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

<div><p>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 (<i>Tursiops truncatus</i>) 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., <i>p</i> = 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 (<i>p</i> < 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.</p></div

Results of a Pearson Chi-Square goodness-of-fit test for selected geographic subgroups during the Active Response and Post-Removal Actions Deemed Complete time periods.

<p>Results of a Pearson Chi-Square goodness-of-fit test for selected geographic subgroups during the Active Response and Post-Removal Actions Deemed Complete time periods.</p

Gauging the influence of increased search effort on reporting rates of bottlenose dolphin (<i>Tursiops truncatus</i>) strandings following the deepwater horizon oil spill - Fig 3

<p>U-Charts displaying the temporal distribution of strandings in the northern Gulf of Mexico from Jan 1996 –Apr 2015 for a) Alabama Mobile County with all records, b) Alabama Mobile County with response-influenced records removed from the Active Response period, c) Alabama—Bon Secour to Orange Beach with all records, d) Alabama—Bon Secour to Orange Beach with response-influenced records removed from the Active Response period, e) Mississippi Barrier Islands with all records, and f) Mississippi Barrier Islands with response-influenced records removed from the Active Response period. Green lines represent the mean monthly stranding rate and red lines are three standard deviations above the mean.</p

Gauging the influence of increased search effort on reporting rates of bottlenose dolphin (<i>Tursiops truncatus</i>) strandings following the deepwater horizon oil spill - Fig 6

<p>Maps showing the results of Getis-Ord Gi* statistical tests on stranding records in Louisiana for the a) Active Response period from May 2010 –April 2014 and b) Active Response period excluding records determined to be influenced by responders from May 2010 –March 2014. Each map shows the Gi* Z-score classified into four categories: non-significant clustering and significant clustering at the α < 0.1, 0.05, and 0.01 levels. Also shown is the sample size for each temporal grouping and labels for Grand Isle (GI) and the Grand Terre Islands (GT).</p

Gauging the influence of increased search effort on reporting rates of bottlenose dolphin (<i>Tursiops truncatus</i>) strandings following the deepwater horizon oil spill - Fig 4

<p>U-Charts displaying the temporal distribution of strandings in the northern Gulf of Mexico from Jan 1996 –Apr 2015 for a) Louisiana Public/Private Beaches with all records, b) Louisiana Public/Private Beaches with response-influenced records removed from the Active Response period, c) Louisiana Remote areas with all records, and d) Louisiana Remote areas with response-influenced records removed from the Active Response period. Green lines represent the mean monthly stranding rate and red lines are three standard deviations above the mean.</p

Stranding observations in Alabama, Mobile County and Baldwin County—Bon Secour to Orange Beach, Louisiana (remote), Mississippi Barrier Islands, and Louisiana (public/private) from 2010 –April 2015.

<p>Green points are oil spill response-related observations and red points either did not specify the reporting party or specifically noted that they were not related to the oil spill response effort.</p

Distribution of stranding observations among observer parties during the Active Response period in the northern Gulf of Mexico 2010–2014.

<p>Distribution of stranding observations among observer parties during the Active Response period in the northern Gulf of Mexico 2010–2014.</p

Results of Comparison of Poisson Rates test used to compare stranding rates during the Active Response to the Pre-Spill time period for selected geographic groups.

<p>Results of Comparison of Poisson Rates test used to compare stranding rates during the Active Response to the Pre-Spill time period for selected geographic groups.</p

Monthly stranding rates (strandings per month) among geographic subgroups during the Pre-Spill, Active Response (all data), Active Response with responder-influenced records removed, and Post-Removal Actions Deemed Complete (RADC) periods in the northern Gulf of Mexico from Jan 1996 –April 2015.

<p>Monthly stranding rates (strandings per month) among geographic subgroups during the Pre-Spill, Active Response (all data), Active Response with responder-influenced records removed, and Post-Removal Actions Deemed Complete (RADC) periods in the northern Gulf of Mexico from Jan 1996 –April 2015.</p

Location of dolphin strandings from 1996–2015 that were used to analyze the influence of increased search effort associated with the Deepwater Horizon oil spill response in the northern Gulf of Mexico.

<p>Location of dolphin strandings from 1996–2015 that were used to analyze the influence of increased search effort associated with the Deepwater Horizon oil spill response in the northern Gulf of Mexico.</p