Riverwatch

A commotion on the shingle

For those about early in the day as summer warms the river, you may just come upon a commotion of fishy activity on shallow and faster runs of shingle. Water may spray as boldly scaled fishy bodies twist and turn in the shallow riffles, often in the tail of weir pools. Sometimes, dense shoals of minnows darkening the shingle may catch your eye, closer observation revealing darker shapes swaying in the current upstream.

It is at this time of year, typically in June but sometimes in May in warm seasons or into the first week of July when cooler, that chub (Squalius cephalus) and barbel (Barbus barbus) seek out these well-flushed waters and their more open gravels in which to spawn.

Chub and barbel, both members of the carp family of fishes, occur towards the north of their broad Eurasian range here in Britain. It is for this reason that they spawn late in the year relative to other fishes such as the cool-adapted dace and pike, and spring spawners including roach and perch.

Both chub and barbel spawn in or on gravel. Unlike trout and salmon, neither chub nor barbel cut redds (depressions in the gravel) into which females deposit eggs. But their communal breeding behaviour can be quite vigorous, gravid females thrashing around with accompanying male fish competing to fertilise the eggs as they are released. This intense activity, often sending spray up into the air, ruffles the gravel surface opening up its pores into which the sticky eggs may fall and obtain at least some level of protection.

Minnows rarely miss a treat. Though potential prey of the larger, frisky fish, they intercept nutrientrich eggs drifting down to them into the current. They frequently risk their lives by nipping into the frenzy to steal eggs at source. After the post-dawn spawning, many of these fish may remain on the gravels. This is particularly true of the minnows, perhaps joined with other smaller fishes such as gudgeon and dace, to pick over the nutritious feast laid out beneath them. The chub and barbel tend to drop back into slightly deeper water for cover, though remain in the vicinity for a few days repeating the spawning riot over a number of successive dawns. Many other animals join this foodrich bonanza, from insects to crustaceans and even some birds.

Chub show no allegiance to their gravelspawning companions in this regard. Barbel generally spawn a little later than chub. At this time, chub are generally attentive, hanging back from the spawning barbel to join the minnow hoard in cramming as many of eggs into their mouths as they can.

After spawning, parent chub and barbel drop back into deeper and slacker stretches of the river, displaying no parental care. Depletion of the fertilised eggs is massive, mainly resulting from high levels of predation. In gravels blinded by silt, eggs may not in fact be able to fall into pores, receiving little protection. Furthermore, water transfer in the sediment-enriched matrix may impede the flow of oxygen-rich water resulting in the eggs suffocating.

However, a small proportion may survive, hatching out after a few days as embryos with yolk sacs still attached. The hatchlings attach to gravel or plant matter for a few days more until the yolk sac is fully absorbed, before then ‘swimming up’ as tiny fry no longer than an eyelash to start to feed on tiny algae and invertebrates. Survival of the tiny juvenile fish is initially poor but, after three or so years, chub and barbel mature and can join the breeding population, living as long as 15 years.

At the north of their geographical range, successful reproduction may not occur every year. Indeed, it can fail entirely in cooler years, or where flooding washes out poorly developed juveniles. However, over a number of years, populations tend to recruit though often with notable ‘clumping’ of year-classes, and hence fish of only certain size ranges. These strong year-classes can generally be traced back to good spawning years when temperatures were high and with favourable foodrich, flood-free ‘nursery’ conditions.

If you are out and about early in the day at this time of year, do look out on the well-flushed gravels for commotions that may just be these fishes completing their challenging life cycles as the waters warm. Mark Everard