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Damaged coral reef in Troms III.
Damaged coral reef in Troms III.
Photo: MAREANO/Institute of Marine Research
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The impact of trawling

Videos taken by the MAREANO project immediately after bottom trawling in the Troms III area show bottom sediments that are stirred up and sponges covered in “dust”. It appears that the trawling destroys the squat lobster’s burrows, and there is also very little fauna visible in the direct trawl path.

By Pål Buhl-Mortensen, Lis Lindahl Jørgensen and Valerie Bellec

MAREANO’s summer expedition went like a clockwork in the fine summer weather. Even though we were hit by gales for a while, the waves never became so high that we had to stop working. We completed the mapping both on the Nordkapp Bank, and the Troms III area. We observed that the fauna was poorer on the Nordkapp Bank than in the study areas to the south (Troms III and Nordland VII), we found species that we have not seen further south, discovered several new coral reefs and once again we saw how widespread the traces of bottom trawling are. In this small article we want to share with you our observations of trawl marks and the damage to benthic fauna.

Widespread trawl marks

Traces of trawling on the sea bottom are common in the areas mapped by the MAREANO project. At 93 percent of the locations videoed during MAREANO’s summer expedition we found trawl marks. At 70 percent of the locations we found more than one trawl mark per 100 metres. The highest density was 5.3 trawl marks per 100 metres. Although this is less than previous observations on MAREANO expeditions to Tromsøflaket, the average values for the two areas are almost identical at 3.2 trawl marks per 100 metres. By comparison the trawling is much less intensive in Troms III than Nordkapp Bank. On average there were 1.4 trawl marks per 100 metres in Troms III, with a maximum value of 4.7.

Trawl marks in Tromsøflaket (map)

Trawl marks observed by the Campod video rig in Tromsøflaket. Previous results from Troms III are also shown on the map.

Absence of fauna after trawling

Not much is known about the impact of bottom trawling on benthic fauna. We do know that large and delicate organisms such as corals, sponges and sea pens can easily be damaged by trawling. However, it is unclear how smaller organisms cope with it. On this voyage we were able to observe what the sea bottom looks like immediately after trawling. At the outer edge of the continental shelf in Troms III we took a video straight after one or two trawlers had passed over the location with their trawls. The first thing we noticed was that the seabed had been stirred up. Visibility was poor, and there was sand on all of the stones and sponges, as well as on the shell of a king crab. The normal brown layer of detritus (consisting of organic material that sinks to the sea floor) was almost entirely absent. We also noticed that there were no burrows of the type usually made by the squat lobster Munida. On the other hand we did observe some squat lobsters, but we think that their burrows maybe were destroyed by the trawling. We also saw a buried sponge and a crushed unidentified organism. Apart from that there was a striking absence of fauna within the trawl path. What impact will that have on the future composition of benthic fauna in the area? Will the small organisms that may have been killed and buried be replaced by new larvae that settle and develop, or will other species take over? Will the sand-covered sponges survive? And what about small filter feeders such as bristleworms and moss animals? As there are few studies into how the fauna changes over time in locations affected by trawling, we are not in a position to answer those questions.


Recently trawled sea floor.

Recently trawled sea floor.

Photo: MAREANO/Institute of Marine Research

Coral reefs

It is easier to understand the impact of bottom trawling on Lophelia coral reefs, as the contact with the trawl leaves easily recognisable scars in the reef, and a number of studies have shown that bottom trawling has a dramatic effect on these reefs. During this voyage we have so far discovered seven new coral reefs, on three of which we found traces of trawling on the reef itself. In one case there was extensive damage, with crushed corals spread out on the sea floor. At the other reefs we did not observe any trawl marks either on the reefs or in the surrounding area. This part of Troms III probably has a reputation amongst fishermen as being a “difficult” area, with lots of coral.



Photo: MAREANO/Havforskningsinstituttet