Albatross response to survey vessels: implications for studies of the distribution, abundance, and prey consumption of seabird populations

The study of marine bird ecology at sea is complicated by the tendency of m any species to follow and otherwise attend vessels. Vessel-attraction likel y biases abundance estimates and blurs the correlation between seabird dist ributions and habitat features over scales of tens of kilometers. Moreover, ship-following behavior inhibits the statistical analysis of seabird distr ibutions because samples too closely spaced in time and space are not indep endent. These biases have important implications when estimating the size o f rare and endangered seabird populations and the prey consumption by abund ant species. This paper illustrates how observations of seabird vessel-atte ndance can be used to mitigate the biases of vessel-attraction and ship-fol lowing behavior. I quantified the degree of albatross attraction to survey vessels off southern California, and estimated that standard 300 m strip tr ansects overestimated their abundance by at least a factor of 3.57. Additio nally, I modeled albatross ship-following behavior, and determined that 95% of recognizable black-footed (Phoebastria nigripes) and Laysan (P. immutab ilis) albatrosses ceased to follow the survey vessel after 60 and 38 min re spectively. Using these models, I estimated that standard survey methods ov erestimated black-footed albatross abundance by a factor of 1.17 due to the ir ship-following behavior. When the vessel-attraction and ship-following b iases were combined, standard survey techniques overestimated albatross squ id consumption off southern California by a factor of 4. Determining the de gree of vessel-attraction and the temporal scale of ship-following behavior will help design and analyze seabird surveys. These improvements will enha nce our ability to monitor the distribution, abundance, and prey consumptio n of seabird populations.