Two of the Antarctic pack ice seals, Ross, Ommatophoca rossii, and leopard, Hydrurga leptonyx, seals, are extremely dificult to study via traditional visual survey techniques, yet are ideal for an acoustic survey as they are highly vociferous and produce an array of underwater sounds during the austral summer. To determine their acoustic occurrence in the Antarctic pack ice, we use their calls, detected within 680 acoustic recordings made between 1999 and 2009 as part of two multinational programmes. Siren calls of Ross seals were detected mainly in January, and 9.88 calls per minute from low siren calls was the highest call rate for this species. High numbers of Ross seal calls were detected close to the ice edge in areas between 0° and 20° E and 60° and 130° E, suggesting these are important summer habitats. Leopard seal calls were detected mainly in December and January, and December had the highest percentage of calls. Call rate of 11.93 calls per minute from low double trills was the highest call rate for leopard seals. Leopard seal calls were detected throughout the Southern Ocean with more calls detected throughout the pack ice. There was little spatio-temporal overlap in call occurrence of Ross and leopard seals, but both species were more vocally active during the day. Longitude and latitude were the most important predictors of Ross seal occurrence, and month of the year highly predicted leopard seal occurrence. This is the irst study to examine the circumpolar acoustic occurrence of Ross and leopard seals in the Southern Ocean pack ice.

The acoustic dataset from the SCAR-APIS Programme consisted of 112 underwater passive acoustic recordings made remotely at fixed points using sonobuoys [Sparton Electronics AN/SSQ-57A, mean sensitivity: − 155.20 (range: − 158.50 to − 151.00) dB re 1 V μPa−1)] in the Southern Ocean from 4 December 1999 to 12 January 2000 within the pack ice between 64° 31′ S; 62° 42′ E and 67° 17′ S; 149° 31′ E (Rogers et al. 2013; Fig. 1). Sonobuoys sampled over 10 to 22,000 Hz and had omni-directional hydrophone that had been deployed to a depth of 18 m below the water’s surface. Signals were received on the survey platform, the mast of the RSV Aurora Australis, using two, 9-element custom-built stainless steel Yaggi antennas (YH09, RF Industries Pty Ltd) with a series of AR2001 receivers (AOR Ltd, AR 2001). Antennas were secured at a height of 30 m above the sea level. The ship was on survey mode during sonobuoy deployment and kept steaming through the sea ice conducting visual search for seals until the radio signal was lost (Rogers et al. 2013). No anti-aliasing ilters were used during data collection. The acoustic signal was recorded using a Sony Digital Audio Tape recorder (DAT TCD-D8) with a frequency bandwidth of 10 and 22,000 Hz ± 3 dB. Recordings were randomly made throughout the day and were from at least 30-min to 2 h in duration depending on the amount of time the ship remained within range of detecting the signal from sonobuoys.

For analyses of acoustic data for calls of Ross and leopard seals performed on SCAR-APIS data, the number of calls within at least 30-min recording period was counted by a manual observer using Signal 3.1 (Engineering Design, Belmont, USA) and SpectraPRO 3.32 (Sound Technology Inc., USA). The first 10-15 min of the SCAR-APIS recordings was ignored, as ship noise (propeller cavitation) masked the signal.

Shabangu FW, Rogers TL (2021) Summer circumpolar acoustic occurrence and call rates of Ross, Ommatophoca rossii, and leopard, Hydrurga leptonyx, seals in the Southern Ocean. Polar Biology https://doi.org/10.1007/s0030 0-020-02795-z

Rogers TL, Ciaglia MB, Klinck H, Southwell C (2013) Density can be misleading for low-density species: Beneits of passive acoustic monitoring. PLoS ONE 8(1):e52542. https://doi.org/10.1371/journ al.pone.0052542