Multi-modal surveys of fin whales in the Sea of Cortez, Mexico

Abstract

A population of fin whales (Balaenoptera physalus), resident to the Gulf of California, Mexico, was studied over two seasons using an integrated approach. Systematic vessel-based visual survey and photo-ID efforts were conducted every 5–7 days to independently estimate the number and distribution of whales within a 10×30 mi2 study area. Some whales were tagged with time-depth-recorders. Sets of 5–6 distributed autonomous seafloor acoustic recorders, operating continuously during each season’s research period, were used to detect, locate, and track vocalizing whales. A 16-element towed array tracked individual vocal whales in real-time concurrently with visual observations, allowing biopsy samples of known vocal animals. Active acoustics was used to collect data on the density and distribution of krill so as to place measured variation in whale numbers, distribution, and behavior within an ecological context. The primary whale activity was feeding. Whale feeding patterns and survey distribution followed prey distribution. Vocal whale distribution followed dial feeding patterns and prey distribution. All biopsied vocal animals were males. Numbers of whales estimated by vessel survey, photo-ID, and passive acoustics were correlated. Results suggest that under certain conditions, vocal activity is a reliable measure of distribution and relative abundance. [Work supported by ONR.]