Persistence of avian carcasses on sandy beaches and marsh edges in the northern Gulf of Mexico.

As part of the natural resource damage assessment for the Deepwater Horizon oil spill, a mathematical model was used to estimate the total number of bird carcasses deposited on shorelines based on the number of carcasses collected and adjustment factors such as detection probability and carcass persistence. Studies of carcass persistence occurred along sandy beaches and marsh edges in the northern Gulf of Mexico to obtain site-specific inputs for the model. We estimated persistence rates for these habitat types and evaluated the influence on persistence of carcass size, location of the carcass on the beach, dominant vegetation type in the marsh, carcass distance into marsh vegetation, and length of time a carcass was stranded on a shoreline. The length of time stranded had the greatest influence on persistence in both habitat types, with persistence initially relatively low and increasing logarithmically. Carcass size and position were weakly influential on sandy beaches. Carcass size had stronger influences along marsh edges, and marsh habitat type also affected persistence. We found evidence of a positive relationship between distance into the marsh and persistence during the first 24 h after carcass deployment.

Detection probabilities of bird carcasses along sandy beaches and marsh edges in the northern Gulf of Mexico.

We estimated detection probabilities of bird carcasses along sandy beaches and in marsh edge habitats in the northern Gulf of Mexico to help inform models of bird mortality associated with the Deepwater Horizon oil spill. We also explored factors that may influence detection probability, such as carcass size, amount of scavenging, location on the beach, habitat type, and distance into the marsh. Detection probability for medium-sized carcasses (200-500 g) ranged from 0.82 (SE = 0.09) to 0.93 (SE = 0.04) along sandy beaches. Within sandy beaches, we found that intact/slightly scavenged carcasses were easier to detect than heavily scavenged ones and did not find strong effects of location on the beach on detection probability. We estimated detection rate for each combination of scavenging state, carcass size, and position along sandy beaches. In marsh edge habitats, detection ranged from 0.04 (SE = 0.04) to 0.86 (SE = 0.10), with detection rates rapidly increasing from small (< 200 g) to medium carcass sizes and leveling off between medium and extra-large (> 1000 g) carcasses regardless of vegetation type (Spartina or Phragmites). Carcasses of all sizes were generally harder to locate in Spartina-dominated marshes than in Phragmites-dominated ones. A subset of the data for which we could adequately assess the effect of distance into the marsh indicated that detection rates generally declined the farther a carcass was into marsh vegetation. Based on power analyses, our ability to identify predictors that influence detection rates would be higher with larger numbers of carcasses, greater numbers of search trials per carcass, or more balanced sampling distributions across predictor values.

Evaluation of the fate of carcasses and dummies deployed in the nearshore and offshore waters of the northern Gulf of Mexico.

In response to the Deepwater Horizon oil spill, federal and state agencies conducted field studies to develop inputs for a shoreline deposition model used to estimate nearshore avian mortality resulting from the spill. A 2011 carcass drift study was designed to generate data on the likelihood that birds that died on the water would deposit along the northern Gulf of Mexico coast (rather than becoming lost at sea). In the case of the Deepwater Horizon oil spill, carcass losses at sea accounted for a significant portion of nearshore avian mortality. We evaluate the data collected during the Deepwater Horizon carcass drift study and compare the results obtained from the use of avian carcasses versus dummy carcasses (dummies) and the differences between those deployed nearshore versus further offshore. We conclude that, although the use of dummies provided valuable confirmation on the drift patterns of dead birds, dummies drifted greater distances, for longer periods of time, and were more likely to be observed beached compared to avian carcasses, with 64.6% of dummies beaching compared to 17.2% of carcasses. In response to future spills, researchers should account for these potential biases when incorporating dummy drift data into estimates of avian carcass loss. Further, none of the avian carcasses and dummies released more than 40 km from the shoreline made it to shore. In the northern Gulf of Mexico, carcasses that die on the waters farther offshore are unlikely to make it to shore to be captured in a deposition model; therefore, it may be appropriate to utilize a separate methodology to estimate offshore mortality. The applicability of these results to other spill events should be evaluated in the context of the specific spill characteristics.