Impact assessment of the effect of a selected range of fishing gears in the North Sea

Discussions in the scientific literature, mainly on the methodology of impact studies, and in the media on the "questionable conclusiveness of the science", make clear that scientific results can easily be interpreted in different ways. It is indeed the case that the conclusions of the different studies can be contradictory if no attention is paid to the specifics of the experiment. The complexity of the marine ecosystem and the complexity of the interactions between fishing gear and life on the seafloor allow many conclusions. Put in the right perspective, however, most of these conclusions will confirm each other.

In this context we cite Gray et al. (2006) in reaction to an FAO report (Løkkeborg, 2005), posing the problem:

"The United Nation‟s Food and Agriculture Organisation commissioned and published a review of the impacts of trawling and scallop dredging on benthic habitats and communities (Løkkeborg, 2005). The main conclusion of this report (p. 47) is that, „„It is difficult to conduct impact studies leading to clear and unambiguous conclusions because knowledge of the complexity and natural variability of benthic communities is rudimentary‟‟. The review speculates further on the utility of grabs and box-corers as sampling tools (p. 9) stating that „„these methods are not suitable for sampling benthic fauna with patchy distribution and low abundance.‟‟ These worrying assertions reflect a profound ignorance of an abundant literature and could lead to inappropriate conclusions by a non-expert reader. "

Making their point clear:
"But how does disturbance affect an ecological assemblage? The „„intermediate disturbance‟‟ hypothesis has been shown to be a useful ecological rule, although it is scale dependent. A moderate disturbance can lead to an increase in species richness as some dominant species are reduced in abundance, so there is opportunity for new species to colonise and for species richness to increase. Trawling may have such an effect in systems characterized by low habitat structure and relatively ephemeral species. However, this conceptual response does not provide evidence that trawling is somehow beneficial simply because it may increase species richness at low levels of disturbance. Damage caused by trawling on a 400 year old cold water coral will take hundreds of years to repair. Such communities dominated by large and long-lived organisms are being impacted by fishing and elegant experiments are not need to prove the obvious immediate effects. Nevertheless we do need to learn about the full consequences of such impacts on biodiversity and define rates of recovery so that the ecological risks can be fully assessed. The intermediate disturbance hypothesis also predicts that as the disturbance persists or increases in strength, or frequency, then richness will decrease. Thus, the response is not a simple one and neither is a simple response expected by experienced researchers."

"Almost certainly the most significant effect of trawling on benthic assemblages is that of habitat homogenisation and/or destruction (a very large literature reviewed in Thrush and Dayton, 2002). This can have important effects on sediment biogeochemical processes as well as modify structure above the sediment surface."

"Natural sedimentary environments are not vast homogeneous plains of sand or mud but contain a variety of three-dimensional structure. These may be caused by natural physical variations in substratum such as isolated stones and patches of different types of sediment. Biological alteration of the sediment is extremely important; shells, animal tubes of a variety of shapes, sizes and durability, faecal piles, holes and pits are all key elements of the structure and functioning of these habitats. Research has shown that such structures are important cues for settlement processes of many organisms, can act as refugia from predators and affect ecosystem processes. Yet trawling tends to homogenise the sediment and reduces three-dimensional structure above and below the sediment-water interface."

The study of environmental impact of fishing gear is complex and with this report we hope to add some clarity in the discussion. We welcome any comment or suggestion at hans.polet@ilvo.vlaanderen.be and jochen.depestele@ilvo.vlaanderen.be. We intend to improve this report so it can be a useful and accessible reference.