How Fast Do Whale Populations Grow? Investigating the Plausibility of Humpback Whale Rates of Increase from Life-history Data
The rate at which biological populations grow is a quantity of considerable interest in conservation and management and is often used in assessments of the status and future of wildlife populations. During the past several years, the rate at which large-whale populations grow has been a matter of debate in the Scientific Committee (SC) of the International Whaling Commission (IWC) because many scientists have questioned the plausibility of some reported rates of increase (ROIs); some are in excess of 15% per year or even as high as 30%! These ROIs were estimated from time series of abundance estimates and were proposed for use in the IWC’s comprehensive assessments of large-whale populations.
The ROI of any population of animals is fundamentally constrained by biological parameters such as survival, age at first parturition, and birth rate. Therefore, the combination of empirical knowledge of these parameters and use of population models provides an opportunity to assess the plausibility of estimates of ROI from time series of abundance estimates.
Previous attempts by the IWC to assess the maximum rates at which humpback whale (Megaptera novaeangliae) populations grow used theoretical values for biological parameters or, because of the lack of empirical estimates, made unrealistic assumptions for certain parameter values. For example, the calf (age 0-1 year) survival probability, which in mammal species is generally lower than that of older age classes, was assumed to be equal to the non-calf survival probability. In addition, no consideration was given to incorporating uncertainty (measurement error) in parameter estimates when computing plausible rates of population growth. Results of previous studies suggested that the maximum ROI in humpback whales was as high as 14.8% per year.
In our paper, Zerbini et al. (2010), recently published in the journal Marine Biology, we recomputed plausible rates of population growth for humpback whales. This study, developed by the National Marine Mammal Laboratory’s Cetacean Assessment and Ecology Program, was based on a review of existing estimates of humpback whale life-history parameters worldwide. The most optimistic estimates, along with estimates of their sampling variability, were used as input parameters in a population model, and a distribution of maximum plausible ROIs was computed using Monte Carlo methods. The most favorable scenario resulted in a mean ROI estimate of 8.6% per year (95% CI = 5.0–11.4%/year).
In our paper, we propose that the upper 99% quantile of the simulated ROI distribution (= 11.8%/year) be established as the maximum plausible ROI for humpback whales, and we recommend that this value be used to ground truth estimates of rates of population growth from time series of abundance estimates. We also recommend this maximum value be established as an upper boundary for growth-rate parameters in population models for humpback whales. Finally, we discuss possible sources of positive and negative biases in the existing estimates of life-history parameters and conclude that it is difficult to evaluate how these biases could affect estimates of ROI without additional data. The methods described in this study are relatively straightforward and can be applied to other species for which life-history parameters are available.
