A visual summary of the key steps to follow when deploying benthic sleds or bottom trawls is shown in Figure 8.1.


Figure 8.1: Images from key steps involved in the use of benthic sleds and bottom trawls for marine monitoring: a) a modified WHOI sled with attached pipe dredges, b) seafloor imagery from towed video and bathymetric grids, c) lowering the AIMS benthic sled, d) sorting animals on the back deck, e) photographing specimens in ship laboratory, f) securely sealed containers to ship animals to museums

Onboard sample acquisition

  1. Use acoustic data or underwater imagery to confirm areas to sample with the appropriate benthic gear (Schlacher et al. 2007, Williams et al. 2010). Do not deploy blind, as this increases the risk of equipment loss and damage, as well as unnecessary impact on potentially vulnerable ecosystems.
  2. Brief crew and sorting staff on potential venomous or otherwise dangerous catch (i.e. cone shell, blue-ringed octopus, some fishes, corals, sponges, urchins).
  3. Ensure the gear is set-up and deployment parameters and procedures are as documented in the gear-specific protocols.
  4. Use netsonde or bottom contact sensor to ensure sled or trawl is suitably deployed along the seafloor [Recommended]
  5. Use USBL System to ensure accurate positioning (Schlacher et al. 2007, Williams et al. 2015) [Recommended]
  6. Mark sled runners or trawl groundline with waterproof pencil or paint to gauge success of seafloor deployment. Also check for polishing on the bobbins or runners. [Recommended]
  7. Record all metadata related to a given tow, specified in Table 8.2.
  8. For rugged slopes (e.g. seamounts), ensure appropriate gear is used and tow downslope to reduce snags.
  9. Maintain speed that is appropriate for the gear and seafloor terrain. Epibenthic sleds and most beam and Agassiz trawls should be towed at 1–2 knots to maintain bottom contact, while faster speeds of 3–3.5 knots are appropriate for otter trawls and other gear dependent on speed to maintain net spreading. See Clark et al. 2016 and Kaiser and Brenke 2016 for details.
  10. Tow into the swell, tide, current and/or wind so that vessel speed and steerage can be better controlled.
  11. A standard fixed tow distance (i.e. bottom time) for monitoring purposes is not practical because because tow distance is highly dependent on gear type and seafloor environment. However, within a given survey, tow distance for each sled or trawl should be standardised to assess relative abundances. It should also be recorded in the metadata (Table 8.2). If the same sled is used on multiple surveys in similar environments, the tow distance should remain the same so that spatio-temporal comparisons can be made. For benthic sleds deployed along the continental shelf over mixed terrain, a tow distance of ~100 m is recommended. Longer tows (commonly 300 m) will be needed in deep waters due to lower density of macro- and megafauna. Information from multibeam data (see point 1) can help inform tow duration decisions.
  12. Assess success of deployment. If there is significant damage to gear, signs of minimal bottom contact, or ripped nets, this should be recorded in the metadata (Table 8.2). The catch from such deployments can be considered for presence-only analyses, species inventories or biological analyses. Inclusion in quantitative comparisons with other tows should only be done after careful consideration of appropriate statistical methods (e.g. transformation, standardisation). In such situations, gear configuration should also be checked after recovery to ensure its correct specification for the next deployment (see point 3).
  13. When the sled or trawl is lifted from the water, follow gear- and vessel-specific protocols for safe release of the catch onto the deck or sorting table.
  14. Record biomass of entire catch using electronics from winch system or onboard scale [Recommended]
  15. Photograph entire catch with station identification placard and make notes of catch composition (e.g. lots of mud or rocks) in metadata sheet (Table 8.2).
  16. Remove all animals from the entire net, including the fore-parts of nets and sleds and not just the codend where most of the catch should have been collected. As soon as practical, begin onboard processing of the samples (next section).
  17. Clean sled of all material and prepare for next deployment.

