American shad

Language English

Onglets

Description

Distinguishing features

American shads are in fact an anadromous type of herring (Alosa sp.), i.e. they spend most of their time at sea but must return to freshwater to spawn. Their silvery body is laterally compressed, with a steel-blue back.

They have a row of 8 or 9 black spots that may be more or less apparent running along either side from behind the operculum. Their belly has a strong keel of saw-toothed scales and their tail is forked.

Reproduction

American Shad reproduces in many rivers along the Atlantic coast of North America, from the St. Lawrence River to northern Florida.

Prior to the early 2000s, only a single spawning ground was known in the St. Lawrence River, at Carillon (Ottawa River), but recent studies made it possible to locate a second spawning ground below a dam on the Des Prairies River, and there may be others.

Like salmon, American shad return to their birthplace, following olfactory clues, and stop feeding during the journey. They make their first migration around 4 years of age.

American shad begin to migrate up the St. Lawrence in the spring, in May or June. They take a break when they arrive in fresh water to acclimatize and then continue towards their natal spawning grounds.

Spawning begins at dusk and continues throughout the night. Several males surround a female to encourage her to release her eggs. The fertilized eggs swell and sink to the bottom, where they settle. They hatch about ten days later, or earlier depending on the water temperature.

The fry and juveniles return to the sea during the summer. They are about 14 cm long by the time they reach the sea in late September.

The reproductive strategy of different American shad populations varies with latitude. Unlike shad in southern populations (in Florida and South Carolina), which reproduce only once in their lifetime, the St. Lawrence shad may return to breed for a number of consecutive years, in some cases up to 7 times.

Up to 86% of the shad that swim upstream in the St. Lawrence has done so at least once before. However, southern shad produce many more eggs (400,000 on average) than do St. Lawrence shad (130,000 on average). This strategy spreads the risks related to the northern climate over several years.

Diet

American shad feed primarily on zooplankton, especially copepods and mysids (two groups of minuscule planktonic crustaceans). They have about sixty long branchiospines on the first branchial arch, forming a fine filtering comb that traps minuscule planktonic organisms and other food particles.

The fry initially feed on zooplankton. As the shadlets (young shad) grow, their diet becomes more varied, including insects and the fry of other fish species.

Predators

The fry and young juveniles are more subject to predation by various freshwater species, including seagulls, pike and walleye.

Historically, striped bass fed on shadlets when they reached the vicinity of the Island of Orleans.

The only predators of adults are large saltwater fish, seals and lamprey.

Habitat

These fish live in schools near shorelines. All the populations that spawn on the Eastern Coast gather off Maine, Massachusetts and in the Bay of Fundy in summer, and off Virginia, North Carolina and Florida in winter.

American shad are found in the Gulf of St. Lawrence only when they are heading to spawn in the St. Lawrence or Miramichi rivers. The young remain in the Atlantic Ocean.

After being introduced to the West Coast, these fish are now found from British Columbia to Alaska.

Ecology, behaviour

American shad and all of the other river herrings (Alosa sp.) populations face the same problem: access to an even (sometimes) complete destruction of their spawning grounds. This problem results from the development and pollution of rivers due to dam construction, filling in of shorelines and deterioration ofwater quality.

The construction of fish ladders does not usually solve the problem caused by dams because shad do not jump. They need to find a laminar water flow, making shad fish ladders very difficult to create.

Like other Clupeidae, american shad are very gregarious fish that travel in schools. This behaviour starts very young, in the juvenile stage (around 4 or 5 weeks), when the shadlets are still in freshwater.

The formation of these compact groups is very common among fish that live offshore, especially filter-feeding fish. This both gives them greater protection against predators and makes it easier for them to identify and obtain their food.

French name

Alose savoureuse

Scientific name

Alosa sapidissima

Phylum

Chordata

Class

Osteichthyes (bony fish)

Order

Acari

Family

Clupeidae

Size

40-60 cm Up to 76 cm

Weight

1-3 kg Record: 5 kg

Life span

14 years in the St. Lawrence; 6 years in rivers of northern Florida

Status

Least concern (IUCN 2013)

At the Biodôme

Because the American shad are a species that essentially feeds on plankton (by filtering), their food must be cut into small pieces (the size of half a shrimp). They may also be fed plankton mixtures (CYCLOP-EEZE).

We are currently introducing training/ implementing a feeding system that will allow us to feed the filter-feeding fish separately from the other, more voracious, fish in the large Gulf of St. Lawrence basin.

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