Assessing total mortality following seabird wrecks given variable data quantity and quality: the Cassin’s auklet die-off

Mass mortality events (MMEs) of seabirds are becoming more frequent as the global climate warms. Often documented via beached bird surveys, methods for estimating event-wide mortality are needed that can accommodate regional differences in carcass deposition and data quality/quantity. We develop a framework for estimating mortality from beached bird counts, extending existing approaches through the novel application of ocean circulation modeling to assess beaching likelihood. We applied our framework to the 2014/15 Cassin’s auklet ( Ptychoramphus aleuticus) MME, which spread across three regions (central California, northern California-through-Washington, British Columbia) with varying data quality/quantity. Our best mortality estimate of ∼400 000 (estimates ranged from 265 000 to 700 000 depending on model uncertainty and extent) places this seabird MME as one of the largest on record. However, we caution that much uncertainty exists surrounding model parameterization and deposition in British Columbia where beached bird data were sparse. We suggest that the application of ocean circulation models, combined with process-based modeling of carcass persistence and detection, can improve estimates of MME magnitude.