There have been different methods used for herring fishing. In late summer and autumn, herring at the waters surface feed on phytoplankton that produce a phosphorescent glow. This was watched for by fishermen in their boats chapping at a particular part of the vessel to cause sound waves to transmit to the water, sometimes the herring would move in reply to the sound, thus displaying the phosphorescent glow of the moving phytoplankton and enabling the fishermen to locate the feeding herring. This phosphorescence is very rarely seen during the wintertime.
The echo-sounder was introduced in the 1930's but not adopted by most vessels until the 1950's. This proved essential equipment for locating schools of fish. The echo-sounder sends sound waves to the sea bed and it reflects back to the sounder where different grades of color identify depth of the objects, it makes it significantly easier to track and locate fish, although it never entirely replaces conventional wisdom and traditional herring fishing methods.
Despite having echo-sounding equipment fishermen now and in the past have to keep a knowledge of submerged rocks, or other dangers, such as running aground, or snagging nets on sharp obtrusive objects on the sea bed. The fishermen through time have passed down the knowledge from skipper to skipper where the 'dry' grounds are so that the vessels are less likely to damage their nets on a trawl. Persisting well into the 20th century, drift-netting as being used centuries before trawling or ring-netting A mile of net, or more would be set in the waters during early evening and the vessel and net would simply drift through the night, if herring were swimming where the nets were drifting then the herring might have been caught. Nets were put out at dusk when the fish were feeding and were left to drift like a giant curtain in the sea. At then end of the night the nets were hauled in and the fish shaken out. Once in harbor the herring were unpacked using baskets and taken to the fish market almost as fresh as when it was first caught. Herring fisheries know the herring recipe to keep it fresh. There have been many changes to fishing in the 20th century. There have been advances in boats, gear and equipment. Herring fishing has declined and over fishing has become a problem.