Mature male rainbow trout have significantly higher relative ventricle masses (RVM, ventricle mass as a percentage of body mass) than do immature males or females. Hatchery-reared maturing male trout had a mean RVM of 0.139%, whereas females had an RVM of only 0.074 %. Moreover, as males matured and their testes grew from 0.07 to 3.92 % of body mass, RVM more than doubled. In female trout no such heart growth occurred; RVM remained unchanged during the period of ovary growth.

Cardiac performance was assessed using an in situ perfused heart preparation. Mature male trout have larger ventricles and could generate significantly greater maximum cardiac power output per kilogram body mass than could immature males or females. This enhanced cardiac performance by the mature males was attributable to delivery of greater cardiac outputs (through larger stroke volumes) and an increased ability of the heart to work against higher output pressures. Power output per gram ventricle mass was similar in both sexes.

Note: Present address: Department of Zoology, University of Queensland, Brisbane 4072, Australia.

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