A 14-year investigation of a North American Acipenseriform fish, the paddlefish Polyodon spathula in the Yellowstone River; Montana, and North Dakota, USA has provided a detailed empirical framework of the life history in relation to the costs of reproduction for that species and potentially for closely related sturgeons. Nearly all aspects of life histories of males and females differed from each other after the immature period. The five periods in the juvenile and adult life histories for males and females, which occur at different ages for each sex, were (I) immature, (2) maturing, (3) somatic growth and reproduction, (4) prime reproduction, and (5) senescence to death. During the first period (immature), fishes exhibit rapid somatic growth as well as accumulation of energy reserves in the form of gonadal fat bodies (GFBs) and other fat deposits. During the second period (maturing), somatic growth slows as production and stored energy reserves are diverted into reproduction. In the third period, fish are allocating energy to both somatic growth and reproduction. Reproductive periodicity is typically close to two years for males and three years for females; gonadal recrudescence is slower than in older fish. The gonadosomatic index (GSI) is increasing, and GFBs are depleted over 2-3 spawns in females and reduced more gradually in males. Fish make longer pre-spawning migrations upriver: In the fourth period (prime reproduction), somatic growth is slow or negative, as energy is routed more strongly into reproduction. GSI is at a maximum; GFBs are completely depleted in females. Reproductive periodicity is typically one year for males and two years for females; the rate of gonadal recrudescence is at its maximum. Fish make shorter pre-spawning migrations upriver: In the fifth period (senescence to death), GSI of some of the oldest females decreases; the oldest males have few energy reserves and are long and lean. Distinct male female differences in life histories were consistent with theoretical models and with empirical observations of the lack of direct male competition for females. The framework outlined for this paddlefish stock may exist among other Acipenseriform fishes and is potentially useful for evaluating their life histories.
