Drivers of declines in common loon (Gavia immer) productivity in Ontario, Canada

Abstract

Common loons (Gavia immer) are top predators that depend on lake food webs to successfully fledge chicks. Common loon reproductive success is consequently recognized as an important indicator of aquatic ecosystem health. Existing evidence points to long-term declines in productivity in portions of the common loon range; however, the reason for these declines is not well understood. Our objectives were to define underlying baseline patterns of loon reproductive success in Ontario, Canada, and to identify drivers of temporal changes in loon productivity. We analyzed 38 years of reproductive data from over 1500 lakes using data from the Canadian Lakes Loon Survey, a citizen science loon monitoring program managed by Birds Canada that has run annually in Ontario since 1981. Overall, we estimated a declining trend in common loon reproductive success of -0.10 six-week-old young per pair per year in Ontario between 1981 and 2018. We assessed the influence of 14 factors on loon reproductive success. We identified low pH and associated higher mercury as factors linked to loon productivity declines. We also demonstrated that lake area, longitude, and April temperatures can predict the number of six-week-old young per pair per year. We hypothesize that climate change-induced stress, acting through multiple interacting pathways involving mercury acidity, fish abundance, lake size, and geographic location, may account for declining loon productivity. These results will be important for focusing future research and conservation efforts to help understand and mitigate threats to common loon populations.

Keywords: Acid rain; Citizen science; Common loon; Fish abundance; Mercury; Reproductive success.