Members of the Working Group of International Pelagic Surveys (WGIPS) International Ecosystem Summer Survey in the
Nordic Seas (IESSNS) post-cruise meeting.
'The swept area of a fisheries survey is the horizontal area covered by the trawl, so it is the distance between the wings of the trawl gear. In our case, that's around 65 metres wide.
A swept area method can be used to estimate the abundance or biomass (total mass of living material is given area) of fish. The method calculates the average density of fish within an area based on the number of sampling stations in that area.
One way to calculate how much mackerel there is in the Northeast Atlantic is to use standardized trawling close to the surface in summer. The mackerel predominantly swims and finds food in the upper 30 meters of the ocean during the intensive feeding period. So we can count how much mackerel we get after towing the trawl from 0-30 m depth for 30 minutes with a five-knot towing speed in selected areas. The trawl stations are set out before we start the scientific survey and we position the trawl stations in randomly selected places inside fjords, coastal areas and in the open ocean where we assume the mackerel is present in summer. It is also important to find the zero lines, which are the areas we do not find any mackerel due to for example too low temperatures (6-7°C) or lack of food.
During the International Ecosystem Summer Survey in the Nordic Seas (IESSNS) we covered 2.7 million km2 and took 250 trawl hauls. The total surveyed area represent more than 60 times the size of Denmark or about 1/3 the size of Europe.'