Finrot of a kind.
Hi Leyton...The cause is a type of bacteria that eats through the bone rays of the fin and can usually be found more commonly on the tail or less common on pectoral fins. As the bacteria eats through the bone rays, the damaged part of the fin will hang, split along the membrane and eventually drop off.
Although some may assume it is otters taking a munch or angling damage caused by line, it is not necessarily the case on most fish.
If the fish is younger, the missing piece can re-generate to a high degree.
If older, then it will just heal over with no re-growth of the fin.
If the bacterium is/are active on part of the fin, it will usually show a red inflammation and sometimes white dots which will show the bacteria's progression path.
If this is the case, then it can be treated with 'Friars Balsam' or other good antiseptic, which will kill the bacteria infection dead in its tracks, seal the wound and the fish will heal itself from the inside... if the water quality is ok.
Re-generation of the fin can be quite noticeable on re-capture (if treated) in quite a short period of time if the fish is generally fit and healthy and depending on the water quality.
Your picture shows that it has lost part of the fin and has healed naturally, so does not need treatment. I don't think that this one will re-generate any further, it has just healed over.
Below are a couple of links where if you scroll through the photos, you will eventually pick out similar pieces of tail missing (top, bottom or both) on various species of fish.
http://www.davistackle.co.uk/fisherynews/royaltynews/147-royalty-fishery-report-30-june-2010
http://www.davistackle.info/Archive_2008.htm
http://www.ringwoodfishing.co.uk/rd...t=Photo Gallery (Barbel)&SPE=barbel&phdisp=20
It the bacteria damage is severe and unchecked, the fish could lose the top, bottom or both, where it will then impair swimming/movement thus leaving the fish open to predation or it will eventually die if it loses all the tail.
The Friars Balsam below only costs 99p-£1.60p from the various Chemists.
Just dab it on ‘neat’ all along the damaged area both sides and in between with a cotton bud and return the fish.
When the fish is caught again at a later stage, you may hopefully see a bit of a miracle.
Hope this helps….Ray