Restricted feeding, spermatogenesis and growth in Arctic charr, Salvelinus alpinus : the identification of two possible gametogenic control points

Experiments were conducted to determine if food restriction during certain critical critical seasons could reduce the incidence of early maturity in Arctic charr (Salvelinus alpinus L.) and to describe the stages of spermatogenesis associated with those critical periods. -- A seven-stage description of the spermatogenic process is presented along with a time series of the progression through the various stages. Spermatogenesis in Arctic charr is initiated in autumn under decreasing photoperiods and temperatures, approximately one year before spawning. The onset is characterized by mitotic proliferation of spermatogonia before changes in GSI are detectable. The spermatogenic cycle is highly asynchronous within the testis of a single fish, and between individuals of a population. Two critical points in the spermatogenic cycle that may be susceptible to nutritional control were identified. The first occurs in autumn, affecting the advancement of testes from the pre-spermatogonial stage to the immature stages; the second in spring, when spermatogonial cysts form and a switch from spermatogonial proliferation to spermatocyte formation ensues. Reduced feeding temporarily interrupts mitotic division of germ cells during the early stages of gonadal development. -- The Arctic charr, aged 1+ in autumn, were subjected to two different restricted feeding regimes. Experiment I, consisting of alternating two week periods of food deprivation with excess feeding (2:2, 14 week duration), at any time from November to June did not reduce the proportions of male fish showing definite signs of maturity by July compared to a control group. In Experiment EC, fish subjected to 6-week periods of continuous starvation (6:0) from September to April exhibited temporary reductions in gonadal activity. By July, final maturation proportions of starved fish were depressed relative to the control group. Restricted feeding had no effect on the degree of gonadal investment in terms of gonadosomatic index (GSI). No females matured during either experiment. -- Temporal changes in fish growth were monitored over the course of both studies by retrospective examination of tagged individuals. After periods of food deprivation, resumption of regular feeding resulted in fish displaying hyperphagic feeding activity and compensatory growth. Growth differentials were most pronounced in fish starved during periods of higher temperature, especially during the time of rapid temperature increase in May/June, ha Experiment I, maturing males continued to grow and increase body condition throughout the winter months, while growth of their immature counterparts remained low until spring. Growth rates of maturing fish were consistently higher than immature fish. In Experiment II, growth patterns of maturing and immature males were similar, however, condition factors of maturing males were slightly above those of immature fish. -- This study lends support to the theory that there are two critical periods in the maturation cycle of salmonids. Food restriction alone, however, is not an effective method of suppressing early maturity in this species owing to the long and flexible duration of the identified critical decision periods