RV Investigator in the Tasman Sea

I just arrived back in Hobart, after spending approximately three weeks aboard the CSIRO RV Investigator, making our way from Brisbane to Sydney. The main goal of the voyage was to characterise the entire Tasman Sea ecosystem as well as possible. Such a holistic approach asks for a rather immense effort from a lot of people with various backgrounds. Data collection included different biological samples with four different kind of nets (to gain information on the species composition of the different depth layers), constant active acoustic recordings at five discrete frequencies (to gain insights into the structure of the water column, including the observation of diurnal vertical migrations, and the detection of distinct biological layers), occasional usage of the Simrad ME70 multi-beam echosounder allowing for the 4-dimensional representation of fish schools in shallow waters, constant acoustic seabed mapping, occasional acoustic sub-bottom profiling, constant logging of acoustic doppler current profiler data (ADCP, providing information on currents, intensity and directivity), constant registration of atmospheric, weather information, regular collection of hydrographic information through a CTD (including data on salinity, temperature, photosynthetic active radiation, turbidity, oxygen and chlorophyll concentrations), occasional usage of the so called Triaxus, a towed device equipped with a CTD, some cameras and a laser optical particle counter, able to deliver information of the presence and estimates of biomass of zooplankton ranging between 500 micrometers and 3 millimetres. All in all this kept most of us pretty busy during the entire voyage, not even mentioning the tremendous amount of work that had to be completed by the crew working in the engine rooms, in the kitchen and made sure all equipment was working as well as possible during the entire voyage.

In Sydney, we were lucky enough to be able to park at the Army wharf, which is right in the city centre, just a few minutes walking from the opera house and just in front of one of  Russell Crowe’s appartments. This certainly made for an impressive entrance into the city.

I will limit the pictures and reporting here to some of my main interests…so by no means will anything here be exhaustive of the communal effort aboard but should rather be looked upon as a little personal impression…

Two examples of mackerel schools around Montague Island, as recorded on the Simrad ME70 and visualised but the Simrad TD50:

My normal view at sea and an example echogram of a few days, showing the diurnal vertical migration of a part of the entire biomass in the water column, with an indication of the time of sunrise and sunset (times in UTC rather than local time):

Pictures of two siphonophores we were lucky enough to pull up in good quality. Siphonophores are pretty special animals, while related to jellyfish, they are colonial, meaning while they appear as a single animal, they are composed of numerous so called zooids, all with a special function, crucial to the survival of the colony. They can take a large variety of shapes and colours, and what makes them special from an acoustic point of view, is that even though they might appear as being small, they often contain small gas bubbles, which will resonate very strongly on the acoustic images and therefore make the general interpretation of the echograms more challenging:

Another such colonial animal are pyrosomes, which we caught in large abundance at a few occasions, while they were virtually absent in other trawls. The pictures might not look that impressive but they are known to put on a rather impressive bioluminescence show underwater. The first picture is just a tray of subsampled pyrosomes and the second picture is a detailed view of one pyrosome, clearly showing the different individuals that form the larger community:

An example of a tiny fish recovered from the net (left) and viewed under the microscope(second from left), a little angler fish with still active purple bioluminescence and a few mesopelagic fish:

Some humpback whales thats were curious about what we were up to and swam with us for a little while:

After the voyage we had a few days left at the Sydney Institute for Marine Science (SIMS), to discuss the voyage and come up with clear plan on how to proceed with data processing and reporting. SIMS is certainly not the worst location for such a meeting, given the stunning surroundings, inviting you for a swim or snorkel.

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