Trawling is one of the most common methods of fishing. Trawl nets are designed to be towed by a boat through the water column (midwater trawl) or along the sea floor (bottom trawl).
Trawl nets are shaped like a cone or funnel with a wide opening to catch fish or crustaceans and a narrow, closed ‘cod-end’. Trawls can be used at various depths and nets differ by their mesh size.
Both bottom and midwater trawls use otterboards/trawl doors to keep the mouth of the net open.
In the Commonwealth Trawl Sector of the Southern and Eastern Scalefish and Shark Fishery, midwater trawl is mainly used to target blue grenadier off the west coast of Tasmania between June and August. It can also be used to target alfonsino.
How midwater trawl works
Midwater trawling involves towing a net behind a boat to catch fish species. The net is connected to the boat by the warp wires and the opening to the net is spread using two large boards known as otter boards. The net is towed off the bottom in depths from just off the bottom to near the surface. Midwater trawl nets are usually shaped like a cone or a funnel with a wide opening to catch fish and a narrow end called a codend where fish are collected.
The sizes of midwater trawl nets vary, however there are minimum mesh size restrictions in place for certain fisheries
Environmental impacts and management
Midwater trawl gear has minimal impact on the environment primarily because it doesn’t come into contact with the seabed. Seals were identified as one of the species at risk from midwater trawling in the freezer boat sector, however AFMA requires these boats to use seal exclusion devices or ‘SEDs’ to protect species such as seals. In 2013 a ‘hydrostatic release’ to keep nets closed until they are too deep to be reached by seals looking for food was tested. This was effective in further reducing the risk of seal deaths.