimental results have shown that the growth rate of juveniles fed on the formulated feed is at least two times higher than that of individuals fed on traditional feed during the 20 to 30 day period. As the accumulation of excess food and faeces increase, harmful germs tend to multiply rapidly and can cause very serious disease outbreaks among the juvenile sea cucumbers, including what is known as the ‘stomach ulcer’. Another disease is ‘white muscle syndrome’ which causes muscle tissues to turn white and rigid.

Transfer of young sea cucumbers to the pond:

 It has been demonstrated that the release of young sea cucumbers measuring 2-3 cm in body length produce the best farming results. These will attain commercial size after 1.5 years. For an optimal growth the culture density should not exceed 10 individuals/m2.

Environmental Factors Affecting Larval Rearing

The ideal temperature is reported to be 27C-29C, Aeration is carried out in larval rearing tanks to maintain oxygen at saturation level (above 5mg/L).  pH of the water may be between 6 and 9. Normal sea water has a pH of 7.5 to 8.6 which is suitable. Normal sea water salinity (32 to 35ppt) is favourable. If salinity falls lower than 12.9 ppt. the larvae shall die.  Most favourable salinity has been shown to be between 26 to 32.7 ppt. Ammoniacal nitrogen should not exceed 500 mg per cubic meters. It tolerates a range of 70-430 mg/m3.

Larve stages

1000,000 juveniles / kg 

6000 juveniles / kg 

600 juveniles / kg 

200 juveniles / kg

Management: 

Sea cucumbers can be farmed with shrimp and certain species of finfish, although they are commonly reared alone.  Prior to stocking the ponds with the hatchery-reared sea cucumber juveniles, It is necessary to clean and sterilize the ponds as well as inoculate the seawater with benthic Diatoms.  These measures will provide an appropriate culture environment and ensure high survival rates. The addition of formulated feed will also enhance growth particularly during spring and autumn. Some field tests have shown that the growth rate of sea cucumbers fed on formulated feed is as high as two times that of non-fed individuals.

Nutritional value of seed cucumber

It is a healthy food. It consists of 21.5% proteins, Mucopolysaachraide, minerals and other biological active substance. Recent medical research proved that the muscle aging is related to reduction of acid Mucopolysaccharide. It has cancer resistant effect, it involved in the enhancement of immune system, and anti blood clotting. It can be used to cure or additional therapeutic method for some desease: such as tuberculosis (TB), stress, erection problems, stomach, duodenum ulceration, diabetes, aplastic anemia.

FISHING DEVICES IN INDIA - Fish Traps and Hook & Lines

India is blessed with abundant water resources in different forms and with diverse fish fauna. So the fishing techniques are also have much importance. Different kinds of fishing techniques are practiced in India from earlier days. These methods are wide in range from hand picking to modern trawl.

The craft and gears used in different areas are different by their design based on the fish species, available materials in the geographic region. Also based on the nature of aquatic resource. The gears are classified in many manner, Including use, materials used to make, shape etc. Fish traps and hook& line are the most primitive type of fishing gears.

Fish Traps

Trapping- an earliest method of fishing. Traps- impounding devices into which an organism is lured and troy which escape is made difficult because of the non-return device is fixed at the entrance

According to Job &Pantulu, traps being fixed engines do not require continuity of attention and vigilance on the part of the operation but can be left to function themselves and secure a catch while the operator is engaged in other occupation. Trap fishing have economic and energy related advantages over the active search and capture fisheries. They require modest investment and due to their efficiency, simplicity and the quality of catch obtained, this method is widely used in all water bodies. 

In trapping the fish remain live or in good condition for a long time. Fish traps are operated in both inland and marine waters throughout India because of it is an eco friendly method. The artisanal fishermen in inland waters use primitive models of trapping. Modern traps are generally made up of plastics replacing the conventional bamboo sticks etc. and having separate part which can be assembled and dismantled easily.

Traps made up off twigs of Palmyra leaf and bamboo splinters are used in Tamilnadu, Kerala,Lakshadweep and Andaman Islands.

TYPES OF TRAPS

PLUNGE BASKET

Plunge basket is otherwise called as cover pots. In Kerala, commonly known as ottal or kuthukoodu. 

It is conical in shape with the size of 50-70 cm height, 40-50 cm width at the lower and 15 cm at the top. Normally 10mm size bamboo splinters or sticks are used for constructing ottal. Split cane or other materials are used to keep the ribs in position.

