Were Multiple Stressors a ‘Perfect Storm’ for Northern Gulf of Mexico Bottlenose Dolphins (Tursiops truncatus) in 2011?

Abstract

An unusual number of near term and neonatal bottlenose dolphin (Tursiops truncatus) mortalities occurred in the northern Gulf of Mexico (nGOM) in 2011, during the first calving season after two well documented environmental perturbations; sustained cold weather in 2010 and the Deepwater Horizon oil spill (DWHOS). Preceding the stranding event, large volumes of cold freshwater entered the nGOM due to unusually large snowmelt on the adjacent watershed, providing a third potential stressor. We consider the possibility that this extreme cold and freshwater event contributed to the pattern of perinatal dolphin strandings along the nGOM coast. During the 4-month period starting January 2011, 186 bottlenose dolphins, including 46% perinatal calves (nearly double the percentage for the same time period from 2003–2010) washed ashore from Louisiana to western Florida. Comparison of the frequency distribution of strandings to flow rates and water temperature at a monitoring buoy outside Mobile Bay, Alabama (the 4th largest freshwater drainage in the U.S.) and along the nGOM coast showed that dolphin strandings peaked in Julian weeks 5, 8, and 12 (February and March), following water temperature minima by 2–3 weeks. If dolphin condition was already poor due to depleted food resources, bacterial infection, or other factors, it is plausible that the spring freshet contributed to the timing and location of the unique stranding event in early 2011. These data provide strong observational evidence to assess links between the timing of the DWHOS, other local environmental stressors, and mortality of a top local predator. Targeted analyses of tissues from stranded dolphins will be essential to define a cause of death, and our findings highlight the importance of considering environmental data along with biological samples to interpret stranding patterns during and after an unusual mortality event.

Fetal distress and in utero pneumonia in perinatal dolphins during the Northern Gulf of Mexico unusual mortality event

“…During and following the oil spill, elevated levels of polycyclic aromatic hydrocarbons (PAHs) were detected in these coastal environments (Allan et al 2012). Exposure to DWH oil spill-associated contaminants has been previously proposed as one of multiple potential contributing factors to the 2011 cluster of perinatal deaths in Mississippi and Alabama (Carmichael et al 2012). Given that the high prevalence of brucellosis and greatest increases in perinatal strandings were not apparent until after the oil spill, there may have been oil-associated factors that led to an increase in failed pregnancies, one of which may have included altered maternal immune system function resulting in increases in illnesses.…”

“…The largest and longest cetacean unusual mortality event (UME) on record in the Gulf of Mexico (GoM) was declared from Louisiana through the Florida panhandle (Franklin County), USA, starting in early tion of the stranded dolphins in Mississippi and Alabama were perinates (<115 cm body length) (Carmichael et al 2012, Venn-Watson et al 2015a. Although perinatal bottlenose dolphin strandings typically increase in the northern GoM from January through April every year due to peaks in dolphin calving, the proportion of perinatal strandings during 2011 was elevated above baseline (Worthy 1998, Mattson et al 2006, Pitchford et al 2013, Litz et al 2014, Venn-Watson et al 2015a).…”

Effects of fluctuations in sea-surface temperature on the occurrence of small cetaceans off Southern California

“…Such trends could indicate a poleward shift in range for both species (MacLeod et al, 2005;Simmonds and Isaac, 2007). In addition, an infl ux of cold freshwater in the northern Gulf of Mexico in 2011 may have contributed to an unusually high mortality rate in bottlenose dolphins (Charmichael, et al 2012).…”

Low reproductive success rates of common bottlenose dolphins Tursiops truncatus in the northern Gulf of Mexico following the Deepwater Horizon disaster (2010-2015)

“…Barataria Bay dolphins also showed reproductive failure rates in the first years after the spill 4 times greater than those from a non-oiled stock (Lane et al 2015). The stranding record for some areas also shows indications of impaired reproduction/early life survival, with periods of excessively high rates of perinatal strandings (Carmichael et al 2012, Litz et al 2014, VennWatson et al 2015b.…”