Diving Deep: Seabird foraging tactics and local prey field

Abstract: Foragers in patchy environments need to decide what patches to use and how to allocate their time between them. Optimal foraging theory describes these choices through the relationship between energy expenditure and energy gain. Very few studies testing this have had spatiotemporally overlapping data on both predator & prey movement, but in this study an autonomous sail drone (USV) equipped with an echosounder was used to map the prey abundance with high resolution around a guillemot colony. Hydroacoustic data was combined with movement tracking data of common guillemots from the colony to determine how prey distribution and abundance affects foraging behaviour. Patch selection was investigated by comparing abundance of different patches with utilization, while GAMMs were used to describe the dive behaviour in patches with different prey abundance and depth distributions. It was found that guillemots mostly utilized patches with shallow prey and that time of day affected the depth and duration of dives, indicating that diel vertical migration patterns of prey influences dive behaviour of guillemots. The total foraging effort per trip did not decrease when dive duration increased, indicating that patches are only of acceptable quality, as total foraging effort did not decrease. It was also found that guillemots targeted patches with more available biomass further from the colony. This study increases the understanding of how guillemots are likely to be affected by further changes in prey populations and as seabirds are good indicators of overall ecosystem health this method can possibly also be used for several different species within different monitoring programmes. Mapping prey distribution with an USV is also a pioneering method that offers a time-and cost-effective method for obtaining data with high spatiotemporal resolution.