It is mainly used to trapping freshwater Prawn (Macrobrachium rosenbergii), other prawns and crabs. The free wide ends are sharpened to push down and fixed temporarily in mud. 

Narrow end is cover with cotton cloth and stitched with cotton or soft materials to avoid damage to operator.  Plunge basket in northeast states mainly in Assam called polo,and Polui  in West Bengal.

BOX TRAP

Box trap have ‘D’ shape or rectangular shape. In north Kerala this is known as chempallikkodu because of catching chembally (Lutjanus argentimaculatus), made up of split bamboo or arecanut tree with around 1.4*0.6*0.6 m size. Bottom piece is rectangular and fabricated used 10-15 strips have 7.4m length and 30-35 with 0.6m length kept perpendicular. It is also used to catch- Etroplus suratensis , Scylla serrata and Epinephelus spp. Etc.

Box traps in northeast are bolotha /pori and tesung purag& hookuri are 

BOX TRAP

FILTER TRAP

Filter trap, locally known as padal made up of the coconut leaf sticks (eerkil) or bamboo splinters in a cylindrical shape. It is about 0.6m in length with a circular mouth of about 0.4m diameter at one end and other end of the slivers are bunched and tried so as to close it. Few traps or creeper stem hoops are fixed inside the trap to give a cylindrical shape. To prevent the slivers from opening 6-7 encircling lacings using coir are also given.Filter traps are set against the receding current in shallow rivulets and pokkali fields.

FILTER TRAP

APRONED FILTER TRAP

This is an improved filter trap popularly known as tharapadal in north Kerala.

This is a simple modification of the cone cage.

APRONED FILTER TRAP

SCREEN BARRIERS

Long leaders of converging screens erected in shallow waters to lead the fishes into the chambers fixed in the end is known as fish fences or screen barriers. This type of trap is fixed during high tide and removed during the next low tide and the fish actively swim up into the barrier.The barrier made up of stone in Gujarat known as vada.

SCREEN BARRIERS

Bamboo barrier

These are large encloses with retarding devices erected in shallow waters where an extensive tract of flooded land is in the process of draining.Life of bamboo screen is about one year where as screen made up of retting may last for several years.

Net barrier

Synthetic netting is cheaper and easy way to trap fishing by screening. The disco net is replacing the traditional theta khonda made of split bamboo and cotton twins.

ARIAL TRAP

These are specialized traps to capture fishes that jump when faced with an impediment. These fishes can be caught on the surface in boxes, rafts, boats and nets (Verandah nets). The fishes are enticed to jump out of water by placing obstacles and are caught in the air by special devices obstructing their jumps. Sometimes the fish are frightened to get them to jump out of the water. Thottilvala- a kind of aerial trap common in Kerala. The commonly used aerial traps in the northeast are Letidiya (Mud trench), and Dolonga (Verandah net).

ARIAL TRAP

A mud trench is constructed in the water channel which blocks the flow of water. The sides of the trench are elevated creating a muddy pit in between it. When the fish encounter the obstruction, they try to jump over the barrier and fall in the mud and collects them by hand. Channa sp. and Puntius sp. are commonly caught through the aerial trapping. It is also used for the trapping of shrimps in Kerala, known as padal changadam. This is a fishing technique based on the shrimp’s reflex action to physical stimulation and is being used for harvesting the shrimps in perennial aquaculture farms in Kerala.

TUBULAR TRAPS

These are the traps or enticing devices exclusive of those made of textile which prevents the escape of fish by means of trap doors provided with non return valves. The traps vary in shapes. The catch comprises of fishes such as Mastacembalusaculeatus, M. armatus, Mysrus spp. Channa, etc. 

SHELTER TRAPS 

Specifically designed and operated considering the nature of fish to take safe shelter are known as shelter traps or habitat traps are provided with tree branches, shrubs, and twigs. They come in various size and shapes: Quadrangular, Conical, Cylindrical and Circular. Placed in low lying areas out of the water after 3 to 5 days and the fishes are taken out after removing the materials provided for shelter. 

These traps are tied to a fixed bamboo pole with a strong rope to prevent displacement in the water. 

The catch mainly- Mastacembelus armatus  (tire track eel), Mystus spp., Puntius spp., Clarias batrachus, Channa spp, Notopterus notopterus (bronze featherback), small prawns.Etc. 

