Purse seining is the process of catching schooling fish near the ocean surface by circling them with a net. Once the fish have been encircled, a wire (purse line) running through the bottom of the net is winched tight to "close the purse" from below. Purse seining for herring can be especially tricky as the entire process is run in the dark of night when herring swim to the surface to feed on rising phytoplankton. No lights are used until the seine is closed.
The entire process starts with the release of the seiner's "bug boat." The bug boat, which holds one end of the net, motors in place while the larger seiner encircles the herring school (generally clockwise by convention in the Gulf of Maine). As the seiner encircles the herring, net is released.
The top of the net stays at the surface, buoyed by a "float line." The bottom of the net is connected to the purse line by large, metal clips called "rings." These also act as weights that sink the net to depths of up to 100 meters.
Once the seiner reaches the bug boat, the bug boat's float line is tied off to the gunwale of the seiner. The bug boat then motors to the opposite side of the seiner, attaches to the seiner using several lines, and powers away from the seine net to insure that the mass of fish and net do not drift below the seiner, pulling the ship over sideways.
When the bug boat is in place, a winch on the deck of the seiner pulls in both ends of the purse line, closing off the bottom of the seine net like a giant drawstring.
Once the bottom of the seine is closed, preventing the herring from escaping, deck lights are turned on and one end of the seine net is run through a large, hydraulically driven "powerblock" located at the top of a crane. As the net is lowered through the powerblock, it is stacked on deck in preparation for the next set. This process slowly pulls the portion of the seine net still in the water shipward, forming a concentrated pocket of herring.
A submersible pump is then lowered into the pocket, pumping fish to a dewatering box then to holds below the deck or to herring carriers waiting nearby. The entire process is repeated as many as five times in a single night with as many as 200 tons of fish captured during each set.
The following excerpt includes the first two stanzas of "The Purse Seine," written in 1937 by west coast poet Robinson Jeffers. 
Our sardine fishermen work at night in the dark of the moon; daylight or moonlight
They could not tell where to spread the net, unable to see the phosphorescence of the shoals of fish.
They work northward from Monterey, coasting Santa Cruz; off New Year's Point of off Pigeon Point
The look-out man will see some lakes of milk-color light on the sea's night-purple; he points, and the helmsman
Turns the dark prow, the motorboat circles the gleaming shoal and drifts out her seine-net. They close the circle
And purse the bottom of the net, then with great labor haul it in.
I cannot tell you
How beautiful the scene is, and a little terrible, then, when the crowded fish
Know they are caught, and wildly beat from one wall to the other of their closing destiny the phosphorescent
Water to a pool of flame, each beautiful slender body sheeted with flame, like a live rocket
A comet's tail wake of clear yellow flame; while outside the narrowing
Floats and cordage of the net great sea-lions come up to watch, sighing in the dark;
the vast walls of night Stand erect to the stars.