Interpreting large-scale experiments on effects of trawling on benthic fauna: an empirical test of the potential effects of spatial confounding in experiments without replicated control and trawled areas

Disturbances due to trawling and dredging is a serious threat to assemblage s of benthic marine animals. We tested hypotheses about effects of trawling on benthic assemblages in a manipulative field experiment, using gear and intensities relevant to future management of trawling in a Swedish fjord. T hree trawled and three control sites were sampled at several times before a nd after trawling was initiated. This paper describes how conclusions about effects of trawling might differ between experiments involving replicate s ites and experiments using only one trawled and one control site, as in sev eral recent studies. Analyses Df selected taxa showed that abundances of ma ny species changed differently among control sites. Differences in temporal change between pairs of single trawled and control sites were also frequen t. Neither the quantitative nor the qualitative nature of differences betwe en treatments could, however, be coherently interpreted among the different combinations of trawled and control sites. This is consistent with results obtained from analyses using all sites, which showed no consistent effects of trawling on any of these taxa. These results provide empirical evidence that spatial confounding may cause serious problems to formal interpretati on of experiments, which use only one control and one trawled area. Such po tential problems can best be solved by ensuring that the study incorporates more than one control site. (C) 2000 Elsevier Science B.V. All rights rese rved.