Dive descent rate as a buoyancy indicator to infer body condition of Weddell seals in the Antarctic

Changes in buoyancy of marine mammals can be used to infer environmental changes. In multiple seal species, how “fast” an animal sinks reveals body condition changes through shifts in buoyancy as the ratio between lean and lipid tissue changes. However, quantifying similar at-sea changes in Weddell seals (Leptonychotes weddellii) has remained unexplored. Here, we present a method of inferring buoyancy of Weddell seals by monitoring descent rates from 4-s time-depth data, to reveal in situ insight of their life cycle. We defined a Buoyancy Indicator Segment (BIS) as the descent rate of a dive segment created with the broken-stick method that was systematically filtered to only include characteristic nonstroking and directed travel segments while excluding lung buoyancy biases. We found that BISs predicted body condition changes in Weddell seals, being a function of dive duration, mean depth, and time-of-year. Descent rates quickened with troughs in early April due to postmolt muscle recovery, early July due to winter conditions, and early September possibly due to pregnancy. Each trough was followed by weight gain, with slowing descent rates reaching peaks in late May, early August, and late October. This new approach showed that determining at-sea condition is possible for Weddell seals, deriving a powerful species and environmental monitoring tool.