Short answers, pulled from the story.
Herring are forage fish belonging to the order Clupeiformes, with most species in the family Clupeidae. The family includes about 200 species such as shads, sardines, and menhadens. Three species in the genus Clupea account for roughly 90 percent of all herrings caught in fisheries worldwide.
The Atlantic herring, Clupea harengus, comprises more than half of all global herring capture, making it the most abundant of the three main Clupea species. In Europe, herring trade has been regarded as the most commercially important fishery in history, and the fish has been called the "silver of the sea".
Herring hunting copepods swim in a grid where the spacing between individual fish matches the characteristic escape-jump length of their prey. When a copepod detects an approaching herring and jumps, the 60-millisecond delay before it can spread its antennae again leaves it vulnerable to the next herring in the grid. A single juvenile herring could never catch a large copepod alone.
A female herring deposits between 20,000 and 40,000 eggs per spawning, averaging about 30,000, depending on her age and size. Eggs are laid on the sea bed and stick to gravel, seaweed, or stones. Greenland populations spawn in 0 to 5 metres of water, while North Sea bank herrings spawn at depths of up to 200 metres.
During the Middle Ages, herring prompted the founding of Great Yarmouth and Copenhagen and played a critical role in the medieval development of Amsterdam. In Scotland, the herring industry peaked in the late nineteenth century, when Shetland and its town of Lerwick became known as the "herring capital of Europe".
Large Baltic herring slightly exceeds recommended limits for PCB and dioxin. Research indicates that the cancer-reducing effect of herring's omega-3 fatty acids is statistically stronger than the carcinogenic effect of those contaminants. Baltic herrings larger than 17 centimetres may be eaten twice a month, while herrings smaller than 17 centimetres can be eaten freely.