By Greg Lutz*
Addditionally, many procedures related to genetic research such as polyploidy, gynogenesis and complex mating designs require precise control over the entire spawning process, from arranging specific combinations of broodstock to exact timing of fertilization.
In many aquaculture situations, including genetic improvement based on simple selection programs, propagation of production stocks may require little more than good nutrition, suitable water quality, reasonably-comfortable surroundings for brood animals and the patience to allow nature to take its course. In other instances, however, it is virtually impossible under hatchery conditions to replicate the natural stimuli and circumstances that trigger some species to spawn. Unreliable captive spawning remains a major stumbling block in the development of successful commercial production systems for a variety of aquatic species.
Frequently, artificial manipulation and control of both external stimuli (through environmental control) and internal physiological processes (through hormonal intervention) is required if captive broodstocks are to be propagated and genetic manipulations are to be successfully applied. The degree to which these techniques can be considered practical, however, often depends on the biology and life history of the species in question and the resources and expertise available.
Over the past several decades many aquatic species have been induced to spawn in captivity, but an objective examination of the numbers often suggests little advantage over natural systems in terms of the total quantity of viable offspring produced. When an advantage in sheer numbers does exist, it is usually at the expense of genetic diversity. This should be an important consideration for any restocking program utilizing hatchery-produced fry or fingerlings to enhance natural populations. However, if fry and fingerlings for grow-out are required, the only alternative to photothermal conditioning and hormonal intervention for acquiring significant numbers of eggs or fry from many aquatic species would be to procure a plankton net and head for the spawning grounds.
Most aquatic organisms have evolved to spawn under conditions that maximize the probability of survival for their offspring. The typical process leading to spawning involves the perception and interpretation of specific environmental conditions that, in turn, trigger a complex pathway of internal physiological developments. Unfortunately, inappropriate stimuli or physiological stressors can easily disrupt this chain of events.
Specific external stimuli that often prompt the reproductive process can include factors such as photoperiod and lunar cycles, ambient temperature or changes therein, precipitation, water flow (current), water depth, changes in barometric pressure, presence and behaviors of other fish, presence of suitable spawning substrate, and various changes in water quality, especially salinity, hardness, dissolved oxygen and pH. When everything works as it should, the appropriate external stimuli are usually sufficient to trigger a series of internal physiological events that result in maturation of gametes, followed by ovulation and spawning. This process can be described as a chain, the links of which involve various organs and hormonal compounds. This chain, or pathway, ultimately leads from the external environment, through the brain, and all the way to the gametes themselves. While the process is similar in many invertebrates, I will focus on finfish in this article.
As external stimuli are interpreted and processed, a specific region of a fish’s brain referred to as the hypothalamus responds by producing substances such as gonadotropin releasing hormone (GnRH) and/or substances such as dopamine that inhibit gonadotropin release. The brain, the hypothalamus and the compounds it produces represent the first internal links in the reproductive pathway. The pituitary gland, typically situated directly below the brain, is stimulated by GnRH and in turn releases gonadotropic hormones (GtH’s). The presence of these compounds in the bloodstream stimulates the gonads (ovaries and testes) to produce steroids that in turn trigger final egg maturation or spermiation, and prostaglandins that are involved in ovulation. When any link in the chain is broken, however, due to stress, injury, inadequate stimuli or other adverse effects, reproduction will not occur.
Under artificial conditions, producers and researchers are often forced to by-pass or even override external stimuli and internal mechanisms within the fish if spawning is to successfully take place. Over the past several decades studies involving numerous species have illustrated that it is often possible to reinforce or replace specific links in the reproductive stimulus pathway through photothermal conditioning or hormonal intervention. As a general rule, the further along the pathway intervention is attempted, the greater the probability of success.
Techniques for photothermal manipulation to simulate annual seasonal cycles have been well-established for a number of aquatic species. Controlled temperature and photoperiod are used to mimic or even temporally compress annual cycles associated with maturation and spawning. Although they may not always provide all the stimuli necessary to lead to full maturation and spawning, these manipulations are often sufficient to stimulate the development of gametes to an advanced stage, often referred to as ‘maturation.’ These systems generally require recirculating pumps and mechanical and biological filtration, as well as equipment such as chillers, heaters or heat-pumps to allow for continuous re-use of temperature-adjusted water. In contrast to artificial temperature regimes, photoperiod manipulation usually requires little more than conventional timers for artificial lighting and a disciplined workforce to avoid inadvertent interruptions of a photoperiod cycle once it has been established. Temperature or photoperiod regimes alone may induce maturation or spawning in some species, but in others both are essential.
In practical terms, hormonal intervention is often a necessity. Holding or conditioning facilities for broodstock may be inadequate, or entirely unavailable, and if wild broodstock must be collected from natural spawning grounds hormonal intervention may be required to prevent the disruption or complete shut-down of maturation processes as a response to capture- and transport-induced stress.
One key to the success of such intervention, however, is an understanding of what links of the chain are in jeopardy and what hormonal substances can be applied to reinforce or replace the natural compounds involved. Historically, the most common and widely-practiced intervention in the maturation pathway of fishes has involved administration of substitutes for the gonadotropic hormones (GtH’s) normally produced by the pituitary. These alternatives can be injected hypodermically, or directly into the musculature or the peritoneal cavity to elicit similar results. One common historical approach involves utilizing actual fish pituitaries; another relies on purified gonadotropins such as Human Chorionic Gonadotropin (hCG) to mimic the GtH that would be produced by the pituitary prior to and during the natural spawning season.
In certain species, however, gonadotropins are entirely ineffective in inducing ovulation or spawning. In these instances, hormonal intervention must target a point nearer the origin in the maturation pathway. The earliest convenient point of intervention is usually the link between the hypothalamus and the pituitary. Luteinizing hormone - releasing hormone (LHRH) from mammals was first used to temporarily replace GnRH in fish, triggering the pituitary release of GtH and subsequent processes. Today, synthetic analogs of these releasing compounds have been used with much more success. These newer compounds are typically more powerful and long-lived, and easier to obtain, characterize and work with. Perhaps one of the most widespread analogues is LHRHa. Similarly, two of the most common GnRH analogues are sGnRH-A (salmon gonadotropin releasing hormone analogue) and GnRHa.
It is often helpful, or even essential, to administer a dopamine blocker such as haloperidol, pimozide, metoclopramide or domperidone in conjunction with these releasing hormones, to prevent natural pituitary inhibition that can limit release of GtH’s following stimulus from the hypothalamus. A commercial preparation combining both GnRHa and domperidone has been widely used on a variety of fishes.
In some circumstances, photothermal conditioning or administration of hormones can foster the reproductive process up to an advanced stage, but the very last links in the maturation chain must be reinforced to induce ovulation and spawning. In rare instances, purified or artificial compounds have been utilized in finfish to replicate the modes of action of steroids and prostaglandins normally produced by gonads. Steroid and prostaglandin compounds have also been implicated in the expression of specific behaviors associated with spawning in finfish, which may serve as a sort of feedback to provide an external stimulus to other broodfish and reinforce the probability of successful spawning.
Dr. C. Greg Lutz is the author of the book Practical Genetics for Aquaculture and the Editor in Chief at Aquaculture Magazine.