HOOK & LINES

Hook and line this might be the one of the oldest and famous fishing method all over the world and one of the most dominant fishing methods. It is very economically viable technique to exploit the large pelagic, column and demersal predator fishes. The principle of line fishing is to offer bait and entice the fish or any other aquatic organism so that it can be lifted from the water together with the bait. In the primitive fisheries there existed curved hooks made of various perishable materials of plant and animal origin like Thorns, bones, tortoise shell, oyster shell and whale bone.Metals are used in modern days. The most important characteristic of hooks are their gap and their spread to ensure that fish shall be unable to spit the hook out with the bait after swallowing it. It should penetrate the mouth of the fish when the bait is taken or the line is pulled so that the fish becomes fast.Modern fish hooks come in a variety of sizes, shapes, and materials.

Two types of hooks:

J-hooks -manufactured with the point of the hook parallel to the shank of the hook creating a J-shape.

Circle hooks -manufactured with the point of the hook turned perpendicularly to the shank forming a circular

Traditionally J-hooks have been used in most fisheries, but recent efforts have been made to promote the use of circle hooks in more fisheries. Both J-hooks and circle hooks can be barbed or barbless. Barb- an additional point that protrudes from the inside of the hook that helps to retain the bait on the hook as well as a fish once it has been caught. Barbless hooks do not have an additional point and thus it is easier to remove from the fish when caught, which is considered less damaging to the fish.

COMMON HOOK AND LINE GEAR

HAND LINES

Hand line may be defined as the simplest form of hook and line gearconsisting of a hand held single line weighted and with one or more hooks spacedalong the far end of the line. Hand lines are popularly known as Choonda orkaichoonda and are operated in all types of water bodies.  Length of the line also varies from 2 to 50 m, depending on the depth of operation. Hand lines can be operated very easily. Fishermen operate lines from shore, canoe or any elevated platforms like bridges and dams. They are dropped into the water at places where the fishes are expected and fishermen feels usually with hands when fish bites. Baited lines having a length of  20-30 m.

hand line

LONG LINES

Long lines can be classified by how they are fished: 

Set long lines: stationary lines that are anchored to the vessel, the seafloor or to an anchored buoy. 

Drift long lines: attached to floats that drift freely with the ocean currents. 

Long lines popularly known as beppe is operated in all places for fish and eel.  Mainline is 200-800 m long and is made of Polypropylene twine of 2 to 3 mm diameter or Poly amide monofilament of l to1.2 mm diameter. Generally round barbed hook of size ranging from number 8-12 are used depending on the target species. Prawns and cut pieces of fish are used as bait.Pelagic species targeted by drift long lines include Tunas, Sharks and Swordfish. Demersal species targeted by set long lines include Cod and Halibut. Long lining is considered one of the most fuel-efficient methods of commercial fishing. 

VERTICAL LINES

In enclosed water bodies the fishing lines can be allowed to drift freely attached to a float. lt can be a tackle with a single hook or a vertical or horizontal long line with several hooks. A kind of vertical line, popularly known as kenichi, is operated in weed infested canals and fields of Alappuzha, Kerala.

Jigger lines are a specialized type of vertical line, fitted with specialized ripped hooks, used primarily in Squid fisheries. Multiple hooks are evenly spaced along the main line, which is hauled in using jerky vertical movements. This movement simulates the realistic movement of common prey species of the targeted species. 

TROLLING LINES 

Trolling lines are lines with baited hooks that are dragged behind trollers as well as other types of vessels. Trolling speeds vary depending on the target species, but generally are between 2.6-8.1 mph. 

A single line or multiple lines may be connected to outriggers that extend from both sides of the boat.

 Targeted species vary in size from small fish like Mackerel to large pelagic species of tuna. 

POLE AND LINE

Pole and line consists of a hook and line attached to a pole. If the line is much longer than the rod it is wound around a reel. Both artificial and natural fish are used to lure the prey. Poles are commonly made out of wood or fiberglass and can be operated by hand or mechanized. Tuna species are commonly caught by the pole and line method in commercial fisheries of Lakshadweep. Pole and line fishing can occur from the surface to great depths, the only limiting factor is the amount of line used.

