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|Herring Biology: Life-Cycle|
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Herring Reproduction | The Larval Stage | The Juvenile Stage | Adult Herring
In the marine realm, reproduction often falls into one of two very different strategies. On the one hand, adults produce limited numbers of offspring in which they invest significant care, nutritional resources, and time to ensure a high probability of survival for each. At the other extreme, adults produce massive quantities of progeny, investing hardly any time or resources in any one offspring. Herring reproduction falls into the latter category, which is basically a number game: the large number of offspring ensures that at least a few survive despite high mortality.
Herring use external fertilization to produce masses of adhesive benthic (bottom-oriented) eggs that develop on the ocean floor and then metamorphose into transparent larvae which join the plankton drifting on ocean currents and eddies.
While some herring species are anadromous, traveling up coastal rivers to spawn, Atlantic herring are fully marine and migrate to coastal and offshore spawning grounds. In late summer and early fall, Atlantic herring aggregate into massive schools and move into coastal waters at various locations in the Gulf of Maine to spawn. They also spawn on offshore banks such as Georges Bank and Brown Bank south of Nova Scotia. Spawning times vary for different populations of Atlantic herring.
In the Gulf of Maine, spawning progresses in general from north to south, commencing in the Bay of Fundy and eastern Maine waters in late July or early August and as late as November and even December on Georges Bank. Spawning takes place in relatively warm (approximately 10-15°C), salty water.
Fertilization is external, as female herring release eggs and male herring release clouds of milt simultaneously. Having feasted for months on the spring blooms of plankton offered up by temperate waters of the Gulf, the herring are fat prior to spawning.
Mature eggs make up a large portion (20%+) of the female's body weight. The fecundity of herring females is typically in the range of 20,000-50,000 eggs per female, although a large female herring can lay as many as 200,000 eggs. Herring are iteroparous, meaning that spawning is not a death sentence and herring generally live to spawn repeatedly for several years. After spawning, their weight declines with the loss of reproductive material (gametes) and associated fat content.
The sticky demersal eggs sink and adhere to the ocean bottom to form dense carpets that can be several centimeters thick. This egg mat contains eggs from numerous individual herring. In one square meter there can be as many seven million eggs. Fertilized eggs hatch into larvae in 7-10 days depending on the water temperature.
In the Gulf of Maine, herring spawn in coastal waters at depths above 100 meters and over a fairly level bottom. Egg beds have been observed in water as shallow as 20 meters. The substrate may be composed of rocks, cobble, gravel, pebbles, beds of seaweed, or fragments of shells. Lobstermen often find herring eggs attached to their lobster traps.
Egg mortality can be quite high. The mass of eggs lying on the bottom attracts an array of predators, for whom the egg mat simply represents a bonanza of protein covering the sea bottom. Cunner, tautog, flounder, cod, haddock, sculpin, and other fish as well as crabs, whelks, and starfish take advantage of this nutritive windfall. 
Location in the egg mat may affect survival, with a trade-off between the effects of predation and oxygen-limitation. Eggs deposited on the upper level of the egg mat are more vulnerable to predatory fish and invertebrates. Alternatively, the top layer of developing eggs benefits from higher oxygen levels as they are more exposed to the currents above.
Preliminary research on herring egg beds off Scotland suggests that the herring eggs located deeper in the egg mat are less viable then those on top. In fact, researchers theorize that small, less developed larvae might hatch from eggs found lower in the egg mat. 
The Larval Stage
Herring eggs that don't succumb to low oxygen levels or hungry predators hatch in about 7-10 days. Early development can take place over a wide salinity range, with the rate of development largely determined by water temperature.
Like many organisms, the larval and adult stages of herring are very different in appearance. Larval herring are elongated, transparent and entirely lacking scales. Larvae are approximately 5 to 7 mm long when they hatch and carry a yolk sac that provides a mobile food reserve.
As they deplete their yolk reserve (+/- 10 days), their tiny mouthparts develop enough to allow them to capture and swallow small prey. The transition to feeding in fish larvae is considered to be a "critical period" in which mortality is catastrophically high. 
Larvae are weak swimmers, but are able to lunge at tiny larval plankton and eggs drifting nearby. Larval herring feed on a variety of tiny plankton. They can attack and fit in their mouths, including the eggs and larval stages of copepods, clams, barnacles, and shrimp.
After hatching, the journey of the pelagic larvae is primarily at the mercy of the prevailing currents, tides and wind. Depending on environmental conditions like water temperature, the larval stage lasts from 3 to 11 (typically 6) months.
