Demographic Clusters Identified within the Northern Gulf of Mexico Common Bottlenose Dolphin (Tursiops truncates) Unusual Mortality Event: January 2010 - June 2013

Abstract

A multi-year unusual mortality event (UME) involving primarily common bottlenose dolphins (Tursiops truncates) was declared in the northern Gulf of Mexico (GoM) with an initial start date of February 2010 and remains ongoing as of August 2014. To examine potential changing characteristics of the UME over time, we compared the number and demographics of dolphin strandings from January 2010 through June 2013 across the entire GoM as well as against baseline (1990-2009) GoM stranding patterns. Years 2010 and 2011 had the highest annual number of stranded dolphins since Louisiana’s record began, and 2011 was one of the years with the highest strandings for both Mississippi and Alabama. Statewide, annual numbers of stranded dolphins were not elevated for GoM coasts of Florida or Texas during the UME period. Demographic, spatial, and temporal clusters identified within this UME included increased strandings in northern coastal Louisiana and Mississippi (March-May 2010); Barataria Bay, Louisiana (August 2010-December 2011); Mississippi and Alabama (2011, including a high prevalence and number of stranded perinates); and multiple GoM states during early 2013. While the causes of the GoM UME have not been determined, the location and magnitude of dolphin strandings during and the year following the 2010 Deepwater Horizon oil spill, including the Barataria Bay cluster from August 2010 to December 2011, overlap in time and space with locations that received heavy and prolonged oiling. There are, however, multiple known causes of previous GoM dolphin UMEs, including brevetoxicosis and dolphin morbillivirus. Additionally, increased dolphin strandings occurred in northern Louisiana and Mississippi before the Deepwater Horizon oil spill. Identification of spatial, temporal, and demographic clusters within the UME suggest that this mortality event may involve different contributing factors varying by location, time, and bottlenose dolphin populations that will be better discerned by incorporating diagnostic information, including histopathology.

Survival, density, and abundance of common bottlenose dolphins in Barataria Bay (USA) following the Deepwater Horizon oil spill

“…The final 'Inter-primary' estimate is considered unreliable due to confounding factors (see 'Results'), and final 'Interval' estimate should be viewed with caution from 1980 to 1987 (Wells & Scott 1990). However, it was not surprising that we found lower survival rates in Barataria Bay dolphins given that an unusual mortality event (UME) in the northern GoM overlapped our study period (Litz et al 2014, Venn-Watson et al 2015. A UME as defined under the Marine Mammal Protection Act (116 USC 1421h) can be declared based on a number of criteria.…”

“…While the UME was declared for the broader northern GoM (Frank lin County, FL, to the Louisiana− Texas border), dolphin strandings were particularly high in Barataria Bay. In fact, Louisiana recorded the highest stranding rates on record between the April 2010 DWH spill and December 2011 (Venn-Watson et al 2015), and a large portion of these strandings were recovered in and around Barataria Bay. The number of dolphin strandings decreased in 2014 and the UME officially ended in July 2014.…”

The Deepwater Horizon oil spill marine mammal injury assessment

“…From 2010 to 2014, the NOAA tracked a cetacean unusual mortality event (UME) in the nGoM (NOAA 2016). In reviewing the UME data, scientists identified several distinct clusters of strandings (Litz et al 2014, Venn-Watson et al 2015b): a cluster of deaths resulting from cold temperatures and low salinity in Lake Pontchartrain, Louisiana and western Mississippi Sound in early 2010; a large number of deaths in southern Louisiana (centered on Barataria Bay) from 2010 to 2011 and a cluster with unusually high numbers of perinates in Mississippi and Alabama in 2011, both of which were attributed to DWH oil (see below); and a Gulf-wide cluster in early 2013 (the authors did not opine on the cause of the 2013 cluster because of insufficient data; Venn-Watson et al 2015b).…”

“…Scientists conducting necropsies found a high prevalence of lung and adrenal lesions in dead dolphins within the DWH oil spill footprint (Venn-Watson et al 2015a). Combined evidence from statistical analysis of the strandings clusters (Venn-Watson et al 2015b), necropsy data from the strandings (Venn-Watson et al 2015a, Colegrove et al 2016, and findings from live health assessments conducted in Barataria Bay following the spill (Schwacke et al 2014 linked these adverse health effects to exposure to the DWH oil after examining and ruling out other potential causes.…”

Assigning stranded bottlenose dolphins to source stocks using stable isotope ratios following the Deepwater Horizon oil spill

“…Given the large number of strandings following the DWH oil spill (Litz et al 2014, Venn-Watson et al 2015b), a subset of available skin samples from stranded animals was selected for stable isotope analysis using several criteria: (1) the strandings oc curred after the beginning of the DWH oil spill (on or after 20 April 2010), (2) the stranding occurred within the geographic range most affected by the DWH oil spill (Louisiana to the western Panhandle of Florida; Michel et al 2013, DWH NRDAT 2016, (3) the total length of a stranded animal was >170 cm to exclude perinatal and young of the year (Fernandez & Hohn 1998) to avoid ontogenetic effects (Knoff et al 2008, Riccialdelli et al 2013, and (4) …”