MULTIPLE HOOK AND LINES

Lines with multiple hooks, a type of jigging line with 20 to 30 branches on one end. The main line is made of PA monofilament having 1.2-1.4 mm dia. and about 80 m length. 20-30 branches each  having a hook. Length of the branch line is about 10 cm and it is tied to the main line at 10-15 cm intervals. A lead piece weighing 150-200 g is attached to the end of the line to keep the line under the current. This type of lines without bait is dropped from the dam and it is continuously pulled and released. 

CULTURE AND SEED PRODUCTION OF CRABS

Among the marine edible crustaceans, crabs, rank third by virtue of their delicacy and demand for human consumption. In India, 600 hundred species of crabs are available, of which only two species are used to the culture. (Scylla serreta and Scylla tranquebarica). S. serreta is smaller and S. tranquebarica are larger species. Scylla species are generally known as mud crabs or green crabs.

 In malayalam they are known as “patcha njandu,kuzhi njandu and kattu njandu”.

Scylla serreta

S. tranquebarica can be easily distinguished from S.serrata by polygonal marking on the carapace.

 Among the marine crabs, mud crabs are the only species which can remain alive out of water for a considerable period of time. In earlier periods crabs are cultivated along with milkfish, but later on great demand, the monoculture is practiced. Major markets for Indian live mud crabs are in Singapore, Malasia, Taiwan, Hong Kong, and China. Hatchery production technology of mud crabs is presently available in Philipines, Indonesia, China and in India.

Scylla tranquebarica

TAXONAMICAL  POSITION OF MUD CRABS

Phylum: Arthropod

Class: Crustaceans

Order: Decapods

Family: Portunidae

Genus: Scylla

Species: serrata

Parts of a mud crab

HABITAT OF MUD CRABS

Adult crabs are found both in marine and estuarine waters. Mud crabs possess a pair of paddle-shaped swimming legs, which help them for fast swimming. Mud crabs in their megalopa stage enter in estuaries, coastal lagoons, and backwaters, grow fast, attain maturity and become berried. For hatching of the larvae, the berried female migrates into the sea waters. They bury under the sandy bottom. These are common among the mangrove forest.

FEEDING

Crabs generally feed on crustaceans, mollusk, small fish, detritus, and plants.But in culture, 18˚c frozen fishes are given as feed, after thawing the feed is given for crabs. 

IDENTIFICATION OF SEX

The immature and mature males of mud crabs have a slender and triangular shaped abdominal flap on the ventral side of the body. The immature females have a broad and triangular shaped abdominal flap. matured females have a semicircular shaped abdominal flap. The male crabs normally grow faster than females and attain bigger size than the females. Based on the gonadal development, 5 stages of maturity are recorded i.e., immature, maturing, late mature, fully mature and spent.

Male and Female abdominal flap

LIFE CYCLE

Mating occurs soon after moulting of a matured female. The male inserts spermatophores into the spermatheca. When the egg matures, it passes through spermatheca where fertilization takes place internally. The fertilized eggs are shed out through the opening of the vulvae and spawning event takes place. The spawned eggs are placed at the abdominal flap of female crab with the help of ciliary action of pleopods. Generally, the crabs spawn near the seawater lagoons, bays and coastal areas. Scylla serata prefers salinity in the range of 28-35 ppt for spawning.

The life cycle of the mud crab is divided into two phases; 

Early post-larval stage and grow out stage.

The eggs hatches into Zoea, which pass through various stages (zoea; 1-5) metamorphose into megalopa and migrate to brackish water areas.Then they enter into the crab instar stage and the entire process takings a total period of 25 days. It continues to grow to become juvenile, sub-adult until it attains adult stage. Upon attaining maturity and mating they migrate to sea for spawning.

Fecundity

The number of eggs found attached to the pleopods of female mud crabs varied from 0.3 to5 million. Breeding season is throughout the year.

SEED PRODUCTION OF MUD CRABS

SELECTION OF BROOD STOCK FROM WILD:

Healthy broodstock is collected from wild, for the immediate breeding programme, only mature female or berried female is collected from wild.

  1. Brood must have; 

  2. hard exoskeleton,

  3.It should be free from all pathogens,

  4 all the appendages are to be intact, 

  5 no damages on the body. 

Berried female

TRANSPORTATION OF BROOD:

After the collection of berried female, chelipeds are tied with jute fiber or banana fiber. 

