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Who lives where on our seabed?

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An assortment of demersal fish and invertebrates about to be released from the net (Picture: Dr Lara Atkinson)
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A photo display representing all invertebrate species caught in trawl number 105, at station number A31476 from 209m depth (Picture: Dr Lara Atkinson)
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The trawl deck of the research vessel FVS Africana (Picture: Dr Lara Atkinson)
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The trawl ramp and doors at the stern of the FVS Africana (Picture: Dr Lara Atkinson)
- Dr Lara Atkinson, Offshore Marine Scientist, SAEON Egagasini Node

 

The honest answer to this question within South Africa’s Exclusive Economic Zone is that we don’t really know.

Conducting research at the bottom of the ocean, on the cold, dark, muddy seafloor has many challenges associated with it, foremost being the simple inaccessibility of this habitat to researchers. Nonetheless, the biological processes driven by the species living in these habitats play an important role in overall ecosystem functioning and there is much we don’t really know or understand about these ecosystems in South Africa.

Highly technical, advanced, innovative tools, like remotely operated vehicles or deep-water cameras that can be used to study the seafloor are often on the wish list of South African benthic (seabed) marine scientists. However, these tools themselves also have limitations in what they can achieve. In situations like this, it is often most pertinent to explore first principals, and in the case of South Africa’s seabed, that would be back more than 100 years to the explorations of South Africa’s first marine scientist, John D. Gilchrist.

Gilchrist’s explorations

In the early 1900s, Gilchrist explored South Africa’s marine environment, largely with the intent of looking for harvestable, marketable marine resources. Fortunately Gilchrist was also an adept scientist who documented, to precise detail, each and every marine species he encountered in his research efforts.

Much of the research conducted by Gilchrist in the early years included trawling, a type of fishing method that involves dragging a large net on or just above the seabed, collecting everything in its path, to be delivered onto the ship’s deck once hauled aboard. This same fishing and sampling technique, called demersal trawling, is still frequently used today for both commercial and research purposes, albeit with substantial technological advancements in gear and ship capacity.

Demersal research surveys

Fisheries scientists from the former Marine and Coastal Management Branch of the Department of Environmental Affairs and Tourism, now based in the Fisheries Branch of the Department of Agriculture, Forestry and Fisheries (DAFF), have conducted dedicated annual or bi-annual demersal research surveys since 1986 along South Africa’s west and south coasts. The primary objective of these surveys is to assess the status of South Africa’s most lucrative marine resource; hake.

During these surveys, approximately 100 research trawls are conducted along each coast, spanning the continental shelf and shelf edge. All vertebrate fish and cephalopods landed on the deck are sorted, identified, counted (or estimated) and weighed by dedicated DAFF staff members. However, along with the fish and cephalopods in the net are also a diverse array of invertebrate species living on the seabed, like starfish, crabs, sea urchins and sponges.

DAFF staff, in collaboration with the newly formed Oceans and Coasts Branch of the Department of Environmental Affairs (DEA) have, in the past, recorded some invertebrate species at a coarse taxonomic level. With the acknowledged increasing importance of biodiversity in regulating global change and a focus on ecosystem based management, there is strong motivation to strengthen South Africa’s long-term marine observations by incorporating all components of the environment, where possible.

Expertise to build capacity

SAEON Egagasini is collaborating with DAFF and DEA to provide expertise to build capacity and incorporate long-term invertebrate monitoring as part of annual demersal research surveys.

It was with this objective in mind that I joined the 17 member research team and set sail from Cape Town harbour on 14 January 2011 aboard the FVS Africana for her 270th voyage. With the seasonal south easterly wind blowing, forming ‘white horses’ across Table Bay and making for a lumpy, bumpy departure, the team set off to traverse the western continental shelf of South Africa.

