Catfish were introduced to British waters in the 1870's and 1880's from Germany. Some were sent by Lord Odo Russell to Frank  Buckland, whose father has been a Professor at Oxford in the 1840's, and others were sent to England by Sir Stephen Lakeman from his Bucharest estates. Seventy were introduced to the Duke of Bedford's lakes at Woburn in 1880 after being presented to the British Ambassador in  Berlin. Some are still there. They are also found in Claydon Lakes, Leighton Lakes, reservoirs at Tring, Herts and the River Wissey, and an increasing number of other locations in England.

It is believed that they do not breed until the water temperatures  reach 20*C., but they can survive in much colder  water. The Danube, where they are found, freezes annually. They are native to much of Europe, and are found as far north as Norway and Sweden, often in lakes with "monster" myths. They are voracious carnivores, mainly nocturnal,  given to lurking in dark places under banks and overhangs. They can live for well over 100 years. They can attain  5lbs by the third year, 180 lb by 24 years and take another 60 years to reach 500 lb. So the 19th century Russian specimens of 15 ft and 750 lb would be substantially older.

If young specimens found their way into Loch Ness in the 1880's, they could have grown to a  substantial size by the 1930's, and could conceivably still be alive today. Here is a plausible scenario.

The middle decades of the 19th century were a time when it was very fashionable for the aristocracy to acquire land in Scotland. Queen Victoria had bought Balmoral Estate, and her German husband, Prince Albert, was also at the fore-front of the craze for innovation, epitomised by the Great Exhibition.  It is entirely possible that some of his aristocratic friends who had introduced the catfish,  or sheet-fish as it was commonly known, to their  English waters could have also attempted to  introduce the commercially important species to  their Highland estates as an "improvement". The reasons for introducing them to England were two-fold; firstly "acclimatisation" of alien species of plants and animals was a popular gentleman's pastime, and secondly, a source of cheap protein offered by the catfish was an attractive proposition for feeding the Victorian working class, on which their wealth depended. "The  Times" newspaper in the 1860's  hailed the new fish as "the most important animal  introduction since the bringing of the turkey". The  flesh was said to resemble "veal with a rich eel-like  flavour, superior to salmon."

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Pursuing this line of thought, two possibilities exist. One, that after introduction, the small catfish escaped the "estate" lochs and found their  way downstream to Loch Ness, where they could eventually grow to prodigious size and age, and were at the same time reckoned to have perished at their original location; and two, that these young catfish lived to breeding age,  unnoticed, in one or more peaty hill-lochs, where the dark water absorbed the sun's heat and reached a  suitable temperature for breeding in the summer months. A proportion of the progeny would then be available to supply Loch Ness with a continuing supply of young catfish. In either event, a Victorian gentleman's experiment could have had significant  long-term consequences for the area's economy.

Highland place-names give a clue to much earlier introductions, possibly from Roman, Viking or Norman times.  "Loch na Beiste" - Loch of the Beast-  is a not-uncommon name for hill-lochs, and might be associated with the catfish's mature size, voracious appetite for anything swimming in the water, and ability to travel overland for moderate distances.

Introductions from continental  Europe could have given rise to the dragon myth in Wales and elsewhere in the UK, and stories  like the "Lambton  Worm" in Weardale, County Durham.

Original caption was:  "Unknown, but successful, angler with 84 lb European catfish." - shows how much I know!
I am informed that this is the the world-renowned specimen angler Kevin Maddocks with one of his smaller catches! I am getting in touch with him to ask permission to use the picture. One of his books is on my list.

And there are stories of much larger ones...