Project 0607/4

Project Title: An ecological approach to determining blue whale critical habitat and seismic impacts in the Bonney Upwelling

Chief Investigators: Dr Peter Gill & Ms Margie Morrice

During 2006 we intended to conduct aerial experiments to investigate the availability bias (g(0)) of blue whales, ie the proportion of time they spend at the surface while foraging and feeding. This is done by finding a group of whales in a given area, and by repeatedly flying a line that passes through the sightings, recording sightings on each pass. We were unable to carry out this work during the 2005-06 season, so planned to do so when opportunity allowed during the 2006-07 season. We would prefer to have carried out one such survey prior to commencing our boat-based surveys, but there were few available weather windows during what proved to be a very windy season.

On 13 April 2007 we flew a dedicated aerial survey for this purpose, expecting to find reasonable numbers of blue whales, as it appeared to be a strong upwelling season with abundant krill surface swarms. However, there were only nine blue whale sightings, well spaced along the shelf with at least 20km between most sightings. The exception was a group of three whales scattered across a 20km span, but when we attempted the experiment with these whales, only one of them could be relocated. The density of whales observed was clearly not sufficient for effective use of this method. We suspect that this result is related to the low incidence of surface feeding observed during the season, both from aircraft and from our boat. Low blue whale densities were also observed during subsequent aerial monitoring surveys for seismic surveys later in April, so a decision was made not to repeat the g(0) experiment until higher densities are encountered next season.

Suction-cup digital tags were deployed on blue whales in the Bonney Upwelling during March 2007. A total of seven (7) tags were deployed and retrieved on separate medium to large adult blue whales over five days, despite extremely bad weather that prevented more time at sea. The tags recorded over 27 hrs of good quality dive, orientation and acoustic data the first of its kind in Australian waters, and the first data from a pair of blue whales worldwide. All tagged whales were focal-followed with continuous recordings made of whale track and surface behaviour. All whales were photo-identified and skin collected for determination of gender. Over 8.5 hrs of hydroacoustic prey survey data were collected while following 5 of the tagged whales. Satellite remote sensing data (SST, chl-a) was acquired for each cloud-free day.

Initial investigation of this data provided important observations on underwater behaviour. For example, tags deployed on both members of a pair of blue whales provided the first data worldwide on the coordination of underwater behaviour by a pair. The acoustic data, from this pair indicated one animal in the pair (the trailing whale) was calling . This information combined with genetic data will indicate whether some of these behaviours and calls were breeding-related. Current evidence suggests that the lead animal was likely to be female, and the trail animal a male.

Data from other tagged whales included dive and call types associated with feeding; however they revealed a relatively low occurrence of feeding dives using the detection method for blue whales from central and southern California. This is despite our recording scattering layers of reasonable density while following tagged whales. No whales were observed surface feeding, with few surface krill swarms being detected. It may be that blue whales that use this southern Australian region adopt different feeding techniques to elsewhere, due to the shallowness of the feeding area, and the related behaviour of the prey. While this was apparently the strongest upwelling season recorded since 2002-03, on close examination of the photographs of whales we noted with concern a number of whales that were either emaciated, with high ecto-parasite loads, and/or evidence of possible skin disease. This included two of the tagged whales. Further analysis on dive kinematics, calls, krill parameters, and habitat data will shed light on whether krill densities recorded may have been below the threshold for successful feeding, or there were other behavioural or environmental explanations for their observed behaviour and condition.

A further benefit of this tagging research is capturing undisturbed behaviour as a baseline for comparison with whales subjected to acoustic disturbance such as seismic e.g. for planned controlled exposure experiments (CEEs) from airguns on blue whales. The above dataset on blue whale surface, underwater and acoustic behaviour during a period when no seismic activity occurred will be used to compare with behavioural studies during CEEs. To account for variability expected in individual whale behaviour and upwelling conditions, we expect to complete Stage 1 after analysis of another field season.