UC Santa Cruz

Weddell seals (Leptonychotes weddellii) are one of Antarctica’s top predators, yet surprisingly little information exists about their year-round movement, habitat, and foraging behavior. Previous attempts to determine the overwinter behavior of Weddell seals were met with limited success due to early tag failure. Conductivity Temperature and Depth – Satellite Relay Data Loggers (CTD-SRDL tags) now make it possible to collect animal movement and oceanographic data on the same scale. These tags are deployed on marine vertebrates with the assumption that oceanographic data collected by miniaturized sensors are on par with standard oceanographic equipment. Movement data collected by the tags, in combination with oceanographic data can be used to study the seasonal habitat preference and foraging behavior of Weddell seals. Tracking data can also be linked to diet to provide a complete picture of the seasonal ecology of the southernmost mammal on Earth. Unlike previous studies which were biased towards prey with indigestible hard parts, stable isotopes can be used to provide a more complete picture of both digestible and indigestible components of prey species.

In the first data chapter (chapter 2), I assess the accuracy of the temperature and conductivity sensors on the CTD-SRDL tags to determine their performance relative to standard oceanographic equipment. I found that CTD tags were not comparable to high-precision oceanographic equipment, but still provided invaluable oceanographic data and were fully capable of identifying water mass characteristics and seasonal changes in otherwise inaccessible areas. This study provides the first independent assessment of CTD tag performance in laboratory (pre- and post-deployment, and after battery replacement) and under in situ conditions (before and during deployment). In the second data chapter (chapter 3), I explain and predict Weddell seal habitat and foraging behavior from a suite of environmental variables as well as examine the relationship between foraging behavior and dive metrics. I found that seasonal sea-ice extent, open water polynyas, and the diverse topography of the Ross Sea were important in determining the habitat preference and foraging behavior of Weddell seals in the western Ross Sea. This work provides insight into how Weddell seals might adjust their habitat preferences and foraging behavior in light of increased climate change. Finally, in the last data chapter (chapter 4), I used stable isotopes (δ13C and δ15N) to examine the diet of Weddell seals over two time scales. I found that Antarctic silverfish (Pleurogramma antarcticum), Antarctic icefish (Neopagetopsis ionah) and several Tremamtomus species were important prey items, but their proportional contribution to Weddell seal diet varied among individuals and across time scales. Overall, this dissertation provides critical insight into the movement, habitat, and feeding ecology of this important top predator of the Ross Sea ecosystem.