Homeward bound

In his last update, NIWA project leader Dr Richard O’Driscoll talks about the challenging search for whales in foggy and icy conditions.

After three weeks of great weather in the Ross Sea, highlighted by a spectacular day in Terra Nova Bay last Saturday, the past 5 days have reminded us of the challenges of working in the Southern Ocean. Persistent snow and fog have made sighting conditions difficult to impossible as we again went searching for blue whales. When I said last week that I hoped the remaining time would put the “icing on the cake” of a successful voyage I didn’t mean putting a snowy-white coating over everything onboard!

Trawl net covered in ice. [Richard O'Driscoll]

After leaving Terra Nova Bay we had a good run north, exiting the Ross Sea polynya along the 177° E longitude on 24 February. During this transit, an aggregation of silverfish was detected and a short acoustic experiment, with accompanying midwater trawl, was carried out.

Blue whale work (Objective 2) resumed on 24 February, but rough seas, wind, and poor visibility made sighting conditions impossible until 26 February. Multiple blue whales were located using the acoustic sonobuoys at about 69° 45’ S 175° 35’ W – about 60 km southeast of where we had found the hotspot on 10-14 February. Unfortunately the current whale location is within the ice, with a mix of brash, first-year ice, much heavier multi-year floes, and icebergs making navigation difficult. We have had several attempts to close in on the blue whales for photo-identification, but the combination of ice and poor visibility has meant that often the whales have moved faster than we have and they have stayed tantalisingly out of sight. Nevertheless we managed to approach and photograph members of a group of at least 8 blue whales on 26 February.

Because of the challenging sighting conditions, our focus over the past week has been on both passive and active acoustic observations. You don’t need to be able to see to hear! The passive acoustic sonobuoys continue to detect calling blue whales, as well as fin whales, humpbacks, and the higher pitched calls of killer whales. To me these killer whale noises sound like kittens meowing! We have had two encounters with groups of killer whales on the voyage so far, and have been able to get some great images for photo-identification.

The active acoustic work has involved using echosounders to map the abundance and distribution of key prey species like krill, silverfish, and lanternfish (myctophids).  Colleagues in the Pelagic Ecology Research Group, at the University of St Andrews, Scotland were recently awarded a Natural Environment Research Council grant to purchase a state-of-the-art wideband (“chirp”) Simrad EK80 echosounder system.  This system was installed on the Tangaroa before the voyage and several experiments have been carried out where the ship’s 70, 120, and 200 kHz transducers were swapped between the narrowband EK60 and wideband EK80 systems while running a repeated line transect. We hope that the wideband echosounder will improve our ability to acoustically distinguish krill and fish.

In support of the active acoustics work we have carried out four mid-water trawls to collect data on the size and composition of the aggregations we are seeing. We have also experimented with dropping a GoPro camera into surface swarms of krill and were successful in recording some nice underwater footage.

NIWA photographer Dave Allen (left) and Sir Peter Blake Trust Ambassador Blake Hornblow (right) deploy GoPro cameras attached to Anchors into the ocean.

This is my last update from the Ross Sea. Early on Tuesday 3 March we will start the long journey back to Wellington. Along the way home we will continue to collect underway data to monitor oceanographic and atmospheric conditions, as well as towing the continuous plankton recorder, deploying surface drifters and ARGO oceanographic buoys, and watching and listening for whales.  

This time next week we should be back in New Zealand waters. We are scheduled to arrive in Wellington at 08:00 NZDT on Wednesday 11 March.

The crew were thrilled when a pod of Orcas decided to pay a visit to the vessel, see the video below:

Pod of Orcas visit RV Tangaroa from NIWA on Vimeo.

Richard