University of Washington Seattle, Washington, United States
Abstract: Understanding the mechanisms that regulate marine survival of salmon has been a challenge for well over a half century. Observational, statistical, and model studies all point to growth and predation being central to population regulation. The consensus, encapsulated in a critical period theory, infers that early period growth is a main determinant of marine survival. In the mechanism, larger and faster growing juveniles reduce their predator exposure by outgrowing the size of gape-limited predators. Conversely smaller, slower growing, counterparts have longer exposures and therefore experience lower survival.
Numerous studies support this scenario but this qualitative description does not explain the complex observations in which the correlation of size, growth and adult survival varies across species and may be absent or opposite in some. Mathematical models describe the growth versus predation dynamic in terms of a prey growing through uniform-sized gape-limited and gape-unlimited predators fields. But such models are not easily fit to observations. Fishery scientists have opined a deeper mechanistic understanding of the processes that control salmon mortality is needed.
This talk introduces a step toward a better mechanistic framework by including a dimension missing from existing theory: the predator size distribution. In this framework prey growing through a distribution of predator sizes experience a continuously declining mortality rate as the fraction of the predators able to consume the prey declines with increasing prey size.
This model presents a complete curve of marine survival of salmon from ocean entrance to adult return by combining growth rate and initial size with the predator size distribution and encounter frequencies with gape-limited and gape-unlimited predators. The 6-parameter model fits data and provides an integrated mechanistic explanation for three diverse datasets of Columbia River Chinook salmon: smolt-to-adult ratio (SAR) vs smolt entrance size; interannual variations of SAR with the pacific decadal oscillation; and the pattern of early marine residence survival.
These three datasets are regulated by the predator size distribution and the encounter frequency with gape-limited predators. Contradicting the existing critical period theory, growth rate is of secondary importance suggesting that increased productivity may act indirectly on salmon by shifting predation to other forage fish population.
An ecologically relevant take home message is that the predator size distribution is an important dimension of predator-prey dynamics that can readily be included in models and estimated directly and indirectly through data.