A Novel Method for Estimating Cetacean Calf Survival Rate From Stranding Records

ABSTRACT

Calf survival is an important demographic parameter, but it can be difficult to estimate for baleen whale species that are infrequently seen with calves. We propose a new method for calculating calf survival by comparing the calf: total ratio in strandings to that expected from an age‐structured population model. Assuming a population with a stable age structure, we estimated a posterior distribution for calf survival using a prior on calf survival and the assumption that observed calf strandings are binomially. We applied this method for estimating calf survival to the last 50 years of blue whale strandings that included 40 calves and 144 non‐calves for records with high species certainty. We estimated a calf survival rate of 0.654 year ⁻¹ (95% credibility interval 0.566–0.736), which is comparable to estimates for other baleen whales. Simulations demonstrate that the method is unbiased and that our dataset is large enough to provide precise estimates of calf survival. Estimates depend on the proportion of calves observed; in more recent years, with higher observed number of calves, survival estimates lower while lower observed number of calves lead to higher estimates. The method can be applied to other marine mammal species with available stranding and demographic data.