Macrofauna van de Noordzee: effect van bodemvisserij op de diversiteit van de benthische fauna
Braeckman, U. (2005). Macrofauna van de Noordzee: effect van bodemvisserij op de diversiteit van de benthische fauna. MSc Thesis. Universiteit Gent. Faculteit Wetenschappen: Gent. 79, appendices pp.
Demersal fisheries affect benthic life not just directl y through the catch of local fish fauna, but indirectly too, through disturbance of the lower trophic levels and their habitat. Communities of epi -and infauna turn out to suffer from both structural and habitat changes as an effect of chronic bottom trawling. As these epi -and infauna are the food source for commercially exploited target species, the knowledge of the ecological consequences of demersal fishing on benthic fauna is essential According to Huston's dynamic equilibrium model (1979, 1994), the effects of disturbance on diversity are dependent on local productivity .Highest diversity will be situated in areas with intermediate disturbance and productivity. If this model appears to be applicable to the situation in the North Sea, predictions on the diversity of local fauna can be made and the model can subsequently be used as a simple tool for advising fishery managers. In the frame ofthe MAFCONS -project, two sampling campaigns on the North Sea (in August -September 2003 and 2004) were organized. Both fish and epi -and infauna were sampled in 35 stations. Weights of the lower trophic levels (epi- and infauna) were converted into productivity by using Edgar's (1990) method. Disturbance was determined from effort data on ICES rectangle scale between 1997 and 2002. Huston' s dynamic equilibrium model has previously been tested with terrestrial and marine data (phytoplankton studies) and pattems similar to the original model were obtained In this thesis, the dynamic equilibrium model couldn 't be proved. This can be attributed to a lack of data or rather to the variation between the values of the model' s parameters that is too small A small community analysis didn 't result in new perceptions in the field of the parameters of Huston's model. Stations only clustered by environmental variables and not by values of local productivity, diversity and disturbance As a result, benthic infaunal productivity and diversity in frequently disturded areas in the North Sea can plausibly be supposed to have become stable and therefore show little variation. Furthermore, data analysis showed high infauna densities, but not necessarily high diversities in muddy sediments. A positive relationship between infaunal productivity and fishing activity was found not truly significant. If this relationship is considered to be significant all the same, fishers can be supposed to frequent high productive areas because of the presence of high densities of demersal target fish. This consideration was confirmed by the significant positive relationship between infaunal productivity and demersal fish density. If the mentioned relationship between fishing activity and infaunal productivity is considered to be not significant, Jennings' (2002) theory saying that chronic trawling disturbance doesn't affect infaunal productivity is confirmed. Total productivity of the trophic level that is foraged by demersal fish turned out to be determined for more than 99 percent by infaunal productivity. This confirms the general theory that small-bodied organisms have a higher metabolism and consequently a higher productivity. By means of this result, studies using bentbic productivity as a parameter can focus on acquiring more accurate and efficient data.
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