Working in a zig-zag fashion across the shelf, 123 random sampling stations stratified by depth zones, were trawled, covering the area from Cape Agulhas in the south to the Orange River mouth in the north. For the next 31 days our lives would be controlled by the increasingly familiar announcements over the loud speaker of "Stand-by Stand-by, over she goes", meaning the trawl net was being deployed, followed 30 minutes later by "Stand-by Stand-by, up trawl" which meant the net was coming up.

It was the latter of these announcements that would activate a flurry of activity from the crew and scientists in trying to hastily complete whatever it was you were doing and get ready for action on the trawl deck. Amidst a cacophony of yelling crew, the groans of the trawl winch and the sounds of the wind and waves, the net is landed on the deck. There is always a moment of anticipation just as the net reveals its catch from the bottom of the sea. Would there be a strange, fascinating creature that none of us had seen before?

Sorting the catch by species

Without hesitation the scientists descended on the marine life piled on the deck and proceeded to sort the catch by species. One DEA team member (Lieze Swart or Deon Kotze) and I were responsible for collecting all invertebrate species (except cephalopods) and trying to prevent them from being squashed underfoot beyond identification or being mixed in with regurgitated fish stomach contents. Not always a pleasant task on a queasy stomach before breakfast at 07:30 on a Sunday morning!

In a relatively short period of time, the deck was cleared and the invertebrate team of two would retreat to the quieter starboard laboratory and carefully sort the invertebrate component of the catch by species. Many of the more common species identifications are known and these are rapidly counted, weighed and documented.

However, many of the more delicate, small and/or rare species proved to be more difficult to identify accurately. The invertebrate team would painstakingly page through the identification guides that had been pre-prepared from previous ad hoc invertebrate surveys and a pile of local and international field guides and taxonomic keys, searching for some clues about the genus or species name of the unknown invertebrate in question. Even documents dating back to the specimens collected during the early Gilchrist surveys are still used to identify the invertebrates from the trawl catch.

"Sponge Bobs"

Sometimes, even after counting tiny spines, claws, legs or feet, it was still impossible to accurately identify some species and these individuals were then photographed, labelled and preserved for more expert identification attempts back ashore. Frequently the invertebrate team (or "Sponge Bobs" as the ‘fish team’ had nicknamed us) would just be completing the last photograph, label or item on the datasheet when the next call for "Stand-by Stand-by up trawl" would boom out, leaving little time for a cup of tea, a toilet break or more importantly, admiring the magnificence of the three albatross species gracefully gliding near the vessel when we were in deeper waters.

The work was intense, back-breaking with not a single day off during the 31 days at sea, but the rewards were great. A total of 132 invertebrate species were identified to genus level and recorded from 123 research trawls completed, ranging from 35 m to 815 m in depth, spanning the west coast of South Africa. Approximately 47 additional species that were unable to be accurately identified at sea were preserved and brought back to Cape Town. Establishment of the invertebrate monitoring programme was substantially advanced during the 2011 west coast demersal survey.

Next steps

The next phase in the project is now to refine the species identifications by checking our preserved specimens against those catalogued in the South African Iziko Museum and employing the expertise of local and/or international taxonomists. Development of the Identification Field guide for South African Offshore Invertebrates is well under way and will now be updated with photographs, descriptions and corrected species names as progress is made.

Preparations for the 2011 south coast demersal research survey have begun and the pressure is on to update and refine the identification guide as far as possible to enable improved species identifications and data collection during the next survey. We have made substantial progress in a relatively short space of time (although 31 days felt like a lifetime out there) and there is long-term commitment to this monitoring programme, however, the urgency of just how much we still don’t know about our seabed habitats, or what lives there remains a pressing issue.

The long-term vision for use of the invertebrate data collected during such research surveys includes long-term monitoring to address an ecosystem approach to fisheries management, mapping of invertebrate biodiversity patterns in the offshore environment and providing data to assist in addressing knowledge gaps about South Africa’s marine benthic habitats and communities.

With a little extra effort, we can gain a wealth of new knowledge.

Related content: Dr Lara Atkinson aims to boost research in offshore marine environment

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