Shoal chub  (Macrhybopsis hyostoma)

The shoal chub has yet to be classified by the IUCN.

The shoal chub (Macrhybopsis hyostoma) is a small, slender freshwater fish named after its preference for relatively shallow, fast-flowing, sand or gravel sections of river, known as shoals (3) (4).

The body of the shoal chub is elongate and tapering, with a somewhat flattened underside (2) (3) (4) (5). The head and snout are moderately rounded (3) (4), and the snout overhangs the mouth, which has one to two pairs of fleshy barbels at the corners (2) (3) (4) (5) (6). The eyes of the shoal chub are set quite high on the head (3).

The shoal chub is generally pale greenish, grey or straw-yellow above, with silvery sides and scattered black spots, which may sometimes form a faint dark stripe. The underside of the body is silvery-white (3) (4) (5) (6) and the fins are usually clear (5). Small tubercles are present on the pectoral fins of breeding male shoal chub (3) (4).

Previously considered to be the same species as the speckled chub (Macrhybopsis aestivalis) (3) (7), the shoal chub can be distinguished from the latter by its rounded rather than conical head, and by various features of its fins and barbels (4).

The shoal chub inhabits streams and rivers in the Mississippi River basin and Gulf Coast of the United States, from Minnesota, Wisconsin and Ohio south to Texas, Louisiana, Mississippi and Alabama (2) (7).

This freshwater species typically inhabits small to large rivers, preferring areas of fast-flowing water over shallow riffles, with a substrate of coarse sand or gravel (2) (3) (4) (7).

The shoal chub is believed to swim over the river bottom in search of food (2) (4), with its diet consisting of a variety of aquatic insects, small crustaceans, and some plant material and detritus (2) (3) (4) (7). Like other fish in the Cyprinidae family, the shoal chub lacks teeth, instead processing food using modified bones in the throat, known as ‘pharyngeal teeth’ (8).

This species spawns between May and August, when temperatures rise above 21 degrees Celsius (3) (7). The shoal chub is thought to be a ‘flood-pulse’ spawner, laying its eggs when the flow of the river peaks after floods (3) (4). It scatters its eggs over the substrate in open water, the eggs then developing as they drift in the current (2) (3) (7). The eggs of the shoal chub hatch after about one day (7).

The lifespan of the shoal chub is around one to two years, with individuals maturing and spawning at about one year old (3) (7).

In the Arkansas River basin, the construction of reservoirs and alteration of river flow patterns is believed to have eliminated the shoal chub from over half of its former range (4) (7) (9). Although it is still common in some areas, this species has also declined or been extirpated from many other river systems throughout its range (3).

Reservoir construction is most likely to affect the shoal chub by creating a barrier to dispersal, isolating its populations and preventing recolonisation should individual populations be lost (9). Other modifications to rivers, such as locks, dams and channelization, may also affect shoal chub populations (3).

There are no specific conservation measures currently known to be in place for the shoal chub. However, this species would be likely to benefit from the protection and restoration of its river habitat, and from further research into its populations and life history (10).

Find out more about the shoal chub:

• FishBase - Shoal chub:
  http://www.fishbase.org/Summary/SpeciesSummary.php?ID=61906&AT=shoal+chub
• Texas Freshwater Fishes - Shoal chub, Macrhybopsis hyostoma:
  http://www.bio.txstate.edu/%7Etbonner/txfishes/macrhybopsis%20hyostoma.htm