At-sea distribution and fine-scale habitat use patterns of zooplanktivorous Cassin’s auklets during the chick-rearing period

Understanding the fine-scale movements and habitat use patterns of marine predators is critical for identifying important foraging habitat and guiding effective conservation planning. Here, GPS loggers were used to track chick-rearing Cassin’s auklets (Ptychoramphus aleuticus) at their largest breeding colony, located on Triangle Island, British Columbia, Canada (50°51′35″N; 129°4′34″W). Analyses were conducted at both the individual and population level to assess whether inter-annual variation in habitat use (2014, 2015, and 2017) could be explained by dynamic oceanographic features, such as sea surface temperature (SST) and chlorophyll a concentrations, or by static bathymetric features. At both scales of analyses, the foraging behaviour of Cassin’s auklets was most strongly influenced by SST. At the individual level, birds spent more time foraging in areas with lower SST, relative to other areas visited over the course of a foraging trip. At the population level, the at-sea distribution of Cassin’s auklets varied across years, with birds using areas northwest of colony in 2014 and areas west of the colony in 2015 and 2017. Furthermore, the probability of foraging across the study area was higher in areas with lower SST, suggesting that SST influences the broad-scale foraging distribution of Cassin’s auklets. Identification of the environmental drivers of habitat use across multiple years can be used to help in predicting suitable at-sea habitat across time, leading to more effective conservation management.