Then they are to be kept in the bamboo basket filled with wet seaweeds /cotton wool/paddy straw/mangrove twigs. Finally, the basket should be covered with wet jute cloth with proper ventilation. In case of berried females, the animal has to be brought in wet condition from the collection point and kept individually in the thermocol container. The chelipeds need not be tied with ropes because it may damage the eggs.

ACCLIMATISATION: 

Once the broodstocks are brought to the hatchery, the animals are carefully removed from the transport container to allow them in an empty basin. Then clean sea water is to be added slowly for acclimatization or it is advised to acclimatize them in clean water at collection point itself in order to maintain fertilized eggs in a healthy condition

QUARANTINE SECTION: 

For removing the pathogens the animals are treated with formalin @150 ppm dosage for 30 minutes.

STOCKING: 

After acclimatization and quarantine section the berried crabs are directly stocked into the black colored FRP spawning tanks of 250-300 liters capacity. Covered with a lid and filled with cleaned and disinfected sea water with aeration. Black tanks are always preferred and should be kept in a spawning room with individual spawner.

WATER QUALITY MANAGEMENT:

Salinity-28-32ppt

pH - 7.5-8.5

Temperature – 28 - 32⁰C

Dissolved oxygen - > 4.0 mg/liter

BROODSTOCK FEED:

Fresh fish, squids, bivalves, mussels etc are given as food. 

The mature broodstock can be fed with 5-10% of body weight (40% in morning and 60% in evening).

 The frozen fishes are thawed first before feeding.

INDUCED BREEDING:

 In crabs, gonadal maturation can be induced by eyestalk ablation. During this process, one of the eyes is ablated to remove gonad-inhibiting hormone-secreting gland situated in the eyestalk. 

Ablated eyestalk has to be treated with iodine ointment for healing the wound and preventing the infection. Spawning would take place between 2 and 8 weeks depending upon the ovarian development.

LARVAL DEVELOPMENT: 

The eggs hatch into Zoea. It has five stages (Zoea 1, 2 3, 4, 5). After hatching of eggs the spawner is shifted into another tank. Before collecting the larvae, the aeration is switched off and the incubation tank is covered with a lid, leaving a small opening. Since the larvae are photo-tactic in nature an electric light is kept near the opening of the lid in the corner to attract the larvae. The accumulated larvae are collected by fry bowl or using siphon net. The collected larvae are transferred into larval tanks. Rearing of larvae is continued in the same tank till it reaches the Zoea – 5 stages. (stocking density 70 numbers/liter).Once the larvae cross zoea 5 sage, its metamorphosis into megalopa stage. (Stocking density 1-2 no./liter). To avoid cannibalism certain hideouts are provided.

Temperature 25-32⁰C, Ph-7.5-8.5 and DO- >4mg/liter are provided for larval tanks

FEED FOR LARVAE: 

For early zoea stages, rotifers (Brachionus plicatilis) are given as feed. After 7 days rotifer with microalgae is provided. Day 8 onwards artemia are given as feed. As an alternative, egg custard, fish meat, squid meat etc., can be given. Megalopa develops into crab instar, and then it develops into sub-adult and adult.

NURSERY REARING:

Hatchery produced crab instars 0.3-0.5cm carapace width are stocked in HPDP Happas in brackish water ponds. Where the salinity range is 15-35ppt. Hideout is provided to avoid cannibalism. The minced fish meal can be given as food. Based on the feeding and management nursery days may vary from 30-40 days to attain the “matchbox” crab let.

CULTURE OF CRABS:

Nursery-reared crablets are used to grow out culture. Normally 6-7 months are required to attain a marketable size of average 500 gms. Another way used to grow out culture is growing of juveniles sizes in small brackish water pond for two months to attain the average size of 50-60gms. 

 The size of the grow out pond can vary from 0.25-1 Ha, with proper inlet and outlet water management. Soil texture with a high slit is not suitable for crab farming. Minimum 1-meter water depth is required. Stocking density 1 juvenile crab/m². Stocking density 1 juvenile crab/m². Stocking of crablets can yield an average survival of 60% and stocking of juveniles yield 80% survival rate.

PEN CULTURE: Crablets can also be cultured in pens with average stocking density is 1no/m². The advantage of pen culture is easy growth, faculitate stocking of different sizes of crabs in different pens which may ultimately increase the survival and yield. Crabs generally feed on mollusks, fish, other crustaceans and annelids.