Abstract
Pygmy sperm whales (Kogia breviceps) and dwarf sperm whales (Kogia sima) are deep diving cetaceans that commonly strand along the coast of the southeast US, but that are difficult to study visually at sea because of their elusive behavior. Conventional visual surveys are thought to significantly underestimate the presence of Kogia and they have proven difficult to approach for tracking and tagging. An approach is presented for density estimation of signals presumed to be from Kogia spp. based on passive acoustic monitoring data collected at sites in the Gulf of Mexico (GOM) from the period following the Deepwater Horizon oil spill (2010-2013). Both species of Kogia are known to inhabit the GOM, although it is not possible to acoustically separate the two based on available knowledge of their echolocation clicks. An increasing interannual density trend is suggested for animals near the primary zone of impact of the oil spill, and to the southeast of the spill. Densities were estimated based on both counting individual echolocation clicks and counting the presence of groups of animals during one-min time windows. Densities derived from acoustic monitoring at three sites are all substantially higher (4–16 animals /1000 km2) than those that have been derived for Kogia from line transect visual surveys in the same region (0.5 animals/1000 km2). The most likely explanation for the observed discrepancy is that the visual surveys are underestimating Kogia spp. density, due to the assumption of perfect detectability on the survey trackline. We present an alternative approach for density estimation, one that derives echolocation and behavioral parameters based on comparison of modeled and observed sound received levels at sites of varying depth.
Highlights
Pygmy sperm whales (Kogia breviceps) and dwarf sperm whales (Kogia sima) are deep diving cetaceans that are widely distributed in tropical and temperate waters worldwide (Jefferson et al, 2015)
Acoustic data were collected in the Gulf of Mexico (GOM) using multiple deployments of High-frequency Acoustic Recording Packages (HARPs) (Wiggins and Hildebrand, 2007) during and following the 2010 Deepwater Horizon oil spill
The signals from four Kogia sp. acoustic encounters captured during broadband recording (160 kHz) at site MC (Figure 3 and Table 1) are helpful for understanding how Kogia spp. signals might appear in recordings limited to 100 kHz bandwidth
Summary
Pygmy sperm whales (Kogia breviceps) and dwarf sperm whales (Kogia sima) are deep diving cetaceans that are widely distributed in tropical and temperate waters worldwide (Jefferson et al, 2015) They are typically encountered along the continental slope and in the abyssal plain (Baird et al, 1996; Baird, 2005). Based on stomach contents and isotope analysis, the two Kogia species may feed at different depths and on slightly different prey (Barros et al, 1998; Willis and Baird, 1998) Kogia may feed both in the water column and at or near the bottom. Their sightings are most frequently reported in water depths between 400 - 1000 m, they are seen in deeper waters (Baumgartner et al, 2001)
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