AGB 252: Animal & Fisheries Biotechnology

Biotechnology in Aquaculture

Biotechnology offers powerful tools to enhance aquaculture practices. Key areas of application include:

• Seed Production Development
• Nutrition & Feed Formulation
• Biosecurity & Disease Control
• Long-term Gamete Storage

Focus: Seed Production

A primary application of biotechnology is the development and improvement of seed production. This area involves several critical manipulations:

• Broodstock Management: Controlling spawning and sex differentiation.
• Selective Breeding: Using genetic tools like gene transfer and marker-assisted selection.

What is "Seed" in Aquaculture?

"Seed" refers to the juvenile fish or fry used to stock aquaculture ponds. The quality of seed is a major determinant for a successful harvest.

It directly affects:

• Growth Rate & Survival
• Final Production Cost

Sources of Seed

Aquaculture farms obtain seed from two main sources, each with its own advantages and challenges.

• Collection from the Wild: Harvesting young fish from natural water bodies.
• Hatchery Production: Breeding fish in a controlled, artificial environment.

Key Hatchery Activities

Successfully producing high-quality seed in a hatchery involves a sequence of carefully managed activities.

• Broodstock Management: Caring for the parent fish.
• Spawning: Inducing the release of eggs and sperm.
• Incubation & Hatching: Nurturing the fertilized eggs until they hatch.

Broodstock Management

Proper care of parent fish (broodstock) is essential for successful breeding. This includes:

• Feeding & Segregation: Providing optimal nutrition and managing a resting period.
• Sex Ratio: Maintaining the correct ratio of males to females.
• Environmental Control: Using light and temperature to influence maturity.

Spawning of Fish

Spawning is the process of releasing eggs and sperm. It can occur in two ways in aquaculture:

• Natural Spawning: Fish breed on their own when environmental conditions are right.
• Induced Spawning: Farmers use hormonal treatments to trigger spawning on a desired schedule.

Incubation and Hatching

After spawning and fertilization, the eggs must be carefully looked after to ensure a high hatch rate.

This final step involves placing fertilized eggs in specialized incubators with controlled water flow and temperature until they hatch into larvae.

Goal: Control of Reproduction

A key objective in aquaculture is to control the reproductive cycle of fish. This allows farmers to achieve sexual maturation and spawning at the most suitable time for production, rather than waiting for nature.

This control can involve advancing or retarding the natural spawning period.

Environmental Cues for Spawning

In nature, specific environmental changes trigger fish to reproduce.

• Catfish: Often spawn seasonally, at the onset of the rainy season.
• Carps (Cyprinus sp.): Their reproductive cycle is highly dependent on temperature, typically spawning in April-May as waters warm.

What is Induced Reproduction?

Induced reproduction uses hormonal regulation to trigger the final stages of gamete maturation (eggs and sperm).

This technique is crucial when the precise environmental cues are absent or untimely in a culture setting, allowing for year-round seed production.

The Hormonal Cascade

When natural cues are present, the fish's pituitary gland releases a hormone called gonadotropin. This hormone triggers the final steps leading to spawning.

In induced breeding, an external source of gonadotropin is injected to start this same hormonal sequence, effectively taking over for the fish's own system.

Advantage of Induced Reproduction

The principal advantage of this technique is that it bypasses the need for specific environmental variables.

By simulating the natural gonadotropin surge with an injection, farmers can induce spawning without having to perfectly replicate factors like:

• Photoperiod (Day Length)
• Temperature Changes
• Rainfall

Hypophysation: Hormone Therapy

The technique of inducing spawning with hormones is called hypophysation.

It is essentially a simple replacement therapy. Gonadotropin from an external source is given to a fish that is not producing enough of its own, triggering the final maturation of eggs and sperm.

Types of Hormones: Purified

Several types of partially purified and purified gonadotropins are used for hypophysation.

• Human chorionic gonadotropin (HCG): Most frequently used.
• Gonadotropin Releasing Hormones (GnRH)
• Luteinising Hormone (LH)
• Follicle Stimulating Hormone (FSH)

Other Hormone Sources

Besides purified hormones, other substances can be used to induce spawning.

• Steroids: Progestational and adrenocortical steroids can be effective.
• Pituitary Extracts: Glands collected from other fish are a common, traditional source of hormones.

Understanding Pituitary Extracts

When using pituitary glands from other fish, the relationship between the donor and recipient is important.

• Homoplastic: The donor fish and recipient fish are the same species.
• Heteroplastic: The donor fish and recipient fish are different species.

Preparing a Pituitary Extract

The process involves carefully extracting the pituitary gland from a donor fish.

The whole gland can be used fresh or preserved. It is ground into a fine powder, mixed with a saline solution, and then injected into the broodstock fish.

Preserving the Pituitary Gland

Since donor fish may not be available year-round, pituitary glands are often preserved for later use.

Common methods include:

• Drying with acetone or alcohol.
• Flash freezing on dry ice (-79°C).
• Temporary storage in alcohol or glycerine.

The Injection Process

A typical hypophysation technique involves a specific injection schedule:

• Females: Receive a small initial dose, followed by a larger second dose 4-12 hours later to induce ovulation.
• Males: Usually receive a single injection at the time of the female's second dose to ensure sperm is ready at the same time.

Drawbacks of Hypophysation

While effective, the traditional use of pituitary extracts has serious drawbacks.

• Lack of Standardization: The hormone potency varies greatly between glands.
• Supply Issues: Demand for pituitary glands often exceeds the available supply.
• Impurity: The extract contains many other pituitary hormones not related to reproduction.

New Challenge: Overpopulation

Some species, like the Nile Tilapia, are highly fecund and breed very frequently.

This presents a challenge in farming, as uncontrolled breeding leads to:

• Overpopulation of ponds.
• Stunted growth due to competition for food.
• Lack of uniformity in size at harvest.

Solution: Controlled Sex Differentiation

To solve overpopulation, farmers create monosex populations. For Tilapia, all-male populations are desired because males grow faster and larger.

This is achieved by treating newly hatched fry with a male hormone (e.g., 17 α-methyltestosterone) in their feed. This turns genotypic females into functional males.

Summary of Approaches

Fish biotechnology provides a range of powerful tools for improving aquaculture efficiency and sustainability.

We've explored several key areas, including broodstock manipulation, induced spawning (hypophysation), and controlled sex differentiation to enhance seed production.