Seasonal movements of Gulf of Mexico sperm whales following the Deepwater Horizon oil spill and the limitations of impact assessments.

As part of the Deepwater Horizon Oil Spill Natural Resource Damage Assessment in the Gulf of Mexico, we conducted a large passive acoustic survey across the eastern Gulf continental shelf edge to assess impacts to sperm whale population. In the months immediately after the spill, sperm whale occurrence was significantly higher in areas closest to the spill. Over the following seasons in 2010-2011, we documented cyclical patterns of decreased and increased occurrence suggesting that this population exhibits a seasonal occurrence pattern in the region, with seasonal movements to other regions, and not likely directly influenced by the oil spill. Unfortunately, a lack of adequately scaled, pre-spill data on sperm whales, along with limitations on the survey duration constrain our ability to infer spill-related changes in sperm whale occurrence. However, our study establishes post-disaster baseline data for continued monitoring, and an expanded study design could provide a model for continued monitoring.

Potential Bryde's whale (Balaenoptera edeni) calls recorded in the northern Gulf of Mexico.

Several marine autonomous recording units (MARUs) were deployed in northeastern Gulf of Mexico from 2010­2012 to study the acoustic ecology of Bryde's whales (Balaenoptera edeni) following the Deepwater Horizon oil spill. However, the acoustic repertoire of this sub-population is poorly documented, presently limiting the efficacy of acoustic monitoring applications. Numerous stereotyped, low-frequency signals from a putative biological sound source were found throughout the recordings. Sounds fell into three categories distinguished by spectral and temporal properties. Multiple calls overlapped temporally on individual MARUs, suggesting that multiple sources produced these sounds. The basic features are similar to those from other mysticetes, but they differ from any previously published sounds. Since Bryde's whales are the most common mysticete in the Gulf and have previously been observed within the recording area on multiple occasions, it is likely that Bryde's whales are the most probable source of these sounds. These results potentially identify a suite of previously undocumented calls from Bryde's whales, which could facilitate future passive acoustic monitoring efforts to better understand the population dynamics and status of this sub-population.