Towing the line: Line-transect based density estimation of whales using towed hydrophone arrays

Towed hydrophone arrays have been used to monitor marine mammals from research vessels since the 1980s. Although towed hydrophone arrays have now become a standard part of line-transect surveys of cetaceans, density estimation exclusively using passive acoustic has only been attempted for a few species. We use examples from four acoustic line-transect surveys that we conducted in the North Pacific Ocean to illustrate the steps involved, and issues inherent, in using data from towed hydrophone arrays to estimate densities of cetaceans. We will focus on two species of cetaceans, sperm whales and minke whales, with examples of beaked whales and other species as needed. Issues related to survey design, data-collection, and data analysis and interpretation will be discussed using examples from these studies. We provide recommendations to improve the survey design, data-collection methods, and analyses. We also suggest areas where additional research and methodological development are required in order to produce robust density estimates from acoustic based data.