The following information won't help members to catch bigger chub, but as the Group are involved in their "study", I think it should at least provide a bit of useful (and little-known) background knowledge. I have certainly learned quite a lot during the process of researching the piece and will be looking a bit harder at the fish I catch in future - just in case.
To most coarse anglers in this country, the word “hybrid" automatically conjures up pictures of the roach/bream version. This is perhaps understandable, as they are probably the most common and easily identifiable of all the naturally occurring hybrids to be found swimming around our rivers and lakes. How many of us though, even within the Chub Study Group, are aware that chub occasionally interbreed with other species? Although for two different species of fish to hybridise with each other, the parent fish must obviously release eggs and milt in the same section of water at the same time, that step is certainly not the only difficult one. It is merely the first in a long line of hurdles to be negotiated if a viable fish is to be formed. Within the fish world, even when both parents are of the same species, the process of spawning is fraught with complications. Some species seem more prone to problems than others and with chub, the conditions allowing for successful fertilisation and hatching of eggs seem particularly strict. This is the reason why populations of chub are often dominated by a single year-class and it is a fact that on many rivers, there may be multiple year-gaps between successful spawnings . Hardly surprising then, that (apart from one exception) hybrids involving chub are not often encountered.
Having alluded to the factors making the process unlikely, I will move on to discuss the three species which are known to cross-breed with chub in British waters:
By far the most frequently occurring chub hybrid is the one involving bleak. These fish are comparatively common in some areas and anyone wishing to do a ‘Dennis Flack‘ and claim a new British record bleak, should first check their fish for signs of chub to avoid disappointment. Any claim involving a fish with less than 14 rays in its anal fin would almost certainly end in tears (chub have 7- 10 rays and bleak 18-23).
Perhaps surprisingly, chub do not seem to hybridise with dace in the wild. One reason for this is that dace tend to spawn earlier than most coarse fish and have usually finished the whole process well before the end of March. Chub invariably spawn sometime during the following couple of months, obviously preferring slightly warmer water and who can blame them - have you ever tried to have sex after a cold shower?
Two of our native British fish, which also spawn between April and June, are roach and rudd and it is with these species that chub hybrids are known to occur. The first time I became aware that any fish was capable of cross-breeding with chub was when I saw a photograph of a chub/ rudd hybrid which had been netted from Pitsford reservoir a few years ago. Initially, I thought it was some sort of ‘April Fool‘ joke. I just wish those people who feel hybrid fish are some sort of worthless second-class life-form when compared to a pure-bred fish had access to this photograph. It had the appearance of a giant chub with very deep, golden flanks and bright crimson fins. A spectacular-looking fish if ever there was one and I would certainly not have been upset to catch it. I believe it tipped the scales at over 7 lbs !
Lastly, we come to the chub/roach hybrid. Again, I have only ever seen photograph ic evidence of one, but at least this time it was a bit closer to home. Group member Phil Sampford caught one of just over 2lb from a tributary of our local river Cam . This fish had the small mouth and red eye of a roach, but the dark tail and body colour of a chub. The shape was intermediate between chub and roach and it would be hard to imagine a fish so obviously "halfway" between the two species. A most intriguing fish and one which caused Alan Owers , Phil and myself considerable excitement when we jointly viewed the photo a while back. The shot is now in the CSG photo album for all to see and it is well worth a look.
Whether chub naturally hybridise with other fish in the British Isles is not clear, but supposedly they don't (yes, hybrids involving chub and various other species of fish – dace included - can be created artificially under laboratory conditions, but for the purposes of this article, we are talking about naturally occurring fish). I have done a little homework in this area recently and although complicated, the subject is very interesting. According to one zoologist I spoke to, it is possible that hybridisation between chub and other species does occur, but that the outward appearance (phenotype) of the resulting fish only reflects the genetic makeup (genotype) of one of its parents. Without getting too technical it is difficult to explain, but it is a bit like certain traits in humans, which only show if both parents carry a particular gene within their DNA. One such "recessive" trait in humans would be ginger hair, for which a gene must be present somewhere in the family of both parents if it is to be apparent in their offspring. Obviously, if two different species are involved, a "double dose" of a gene, which is only found in one of the species, cannot occur and the "dominant" gene will always govern the appearance of the resulting fish.
Pharyngeal (throat) teeth are very characteristically shaped for each separate fish species and are therefore an oft-used marker for detecting whether a fish is pure-bred or not. However, it is just possible that even this seemingly infallible test may not pick up a hybrid in some cases. If the pharyngeal teeth look "right" for one species or another, the fish might not be recognised as a hybrid and this is where things start to get very hazy. Although its genetic makeup may contain equal amounts of DNA from two different species, if there is no outward manifestation of one of the parent fish, their offspring could easily be characterized as pure-bred.
In theory, fish should be able to hybridise more readily with their most closely related cousins, but this statement is complicated by the fact that zoologists have had particular problems in working out the order in which fish species developed. The Latin names used in current taxonomy may therefore not accurately trace their evolutionary progression. This means that two species from the same Genus may not actually be as closely related as a pair with different Genus names.. . Witness the fact that Leuciscus leuciscus (dace) don't cross breed with Leuciscus cephalus (chub), whereas Rutilus rutilus (roach) do!
Another anomaly is the fact that although roach/bream hybrids are common and chub cross-breed with roach, they don't appear to do so with bream. This may seem very strange but something obviously doesn't allow for a viable fish to develop in this particular case. Perhaps two of the genes when mixed together cause a fatal error which prevent the newly- fertilised egg from developing any further. Nature only allows for evolution to take place at a fairly slow rate and even subtle changes take many generations to establish themselves. Although hybrids involving chub and (the similar-looking) ide can be produced under laboratory conditions and occasionally occur naturally elsewhere in Europe, such fish are not thought to arise in the UK due to the lower temperature at which ide spawn. Ide have a smaller mouth and more scales than a chub, but hybrids could obviously be very difficult to identify.
Hybrid fish are often sterile and incapable of reproducing. This sometimes allows individuals to grow larger than is usual for either of the two parent species. Part of the reason for this is that none of the energy obtained from their food is "wasted" in the production of spawn or eggs and, therefore, more becomes available for increasing the bulk of such a fish. This phenomenon partly explains the huge sizes being attained by some of the sterile fish, which have been seeded into various trout fisheries across the country over recent years. As the original purpose of the deliberate genetic manipulation of these fish by man was to create a fast-growing strain of fish for the table (i.e. make more money), the technique is unlikely to be applied to chub in the future as even in Izaak Walton's day, chub were known to be virtually tasteless. Thankfully, finance for similar projects is only likely to be found for propagated game fish and other edible species. As far as I am concerned, this is great news, as it means that our native chub record will never be confused by a fish of artificially-engineered origin. Any hybrids landed will remain rare and very special fish, which their captors should treasure as such. Then, as now, you will never know when one might turn up, so just be aware of their existence and keep your eyes open!