A mechanism for the juvenile-to-adult transition in fishes!

The conventional view (Hoar, 1970; Liley, 1970; Harvey and Hoar, 1979) of the sequence of events which, in fish, leads to first gonadal maturation and subsequent spawning, starts with "environmental stimuli" which the brain and pineal gland process and pass on to the hypothalamus, which passes its information to the pituitary; gonadotropic hormones are then synthesized and released which leads to the maturation and release of gametes, given suitable environmental conditions (including a mate). Other more elaborate schemes allow for feedback loops (Kuo and Nash, 1975), but share with the linear sequence outlined above the feature that they invariably start with "environmental" or "external" stimuli. The conventional view of the link between growth and reproduction in fish is that of a "reproductive drain", i.e., the notion that fish, once they have reached sexual maturity, have a slower growth because the "energy" previously available for growth is, in mature fish, redirected toward the production of gonadal material (Jones, 1976; Lagler et ai., 1977). It is argued here that these conventional views can be replaced by a single hypothesis with the help of which a number of otherwise irreconcilable observations on the growth and reproduction of fish can be explained. b) a medium time scale (weeks to months) during which gonadal maturation takes place, as triggered by environmental stimuli; and c) a long time scale, in which the age and the size at which a fish becomes sexually mature for the first time are determined.