Alewives have an overall silvery color with a grayish green back. A black spot at the eye level is directly behind the head. Adults have longitudinal lines that run along the midline of the body. Small alewives have a violet sheen on the sides while adults have a golden cast on their heads and upper parts. They are colored very much like skipjack herring. Scales along the middle of the belly form a raised edge. Females are bigger than males and generally live longer. The body is very rounded and flattened. The head is broadly triangular. The eyes are large and have structures that look like eyelids. The single dorsal fin usually has 13-14 rays but may have 12-16. The tail fin is forked. The anal fin is short and wide with 15-19 rays. Alewives that live at sea tend to be longer, ranging from 255 to 355 mm. Alewives that live in freshwater are about 150 mm in length and mature faster than alewives that live at sea.
Alewives are native to the Atlantic Ocean and the lakes and streams that drain to it from Newfoundland to North Carolina. They are "anadromous", which means that they live most of their lives in the sea but migrate into freshwater rivers and streams to breed. They are present, but non-native, in all of the Great Lakes and many lakes in northern New York. In the Great Lakes they are now abundant in Lake Huron and Lake Michigan. Alewives don't do well in Lake Superior because the water is too cold and lake trout prey heavily on them there. They don't do well in Lake Erie because the lake is too shallow to provide suitable overwintering grounds for large numbers of fish.
Several theories exist on how alewives could have become established in the Great Lakes. Scott and Crossman suggest that alewives may have been accidentally included in a batch of American shad that were used to stock Lake Ontario. The alewives also could have migrated from Lakes Seneca and Cayuga in New York to the Great Lakes through the St. Lawrence River. A third argument for the current distribution is that alewives were native to Lake Ontario in small numbers but went unnoticed until a population explosion in 1873.
Alewives spend most of their time in coastal waters and most are caught in water 56 to 100 m deep at about 4°C. They are sensitive to light and tend to be in deeper waters during daylight hours. They follow the daily movements of zooplankton in the water column. Adults can withstand temperatures up to 25°C and young of the year can live in waters up to 30°C.
Fertilized eggs are about 0.9 mm in diameter. Three to five days after hatching, the larvae begin to feed. They slowly transform into juvenile fish and remain in fresh water until the fall, growing to 3.8 to 12.5 cm.
Alewives spawn in the spring. The young swim to sea or to deeper parts of large lakes (including the Great Lakes). Migration to spawning grounds happens earlier in warmer areas. Spawning generally starts in April in the south and lasts until the end of May farther north. In all populations, females reach the spawning grounds first and older fish are the first to spawn. The oldest fish recorded at spawning sites were to 10 years old. Spawning occurs in groups of 3 or in pairs.
Females lay their eggs and males fertilize them at the same time. The eggs are sticky at first and may stick to plants or rocks. Within a few hours they settle to the bottom of the lake or river. Alewives lay their eggs over any type of substrate. The number of eggs per female may be 10,000 to 360,000.
In populations that migrate to the sea, adult alewives spend most of their lives at sea but spawn in inland streams. Although they cannot jump obstacles such as dams, they can swim over rapids. Fish in these populations mature at 3 years for males and 4 years for females.
In freshwater populations, fish mature at 2 years for males and 3 years for females. These fish move close to shallow beaches or into streams to spawn. Eggs hatch in 3 to 6 days, depending on how warm the water is.
Alewives do not have any parental investment in their young beyond spawning. The adults leave immediately after spawning in the spring and the young move to the open water in the fall.
Young alewives have a very high mortality rate. Less than 1% survive to migrate into the sea or large lake. About 70% of adult alewives die each year also. Most die during or shortly after the spawning season. Few alewives in freshwater populations live longer than 5 years.
There is little information on the social behavior of alewives except for spawning behaviors and feeding patterns. They migrate up streams or to shallow waters to spawn. They make daily movements in the water column, moving up (closer to the surface) at night and down (into deeper waters) during the day. They generally travel in groups.
Alewives are not territorial.
We don't know much about how alewives might communicate. Their large eyes probably help them find other alewives, their prey, and stay alert for predators.
Little is known about the feeding habits of alewives that live at sea. Alewives in freshwater populations eat mostly zooplankton, especially small crustaceans such as copepods, cladocerans, mysids, and ostracods. When they grow larger than 11.9 cm, they feed mostly on larger, bottom dwelling crustaceans. Some spawning adults eat small fish or fish eggs when in shallow waters.
Alewives have many predators. In freshwater, their main predators are burbot, lake trout, eels, bass, walleye, and whitefish. Introduced predators include chinook and coho salmon. Little is known about the predators of alewives that live at sea but their hatchlings have a high mortality rate. As few as one out of 80,000 will reach the sea.
Alewives are now the most abundant planktivores in Lake Ontario and are the main prey of salmon in the Great Lakes. Their presence in the Great Lakes has caused the decline of many native fish species because of competition.
There isn't much known about the parasites of alewives, but acanthocephalans, cestodes, trematodes, copepods, and nematodes have all been found in populations that live in the sea. Alewives that live in freshwater lakes don't have as many parasites.
Alewives have been considered a nuisance in the Great Lakes since their population explosion in 1873. Live fish can clog industrial intake pipelines and sometimes die in large numbers, resulting in lots of dead fish on beaches in the spring. Coho salmon were introduced in the Great Lakes to control alewives and are now important sport fish in the region.
Since alewives feed mainly on plankton and bottom-dwelling crustaceans, alewives are particularly good at accumulating DDT and other toxins in their fatty tissues. For this reason it is best not to eat too many of the fish predators of alewives, such as salmon from the Great Lakes.
Alewives support an important commercial fishery in the Atlantic Ocean. They are packaged fresh, smoked, salted, or pickled for human food and are often sold as “river herring.” Fishermen use weirs, traps, gill nets, and dip nets for alewives, which they consider one of the easiest fish to catch. Alewives have other uses, including pet food, lobster and snow crab bait, and processing into fishmeal and fish oil. Alewives are not usually fished in the Great Lakes since these are small and too bony to eat. However, recently there has been a trend to use them for pet food and fish meal.
Alewives not listed as an endangered species, but in many places in their natural range, their habitat is threatened by dams along spawning rivers. On the other hand, their introduction into the Great Lakes and other areas resulted in declines in native fish in those areas.
Alewives have some interesting cultural and historical connections. Alewives are the fish the Native Americans in New England buried with crops as fertilizer (Grosvenor, 1965). The silvery coating on the scales is sometimes used in making costume jewelry and is called pearl essence by the jewelry industry (Grosvenor, 1965). (Grosvenor, 1965)
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