Aim: To develop a more ecologically realistic approach for estimating the population

size of cetaceans and other highly mobile species with dynamic spatial distributions.

Location: California Current Ecosystem, USA.

Methods: Conventional spatial density models assume a constant relationship between

densities and habitat covariates over some time period, typically a survey season.

The estimated population size must change whenever total habitat availability

changes. For highly mobile long-lived species, however, density–habitat relationships

likely adjust more rapidly than population size. We developed an integrated population-redistribution model based on a more ecologically plausible alternative hypothesis: (1)

population size is effectively constant over each survey season; (2) if habitat availability

changes, then the population redistributes itself following an ideal free distribution

process. Thus, the estimated relationship between densities and habitat covariates

adjusts rather than population size. We constructed Bayesian hierarchical models corresponding

to the conventional and alternative hypotheses and applied them to distance

sampling data for Dall’s porpoise (Phocoenoides dalli), a highly mobile cetacean

with distribution patterns closely tied to cool sea-surface

temperatures.

Results: The Dall’s porpoise data provided strong support for the hypothesis based on

an ideal free redistribution process. Our results indicate that the population size of

Dall’s porpoise within the survey region was relatively stable over each summer/fall

survey season, but the distribution expanded and contracted with the extent of suitable

habitat. Over multiple survey seasons, the model partitioned variation in observed

densities among three sources: variation in population size, the density–habitat relationship

and measurement error, leading to lower and more ecologically plausible estimates

of interannual variation in population size.

Main conclusions: We conclude that the integrated population-redistribution

model (IPRM) presented here represents an ecologically plausible model for use in future assessments of the population size and dynamics of cetaceans and other highly mobile

long-lived species with variable spatial distributions.

Keywords: Bayesian hierarchical model, California Current, Dall’s porpoise, distance sampling, habitat model, spatial density model

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