Effects of oyster dredging on the distribution of bryozoan biogenic reefs and associated sediments in Foveaux Strait, southern New Zealand

Abstract

Foveaux Strait has been commercially fished for oysters for over 100 years, focusing principally upon bryozoan biogenic reefs that once covered large areas of the strait. Bryozoan biogenic reefs consisted of linear swards of bryozoan patch reefs, paralleling the peak tidal current, and were formed by the frame-building bryozoan Cinctipora elegans and associated epifauna. Two side-scan surveys one in the late 1970s and the other in the late 1990s and estimates of distribution of oyster density and distribution of fishing effort, show how fishing has effected the distribution of habitat, oysters and sediments over the last 30 years. Fishers dredged the biogenic reefs for their oysters, damaging the framework structure, removing epifauna and exposing associated sediments, which were then reworked and transported down-current in the strong tidal flow. Large volumes of biogenic sediments released in this process formed thick sheets and dune bedforms. When dredging ceased locally (after removal of biogenic reef habitat and oysters), sediment supply downstream dwindled and ceased, bedforms diminished and the seafloor ultimately reverted to relict pebble gravel. Sediment derived from the dredging of biogenic reefs in northern Foveaux Strait was mostly transported and deposited in deeper water to the east, however, sediment derived from biogenic reefs in southern Foveaux Strait was transported in the opposite direction, giving rise to a group of large dunes in the southwest that in 1999 contained ∼58×10 6 m 3 of coarse biogenic sediment. By 1998 none of the original bryozoan biogenic reefs remained. We interpret sediment distribution in Foveaux Strait in the light of these processes, and compare with earlier data to provide snapshots of human-induced modification of the benthic environment. We also explore evidence for regeneration of simple biogenic reefs in areas no longer fished, and discuss the conditions in which this has occurred. Communities dominated by byssally attached Modiolus appear to provide the early framework and shelter for development of new patch reefs. However the recent discovery of regenerating colonies of the reef-building bryozoan Cinctipora in eastern Foveaux Strait suggests that bryozoan patch reefs may also be capable of re-establishing where conditions are suitable. We favour helical circulation cells in the strongly linear tidal flow of Foveaux Strait as a mechanism for controlling, not only the linear aspect of the reef frameworks, but also for concentrating nutrients, propagules and the settlement of larvae. Our observations of regenerating epifaunal habitat on the fishery-modified seafloor suggest that careful management could reverse much of the deleterious effect of fishing. (4 0 6)