Net, trawl and sledge system capability

BAS owns a number of bespoke, custom-built nets, trawls and sledges to sample ecosystems in the water column and on the seafloor.

RRS Sir David Attenborough will be capable of deploying this specialised scientific equipment.

Science equipment

 Agassiz Trawl (AGT)

The Agassiz trawl is a device for collecting seafloor samples. Because it has no electronic or pressure-sensitive components and is very stable and robust, it can be deployed at depths up to several thousand metres. The Agassiz trawl consists of a metal frame that pulls a long net which collects samples of organisms living on or just above the seafloor.

Recovery of the Agassiz trawl onboard Cruise JR179 in the Amundsen Sea. Photograph by Rob Larter (British Antarctic Survey)
Recovery of the Agassiz trawl onboard the RRS James Clark Ross in the Amundsen Sea. Photograph by Rob Larter (British Antarctic Survey)

Epibenthic sledge (EBS)

An epibenthic sledge has a finer net than the Agassiz trawl to collect smaller organisms. As the sledge passes over the seafloor, it stirs up the top layer of sediment and collects species which live just above the seafloor. A bucket at the back of the sampling net catches the animals safely. The sledge also has doors that close during the passage up or down through the water column to ensure sampling only occurs when the sledge is on the seafloor.

The epibenthic sledge can be equipped with additional equipment including:

  • A deep-water camera system custom-built for BAS, allowing researchers to monitor each tow in high-resolution colour imagery.
  • A small CTD to collect data on the water depth, temperature and salinity

Rectangular midwater trawl (RMT)

This pelagic trawl system is available in different configurations for specific scientific requirements. The RMT systems are operated in combination with the down-wire net monitor system (see description below). The RMT system consists of two nets of the same size, which can be opened and closed independently, allowing for targeted sampling in specific swarms or water layers, down to 1000m.

The smallest version of the RMT net system (RMT1) has a 1 square metre mouth opening and is mainly used to catch zooplankton.

The mid-sized RMT (RMT8) is best suited for catching krill with a mouth opening of 8 square metres. Its relatively small size means that it can be deployed in an efficient and targeted manner.

The biggest RMT system (RMT25) has a mouth opening of 25 square meters and is best suited for catching fish.

Weighing nearly 1,000kg, the RMT25 can be unwieldy in rough conditions. A custom-built support stand constructed at BAS means that the net and its frame can be pre-assembled and kept in one piece during an entire cruise, facilitating and accelerating deployment of the RMT25 system.

Deploying Rectangular Mid-water Trawl (RMT8) on the James Clark Ross, 2015. Photographer: Sarah Chapman (BAS)
Deploying Rectangular Mid-water Trawl (RMT8) on the James Clark Ross, 2015. Photographer: Sarah Chapman (BAS)

Longhurst-Hardy plankton recorder (LHPR)

This system samples zooplankton from depths of up to 1,000m between two layers of gauze. The LHPR operates in combination with BAS’s custom-built down-wire net monitoring system to yield greater sampling accuracy. It can collect up to 100 discrete, sequential samples, producing high spatial resolution within the water column.

The LHPR is deployed during darkness from the James Clark Ross off South Georgia LHPR - Longhurst-Hardy Plankton Recorder.

“Mammoth” MultiNet Multiple Plankton Sampler

This multi-net plankton sampler is a world-leading system for horizontal, oblique and vertical sample collections in successive water layers, down to depths of 3000m. Nine nets with 1m² openings are attached to a stainless steel frame can be opened and closed remotely via a deck command unit. This unit also provides depth readings from the integrated pressure sensor.

Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS)

This system of nine nets for zooplankton is towed by a metal frame in open water at depths of up to 1,000m. The frame also carries a CTD system to record the water properties and a current meter to measure how much water has passed through the nets.

The MOCNESS system has a live data stream, so an operator can control it from the ship. This allows for accurate control over the location and timing of samples, which is of crucial importance in plankton studies. Closing one net triggers the opening of the next one, so samples are fully sequential. Buckets are situated at the end of each net to prevent zooplankton samples from being crushed by the current moving through the system.

Bongo nets

Three different types of Bongo nets are available for the RRS Sir David Attenborough.

  • Bespoke motion-compensated Bongo net – it corrects for the rolling of the ship using a spring mechanism that helps maintain a constant upward velocity of the net through the water. This avoids damage to equipment or samples from sudden acceleration. Using these nets the zooplankton is in much better condition, compared to bongo nets without motion-compensation. This is important as it helps scientist when identifying the sampled animals.
  • Towed Bongo net – used for sampling specific water depths or strata
  • Mini-Bongo net – small and lightweight, this net can be deployed from smaller boats, such as the Erebus workboat, using smaller winches. It is mainly used to sample small zooplankton in good conditions from depth of up to 100m.

Neuston sledge

The Neuston sledge samples plankton, fish larvae and micro-plastics at the sea surface. A slender catamaran body with a relative high draught supports the net at the surface at an adjustable height, ensuring stable positioning. The system is deployed over the side, at as great a distance from the ship as possible in order to achieve an undisturbed sample.

Shallow Underwater Camera System (SUCS)

The relatively shallow Antarctic continental shelf regions are of increasing interest to researchers from various disciplines. The SUCS system provides a high-resolution colour video feed from depths of up to 1,000m to facilitate exploration of the seafloor.

