Lansdall-Welfare, T. et al. (2016). “Content Analysis of 150 Years of British Periodicals”. In: Proceedings of the National Academy of Sciences.
As the title says, they scanned every periodical they could find in Britain from 1800 to 1950, made a giant database of the million most common words, and put all their results on line. This is exactly the tool I need to address the Tuna question, without American noise drowning out the signal I’m looking for.
First discovery: “tunny” didn’t make the cut. It appears in books; apparently not in newspapers or magazines. But there are, as the saying goes, plenty of fish in the sea. The most-mentioned species are cod and haddock. Trout leads the freshwater contingent. Tuna is lower in frequency than these by an order of magnitude, down among words that I don’t believe I’ve ever typed before like “pilchard” and “kipper”.
Second discovery: “tuna” doesn’t show an increasing trend over time. The FindMyPast team uses an appearance-per-year metric (as does Google Ngrams), so the growth in references is corrected for the growth in the number of publications. English periodicals are more likely to talk about trout and kippers since the end of the 19th Century, but not tuna or pilchards. All the growth in the Google result seems to have come from the USA.
For what it’s worth, the big spike in “trout” in 1897 coincides with the re-publication of Izaak Walton‘s The Compleat Angler, edited by Andrew Lang of fairy-tale fame.  Was there a surge in interest in fishing, on which Lang capitalized? Or was the book the reason for the increase in trout-mentioning?
Summing up the facts we have:
- “Tuna” was not prominent in texts in the UK at the time when Tolkien was writing The Silmarillion.
- There’s only a fifty-percent chance that people would have called that fish a “tuna”, anyway.
- Tolkien could certainly have known the Americans were making tuna into a household word.
- There is no sign that the word “tuna” would have intruded upon Tolkien’s notice from external sources as he was writing.
It is highly unlikely that JRRT would have thought the word “tuna” might have humorous resonances among his audience, but there is no scholarly merit to wondering about this issue. Idiosophers just like playing with databases.
 There — a connection with speculative fiction, at last.