From presence/absence to reliable prey proportions: A field test of dietary DNA for characterizing seabird diets

Climate change and human impacts are causing rapid shifts in species distributions and abundance, potentially disrupting predator-prey relationships. Monitoring animal diets can elucidate these relationships and potentially provide a mechanistic understanding of population declines, but efficient methods to monitor diets are needed. Dietary DNA can be used to gather data on species diets with high-taxonomic resolution, but the question of whether it can provide quantitative information on animal diets has so far limited its applications. Here we show that dietary DNA can efficiently provide quantitative information on the relative proportions of fish in the diets of piscivorous seabirds. Over three breeding seasons, we observed common tern chick feeding events and collected fecal samples from chicks. We compared the frequency of occurrence (FOO) and relative read abundance (RRA) of fish prey in the fecal samples to the relative biomass recorded through visual observations and found a high correlation (R = 0.94) between RRA and relative biomass at the colony level. We also found that RRA outperformed FOO in capturing interannual changes in relative prey biomass, and that FOO systematically over-estimated the relative importance across all prey categories. The high taxonomic resolution provided by dietary DNA and the high correlation between RRA and relative biomass we found here suggest that dietary DNA can be used as a quantitative metric to monitor seabird diets. This has important applications for studying the impact of changing forage fish availability on seabird populations, but also for harnessing seabirds as sentinels of ecosystem health. Since quantitative diet data can be collected highly efficiently using dietary DNA, this method could provide key information about food web dynamics and forage fish availability as we move towards ecosystem-based fisheries management.