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|Herring Biology: Ecology|
What do herring eat?
Herring survive entirely by feeding on plankton - the tiny drifters of the ocean. There are two basic types of plankton: phytoplankton (plants) and zooplankton (animals). The zooplankton community, comprised of invertebrate and fish larvae as well as many species that remain drifters for life (holoplankton), is thought by some scientists to be the biggest source of protein in the world's oceans and is, not surprisingly, an important food source for many organisms. Although herring are opportunistic feeders, they feed primarily on small holoplanktonic crustaceans called copepods.
Copepods and other tiny crustaceans are the predominant members of the zooplankton in the coastal waters of the Gulf of Maine.  Two copepod species, Pseudocalanus minitus and Calanus finmarchicus comprise more than two thirds of the herring's diet in winter through summer, with three other copepod species the dominant food sources in the fall. The larvae (nauplii) of copepods make up a major part of the diet of larval herring.
The survival and distribution of herring in the Gulf of Maine depends largely on the distributions their major prey, the copepods. Herring growth rates vary with copepod availability and abundance. The copepod Calanus finmarchicus, perhaps the most abundant copepod in the Gulf of Maine, is vital to the success of Atlantic herring populations. Atlantic herring time their migrations around the presence of this copepod. The herring are dependent on these food sources to such a degree that herring scientists have proposed that information on prey species may provide a method to predict the location of herring stocks.
Herring feed at night in the upper water column, following the massive vertical migrations of zooplankton that inhabit deep waters by day and surface waters by night. Research has shown that herring feed on zooplankton in several ways. Much like the way a whale strains zooplankton from the water with its baleen plates, herring are able to use their gill rakers to filter-feed. Herring can also visually detect larger prey, such as an individual copepod or a mysid shrimp, and execute directed attacks on these targets. 
What eats herring? (Short answer: almost everything)
The Atlantic herring is one of the most important pelagic species in the Gulf of Maine and throughout the North Atlantic. Many species of fish, bird, and marine mammal rely on herring as a source of food. Herring, which filter tiny zooplankton such as the copepods out of the water, are consumed by top marine predators such as birds and seals. Herring therefore form a critical link between the base of the food web (plankton) and other marine organisms. In the northeast Atlantic, the herring is a key species in the marine environment, forming layers of 40 billion individual herring that cover several square kilometers of coastal inlets.  Predators as large as killer whales directly feed upon these herring.
Herring in the Gulf of Maine have many predators, including fish, sea birds, dolphins, harbor porpoises, seals, whales, and man. Cunner (Tautogolabrus adspersus), small finfish, have been observed feeding on patches of herring eggs in the Gulf of Maine. 
Cod feed not only on herring eggs but also on larvae, juvenile, and adult herring. Sculpins also feed on herring eggbeds. Hake, dogfish and other shark species, tuna, bluefish, pollock, and striped bass feed on herring at all life history stages.
The Harbor porpoise (Phocoena phocoena), the smallest cetacean found in North Atlantic waters, has been nicknamed herring porpoise by fishermen because of their close association with herring.
Harbor porpoise are common throughout the waters of the Gulf of Maine and Bay of Fundy area, especially in summer when their primary prey species, herring, moves inshore in large schools. Harbor porpoises use echo-location to find and follow herring and other schooling fish.
Other marine mammal herring predators in the Gulf of Maine, Harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus), detect herring schools through their vibrissae (whiskers) that serve as important sensory organs. [REFERENCE???] Harbor porpoises and seals are so closely associated with the schools of herring they hunt that they can become accidentally trapped in herring fishing gear.
Seabirds consume large numbers of larval and juvenile herring. Despite their name and contrary to popular belief, herring gulls do not actively hunt herring. They will plunge into the water after young and adult herring they might notice from above, but they are primarily scavengers, showing up, often in great numbers, whenever herring fishermen haul in their catch.
Atlantic puffins (Fratercula arctica), razorbills (Alca torda), common terns (Sterna hirundo), and Arctic terns (Sterna arctica) are true herring predators. Terns and puffins actively pursue 0-1 year age class herring in the Gulf of Maine. Herring are a critical component to the annual diet of these seabirds with much of their catch being fed to their young. After quantifying seabird predation on certain herring stocks, scientists have been able to correlate the amount of herring in a puffin chick's diet to herring recruitment in the North Sea.
There may be some safety in numbers. Schooling behavior has several potential benefits, one of which is predator evasion. Schools of brit herring demonstrate elaborate schooling behavior patterns for predator evasion. Newly- metamophosed herring under attack by puffins form complex school-level patterns with fascinating names such as vacuole, hourglass, pseudopodium, herd and split. 
Schooling herring may also benefit from a large number of individuals to detect incoming predators. Ironically, schooling behavior is exploited by several top predators with complex hunting behaviors. Hake, a predatory fish, use cooperative hunting strategies to herd and drive schools of herring into shore while killer whales in the Northeast Atlantic drive schools to the surface. Squid encircle herring schools and dart in to prey on the corralled prey. Baleen whales can corral herring using bubble nets and, of course, humans have devised elaborate methods of trapping and netting herring schools. The strong schooling behavior of herring is one reason the herring species has long been a focus of the fishing industry.
 Sherman, K. (1986) Seasonal and Areal Distribution of Zooplankton in Coastal Waters of the Gulf of Maine. NMFS, NE Fisheries Science Center, RI.
 Fisheries and Oceans Canada Website. http://www.nwafc.nf.ca/english/species/at_herring.html.
 Nottestad, L., T. Simila. (2001) Killer whales attacking schooling fish: why force herring from deep water to the surface?. Marine Mammal Science. 17(2): 343-352.
 Cooper, R. A. (11/2000) Personal Communication. University of Connecticut, Avery Point Marine Lab.
 Axelson, B.E., T. Anker-Nilssen, P. Fossum, C. Kvamme, and L. Nottestad. (2001) Pretty patterns but a simple strategy: predator-prey interactions between juvenile herring and Atlantic puffins observed with multibeam sonar. Canadian Journal of Zoology. 79: 1586-1596.