Review of historical unusual mortality events (UMEs) in the Gulf of Mexico (1990-2009): providing context for the multi-year northern Gulf of Mexico cetacean UME declared in 2010.

An unusual mortality event (UME) was declared for cetaceans in the northern Gulf of Mexico (GoM) for Franklin County, Florida, west through Louisiana, USA, beginning in February 2010 and was ongoing as of September 2014. The 'Deepwater Horizon' (DWH) oil spill began on 20 April 2010 in the GoM, raising questions regarding the potential role of the oil spill in the UME. The present study reviews cetacean mortality events that occurred in the GoM prior to 2010 (n = 11), including causes, durations, and some specific test results, to provide a historical context for the current event. The average duration of GoM cetacean UMEs prior to 2010 was 6 mo, and the longest was 17 mo (2005-2006). The highest number of cetacean mortalities recorded during a previous GoM event was 344 (in 1990). In most previous events, dolphin morbillivirus or brevetoxicosis was confirmed or suspected as a causal factor. In contrast, the current northern GoM UME has lasted more than 48 mo and has had more than 1000 reported mortalities within the currently defined spatial and temporal boundaries of the event. Initial results from the current UME do not support either morbillivirus or brevetoxin as primary causes of this event. This review is the first summary of cetacean UMEs in the GoM and provides evidence that the most common causes of previous UMEs are unlikely to be associated with the current UME.

Vulnerability to climate change of United States marine mammal stocks in the western North Atlantic, Gulf of Mexico, and Caribbean.

Climate change and climate variability are affecting marine mammal species and these impacts are projected to continue in the coming decades. Vulnerability assessments provide a framework for evaluating climate impacts over a broad range of species using currently available information. We conducted a trait-based climate vulnerability assessment using expert elicitation for 108 marine mammal stocks and stock groups in the western North Atlantic, Gulf of Mexico, and Caribbean Sea. Our approach combined the exposure (projected change in environmental conditions) and sensitivity (ability to tolerate and adapt to changing conditions) of marine mammal stocks to estimate vulnerability to climate change, and categorize stocks with a vulnerability index. The climate vulnerability score was very high for 44% (n = 47) of these stocks, high for 29% (n = 31), moderate for 20% (n = 22), and low for 7% (n = 8). The majority of stocks (n = 78; 72%) scored very high exposure, whereas 24% (n = 26) scored high, and 4% (n = 4) scored moderate. The sensitivity score was very high for 33% (n = 36) of these stocks, high for 18% (n = 19), moderate for 34% (n = 37), and low for 15% (n = 16). Vulnerability results were summarized for stocks in five taxonomic groups: pinnipeds (n = 4; 25% high, 75% moderate), mysticetes (n = 7; 29% very high, 57% high, 14% moderate), ziphiids (n = 8; 13% very high, 50% high, 38% moderate), delphinids (n = 84; 52% very high, 23% high, 15% moderate, 10% low), and other odontocetes (n = 5; 60% high, 40% moderate). Factors including temperature, ocean pH, and dissolved oxygen were the primary drivers of high climate exposure, with effects mediated through prey and habitat parameters. We quantified sources of uncertainty by bootstrapping vulnerability scores, conducting leave-one-out analyses of individual attributes and individual scorers, and through scoring data quality for each attribute. These results provide information for researchers, managers, and the public on marine mammal responses to climate change to enhance the development of more effective marine mammal management, restoration, and conservation activities that address current and future environmental variation and biological responses due to climate change.