Life history patterns of a livebearing fish in contrasting environments

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Abstract
The population dynamics and energy allocations of the gila topminnow, a small livebearing fish, were studied in 2 contrasting environments, a spring run of constant characteristics and a fluctuating desert wash. Topminnows grew and matured in 2 basic patterns. Many fish in both areas matured the year after their birth. Spring fish born early in the breeding season grew rapidly, bred within 5 mo. and died by 8 mo. of age. Although spring fish assimilated more energy, wash fish actually expended more calories for growth and reproduction, partly because of lower maintenance costs. Reproductive effort of long-lived spring fish varied with age between 3.1 and 65%, while efforts of short-lived spring and wash fish increased steadily with age to 5.2 and 9.8%, respectively. Although spring fish produced eggs of higher energy content, females in both areas varied their investment per offspring, apparently tracking seasonal changes in the availability of food for fry. When long-lived spring fish experienced food shortage, they allocated less energy to both growth and reproduction. Wash and short-lived spring fish under similar conditions reduced only their growth allocation. The reproductive mass in spring fish may be limited by food availability, incompletely filled the abdominal space and reflected no tradeoff between fecundity and investment per offspring. Reproduction by wash fish may be limited by body space and was characterized by a tradeoff between fecundity and egg size.

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