Avian vomitomics: using seabird regurgitations to assess forage fish size and age distributions as a complement to traditional survey methods

Abstract Assessing fish size and age distributions is essential for understanding recruitment dynamics, yet traditional survey methods may underrepresent early life stages of pelagic species due to methodological constraints. This study explores the potential of avian vomitomics—the systematic analysis of seabird regurgitations—as a complementary and non-invasive approach for estimating the fork length (FL) and age of Atlantic mackerel (Scomber scombrus). Using partial fish remains collected from northern gannets (Morus bassanus) and a comprehensive reference dataset from Fisheries and Oceans Canada, we developed predictive models that accurately estimate FL from incomplete specimens and infer age from FL. Our results reveal a sequential incorporation of young-of-the-year (YOY) mackerel into gannet diets, with juveniles becoming more prevalent later in the season compared to older age classes (one-year-old [OYO], two years and older [2Y+]). While seabird-derived data are shaped by prey selectivity and localized foraging behavior, these very biases offer a focused lens on early life stages that are typically underrepresented in standard stock assessments. Integrating avian vomitomics with fisheries surveys provides a more nuanced and ecologically grounded view of mackerel population dynamics, supporting ecosystem-based fisheries management. This approach also opens new avenues for characterizing the spatiotemporal distribution of mackerel age classes by linking diet composition with fine-scale dive location and depth data from northern gannets. While the current method is species-specific, the underlying framework can be adapted to other prey species, provided that adequate morphometric references are available.