PhD thesis of Alexandre Robert (Agrocampus Ouest) defended on 30/05/2017
Bottom trawling is a widespread fishing activity that covers most of the North-European continental shelves. This activity is likely to alter the benthic habitats, the structure and biodiversity of benthic communities as well as the ecosystem functioning. However, bottom trawling effects vary according to the habitats and, specifically according to the magnitude of the natural sources of disruptions. Consequently, it is difficult to predict the effects of the fishing activity. So, a specific investigation has to be conducted in each fishery in order to characterize the effect of bottom trawling.
The Bay of Biscay (BoB) is fished for Norway lobsters and hake. Bottom trawling is one of the most important fishing activities, both economically and regarding to the fishing effort. Nevertheless, the benthic ecosystem of the BoB is still relatively unknown. The present thesis participates to fill this lack of knowledge by investigating the role of natural factors and bottom trawling on the structure and functioning of the benthic communities. We focused on a soft bottom area called "Grande Vasière" (GV) that crystalizes most of the fishing effort within the BoB.
At the GV scale, our results suggest that bottom trawling is the main factor that drives the megabenthic structure (> 10mm). However, this hypothesis has not been formally verified because of the difficulty to dissociate the effects of the fishing intensity and from those of certain environmental characteristics (depth, sediment classes). Hence, our further works have be conducted on a restricted part of the GV, chosen in order to minimize the environmental variations while conserving the gradient of trawling intensity that has been previously observed at the GV scale. Results obtained on this small area suggest that bottom trawling modifies the structure of both mega- and macrobenthic (> 1mm) communities. However, the effects were seasonal and transient. They could be attributable to an increase of food availability for megafaunal predator-scavengers and to changes of the sedimentary characteristics for macrofauna. A contrario, the investigation of functional diversity thanks to a biological traits analysis, suggests that changes in the structure of benthic communities have minor repercussions on the benthic ecosystem functioning.
We concluded that the benthic ecosystem functioning of the GV has been adapted to several decades of intensive trawling.