Gauging the influence of increased search effort on reporting rates of bottlenose dolphin (Tursiops truncatus) strandings following the deepwater horizon oil spill.

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.
Competing Interests: Authors JLP, EEP, AH, AMA, and MS are employed by the Institute for Marine Mammal Studies (IMMS). Author MG is employed by NOVACES LLC who is contracted to Swift Worldwide Resources. Swift Worldwide Resources had a contract with BP to provide operational support for the Deepwater Horizon oil spill response and cleanup. BP provided Shoreline Cleanup Assessment Technique and other operations data used in the study. IMMS and MG were involved in the study design and write-up of the manuscript, and BP had an opportunity to review and comment on it prior to submission. BP did not have a role in the analysis of data or the decision to publish the paper, which was solely vested in IMMS. The findings and conclusions are those of the authors alone, and may not reflect the views of BP. There are no patents, products in development, or marketed products to declare. This does not alter the author’s adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors.