The Control of Reproduction in Rainbow Trout - Commercial and Technical Aspects

Abstract

The thesis provides a detailed study of the endocrine control of reproduction in the rainbow trout. By using photoperiod and temperature manipulations and hormonal treatments, designed to interfere with this natural control, possible solutions are provided to the two major problems related to reproduction in the farming of rainbow trout, i.e. the limited availability of eggs, and the regressive changes which accompany maturation amongst production stock. During the normal spawning cycle, sequences of changes in serum levels of gonadotropin, oestradiol 178, phosphoprotein phosphorus and total calcium in females, and gonadotropin and testosterone in males were observed. Under shortened seasonal photoperiod cycles, these changes occurred earlier in the year although the basic sequences remained unchanged. As a result, spawning was advanced by up to four and five months in the female and male fish respectively. Injection with oestradiol caused similar changes and thus established the importance of this hormone in the control of vitellogenesis. At low temperatures, the egg incubation period was increased by 50 days, thus providing a further method of extending the availability of eggs. To exploit a supply of eggs throughout the year, a computer model was formated to predict growth under a variety of environmental conditions. This information could enable the trout farmer to design an optimum stocking programme. Treatment of first-feeding fry with high doses of methyl testosterone and with oestradiol 178 produced sterilization and feminisation respectively without any adverse effects on growth and mortality. Pairing masculinised females with normal female fish demonstrated that the female rainbow trout is the homogametic sex and that by using such sex-reversed fish monosex populations could be produced. Clearly, the production of eggs at any times as specified by computer assisted stocking schedules, together with the potential for producing sterilized or feminised stock is a joint approach which will be of considerable benefit to fish farming in the U.K.