The Natural History of the River Herrings
(Alosa pseudoharengus and Alosa aestivalis)
Alewife and blueback herring, collectively referred to as river herring, are relatively small members of the herring family. Distinguishing between the two species externally is difficult. The most reliable identifying characteristic is the color of the tissue lining the body cavity, dark brown or blackish in the blueback herring and grey or silvery in the alewife. The alewife occurs from Newfoundland to South Carolina, while the blueback herring occurs from Nova Scotia to the St. Johns River in Florida. Both are a schooling species; adult blueback herring typically occupy a narrow band of coastal water entering fresh or brackish water to spawn while adult alewife oceanic movements are apparently restricted to coastal areas proximal to natal estuaries.
The onset of spring spawning is related to temperature, and thus varies with latitude. Alewife spawn in rivers and tributaries from northeastern Newfoundland to South Carolina, but are most abundant in the mid-Atlantic and northeastern states. Blueback herring spawn from Nova Scotia to northern Florida, but are most numerous in warmer waters from Chesapeake Bay south. Alewife usually spawn three to four weeks earlier than blueback herring in the same watershed. Alewife initiate spawning when water temperatures reach 51 o F while blueback herring commence spawning at water temperatures of 57 o F.
Alewife spawn in a diversity of habitats that includes large rivers, small streams, and ponds, over a range of substrates such as gravel, sand, detritus, and submerged vegetation. Blueback herring prefer to spawn in swift flowing sections of freshwater tributaries, channel sections of fresh and brackish tidal rivers, and coastal ponds, over gravel and clean sand substrates, especially in northeastern rivers where alewife and blueback herring coexist.
Mature river herring broadcast their eggs and sperm simultaneously into the water column and over the substrate. Larvae begin to feed externally three to five days after hatching, and transform gradually into the juvenile stage. Juveniles remain in freshwater nursery areas in spring and early summer, feeding mainly on zooplankton. As water temperatures decline in the fall, juveniles move downstream to more saline waters and eventually to the sea. Little information is available on the life history of subadult and adult river herring after they emigrate to the sea as young-of-year or yearlings, and before they mature and return to freshwater to spawn.
Immediately after spawning adults migrate rapidly downstream. While in the ocean, both species migrate seasonally, possibly in conjunction with changing water temperatures. Alewife are most abundant at depths greater than those where the largest concentrations of blueback herring are found. The alewife is captured most often at depths of 184 -361feet at a temperature range of 37 to 63 o F, while the blueback herring is more frequently captured at 86 - 180 feet. In summer and fall, both species are confined to areas north of 40 degrees north latitude, particularly to Nantucket Shoals, Georges Bank, and the Gulf of Maine. Winter catches are made between 40 and 43 degrees north latitude and spring catches over the entire continental shelf area.
For both species, growth rates, age at sexual maturity, and longevity vary greatly according to geographic location. Few individuals of either species exceed 12 inches in length or about 2/3 of a pound in weight. Throughout their ranges, alewife tend to be longer than blueback herring of the same age. Within each species, females tend to grow somewhat faster than males. Age at sexual maturity for both species is primarily ages 3-5 in the northern portion of their ranges. Fecundity increases with size and age. Alewife produce 48,000-360,000 eggs per season and blueback herring produce a similar amount (45,000 to 350,000 eggs per season). River herring suffer relatively high rates of mortality throughout their life cycles. Fewer than 1% of the eggs survive early life stages to migrate to the sea as juveniles. Total annual mortality of adults is about 70%. As many as 90% of all adults die annually during, or after, spawning migrations and reproduction.
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