Net, trawl and sledge systems
RRS Sir David Attenborough
A new polar research ship for Britain
The RRS Sir David Attenborough will be equipped with a range of nets, trawls and sledges to sample ecosystems in both the water column and on the seafloor.
Three different types of Bongo nets are planned for the RRS Sir David Attenborough. The bespoke motion-compensated Bongo net corrects for the pitching and 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.
The same net can also be configured as a towed Bongo net in order to sample specific water depths or strata.
Additionally, a small and lightweight mini-Bongo net can be deployed from smaller boats using smaller winches. It is mainly used to sample small zooplankton in good condition from depths of up to 100m.
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.
Epibenthic sledge & nets
An epibenthic sledge is 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 organisms living just above the seafloor. A bucket at the back of the sampling net catches the animals safely, ensuring that they don’t get squeezed against the back of the net as it moves forward. The sledge also has doors that can be closed during passage up or down through the water column to ensure accurate sampling.
The epibenthic sledge can be equipped with a deep-water camera system custom-built for BAS. Mounted at the front of the sledge, it allows researchers to monitor each tow in high-resolution colour imagery.
Rectangular midwater trawl (RMT)
This pelagic trawl system is available in different constellations for specific scientific requirements. The RMT systems are operated in combination with the down-wire net monitor, a custom-built system that enables two-way communications between the net and controllers aboard the ship. 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. Two nets can be opened and closed independently, allowing for targeted sampling in specific swarms or water layers.
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.
Longhurst-Hardy plankton recorder (LHPR)
This system samples zooplankton from depths of up to 1,000m between two layers of gauze. It performs best in waters where larger organisms like jellyfish or phytoplankton have a lower risk of clogging the net.
The LHPR operates in combination with BAS’s custom-built down-wire net monitoring camera system to yield greater sampling accuracy. It can collect up to 100 discrete, sequential samples, producing high spatial resolution within the water column.
MultiNet Mammoth Multiple Plankton Sampler
This multi-net plankton sampler is a world-leading system for horizontal, oblique and vertical sample collections in successive water layers. Nine nets with an aperture of 1m² and a length of 550cm are attached to a stainless steel frame can be opened and closed remotely via a deck command unit. This unit also shows readings from the integrated pressure sensor, allowing for supervision of depth; the MultiNet sampler is rated to a depth of 3,000m.
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 and a current meter to measure how much water has passed through the nets.
The MOCNESS system has a live data stream, allowing an operator to 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.
The Neuston net samples plankton and fish larvae 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 and at as great a distance from the ship as possible in order to achieve an undisturbed sample.
Sieving table for benthic samples
Benthic samples are often retrieved in a muddy state and require significant cleaning. BAS have a sieving table for benthic samples to facilitate the cleaning process, which was traditionally very labour- and time-intensive. A constant flow of water and an outlet at the bottom help keep separate mud from samples and keeps the ship’s aft deck cleaner. Three increasingly fine levels of sieves mean that the sample can be pre-sorted during the cleaning process.
Underwater video inspection camera
This lightweight camera system can be deployed to a depth of 50m from a RIB or other smaller boat. It consists of a webcam in a aluminium casing powered by an USB connection to a laptop on the surface. The laptop shows the live picture from the webcam, allowing researchers to decide whether to commit to the dive based on the conditions or the organisms present.
The inspection camera is also a simple tool to choose the best dive sites before entering the water – an advantage that becomes even more pronounced in Antarctic waters, where dive times are limited and dive operations are complex due to the remote location and cold water.
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.