Onboard sample processing

  1. For very large catches, implement the agreed sub-sampling protocol if applicable (see Pre-Survey Preparations).
  2. Consider retaining material on ice or in an ice slurry while awaiting sorting to ensure material remains in best condition to assist accurate and consistent identification.
  3. Separate large easily visible taxa into sorting trays by coarse groups: fish, sponges, soft corals, echinoderms, molluscs, ascidians, bryozoans, annelids, other. Weigh each group. Discard severely damaged organisms and non-biogenic material, unless otherwise needed. It can be useful to record the weights, descriptions, and images of rock, coral rubble and other non-biogenic material as this gives useful information on substrate type. Add a label to each sorting tray with Tow ID so as to avoid confusion when multiple tows are being processed.
  4. Follow Animal Ethics procedures to euthanize animals where applicable
  5. Place fragile organisms in seawater in the sorting trays. Use chilled seawater for deep-sea and polar samples to minimise sample degradation during sorting time.
  6. Transfer groups to the sorting station, if not already there. See Coggan et al. (2005) for practical advice on setting up a sorting station.
  7. Based on previous decisions about onboard level of sorting (Section 8.5), progressively sort organisms into finer taxonomic groups, as much as time or expertise allows, with OTU (operational taxonomic unit) or species representing the finest taxonomic level.
  8. Weigh, count, and photograph each of the final groups, including a scale bar and unique identifying sample number. Ensure this is done in a way that doesn’t destroy the DNA in the specimens (e.g. pericards need to be kept chilled and moist). Refer to Schiaparelli et al. 2016 for suggestions on specimen photography.
  9. Record data against a unique station identifier for the data base and keep a label with the same unique identifier with the specimen(s) (Table 8.3). At this stage identify specimens (or subset of specimens) for analyses purposes (whole specimens for taxonomy/isotopes/genetics etc.) or where appropriate (and pre-determined in plan) take tissue samples for analyses (genetics, isotopes etc.) If there are large numbers of the same species or OTU, only a sub-set may need to be preserved for museum collections; this should be established during Pre-Survey Planning in consultation with taxonomists or curators. In this case, record the total number collected (i.e. number caught) as well as the number in the collection container (i.e. number preserved).
  10. If applicable relax and fix specimens according to survey objectives and taxonomists’ preferences (e.g. samples for genetic analysis should not be fixed in formalin).
  11. Preserve specimen according to methods decided in Pre-Survey Preparations, and place into container. See Rees (2009) and Schiaparelli et al. (2016b) for comprehensive description of fixatives and preservatives used for marine invertebrates.
  12. Place solvent-hardy label with unique identifier in each sample container. It is not sufficient to label only the outside of the container, as this can easily rub off. See Box 15.6 in Schiaparelli et al. 2016 for suitable label characteristics.
  13. Place container in large sealable container (i.e. lidded drum) with other samples preserved using the same chemicals (e.g. ethanol) or method (e.g. freezing). It saves time in post-survey sample distribution if taxa are grouped together in containers rather than by station.

Onboard sample storage

  1. Store large labelled drum onboard in the freezer or in an approved storage area for hazardous chemicals.
  2. Transcribe metadata from Tables 8.2 and 8.3 into digital format as soon as possible to minimise the build-up of data entry. This must be done onboard preferably during the same shift because it provides a back-up and an immediate check of the record, as well as facilitating timely metadata release.
  3. Check the data entry is correct by cross-checking field sheets with database. This is best done by a person who didn’t enter the data [Recommended].
  4. During demobilisation, ensure samples and drums are properly labelled and closed, and implement shipping according to decisions made during pre-survey planning.

Table 8.2: Sample field datasheet to record metadata (i.e. deployment or event data) from each sled or trawl haul. Waterproof paper and pen/pencil is required.

Gear in water Gear on bottom Tow speed Wire out (length)8 Wire out (angle)8 Gear off bottom Gear out of water Total catch biomass9 Notes10
Tow ID Lon Lat Time Lon Lat Depth Time11 Lon Lat Depth Time11 Lon Lat Time11
(e.g. equipment configuration changes during survey, torn net, etc):

8 Record the length and angle of wire payed out during seafloor contact. This is required if deep water survey with no USBL; otherwise recommended.
9 Include units (e.g. kilograms)
10 Record person entering data, spread of trawl doors if applicable
11 UTC timezone

Table 8.3: Sample field datasheet to record metadata from each sorted biological sample. Waterproof paper and pen/pencil is required.

Tow ID Sample ID Phylum Class Order Family Genus, Species / Common Name Weight Abundance Preservative / Quantity Photos Notes