Capturing the Life Cycle of Pacific Salmon and Its Variations in a Single Modeling Framework

Pacific salmon use multiple habitats during their life cycle making it difficult to isolate how environmental conditions experienced during a particular life stage impact the growth, development and survival of an individual fish. It is even more difficult to determine how sublethal stresses experienced during a particular life stage will manifest later during in their life cycle. In theory, the direct effects of sublethal stresses on juvenile salmon in a river could be assessed and mitigated through water-management actions. But to better understand salmon population dynamics and evaluate management actions, we need mechanistic modeling tools that describe how stresses in one stage are expressed in later stages and in offspring.

We have developed the first full life cycle model of a Pacific salmon, from an egg to an adult female and its eggs, that can account for these sublethal effects with delayed consequences. Our model is developed in the framework of Dynamic Energy Budget (DEB) theory. To our knowledge, the standard DEB model is the simplest bioenergetic model that predicts development, growth and reproduction of an organism in a dynamic environment, while also linking the adult female state to egg traits. To reproduce the life cycle of a Pacific salmon, we supplement the standard DEB model with a limited number of assumptions on anadromy and semelparity.

We first show that the body-size scaling relationships implied by DEB theory capture most variations in life-history traits (egg size, fry size and fecundity) among five species of Pacific salmon: pink, sockeye, coho, chum and Chinook. These results are then used in a more detailed analysis for Chinook salmon, using a Bayesian approach for parameter inference. This model for Chinook allows us to study how variations in environmental conditions affect age and size of spawning adults. We reproduce the observation that fast growing individuals migrate back to the river to spawn at an earlier age and smaller size than slow-growing individuals. We finally discuss how the model can be further extended to describe the effects of oxygen stress on embryonic development and the growth of the youngest fish by coupling the DEB model to a high-resolution temperature model of the Sacramento River in the Central Valley of California. This modeling framework can be use by water and fisheries managers to optimize water allocations to protect endangered salmon.