Hybrids are not true members of any species but are the result of a cross between two different species. Most coarse anglers are interested in hybrids only when it affects a record — you can’t claim a new British rudd record with a roach/rudd hybrid! Hybrids occur between members of the carp family such as bleak and chub and some have been recognized for a long time. However, it is only quite recently that as many as nine different crosses have been found.
What a hybrid looks like obviously depends on its parents. The offspring of species that are quite different, such as roach and bream, look like a mixture of the two. This can make them quite easy to pick out.
Hybrids from other pairings are often trickier to spot. Roach and rudd are very similar and telling them apart is sometimes hard even for experienced anglers. Hybrids can make the confusion even worse. There is one cast iron test, but unfortunately this involves killing the fish to look at its pharyngeal (throat) teeth.
With some crosses there are other tests you can try. Roach/bream hybrids have 15-19 rays in the anal fin, half way between roach with 9-12 rays and bream with 24-29.
Hybrids probably also behave a bit like both parents, especially in their feeding habits. However, their growth patterns tend not to be similar to their parents. They grow quickly and to a large size — this is known as hybrid vigour. For example, roach/bream hybrids tend to be more bream-size than roach-size. Specimens up to 8 lb (3.6kg) have been taken on rod and line.
It is noticeable that some waters seem to contain a far higher proportion of hybrids than others – even though the species involved are the same. It seems that hybrids are most likely to occur in waters that have been altered by man.
Dredging, channelization, weed cutting, species introductions and fish stocking all tend to increase competition for available spawning sites. This brings the various members of the carp family together during spawning, increasing the chances of a hybrid or two.
Intensively managed waters, such as reservoirs, usually have a high incidence of hybrids, whereas there tend to be very few in an unaltered river.
Many hybrids are fertile. This makes it possible for them to breed – further complicating things and producing second generation hybrids. Most other hybrid animals, such as the mule which is a cross between a male donkey and a female horse, are incapable of breeding.
Crossbreeding may have happened in some waters, making it all but impossible to identify a second or third generation hybrid. It also makes it hard to be sure that a record roach is a pure roach, and that there isn’t a hint of bream somewhere in its genes.
Indeed, we may find that none of the hybrid-producing species of the carp family is pure – that faster growing strains are all descended from hybrids somewhere down the line. At present, too little is known about the ways hybrids interact with each other and with their parent species at spawning time to be able to come to any definite conclusions.