Benthic sleds (also called sledges) and bottom trawls both use nets to collect organisms while they are towed across the seafloor. While trawls use free nets with doors or beams to spread the net, sleds use frames and runners to protect and secure the net (Eleftheriou and Mcintyre 2005). Benthic sleds target sessile or sedentary macrofauna and megafauna with some designs able to be deployed over rugged terrain, while bottom trawls are typically more successful in collecting demersal or mobile fauna and are deployed over smooth but compact terrain or soft sediments.
There is no one type of sled or trawl suitable for all habitats and depths, and selection of the most suitable gear type depends on scientific objectives, previous knowledge, targeted fauna, environment, depth, and vessel capabilities (Clark et al. 2016, Kaiser and Brenke 2016). Acquired data are often described as semi-quantitative (Table 2.1 in Schiaparelli et al. 2016a) due to inconsistencies in gear path, swept area, and movement (e.g. sled skipping along seafloor), as well as taxa targeted by the gear (e.g. avoidance by highly mobile megafauna, herding effect in some fish). Imagery of the seafloor helps enormously with sled choice and deployment techniques. Imagery and geospatial positioning can be obtained with available technology and can aid in the success of each deployment. In the absence of imagery, bathymetry and backscatter can also provide a good indication of gear suitability. The use of multiple types of sleds and trawls may be most appropriate for surveys trying to quantify overall biodiversity in a given location (Williams and Bax 2001, Clark and Roberts 2008), while a single sled or trawl type may be more efficient for quantifying species in a particular location or habitat for monitoring purposes (Przeslawski et al. 2015). For these reasons, this manual does not mandate specific gear types, although sled and trawl types historically used in Australian waters are listed in Table 8.1 to help facilitate decisions regarding equipment for a given marine survey. Nevertheless, for monitoring purposes, it is preferable to maintain consistent gear in time and space, and we therefore recommend this where possible.
For further information on the advantages and disadvantages of sleds and trawls compared to other benthic sampling platforms, refer to Comparative assessment of seafloor sampling platforms Przeslawski et al 2018).
Table 8.1: Types of benthic sleds and trawls deployed in Australian waters and their associated characteristics as of 2018. See reviews on benthic sleds and trawls for information about gear deployed elsewhere in the world (Clark et al. 2016, Kaiser and Brenke 2016). Unavailable indicates information that was unable to be obtained for this manual.
| Type | Dimensions (mouth, h x w, mm) | Weight | Target taxa | Cod end | Other features | Suitable terrain | Ref |
|---|---|---|---|---|---|---|---|
| Scampi trawl | 3200 width | 460 kg | Benthic and demersal fish, invertebrates | 50 mm mesh | 4 otterboards, groundgear includes footrope-bound chain | Various shelf substrates | Althaus, personal communication |
| Sherman (CSIRO-SEBS) sled | 600 x 1200 |
860 kg (excluding modifications from Lewis 2009) | Benthic invertebrates and fish | Polyethylene twine, 3.2 m long, 25 mm mesh | Reinforced frame, weak link chains, chaffing mat, net sonde, optional infaunal or 1 mm net | Seamount, rugged terrain, hard substrates | (Lewis 1999, 2009) |
| Rainer sled | 2900 mm width | 590 kg | Benthic invertebrates | 25 mm stretch mesh | Sled divided into epibenthic and infaunal halves | Various shelf substrates | (Bax et al. 1999) |
| AIMS sled | 1500 x 1000 | Large benthic invertebrates | 45 mm stretch diamond mesh | Various shelf substrates | (Colquhoun et al. 2007) | ||
| SARDI sled | 600 x 1800 | Sessile and sedentary epibenthos | 50 mm mesh | Soft sediment shelf ecosystems | (Ward et al. 2006) | ||
| NIWA seamount sled | 1130 x 380 | 400 kg | Sessile and sedentary epibenthos | 28 mm mesh | Reinforced frame, weak link chains, location beacon, anti-chafing net, smaller model available (250 kg) | Seamount, rugged terrain, hard substrates | (Clark and Stewart 2016) |
| Brenke Sledge (MNF) | 1300 x 1240 | unavailable | Benthic macrofauna | 0.5 mm mesh | Dual nets, nodule exclusion mesh, insulated cod end | Smooth terrain | (Brenke 2005) |
| MAPS sled | 300 x 500 | unavailable | Planktobenthos | 100, 500, and 1000 µm | Concurrent planktobenthic and benthic sampling, tri-layered net | Smooth terrain | (Przeslawski and McArthur 2009) |
| Scaled down Woods Hole sled | 300 | unavailable | unavailable | unavailable | unavailable | Estuaries | (Hirst 2004) |
| CSIRO beam trawl | 500 x 4000 | unavailable | unavailable | 25 mm mesh | Tickler chains, triple tow bridle, chaffing mat, pivot points | Flat to low relief terrain, soft substrates | (Lewis 2010) |
| Orange roughy trawl (ORH) | 26 000 x 6 500 m | 3 t in water | Large mobile fauna | Various depending on cod-end fitted (40 mm common) | Small attached cone nets to sample small animals, otter boards, heavy duty high ground gear | Rough bottom, including seamounts | (Clark et al. 2016) |
| Full-wing bottom trawl | 28 000x 3 500 m | 3 t in water | Mobile fauna, demersal and benthic species | Various depending on cod-end fitted (40 mm common) | Otter boards | Smooth terrain | (Clark and Roberts 2008) |
| NORFANZ beam trawl | 300 x 4000 mm | unavailable | Slower-moving demersal fish, benthic invertebrate mega-fauna | 10 mm | Chaffing mat | Smooth terrain | (Clark and Roberts 2008) |
| Florida flyer shrimp trawl | unavailable | unavailable | Mobile fauna, demersal and benthic species | unavailable | unavailable | Smooth terrain | (Wassenberg et al. 1997) |
| McKenna market trawl (CSIRO) | 19 000 x 5000 mm | unavailable | Mobile fauna, demersal and benthic species | 15 mm | Weighted bottom line, floats hold up the upper line, doors keep the net | Smooth terrain | SEF voyages, NWS voyages, RV Investigator deep-sea |