Erik B. Muller, Konstadia Lika, Roger M. Nisbet, Irvin R. Schultz, Jérôme Casas, André Gergs, Cheryl A. Murphy, Diane Nacci, Karen H. Watanabe

Female rainbow trout in experimental holding tank. Picture by Irvin Schultz.
Female rainbow trout in experimental holding tank. Picture by Irvin Schultz.

Organisms need energy and nutrients for sustenance, growth, development and reproduction. In bioenergetics, we aim to find quantitative descriptions that relate those needs to the acquisition of energy and nutrient resources (e.g. feeding in animals). Those relationships depend on environmental conditions, such as food availability, temperature and pollution levels. They also depend on the state of the organism, such as its size and life stage. Although organisms vary enormously in their resource and dietary requirements, tolerance for environmental stress, reproductive strategy, body size, life span, and so on, they all need to acquire resources to cover those basic needs. This universal principle underpins mathematical bioenergetics models. Models that are applicable to many species are particularly important, e.g., for environmental risk assessment, since it is infeasible to develop models for each species and stressor separately.

In this paper, we aim to develop mathematical models that describe how a female rainbow trout allocates resources to the production of eggs during her annual reproductive cycle. Eggs develop throughout most the year, but the bulk of egg mass is formed during the last two or three months before spawning. The trout in our study were fed daily, implying that the fraction of resources allocated to egg formation changed over time. This allocation is under the control of hormones, in particular estradiol. Our models describe how the amount of egg mass already present modulates the estradiol levels in blood plasma and how this, in turn, determines the allocation of resources to reproduction (and other basic destinations, such as growth).

Models describing the reproductive effort of this common and commercially important species have intrinsic value, and the inclusion of hormonal control of resource allocation is crucial for evaluating the impact endocrine disruptors may have on reproduction (endocrine disruptors are pollutants that disturb hormonal regulation mechanisms). Since the postulated control mechanisms are designed to be generic, the models are potentially relevant for a wide range of species, and we verified that our models could describe growth and reproductive patterns in a species very different from rainbow trout, namely the common bean, an annual plant that grows, sets seed and then dies.

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