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.
, and the interns who assisted with data collection. We extend special thanks to veterinarian Debra Moore for offering helpful counsel during the assessment of the malformations observed, and Eric Pulis for providing constructive comments on previous drafts of this manuscript. All field research was conducted under NOAA, National Marine Fisheries Service Permit GA LOC #18185. We would also like to thank the IMMS for funding these research efforts.
Bottlenose dolphins’ (Tursiops truncatus) foraging strategies in the Biloxi Marsh have received little attention when compared with other comparable habitats in the southeastern United States. Previous reports of dolphin foraging in this region have included sophisticated strategies such as strand feeding but have not included a detailed analysis of the observed behavior. Dolphins were observed performing a unique solitary foraging strategy, termed marsh bank feeding. Although this behavior was observed to occasionally result in stranding upon the marsh bank, it is argued here that the observed behavior is fundamentally different than the definitions for strand feeding, as well as other prominent shallow water foraging strategies. Video footage resulting from observations made between 2013 and 2019 were analyzed in order to establish the components of the behavior, enabling a detailed comparison to other dolphin foraging strategies.
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