Investigating blue and fin whale populations through passive acoustic monitoring using ocean bottom seismometers

Abstract

Many large baleen whale species are endangered due to decades of commercial whaling before its international ban in the 1980s. Monitoring the recovery of their populations is difficult to study with traditional animal survey methods. Visual transect methods using ships and planes cover only small portions of baleen whales’ vast migratory ranges that span thousands of kilometers and are incomplete because they only record surfacing whales. GPS tagging studies are invaluable for studying behavior but are limited to small numbers due to cost. Passive acoustic monitoring (PAM), which detects whale presence through recording calls, is a complementary method that is not sight-limited and can provide long-term continuous data on a large spatial scale. Ocean bottom seismometers (OBSs), geophysical instruments used to study earthquakes through recording ground motion, are opportunistic PAM sensors that record large baleen whale calls. There are large amounts of publicly available OBS data, but these datasets have so far been underutilized as tool to study whales. This dissertation develops new methodologies for PAM using OBSs and highlights their utility for characterizing populations and studying behaviors of fin and blue whales. In Chapter 2, I develop, implement, and evaluate a method for ranging to fin whales using multipath reflected arrivals of calls on solitary OBSs deployed in different regions of the abyssal ocean. I demonstrate how to account for various site properties and discuss how the method might be implemented in different environments. In Chapter 3, I employ the multipath ranging method and point-transect distance sampling methods to estimate fin whale call densities using OBSs deployed in the Marianas region. This chapter demonstrates the utility of single-OBS ranging for estimating populations through PAM. In Chapter 4, I employ a previously developed acoustic localization algorithm to track calling northeast Pacific (NEP) blue whales with a pair of large OBS networks off the Pacific Northwest coast. I characterize the behavior of an understudied subpopulation of NEP blue whales that skip southward migration to investigate how they are using the temperate habitat while they overwinter. Behavioral metrics indicate that overwintering whales likely forage off the coast of Washington and Vancouver Island throughout the fall and early winter.