Preassessment Data Report #6: Detection probabilities for bird carcasses on beaches of Unalaska Island, Alaska, following the wreck of the M/V Selendang Ayu

Bird carcass surveys were conducted as part of the pre-assessment phase of response to the M/V Selendang Ayu oil spill off the coast of Unalaska Island. The probability a survey team would detect an oiled carcass present on the beach was expected to be less than 100%. To estimate the detection probability a field study was conducted in which two survey teams each made two passes on a number of beaches. The probability of a single team detecting an oiled carcass on a single pass was then estimated using standard mark-recapture models. Less than 50% of all carcasses present on a beach segment were detected in a single pass by a survey team. The relatively low detection probability and low persistence rates (Byrd and Reynolds 2006b) indicate that only a very small proportion of all deposited carcasses were counted during beach searches on Unalaska Island. INTRODUCTION On 8 December 2004, the M/V Selendang Ayu ran aground and broke in half in rough seas off Unalaska Island, Alaska (53°38'N, 167° 07'W). An estimated 354,218 gallons of oil (339,538 gallons of bunker oil [IFO 380] and 14,680 gallons of marine diesel and miscellaneous oils) were discharged. Numerous bird carcasses were recovered during beach searches in the months following the spill as part of the typical pre-assessment phase of response to an oil spill, which includes collecting data from which to estimate the total number of animals affected by the event (Ford et al., 1987; Page et al., 1990; Piatt et al., 1990; Burger, 1992a, Flint et al. 1999). Recovered carcasses represent only a fraction of the total number deposited on beaches (Flint et al. 1999) Carcasses found by observers on a specific beach at any given time after a mortality event are a result of three processes: (1) the rate at which carcasses are deposited on beaches, (2) their persistence once on the beach, and (3) the probability of observers detecting them when surveying the beach (Flint et al. 1999). These components vary depending on site and environmental characteristics present in each specific mortality event. For example, carcass deposition rate can vary with factors such as beach type (Bodkin and Jameson, 1991, Flint et al. 1999), persistence rates can vary with factors such as beach type, time since deposition, weather, tidal activity, carcass size and scavenger activity (Bodkin and Jameson, 1991; Burger, 1992b, 1993; Van Pelt and Piatt, 1995, Ford et al. 1996, Fowler and Flint 1997, Ford in press), and detection probabilities can vary with factors such as beach type and weather conditions like presence or absence of snow (Fowler and Flint 1997). Detection probability is specifically defined as the probability that a carcass known to be on the beach will be found by a searcher. Detection probabilities of carcasses can be estimated using mark-recapture (Pollock et al., 1990). This report summarizes carcass detection probability estimates for beach surveys on Unalaska Island, Alaska in January 2005. Carcass persistence rate estimates are reported in a companion document (Byrd and Reynolds 2006b).