Though they are so small, all three of these fish play a role in the ecology of the river — mopping up tiny items of food from the bottom, competing for food with the young of potentially larger fishes or by being eaten themselves by fish or other predators such as birds.
A river trio
Most small streams and rivers have populations of one or more of these fish, as do some lakes. A few canals have them – the Grand Union, for example. All three are easy to identify, even if the two loaches are superficially similar.
The stone loach is slender but its body is round in cross-section, flattening out towards the tail. It has six fairly long barbels along the upper lip. In colour it is a muted greeny-brown above and yellowish brown on the sides, with irregular darker blotches on both back and sides. The spined loach, in contrast, has a very clear colour pattern – conspicuous rows of rounded dark blotches along its back and on the sides. The back and upper sides are light sandy brown while the underside is creamy. Like the stone loach, the spined loach is slender, but its sides are flattened from head to tail. It too has six barbels on the upper lip, but they are short and inconspicuous.
The features which give the fish its name are the spines beneath each eye. These have double tips and point backwards but are hidden in the skin. They become obvious if you run your finger from tail to head just beneath the eyes! The bullhead is not related to the loaches, though it shares the same habitat as the stone loach. Its head, broad and flattened from above, has a small curved spine on each side. The two dorsal fins and the pectorals are large and fan-shaped. Colour varies according to habitat, but is often brownish green with darker mottling.
The bullhead and stone loach live in rivers with a moderate current, usually hiding under small rocks, in dense weed beds or among plant roots at the edge of the bank.
The spined loach occupies a very different habitat, although it too remains hidden. It lives in slow-flowing rivers and drainage canals, burrowing into the surface mud and lying with just its head exposed, or hidden in the dense blanketweed that grows when flow is low during the summer.
The stone loach and bullhead are widespread in Britain, except in Scotland and the extreme west. The stone loach has been introduced to Ireland. The spined loach occurs only in the Trent, its tributaries and the river systems of Lincolnshire, Norfolk, Suffolk and Cambridgeshire.
Follow the feeding
All three fish feed on small food items. The spined loach eats minute bottom-living crustaceans. Its small mouth makes it impossible to eat larger food.
Both the stone loach and the bullhead take larger creatures, such as freshwater shrimps and insect larvae. The bullhead is sometimes found in the gravel of salmon or trout redds, giving rise to the suspicion that it eats their eggs. There is, however, little evidence that this is the case.
The spined loach spawns from April to June, laying its eggs on algae on the bottom. Little is known of its life-history. The stone loach spawns at the same time, shedding its eggs in batches among stones and plants. The eggs hatch in 14-16 days, depending on temperature. The loaches seldom protect their eggs other than by laying them in out of the way places.
By contrast, the bullhead protects and guards its eggs, which are laid from March to May in a cavity under a stone or tree root in the water. Here they stay for three to four weeks before they hatch. After hatching, the young disperse downstream.