Ocean floor video to track trawlers’ footprint

30 June 2014

A pioneering experiment being conducted in the Atlantic Ocean 100 nautical miles off the west coast of South Africa will tell marine researchers from the University of Cape Town (UCT) how long the seabed and its ecosystem takes to recover after hake trawling operations.

The Benthic Trawl Experiment is a joint research project between UCT, the South African Deep Sea Trawl Industry Association, the South African Environmental Observation Network (Saeon), and the Department of Agriculture, Forestry and Fisheries.

It’s the first in a series of experimental surveys of the impact of bottom trawling on the benthic communities – organisms that live in or on the seafloor – of the outer shelf of South Africa’s west coast, says Associate Professor Colin Attwood, the chief scientist on the project.

These will be conducted annually, though Attwood says it will take longer than the allotted four years for previously trawled lanes to bounce back.

Recovery among sea-floor species

Saeon’s Lara Atkinson, who is managing the project, says: “We have been planning for a long time to get this experiment under way, and we’re very excited about this opportunity to be able to monitor for any changes in the benthic communities in the areas where trawling has stopped.”

The researchers have established a 6-by-15 nautical mile block within one of the fisheries’ commercial blocks, and divided this into five lanes, two of which will remain open to trawling. The remaining three will be closed over the project’s lifespan.

Data recovered during repeated surveys over this time will give researchers a clearer idea of recovery among seafloor species – and of the ocean floor itself, which becomes churned up by metal trawl “doors” (some weighing up to 10 tons) angled to hold open the net mouth as it is pulled along.

A towed camera and a Van Veen grab (instrumentation used to collect sediment) will be used to sample benthic epi-fauna and in-fauna respectively.

Marine life not previously seen on the seabed

Two-and-a-half hours of video footage of the seabed, shot during the first of the experimental surveys on board the research vessel Ellen Khuzwayo in February, show marine life previously not seen on the seabed: jacopever, spiny eels, starfish, anemones and other crustaceans – and hake.

“The footage is very clear; it’s the best and most systematic video we have of life in the trawl lanes,” says Attwood.

Hake live in deep water during the day, mostly at depths of 350 to 600 metres, and because of their inaccessibility they aren’t often observed in their wild habitat. But they migrate vertically, coming up to feed at night. Most trawling is therefore done during the day when the fish are near the seabed.

“Mass food production has an environmental impact, and it’s great that we have the fishing industry’s co-operation on this experiment,” says Attwood. “They understand that we need to limit harvest to replenishable numbers, and that it’s important to work with the ecosystem.”

Source: University of Cape Town