SOME EFFECTS OF ENVIRONMENT ON EGG PRODUCTION IN

Brown trout collcctcd from fcrtilc and infcrtilc streams in l’cnnsylvanin wcrc compared to dcterminc diffcrcnces in fecundity and in the age at First sexual maturity. Trout from iufcrtilc streams wcrc older at first sexual maturity and produced fcwcr eggs hccause OF smaller avcragc size than trout from fertile streams. Fish of the same size produced a smaller total weight of eggs in inbrtilc waters than in fertile waters. Sexual maturation clcpendcd on both size and age. It was concluded that in waters of low basic productivity few eggs arc produced per adult trout in the stock and that this dcprcsscd rate of rcpro-duction is important in keeping brown trout populutions in equilibrium with the low basic productivity of the environment. INTRODUClTON The numerical density of a fish popula-tion at any given time is dctcrmincd by the combined effects of natality, mortality, im-migration, and emigration. In another sense, the density of a population is thought to fluctuate about an cquilibrimn level dc-termined by interactions bctwccn these population parameters and the cnviron-mcnt. If this is so, populations of trout living in widely differing cnvironmcnts may differ in one or more of the above parameters. We consider hcrc possible modes of interaction bctwecn natality and environmental fertility. The objective is to determine whether or not variations in egg production that occur in natural stocks OF brown trout (Snlmo trzctta) are corrclatcd with environmental productivity. The choice of natality of the population and environmental fertility as factors to be investigated was made bccausc of their as-sumed importance in determining popula-tion size. Evidence in support of this as-sumption follows. In several salmonids, the success of natural fertilization of the eggs oEten exceeds 9O%, and the survival t