Fish - Inducing Reproduction - Anatomy & Physiology

Introduction

Among the most significant advancements in the field of aquaculture during recent decades is the development of techniques to induce reproduction in fish. These techniques have allowed farmers to profitably breed and raise species that do not naturally reproduce in captivity, and to manipulate the timing of reproduction to suit production cycles.

Some species will not readily breed in captivity due to environmental or culture conditions that are different from those found in nature, such as water temperature or substrate type. These conditions may cause stress or may not provide the cues needed to complete the reproductive process.

Fish in captivity may not always reproduce at the most advantageous time, and alteration of the spawning cycle may be desirable. This allows a farmer to:

• Obtain fish outside of the normal spawning season either to lengthen time for grow-out or to produce hybrids with other species.
• Improve efficiency by getting fish to spawn on a predetermined date.
• Maximize survival by fertilizing and incubating eggs under hatchery conditions.

Where successful, techniques for altering the spawning cycle of fish have become a valuable tool.

Strategies

There are two main strategies used to induce reproduction. Often the two strategies are used sequentially, the first to manipulate maturation, then the second to induce ovulation.

1. Provide an environment similar to that in which spawning occurs naturally.

• Catfish, for example, like to spawn in enclosed spaces such as hollow logs. A farmer can simulate this by putting milk cans in a pond.
• The presence of vegetation and an increase in temperature will usually work for goldfish. *
• Changing the photoperiod in a hatchery can accelerate or delay maturation and ovulation in many salmon and trout species.

2. Inject the fish with one or more naturally occurring reproductive hormones or their synthetic analogues.

• This is only effective in fish that are already in breeding condition and have mature eggs in which the germinal vesicle has migrated.

Induction using Hormones

Numerous hormones have been used to induce reproduction. Two methods have emerged over the past few years that seem to offer the best chance for success at the least expense. They are injection of a GnRH analog with dopamine antagonist, and injection of gonadotropin.

GnRH Analogue with Dopamine Antagonist

• Leutinizing Hormone Releasing Hormone (LHRH) is the name of a mammalian hormone that has been employed successfully to induce the reproductive hormonal cascade.
• Recently, synthetic analogues of LHRH (LHRHa) have been developed that are more effective.
  • More pure and are not rapidly metabolized by fish.
  • Remain active for longer periods.
• Under natural conditions, there is a feedback mechanism in fish that limits the release of gonadotropin.
• This mechanism uses dopamine, which inhibits the action of LHRH.
• When dopamine is present in the fish, even LHRHa will have only limited success.
• A dopamine antagonist is often used to limit the effects of dopamine.
• When LHRHa and a dopamine antagonist are used in conjunction, reproductive success dramatically increases.