Catching prawns in the abyss

The deep sea is the most common habitat on our planet – but we know more about the moon than our own ocean. Dr Penny Berents, Senior Fellow at the Australian Museum, who was recently onboard the CSIRO research vessel (RV) Investigator, delves into the complexities of sampling in the deep sea.

Imagine you have been asked to find out what lives on the floor of a rainforest. Easy, you go for a walk and look for animals and plants living on the forest floor, in the leaf litter and under logs. Now imagine that you have to do this from a helicopter 5 kilometres up in the air, in the dark with no view of the forest floor. This is the challenge of sampling in the deep sea.

We have recently completed our voyage on the CSIRO research vessel (RV) Investigator, where scientists from CSIRO, the Australian Museum, Museums Victoria Research Institute and Western Australian Museum studied the biodiversity of seamounts from 100 metres to abyssal depths of more than 5000 metres. These surveys are vital to understand the unknown biodiversity of newly established marine parks near the Cocos Keeling Islands and Christmas Island in Australia’s Indian Ocean Territories, and to inform current and future conservation policies.

The sea floor is 5000m below in dark, cold water. However, we were not completely in the dark – we were able to map the sea floor before we sampled, thanks to the multi-beam mapping capability of the RV Investigator. Multi-beam mapping uses sound waves to map the sea floor. Sound waves are transmitted in a fan shaped pattern from an array under the ship, and depth is calculated by measuring the time taken for the sound to leave and return to the array. The intensity of the signal reveals information about the nature of the seafloor, such as a muddy or rocky bottom.

Before sampling with a net or a dredge, the area of interest was checked to find a suitable site for study. Rocky or uneven bottom topography was not suitable for sampling with a beam trawl (which could become tangled or caught). The beam trawl is a small net held open by a four-metre beam, which is pulled along the seafloor behind the ship. Tickler chains scraped the seafloor and disturbed animals which swam up into the water column, where they were caught in the cod end of the net. A second smaller mesh (500 microns), called a Gandalf net, was mounted on the top of the beam trawl to catch some of the smaller animals which were washed through the larger mesh of the beam trawl.

Another sampling tool we used on board was the Sherman epibenthic sled, which scraped the sea floor and caught animals in the cod end of a net strung out behind. The ship towed the sled or trawl at a speed of about 2 knots for a distance of 1-2km along the sea floor. Huge winches carrying kilometres of wire were required. For example, more than 7000m of wire must be paid out to deploy the beam trawl at 5000m. A successful trawl tests the skill and teamwork of the ship’s crew, the mapping team and scientists to keep the trawl or sled on the bottom and to avoid tangles and underwater features that could foul the net or sled.

The position of the trawl or sled on the seafloor was tracked with an Ultra-Short Baseline system (USBL). A transponder mounted on the sampling device received an acoustic pulse from a transceiver under the ship and sent an acoustic pulse back to the ship. The distance and bearing of the sampler could then be calculated. The trawl or the sled was winched up from the sea floor and the catch brought on deck.

Once the gear was safely secured by the deck crew, the scientific team eagerly examined the catch on deck. The catch, which included fish, corals, prawns, shrimps, worms, jellyfish, crabs and echinoderms was sorted as quickly as possible and carried below deck to the lab for identification and preservation. To prevent tissue and DNA degradation, it was important to keep the animals cool when the temperature on deck was 27ºC and the animals had come from abyssal depths of 5ºC. The samples were kept on ice while sorted, identified, counted, registered, labelled and photographed. The lab was a production line, with each scientist in the team having a specialised role in identifying particular animals but also assisting with all the steps in the process. A big sample could take our invertebrate team of seven scientists up to six hours to complete the processing. The fish team had a similar system to identify and register the fish collected in the beam trawl.

The collections will be deposited in the Australian Museum, Museums Victoria, Australian National Fish Collection, Western Australian Museum, South Australian Museum and the Tasmanian Museum and Art Gallery, where they will be used by scientists to study the biodiversity of seamounts and the abyss of the Indian Ocean Territories.

Dr Penny Berents, Senior Fellow, Marine Invertebrates, Australian Museum.