Unit 3: Fish and Fishery

1. Types of commercial fishes

Fish farming (or pisciculture) is the commercial breeding of fish in tanks or artificial enclosures like fish ponds, primarily for food. Worldwide, the most important fish species produced include carp, catfish, salmon, and tilapia.

In the context of India and Assam, commercial fishes are often grouped as:

• Major Carps: These are the most popular and commercially viable freshwater fish.
  • Indigenous: Catla (*Catla catla*), Rohu (*Labeo rohita*), Mrigal (*Cirrhinus mrigala*).
  • Exotic: Silver Carp, Grass Carp, Common Carp.
• Catfishes: Air-breathing (like Magur, Singhi) and non-air-breathing (like Pangas) species.
• Tilapia: A hardy and fast-growing exotic fish.

2. Construction and management of ponds for fish culture

Successful fish farming depends on a well-constructed and well-managed pond ecosystem.

Construction of a Fish Pond

1. Site Selection: The site should have good water retention capacity (clay soil is best) and a reliable water source (like a stream, well, or diversion canal).
2. Marking and Clearing: The boundaries of the pond are marked, and the site is cleared of all vegetation and debris.
3. Constructing Dikes: Watertight dikes (embankments or walls) are built around the marked area to hold the water.
4. Inlet and Outlet Structures: A pond must have structures to manage water flow.
   • Inlet: Allows fresh water to enter the pond.
   • Outlet (Drain): A crucial structure that controls the water level and allows the pond to be completely drained for harvesting. The pond base should have a gentle slope towards the outlet.
5. Depth: Ponds are typically shallow, often with a maximum depth of 1.5 meters. The shallowest part should be at least 0.5 meters to control weed growth.

Management of a Fish Pond

• Water Quality: This is critical. Key components include:
  • Biological Filter: Helps break down toxic waste (ammonia) into harmless nitrates.
  • Oxygenation: Increasing the contact between air and water (e.g., through a waterfall or aerator) increases dissolved oxygen.
  • Pond Skimmer: Removes floating debris to keep water clean.
• Aquatic Plants: Help regulate water quality, provide oxygen, and enhance the pond's aesthetic.
• Predator Control: Fencing (including buried netting) is essential to prevent predators like otters, birds, and snakes from entering the pond.
• Gravel/Rocks: Provide a surface for beneficial bacteria (part of the biological filter) to grow on.

3. Composite fish culture

Composite fish culture is an advanced aquaculture technique where multiple (usually five or six) different species of fish are reared together in a single fish pond.

The Concept:

The species are chosen very carefully so that they do not compete with each other for food. Each species has a different feeding habit, allowing them to utilize all the available food resources in the pond's ecosystem.

A Typical Species Combination:

• Surface Feeders:
  • Catla: Feeds on zooplankton in the surface zone.
  • Silver Carp: Feeds on phytoplankton in the surface zone.
• Column Feeders:
  • Rohu: Feeds on detritus and plants in the middle (column) zone.
• Bottom Feeders:
  • Mrigal: Feeds on detritus and mud at the bottom.
  • Common Carp: An omnivore that feeds on bottom-dwelling insects and detritus.
• Weed Feeder:
  • Grass Carp: Feeds on and controls aquatic weeds.

4. Ornamental fishes

Ornamental fishes (or aquarium fishes) are species kept for their aesthetic appeal in home aquariums or garden ponds. They are valued for their bright colors, unique shapes, and interesting behaviors.

Common Types of Ornamental Fishes:

They are often grouped by their breeding habits:

• Livebearers:
  • Definition: Give birth to live, free-swimming young ones. They are relatively easy to breed.
  • Examples: Guppy (*Poecilia reticulata*), Black Molly (*Poecilia sp.*), Swordtail (*Xiphophorus helleri*), Platy (*Xiphophorus maculatus*).
• Egg-Layers:
  • Definition: Lay eggs. This group is very diverse.
  • Examples:
    • Barbs: Rosy Barb, Striped Barb.
    • Tetras: Small fish from South America, like the Black Widow Tetra and Neon Tetra.
    • Goldfish (*Carassius spp.*): One of the most popular, with many varieties like the Comet, Lion Head, and Oranda.
    • Nest Builders: Males build nests (often bubble nests) to incubate eggs. Examples include Gouramis and the Siamese Fighter (*Betta splendens*).
    • Angelfish: A popular, graceful fish with many varieties.