Modeling population status and demographic rates of baleen whales using historical whaling data

Abstract

Commercial whaling in the 20th century decimated the populations of many baleen whale species. Blue whales (Balaenoptera musculus) were especially impacted, with multiple populations whaled to near extinction, resulting in their current listing globally as Endangered by the IUCN. During whaling, extensive biological data were collected that, when combined with contemporary statistical methods, can be used to answer long-standing questions about baleen whale demography and population status. In this dissertation, I use historical data from whaling and contemporary Bayesian statistical methods to model baleen whale population dynamics and demography, with a particular focus on the highly exploited blue whale. In Chapter 1, I used a multi-state mark-recovery model to investigate population structure in Antarctic blue whales using historical mark-recovery data and found that they move frequently in the Southern Ocean, suggesting they are a single well-mixed circumpolar population. In Chapter 2, using the extensive fetal sex ratio data collected during whaling, I investigated ecological theories about adaptive sex ratio behavior, finding that longer rorqual (family Balaenopteridae) whale mothers have more female offspring. This suggests there is an advantage to being large that larger mothers pass on their daughters, likely stemming from the high costs of gestation and lactation for female baleen whales. In Chapter 3, I combined historical catch data and contemporary abundance estimates to build a population assessment model for Antarctic blue whales, finding that at the end of whaling they were at just 0.2% of pre-whaling levels, although their population size is currently increasing. Despite increasing, they are currently at less than 2% of pre-whaling levels so they still have many decades before they recover from whaling. In Chapter 4, using a global compilation of blue whale aging and reproductive data, I modeled age-length relationships and estimated reproductive rates for Antarctic, pygmy, and eastern North Pacific blue whales, finding that asymptotic lengths were longer for females than males across subspecies, and that Antarctic blue whales were the largest while pygmy blue whales were the smallest. In addition, I estimated pregnancy rates and age of sexual maturity for female eastern North Pacific and pygmy blue whales and, in Chapter 5, estimated natural survival for pygmy blue whales. These projects provide a deeper understanding of blue whale population dynamics and demography and lay the groundwork for age-structured stock assessments of blue whales.