Pygmy blue and Antarctic blue whale presence, distribution and population parameters in southern Australia based on passive acoustics

Abstract

Passive acoustic recorders set around Australia since 2004 have been used to study blue whales. Calls from New Zealand pygmy blue whales occur predominantly eastward of Bass Strait (145.8° E), calls from Eastern Indian Ocean pygmy blue whales (EIO PB) west of Bass Strait, while Antarctic blue whale calls occur along the entire southern Australian coast. The only location all calls have been detected within a short space of time in the same season is Bass Strait, where inter-species mixing commonly occurs. The EIO PB shows three migratory stages: "southbound" from November to January during which whales travel down the Western Australian coast; "southern Australian" where animals spread across the Indian and Southern Oceans spanning longitudes from 74° to 146° E down to at least a latitude of 55° S searching for food and feeding; then a northern migration where they return north to Indonesian waters post April-August. Along the southern Australian coastline EIO PB whales are more often detected towards the east, favouring an area where late summer to autumn upwelling occurs mostly over ~ 134-145° E longitude. Receivers on the shelf break south of Portland (141.2° E) showed 10–181 times greater EIO PB whale calling when integrated over their three month season than at sites located around the Great Australian Bight to the west. Within a season the Portland site did not show consistency of EIO PB whale presence or number of calling individuals, but when the number of calling individuals was integrated across a season and correlated with an upwelling index of seasonally integrated seabed water temperature across seven seasons, the upwelling index predicted 83% of the seasonal variance in whale presence. The strength of this correlation will be reduced by population growth of whales. By correcting EIO PB whale call rates for a variety of population growth rate values, we found a 4.3% growth rate for the population proportion visiting the Portland area, gave the maximum regression, correlation coefficient of r2= 90%. Time averaged levels of squared pressure produced at the upper chorus frequency of the Antarctic blue whale z-call and received via deep sound channel propagation at Portland showed a 12.6% increase rate. This is indicative of population growth rate for Antarctic blue whales using waters south of Australia, although the value of using pressure squared in this fashion and the assumptions underlying the technique need to be explored.