Underwater calling rates of harp and Weddell seals as a function of hydrophone location

Stereo recordings of harp seal (Pagophilus groenlandicus) and Weddell seal (Leptonychotes weddellii) underwater vocalizations were made near breeding groups using separations between two hydrophones of 159–339 m. Within a large herd, harp seal call numbers varied slightly between channels. Counts of Weddell seal calls were higher near a small herd on the ice than 159–180 m away. Repeat counts of harp seal calls by a single observer differed significantly. Source levels of Weddell seal calls varied and higher amplitude calls would be detected up to 2 orders of magnitude farther away than quieter calls. Hydrophone location (especially near small groups), observer variability and call source level differences will bias the use of monitoring underwater seal vocalizations to index locations, population size or underwater behaviours. Using hydrophone arrays and multiple observers may mitigate these problems. Accepted: 15 September 2000