Symposium Abstract: Detecting the Effects of Fishing on Seabed Community Diversity: Importance of Scale and Sample Size
M. J. Kaiser
I investigated the importance of the extent of area sampled to the observed outcome of comparisons of the diversity of seabed assemblages in different areas of the seabed that experience either low or high levels of fishing disturbance. Using a finite data set, within each disturbance regime, samples of the benthic communities were pooled at random. Thus while individual sample size increased with each additional level of pooled data, the number of samples decreased accordingly. Detecting the effects of disturbance on species diversity was strongly scale dependent. Despite increased replication at smaller scales, disturbance effects were more apparent when larger, but less numerous, samples were collected. The detection of disturbance effects was also affected by the choice of sampling device. Disturbance effects were apparent when using pooled anchor-dredge samples, but were not apparent for pooled beam-trawl samples. A more detailed examination of the beam-trawl data emphasised that a whole community approach to the investigation of changes in diversity can miss responses in particular components (e.g. decapod crustacea) of the community. The latter may be more adversely affected by disturbance in comparison with the majority of the taxa found within the benthic assemblage. Further, the diversity of some groups (e.g. echinoderms) actually increased with disturbance. Experimental designs and sampling regimes that focus on diversity at only one scale may miss important disturbance effects that occur at larger or smaller scales.