Physiological Changes with Age in Fish 193 



known sample volume is counted. The fecundity is deter- 

 mined by direct proportion. The dry- weight and wet- weight 

 methods are essentially the same except that the counts are 

 made either on eggs which have been dried to constant weight 

 or on eggs taken directly from the preserving fluid. 



Like those of other animals, the ovaries of fishes contain 

 eggs in various stages of development. Early workers debated 

 the issue as to which eggs would be spawned. By measuring 

 the diameter of the ova in fish just ready to spawn, by des- 

 cribing their external appearance, and by examining histo- 

 logical sections of the ovary, various categories of eggs were 

 established. A comparison of these observations with those 

 on the ovaries of spent fish was the basis for deciding which 

 eggs to count for an estimate of the fecundity of an individual. 

 These observations have been made in great detail, and they 

 have resulted in learning that not all ripe eggs are shed dur- 

 ing the spawning season. Usually the numbers retained are 

 insignificant compared to those that are liberated. Ripe ova 

 which are retained in the ovary quickly degenerate and are 

 resorbed. Such observations have also been responsible for 

 discovering that some species, like the long rough dab, do not 

 spawn every year (Bagenal, 1957a) while in others, like the 

 yellowfin tuna {Neothunnus macropterus), several batches of 

 eggs may mature during a single, long spawning season (June, 

 1953). 



Herring. Wynne-Edwards (1929) immediately set about 

 testing Orton's hypothesis about a possible disproportionate 

 growth of the gonad in relation to body weight in the herring 

 {Clupea harengus). Both ovary and testes reach maximum 

 weight during the spawning season and decrease to an 

 insignificant size immediately after the sexual products have 

 been shed. The cycle of gonad growth then begins again in 

 preparation for the next spawning. Since the reproductive 

 tissue makes up about 20 per cent of the weight of the body 

 at maturity, a considerable amount of the anabolic process is 

 concentrated on the maturation of the sex organs. 



AGEING — ^V — 7 



