Purse seiners capture shoaling fish that aggregate into large, dense concentrations near the surface by surrounding the shoal with a deep curtain of netting supported at the surface by floats. The net is then pursed under the shoal by heaving on a wire that runs through rings attached to the bottom edge of the net.
Scottish purse seiners range from 30-60 metres in length and while the main target species of mackerel and herring are caught for human consumption, industrial species such as blue whiting and scad can also be taken by pelagic trawl when conditions are favourable.
Mid-water or pelagic trawls are towed at the appropriate level in the water column to intercept shoaling fish such as mackerel (Scomber scombrus), herring (Clupea harengus) or sprats. The depth of net relative to the surface is indicated by an underwater instrument known as a net monitor or net sounder. Gear depth can be controlled by changing the length of towing wires and/or altering towing speed. As with bottom pair trawls, the absence of otterboards allows two vessels of modest horsepower to tow a relatively large mid-water trawl.
Scottish vessels using this method range from 15-50 metres with combined horsepowers of 500-6,000hp.