Surveys of deepwater benthic communities using a benthic sled in Ningaloo Marine Park, Western Australia (WAMSI Node 3 Project 1 Subproject 3.1.1)

Full description

Towed video footage and single beam sonar allowed for the detection and targeting of different benthic communities with the benthic sled, so species and functional groups could be identified and their distribution, abundance, biomass and size composition investigated.50 sites were sampled in 2006 and a further 12 sites were sampled in 2007. Sites were located at various depths in different habitats with varying degrees of benthos.
Due to the large size of some of the benthos collected (i.e. sponges and gorgonians) in the 2006 field work, the dimensions of the CSIRO designed benthic sled were sometimes inadequate for the purposes of our survey and a new sled was designed and built for the 2007 field work. The new steel benthic sled is based on the design of a commercial fishing Tri-gear Trawl Beam, slightly modified for our purposes. Traditionally a Tri-gear Trawl Beam is used to locate, track and sample scallops and deep water prawns in depths less than 1000 metres. The dimensions of the sled used at Ningaloo are 1.5 m wide x 1 m high. The net and cod end are made of 48 ply 2" Amican TM. It is a 4 seam net; top and bottom panels 100 meshes wide with wing panels 50 meshes high. It has a 9 bar 1 point taper with panels 75 meshes long with dimensions of 55 meshes long and 100 meshes round. The net and cod end are protected by rubber chafe matting designed to minimise damage from abrasive substrates (i.e. rock, coral). There is a double cod end to help secure the sample from being lost from a torn net.Trials indicated the new sled's sampling was an improvement on 2006 and all the macro benthos shown on towed video was being sampled. Trials also indicated that the most representative and manageable sled sample distance, out of several tested, was 50 metres in all the habitats sampled. Fifty metre long samples will be adopted as the standard for future samples to be carried out in 2008.

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Statement: Distance fished was estimated by the winch operator, recording the GPS position when the sled touches the bottom and when it leaves the bottom. GPS positions provide the distance travelled in metres of all the samples conducted. To ensure more accurate fishing times for the sled in the future a depth sensor, time synchronised to the ships GPS, will be attached to provide a downloadable profile of fishing time and distance. The improved benthic sled design and fishing time/distance measurement capabilities to be fitted before 2008 will allow us to quantify more accurately the abundance and biomass of phyla that make up the benthic communities in different areas of the marine park. An improved processing protocol for each sample has been established, with the assistance of the West Australian Museum, and will be further developed and documented. Overall weight of each sample was calculated by weighing the empty and full cod end with a 200 kg capacity digital scale attached to the end of a crane. A standard fixed lifting point was made at the end of the cod end. Weighing both empty and full allowed for the total biomass of each sample to be calculated in varying conditions on board the vessel. The whole sample was then emptied onto the sorting tray. Each phyla represented was separated. The dominant taxa from the main habitats were selected for priority identification i.e. Porifera (sponges), Alcyonaria (gorgonians, whipcorals). Overall sponge weight in each sample was measured. Individual sponge weights in each sample were measured. The weight of other dominant sessile phyla was measured (i.e. bryozoans, soft corals etc). The weights of the dominant mobile phyla were mearured. Trials indicated that both a higher capacity scale >200 kg and lower capacity scales

Notes

Credit
Colquhoun, Jamie, Mr (Principal Investigator)