The system consists of a camera in an underwater housing and a stand-alone light, both supported by a tripod which is connected directly to the fibre optic link with the surface. On deck, the SUCS system is spooled on a small winch and is connected to a PC running a MatLab graphical user interface to control the camera’s functions.

Deploying the custom-built remote camera system to monitor the seafloor while taking benthic samples. Photography by Dr David Barnes (British Antarctic Survey)
Deploying the custom-built SUCS system to monitor the seafloor while taking benthic samples. Photograph by David Barnes (British Antarctic Survey)

Operational equipment to support this capability

Sieving table for benthic samples

Benthic samples from nets and trawls are often retrieved in a muddy state and require significant cleaning. Sieving table on board the ship facilitate the cleaning process, which was traditionally very labour- and time-intensive. A constant flow of water and an outlet at the bottom helps to separate mud from the samples and keeps the ship’s aft deck clean. Three increasingly fine levels of sieves mean that the sample can be pre-sorted during the cleaning process. This speeds up processing, so scientists have more time to study the samples collected.

Dr Alexis Janosik sieving sediment from the Marguerite Bay (Antarctica) seabed for animals on RRS James Clark Ross‘ sieving table. Photograph by Pete Bucktrout (BAS)

Down-wire net monitoring system

The down-wire net monitoring system is purpose-built in-house system at BAS to enable live two-way communication, between the pelagic net systems and controllers aboard the ship, via a single conductive towing cable. It has an array of sensors attached to the underwater unit, which feed back information such as depth, temperature, salinity, currents and light penetration. This system allows researchers to get live information about the physical conditions around the net and to control their nets by opening and closing them at will.


Nets, trawls and sledges are used for a wide range of biological and ecological research. Food webs and ecosystems in the polar oceans are often complex, with organisms ranging in size from microscopic plankton to blue whales. Studying the interactions within and between ecosystems helps us understand their relationship with the ocean, and how they, in turn, are affected by climate change and industrial fishing activity.

Highly specialised equipment is often required to sample the wide variety of habitats in the polar oceans, which can range from shallow waters to the seafloor, and even underneath permanent ice shelves.

Case study: Polar blueing – a negative climate feedback

Emissions of carbon dioxide and other greenhouse gases – from both human and natural sources – are among the most powerful known drivers of climatic change. They can also affect the climate system indirectly, causing poorly understood cycles of change known as feedback loops. Such feedback loops are one of the reasons why it is so hard to predict what the climate of the future will look like.

Most polar feedbacks we know of are positive: as it gets warmer, the consequences produce even further warming.  However, there are also some negative ones, which help offset the impact of greenhouse gases instead of amplifying it. Dr David Barnes, a marine ecologist at British Antarctic Survey, has used seafloor trawling equipment and remotely operated vehicles (ROVs) aboard the RRS James Clark Ross to study one such negative feedback related to the “blueing” of the polar regions as sea-ice cover diminishes. In a recent paper he published results showing that tiny seafloor dwellers called bryozoans are removing more carbon from the ocean as summer ice-free periods become longer.

Bryozoans collected during a research cruise on the RRS James Clark Ross. Picture by Dr David Barnes (British Antarctic Survey)
Bryozoans collected during a research cruise on the RRS James Clark Ross. Picture by David Barnes (British Antarctic Survey)

Bryozoans are tiny filter-feeders that live on the relatively shallow seafloor of the continental shelf and depend on food sinking down from the surface. Like many benthic organisms, bryozoans grow their exoskeletons using carbon that they extract from the ocean and convert to calcium carbonate. This process is known as carbon immobilisation. When the organism dies, its exoskeleton may be buried by sediment, removing the carbon fixed in it from the global carbon cycle for millions of years.

Because bryozoans depend on food dropping down from above, they can only grow when the sea surface is ice-free, allowing phytoplankton blooms to grow and providing food for the seafloor community.  Over the last three decades, the duration of sea ice cover has shortened by an average of 80 days on 1 million square km of ocean – that’s roughly four times the size of the UK. This huge area can now support phytoplankton – and bryozoan growth – for much longer every year. Where permanent ice shelves collapse, plankton and benthos can also colonise entire new areas previously uninhabitable to them. As a result, Antarctic benthos can immobilise and fix far more carbon than before as the region’s climate changes.

The future: carbon immobilisation around the Antarctic

A recent study by Dr Barnes and colleagues at BAS and the National Oceanography Centre in Southampton found that carbon immobilisation is particularly pronounced around the remote South Orkney Islands, 300 miles off the tip of the Antarctic Peninsula. In 2010, this area became the first marine protected area located entirely in the high seas (as opposed to a country’s territorial waters).

In order to map the carbon immobilisation effect of benthic communities around the Antarctic, Dr Barnes and his colleagues will be undertaking several more research cruises around the entire Antarctic continent in the coming years. They will be able to visit very rarely studied areas and add to our understanding of the complex relationships between Antarctic ecosystems and climate change.

Carbon immobilization in the Antarctic

SCIENCE IN THE SEA: You study what?

28 February, 2018 by Mel Mackenzie

Dr Mel Mackenzie, a Collection Manager of Marine Invertebrates at Museums Victoria in Melbourne, is living and working on board the BAS research ship RRS James Clark Ross for the …