a small summary from the University of Oregon:

The seals, the world’s southernmost-ranging mammal, regularly produce chirps, whistles and trills at frequencies that are inaudible to humans. […] The nine new ultrasonic call types featured single or multiple vocal elements. Eleven of the elements, including chirps, whistles and trills, exceeded 20 kHz. Two were over 30 kHz and six were constantly above 21 kHz. One whistle reached 44.2 kHz and descending chirps in another call type began at about 49.8 kHz. The overtones of some calls exceeded 200 kHz. […] How Weddell seals use their ultrasonic calls is unknown. They are among 33 species of fin-footed mammals grouped as pinnipeds. Until now, pinnipeds, which also include sea lions and walruses, were believed to vocalize only at sonic levels.