The gleaming shoal of fish in the water at night is a beautiful scene, the fish completely natural in their phosphorescent glow. Jeffers describes the scene as beautiful and "a little terrible" as the fish are confined by the net (purse-seine).
Jeffers compares the capture of the fish to the confinement and capture of people in cities and the urban landscape. Thus, the fish symbolize the people caught up in modern cities. Likewise, the city is beautiful and "a little terrible" because the cities, for Jeffers, are like prisons, as was the seine-purse for the fish.
There is no escape. We have gathered vast populations incapable of free / survival, insulated / From the strong earth, each person in himself helpless, on all dependent. / The circle is closed, and the net / Is being hauled in.
Jeffers seems to be less critical of the fisherman taking the fish from their natural habitat than he is of government/anarchy and "Progress" taking people from the natural world and confining them, like caught fish, in the physically cramped space of the city and the potential trappings of industrialization and civilized control that we usually associate with "Progress." So, the net symbolizes progress and industrialization. Jeffers, in this poem, deems these things as largely detrimental because they imprison people, making them dependent upon technology ("machines") and this will inevitably lead to "mass-disasters."
Jeffers ends the poem by asking if the reader is startled by his cynicism. But he says he is not amazed by the foreboding outlook he has for human civilization. Thus, he is calm, albeit frustrated, at his bleak outlook on the future of progress and civilization. He considers the fact that everyone dies and all civilizations decay, so there's no reason to be amazed at his "troubled verse."