Not surprisingly, mortality levels are high during this vulnerable life-history stage - the odds of survival are stacked against these odd-looking transparent drifters. The pelagic phase is long and the young fish are potentially dispersed over a huge geographical area. Some researchers estimate that only about 1% of herring larvae survive to become juvenile fish.
Herring larvae are found throughout the Gulf of Maine and Georges Bank. Larvae that are produced on Georges Bank are retained in a local gyre of water, while larvae originating from spawning grounds in eastern Maine are transported in a westerly direction along the Maine coast. The larvae are caught up in the Eastern Maine Coastal Current, a dominant cold-water feature of the Gulf of Maine that sweeps from the Bay of Fundy west along the coast at about 17 km/ day. The nutrients and blooms of plankton in the current provide the ideal conditions for larval herring growth. Research conducted by the Maine Department of Marine Resources showed that larvae that originate from eastern Maine spawning grounds are transported as far west as the Sheepscot River, in central Maine, a distance over 150 km.
As the herring larvae are passively swept along in the coastal current, they rise and fall, drift and feed. In the vicinity of Penobscot Bay, a fraction of the coastal current sweeps offshore in a plume of cold water. A study by oceanographer Dave Townsend at the University of Maine found that some larvae are spun into Penobscot Bay and along the coast while others are swept off into central Gulf of Maine waters. Variability in ocean currents, or conditions that affect over-wintering larvae in coastal estuaries and embayments, might have a significant effect on larval recruitment (the number of young fish surviving to join the fishery) from year to year. 
The Juvenile Stage
In the spring, larvae metamorphose into juvenile herring. Scales form, their bodies begin to deepen and flatten, and they are no longer transparent, taking on the silvery blue-green colors characteristic of adults. In short, they begin to look like herring.
The young herring, now termed "brit," are about 40 millimeters long. The brit migrate shoreward, collecting in dense schools near the surface. Density-dependent growth is evident in schools of juvenile herring; if the schools exceed an optimal number, growth rates decline.
Aggregations of brit herring enter shallow bays and inlets, where they migrate vertically in the water column in response to light cycles. Dispersed throughout the water column during the day, they collect in surface waters at night to feed on their zooplankton prey. In the late summer and fall, when adults are migrating onshore to spawning grounds, the brit move offshore to spend winter close to the bottom.
Brit comprise an important food source for many predators and, as a result, are commonly spotted hiding under docks and piers. Mackerel, striped bass, and many sea birds including puffins, gulls, and terns focus their attention on brit herring schools.
Atlantic herring mature when they reach three to four years old, at which time they measure approximately 23-26 cm in length. In the Gulf of Maine, they can live for 12 years or so and reach a maximum length of approximately 40 cm at a weight of about 1.5 pounds.
Atlantic herring migrate over great distances of open sea. They migrate to feed. They migrate to spawn. They migrate to their winter grounds. The spawning migrations of herring in the Gulf of Maine have been studied by researchers and fishermen alike.
In general, the spawning pattern of herring conforms to the typical "triangular" migration pattern common in pelagic schooling fish. Adults migrate against currents from feeding grounds to spawning grounds. Larvae drift passively away with currents, and juveniles eventually swim back out to join adults at feeding grounds. 
Adult herring that spawn on Georges Bank travel southwest to the mid-Atlantic/Chesapeake Bay area in search of warmer waters in winter, and return to the Gulf of Maine in the spring. Herring that spawn off the southwest coast of Nova Scotia near Yarmouth winter in Chedabucto Bay in northern Nova Scotia, a sea migration of over 500 kilometers. Although some Gulf of Maine herring may over-winter in Massachusetts Bay, most migrate around Cape Cod after spawning, joining the fish that are migrating south from Georges Bank and Nantucket shoals.
 Cooper, R. A. (11/2000) Personal Communication. University of Connecticut, Avery Point Marine Lab.
 Stratoudakis, V., A. Gallego, and J. A. Morrison. (1998) Spatial Distribution of Developmental Egg Ages Within a Herring Clupea harengus Spawning Ground. MEPS. Inter-Research Science Publisher, Oldendorf/Luhe. 174:22-32.
 Hjort, J. (1914) Fluctuations in the great fisheries of northern Europe viewed in light of biological research. Rapp. Proc.-Verb. Reun. Cons. Int. Explor. Mer. 108:1-8.
 Townsend, D. (1992) Ecology of larval herring in relation to the oceanography of the Gulf of Maine. J. Plankton Res. 14(4): 467-493.
 Moyle, P.B. and J.J. Cech. (1992) Fishes, An introduction to Ichthyology, 2nd Edition. Prentice Hall, New Jersey. 559 pp.