An environmental assessment of a proposed marine farming zone extension at Flathead Bay, in the D’Entrecasteaux Channel

An environmental assessment of a proposed marine farming zone extension at Flathead Bay, in the D’Entrecasteaux Channel, was conducted in February 2014. The site covered 3.24 km² (324 ha) and was orientated in a northerly direction perpendicular to the coastline. The benthic habitats within the zone assessment area comprised of silt (69.5%) and silty sand (30.5%).

Environmental data on substrate type, habitat distribution, bathymetry, and benthic flora and fauna were assessed. Within the proposed zone the substrate was characterized by silt and silty sand. The bathymetry ranged from -17.1 m to -45.9 m and was characterized by a gradual slope from the shoreline to the east with two shallower regions, one in the north east and one on the southern boundary.

The silty sand substrate was brown to yellow in colour with shell grit, evidence of active bioturbation; live and dead New Zealand screwshells (Maoriculpus roseus), brittle stars, squat lobsters (Munida haswelli) and polychaete worms were common across the whole site. Other features on the substrate included drift algae (Ecklonia radiate) and (Phyllospora comosa), Northern Pacific sea stars (Asterias amurensis), gobys (potentially Nesogobius sp.), zoanthids (potentially Parazoanthus sp.) and a Tasmanian numbfish (Narcine tasmaniensis).

One notable feature that was identified from the hill-shaded bathymetric image was “pock marks” occurring in the soft sediment on the western boundary of the zone assessment area. The six pock marks that appear on the eastern boundary, inside the zone assessment area were around 20 m in diameter and 1- 2 m deep and occurred in the basins and not on the terraces.