Abstract
The co-occurrence of the Deepwater Horizon oil spill and the northern Gulf of Mexico cetacean Unusual Mortality Event have raised questions about the stability of inshore bottlenose dolphin (Tursiops truncatus) populations throughout the region. Several factors could have contributed to the ongoing event, but little attention has been paid to the potential effects of increased search effort and reporting of strandings associated with oil spill response activities, which were widespread for an extended period. This study quantified the influence of increased search effort by estimating the number of bottlenose dolphin strandings reported by oil spill responders and comparing monthly stranding rates with and without response-related records. Results showed that response teams reported an estimated 58% of strandings during the Active Response period within the study area. Comparison of Poisson rates tests showed that when responder-influenced stranding records were removed, the monthly stranding rates from the Active Response period (May 2010 –April 2014) were similar to the Post-Removal Actions Deemed Complete period (May 2013 –March 2015) (e.g., p = 0.83 for remote areas in Louisiana). Further, analyses using the Getis-Ord Gi* spatial statistic showed that when response-related stranding reports were removed from the Active Response period, significant spatial clustering of strandings (p < 0.05) was reduced by 48% in coastal Louisiana. Collectively, these results suggest that increased search effort resulting from the Deepwater Horizon oil spill response throughout remote portions of the Unusual Mortality Event geographic region had the capacity to increase reporting and recovery of marine mammal strandings to unusually high levels. To better understand how stranding data relates to actual mortality, more work is needed to quantify dolphin population size, population trends, and carcass detection rates including the role of search effort. This is vital for understanding the status of a protected species within the northern Gulf of Mexico.
Highlights
The longest running significant mortality event ( known as unusual mortality event (UME)) of marine mammals on record in the northern Gulf of Mexico occurred from March 1, 2010 to July 31, 2014 and included 1,141 cetacean strandings over a region extending from the western border of Louisiana to Franklin County, FL [1]
This area was within the geographic boundaries of the northern Gulf of Mexico (nGOM) UME that was declared by the National Oceanic and Atmospheric Administration (NOAA) National Marine Fisheries Service (NMFS) that began in March, 2010 [5]
The results presented here provide evidence to suggest that increased search effort during the Deepwater Horizon (DWH) oil spill, search effort associated with the DWH response, increased the frequency and the spatial and temporal extent of dolphin stranding reports in the nGOM
Summary
The longest running significant mortality event ( known as unusual mortality event (UME)) of marine mammals on record in the northern Gulf of Mexico (nGOM) occurred from March 1, 2010 (start date declared in December 2010) to July 31, 2014 (ending date declared in April 2016) and included 1,141 cetacean strandings over a region extending from the western border of Louisiana to Franklin County, FL [1]. This UME was the longest on record and the geographic extent was larger than the majority of declared UMEs [2]. The role of increased search effort has been acknowledged as a potential factor that could increase carcass recovery rates [10,13], this has not been formally studied in the Gulf of Mexico since the DWH oil spill
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