Diving deeper

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ZSL's Chris Yesson, Mona Fuhrmann and Stephen Long have just returned from a research cruise in Arctic waters of West Greenland. Following their previous work to monitor seafloor habitats in the  Marine Stewardship Council (MSC) certified prawn fishery, they explored the seabed around the offshore MSC Greenland halibut fishery in the Davis Strait.

This time we went deeper than ever before, which was both exciting and challenging. The offshore Greenland halibut fishery trawls the seafloor at depths ranging from 800 to 1500m. Our mission was to explore these never before seen habitats. We joined the annual stock assessment conducted by the Greenland Institute of Natural Resources (GINR) aboard the research vessel Paamiut. Using towed underwater camera and a benthic dredge to physically collect samples we were able to investigate areas inside and outside of the fishery. The aim is to be able to determine how trawling is impacting the deep-sea habitats.

At the beginning of the cruise, heavy wind and waves made life on board extremely difficult and dangerous, preventing us conducting surveys. Luckily, after a few days the sea calmed down and allowed us to start lowering our video equipment to the deep sea, beyond the reach of sunlight.

What we found definitely met up to our expectations: despite cold temperatures, pressure and no light, the biodiversity down there was amazing! We found beds of seapens on muddy bottoms, cup corals, sea anemones and large sponges to name only a few. Every time we would encounter a stone, it was like a little oasis, providing a hard surface for life to thrive! Sampling some of the specimens with a benthic dredge and actually holding them in our hands helped us to identify the species we saw in our videos.


Black corals on drop stones
Glacial drop stones offer attachment to a variety of species, i.e black corals and sponges, 1300 m.

Sadly, but not surprisingly, we also saw signs of trawling and habitat destruction in areas of heavy fishing. The heavy gear flattens out the seafloor and causes visible tracks, removing most larger and erect fauna, such as sea pens, sponges and corals.

Greenland, trawling impact
Trawl marks on the seafloor in areas of high fishing effort. Here at a depth of 1250 m, next to the abundant flat sea urchin.

Only a small fraction of the seafloor life that is impacted by nets actually makes it to the surface. Nevertheless, we are able to find out useful information by inspecting the bycatch in the stock assessment trawl nets. On one occassion we caught a huge amount of sponges likely a common bycatch in fisheries. Sponges are habitat forming species, important for other invertebrates and of course fish, their removal by fishing will have knock on affects through the food chain. Also vulnerable to bycatch are delicate seapens, these beautiful coral related species stand up to 1 m above the floor, filtering tiny particles from the water column.

Benthic bycatch, Greenland, halibut fishery
Mona and Chris looking at some of the benthic bycatch in halibut trawls

One of the personal highlights was definitely being followed by northern bottlenose and sperm whales’ says Mona Fuhrmann. These whales dive deep down and then follow the trawl to the surface, picking their share of halibut from the net on the way up.

Northern bottlenose whales following trawl haul, West Greenland
Northern bottlenose whales following in the trawl haul.

Further analysis of video footage and effort data will quantify the effect of trawling in this area. These findings will be shared among stakeholders contributing to management and annual re-assessment of sustainability as part of the Marine Stewardship Council’s certification program.

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