Field experiments were conducted on 2 subtidal sandflats to identify the short-term impacts of commercial scallop dredging on macrobenthic communities. The 2 sites (1400 m2) were situated 14 km apart, both at about 24 m depth, with similar exposure aspects and were characterised by infaunal communities dominated by small and short-lived species. Prior to dredging, preliminary sampling failed to reveal significant differences in the density of common macrofauna within each site, although community composition was distinctly different between sites The experiment was initiated by using a commercial scallop dredge to dredge half of each study site. Macrofauna samples were collected in both the dredged and adjacent reference plot at each site irnmed~ately after dredging and again 3 mo later. The density of common macrofaunal populations at each site decreased as a result of dredging, with some populations still significantly different from the adjacent reference plot after 3 mo. Significant compositional differences in the assemblage structure between dredged and reference plots were also recorded at each site over the course of the experiment. The f~n d~n g s of this experiment are considered a conservative assessment of bottom disturbance by fishing because of the area of seabed used, the types of community present and the intensity of disturbance used in the experiment. The findings of this and similar short-term experiments are discussed in light of the need to predict and assess possible large-scale changes to benthic communities as a result of habitat disturbance by fishing.

Field experiments were used to assess the influence of density vanatlons of adult bivalves on recruitment of juveniles. The generality of these results was tested by running the experiment concurrently at 2 sltes of different sediment grain size and wave exposure. Adults of 2 bivalve species, the deposit-feedlng tellinid Macomona lillana and the suspension-feeding venerid Austrorienus stutchburj~i, were used in the experiment. The experiment consisted of 14 treatments of vanous combinations of densities of live adults or the empty articulated shells of the 2 species. Three species of juvenile bivalve (M. liliana, A. stutchburyi and Nucula hartvigiana) were sampled from the experiment on 3 occasions over a 9 wk period. Treatment effects were consistent over lime, but reflected only small changes In the mean density of juvenile bivalves. The presence of empty articulated shells of adult M. liliana or A stutchburyi, positioned at localities within the sediment which reflected normal living conditions, had no detectable influence on the density of juvenile bivalves. Treatments ~nvolving adult A stutchburyi ~nfluenced only the density of juvenile conspecifics at the muddy-sand site. Treatments involving adult M. liliana produced significant effects on the density of both M. l~liana and N. hartvlgiana juveniles a t both sites. However, these effects occurred in opposite directions a t the 2 sites. At the muddy-sand site, highest juvenile densities were associated with high adult densities, but, at the sandy site, they were associated with the absence of adult M. liliana. The identification of subtle density changes a t the sandy site was unexpected because sediment transport is frequent at this site and associated passive movement of juveniles has the potential to mask local biotic interactions. The change found in the direction of interactions between adult and juvenile bivalves between sltes has irnplications for the generality of such interactions: in particular it highlights the influence that habitat may have on ecolog~cal interactions.

Descriptions of patterns of community or population varlation in space can provide a stepping stone to inferring the relevant processes affecting spatial patterns. However, matching patterns to particular processes has proven difficult. In this study we make a pnori predictions about the form and intensity of spatial patterns in abundance of the infaunal bivalves A4acomona liliana and Austrovenus stutcliburyi based on size, feeding mode and mobility. These predictions are tested by describing Ihe spatial patterns found for a range of scales (5 to 33 cm and 33 cm to 6 m). In order to incorporate environmental variation, patterns were studied at 2 sites of different sediment grain size and hydrodynamic regime. Most of the spatial patterns found were the same in intensity and patch size a t both sites, suggestlng that biological rather than environmental proccsses were important. In particular, patch size appeared to be a function of mobility for all but M. llllana juveniles. Over the scales investigated in this study, individual mobility and inter-individual interactions appeared important processes influencing the observed spatial pdtterns of both juveniles and adults of these 2 functionally different species. This has important implicat~ons, as the ability to predict the reldtive Importance of different processes through an understanding of an organism's natural history allows a much better appreciation of the effects of generalising results from small-scale studies