Diel Vertical Migrations and Climb-and-Glide Ascents in Veined Squid (Loligo forbesi)'

Many squids undergo daily vertical migrations to inhabit shallower, warmer waters at night. Fast-moving, energetic squid such as Loliginids and Ommastrephidae are often negatively buoyant and must continuously swim to maintain their position or ascend in the water column. This suggests that such species would be characterized by high rates of energy expenditure and thus might be particularly vulnerable to changes in prey distribution and abundance that offset their energetic balance. Yet despite their critical role in open-ocean food webs, little is known about in-situ swimming movements, patterns, energetics, and orientation during diel behaviors and vertical ascents. To examine broad and fine scale vertical movements, we affixed 11 Loligo forbesi (mantle lengths of: 44-58 cm) with tags (ITAGs) containing inertial measurement units (100 Hz), as well as light, depth, and temperature sensors (1 Hz). We also tagged five L. forbesi with VEMCO tags, which give lower-resolution acceleration, depth, and a location estimate when within range of an underwater receiver array. The 11 ITAG deployments totaled 137 recording hours, including five tag deployments with ca. 24-hour continuous sampling. Each animal underwent upward and downward migrations closely associated with sunset with sunrise, respectively. Daylight depths were 250-400 m while night depths were 100-175 meters. High-resolution inertial measurement unit data provide detailed orientation estimates to examine their fine-scale behavior. ‘Climb-and-glide’ ascents commonly consisted of repeated 6-8 meter climbs with 30-60 second glides that potentially allowed animals to ‘rest’ during ascents. Descents were more continuous and did not have repeated swimming movements. Two tagged animals are thought to have been predated upon during their upward migration, reflecting the ecological pressures of such behaviors. Our results demonstrate that L. forbesi enact complex vertical swimming behaviors and likely compensate for their negative buoyancy with variable vertical movements. These data provide a unique insight into the fine-scale behaviors of a key ocean trophic link.