Herring spawn colours the near shore waters an aqua blue.

Conditions that trigger spawning are not altogether clear, but after spending weeks congregating in the deeper channels, both males and females will begin to enter shallower inter-tidal or sub-tidal waters. Submerged vegetation, especially eelgrass, is a preferred location for spawning. A single female may lay as many as 20,000 eggs in one spawn. However, the juvenile survival rate is only about one resultant adult per ten thousand eggs, due to high predation by numerous other species. ~ Wikipedia