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Technical factsheet on the different methods of controlling puberty in fish

© Euroceans
Pubertal maturation of the male reproductive system is associated with development to functional competence of the brain-pituitary-gonad axis, the endocrine system that regulates reproductive processes. Puberty is the development period covering the transition from an immature juvenile to a mature adult reproductive system. The initiation of puberty in males is marked by the onset of spermatogenesis and in females by the onset of oogenesis.

What defines the onset of puberty?
A variety of biotic and abiotic environmental cues (e.g. availability of food, population density, predators, photoperiod, temperature, water quality, pollutants) are involved in the timing of puberty. Puberty is a key component in life-history strategies of many fish species and has evolved to allow maximal reproductive fitness.
In salmonids, the timing of puberty is known to be linked to growth and metabolic status. For example, higher muscle lipid content positively influences sexual precocity and advances the initiation of gametogenesis during the first reproductive cycle of rainbow trout.
In the sea bass, genetic and environmental factors influence the age of sexual maturation, with photoperiod being one of the most important cues in triggering puberty and reproduction.
Why is puberty a big issue for the fish farming industry?
Under farm conditions some species (e.g. cod, sea bass, salmon) enter puberty precociously, leading to decreased flesh quality, growth performance and feed conversion, as well as low resistance to infectious diseases and an unattractive appearance to the consumer - all resulting in a lower commercial value.
Early maturation is considered to be one of the greatest problems in, for example, cod, sea bass and salmon farming. Fish stop growing during spawning and food fed for somatic growth is lost through spawning, resulting in elevated feed costs. Methods to delay puberty, including photoperiod control, have been developed. However, these protocols are not always fully effective; for example, there is limited knowledge on the activation of the brain-pituitary-gonad axis during puberty in salmon, which is essential for further development of some key genes during puberty under different photoperiod regimes.
The dominance of male fish in culture conditions is an additional hurdle, for example in sea bass, where females naturally reach puberty a year later than males and are generally larger in size. Farmed sea bass populations are typically 90% male, with over a quarter attaining puberty precociously.
How can this knowledge help protect wild stocks?
Some species of commercial interest cannot currently be reared in captivity (they are simply fattened in fish farms), such as the bluefin tuna and the eel. The bluefin tuna fattening industry has stimulated an increase in fishing pressure and a consequent significant depletion of the wild stock. The eel, which does not normally undergo gonadal development in captivity, faces a significant threat to its natural populations due to the demand for elvers from eel farms.
An improved knowledge of the processes controlling puberty will aid in enabling the maintenance of captive broodstock, for restocking dwindling natural populations and for a sustainable fish farming industry.
How is puberty artificially controlled?
Puberty is delayed or prevented by continuous light conditions and by induction of sterility by triploidy.
Induced triploidy as a means of producing sterile populations for aquaculture has been investigated extensively for Atlantic salmon and other salmonids, as well as for sea bass, sea bream and turbot, but not for any gadoids (e.g. cod). The production of mixed-sex triploids involves either pressure or temperature shocking zygotes at specific intervals, post-fertilisation (e.g. in sea bass). Monosex female triploid offspring may be produced by initially administering androgen hormones (e.g. for salmonid species) to sex reverse broodstock females into "neo-males", which are then crossed with normal females; the resulting zygotes are then pressure or temperature shocked in the same way to induce triploidy. Triploidy induces sterility in both males and females, although in some species triploid males may produce small quantities of aneuploid sperm that results in unviable progeny. In most fish species juvenile triploids grow at an equal or lower rate than diploids during the pre-maturation period, whereas, in general, adult triploids are heavier than diploids during the post-maturation period. However, the advantage of producing triploids for fish farming is species specific, and should be assessed accordingly.
Continuous light treatment can delay or stop sexual maturation, but this is not fully effective in outdoor situations. In sea bass it has been shown to be highly effective in inhibiting the appearance of precocious males and subsequently to increase size at age of commercialisation. Continuous light treatment both prevents the reallocation of energy resources towards gonadal development, and directly affects growth performance. This may be a realistic therapy on a commercial scale.
A method currently used in salmonids to obtain large female individuals for the market, and to avoid the problem of precocious male puberty is the production of female monosex populations. Fish sex is manipulated by the direct application of hormones. Within the European Union only the use of androgens (methyltestosterone) is permitted in fish farming under certain derogations (see legislation section for further details). This treatment produces genetically female, but phenotypically male fish or "neo-males", which are then crossed with normal females in order to obtain monosex female populations (i.e. the treated fish are not consumed directly). It is also a promising option in sea bass culture. Further details on monosex populations can be found in the biotechnology factsheet.
The application of antireceptor vaccinations, which have specific antagonistic effects on gametogenesis, is also in development.
What are the future requirements for research?
There is increasing evidence that the incidence of early sexual maturation is influenced by somatic growth and / or energy stores, however the underlying endocrine mechanisms are still poorly understood.
Although it is known that increased photoperiod is effective in reducing the incidence of early maturation, it is not clear how this occurs and how many factors are involved. For example, protocols currently in use for photoperiod treatments to delay puberty in cod are not fully effective in arresting sexual maturation and need refining.
The control of the uneven sex ratio in cultured populations of sea bass relies upon a better understanding of the basic mechanisms of sexual differentiation and the influence of genetic and environmental factors.
Triploidy has been shown to produce sterile populations of sea bass, however this method requires further experimentation in commercial production conditions (sea cages).
Improvement of the immunisation protocol for an antireceptor vaccination strategy is required, so that it is more potent and practicable by the industry.
This factsheet was prepared with the expert assistance of scientist Silvia Zanuy.

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