Assessing trophic links, individual movement, and habitat use of non-breeding Northern Gannets

Abstract

**Abstract:** Marine ecosystems are complex, with interconnected natural and anthropogenic drivers influencing spatiotemporal patterns of marine predators. Resource availability is a key factor driving seabird movement and distribution, but information on prey availability is difficult to obtain at useful scales for understanding predator ecology. By using new techniques for estimating prey availability from digital aerial survey data, which capture the location and size of surface-level forage fish shoals, and estimating forage fish occupancy from bottom trawl survey data, we have new methods to describe prey availability for seabirds. This study explores links between seabirds and prey availability by examining Northern Gannets (Morus bassanus) movement in the U.S. Atlantic during the non-breeding season. Using satellite telemetry data from adult and juvenile Northern Gannets (n=75), we employed hidden Markov models to examine behavior states in relation to forage fish availability and environmental covariates. Here, we tested the importance of four types of environmental covariates: static (e.g., depth), dynamic (e.g., sea surface temperature), frontal (e.g., sea surfaced temperature fronts), and behavior-modulating (e.g., wind speed) in Northern Gannet transitions between behavioral states (resident and transiting). Our results suggest that static and dynamic environmental variables played significant roles in movement behavior, with chlorophyll a and depth influencing transitions between resident and transiting states. While frontal features were not as important overall, sea surface temperature fronts specifically played a role in facilitating behavioral transitions. While we expect many of these environmental covariates to be related to forage fish availability, the next step is to use the forage fish occupancy and availability estimates to determine if they improve our understanding of Northern Gannet behavior. Disentangling trophic relationships is critical to understanding the mechanisms driving seabird behavior and space use, particularly in ecosystems that are rapidly changing due to climate and energy development. **Authors:** Julia Gulka¹, Evan Adams¹, Alicia Berlin², Kevin Friedland³, Andrew Gilbert¹, Chandra Goetsch¹, William Montevecchi⁴, Matthew Perry², Iain Stenhouse¹, Kate Williams¹ ¹Biodiversity Research Institute, ²U.S. Geological Survey, Patuxent Wildlife Research Center, ³National Marine Fisheries Service, ⁴Memorial University