Summer circumpolar acoustic occurrence and call rates of Ross, Ommatophoca rossii, and leopard, Hydrurga leptonyx, seals in the Southern Ocean

Abstract

Two of the Antarctic pack ice seals, Ross, Ommatophoca rossii, and leopard, Hydrurga leptonyx, seals, are extremely difficult 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 first study to examine the circumpolar acoustic occurrence of Ross and leopard seals in the Southern Ocean pack ice.