LB 3-23 - Sleeper shark activity patterns reveal possible predatory behavior through archival tag data from the Gulf of Alaska

Pacific sleeper sharks (Somniosus pacificus, PSS) are considered to be sluggish, bottom-dwelling scavengers. Yet, recent evidence suggests they are a common and likely active generalist predator with diverse diet that may play a key but completely ignored role in marine polar ecosystems. Stomach content and stable isotope analyses have previously shown sharks in the genus Somniosus to exhibit ontogenetic diet shifts to larger and higher trophic animals, and telemetry data revealed frequent active shark movement through a wide range of depths. These life history related changes in foraging and movement behaviors would not be expected if scavenging were the sole strategy. Telemetry data from juvenile Steller sea lions have also provided indirect evidence in support of active predation by PSS in the GOA region. Here we describe multiple acceleration events that indicate active predation behavior from recently recovered pop-up archival satellite transmitters (miniPATS) deployed on six PSS. These events are characterized by a change in orientation of the flexibly, externally attached mini-PATs from near vertical floating that occurs during >99.9% of data records, to a more horizontal, trailing orientation likely associated with a momentary acceleration and increased swimming speed. At the same time, the depth change rate increased from the very slow but persistent 0.1-0.3 m/s to an average of 0.8 m/s, (range 0.6-1.2 m/s). The peak individual depth change rates averaged 1.2 m/s (range 0.8-1.7 m/s) across the six sharks. All 104 such events that were identified during the first 4-12 deployment days coincided with an ascending movement, at an average depth of 194 m (range 73-272 m), and typically lasted 30-60 seconds. Events occurred on average 1.9 times per 24 hours (range 0.5-4.9 events per 24 hours). Verification of these events will be pursued through additional direct observations, ultimately enabling us to confirm whether these large vertebrates have an active role as predators in marine ecosystems.