Yes, the substitution of the word “tuna” for the more traditional English word “tunny” to refer to the tuna-fish does seem to have been influenced by American Spanish-speakers, although apparently not with any reference to the tuna cactus:
No, the combination of the word “tuna” with the word “fish” by English speakers did NOT originate as an attempt to distinguish the tuna fish from the tuna cactus. Speakers of Germanic languages, including English, have been using that combination (“tunny-fish”, “Thunfisch”, etc.) for several centuries. The term “tonny fishe” appears, for example, in an English translation of Plutarch by Sir Thomas North published in 1579.
Your hypothesis is disproven by the evidence given above. English speakers have been commonly using the term “tunny fish” for tuna for centuries before English speakers became aware of the existence or name of the tuna cactus.
It should be noted that the word in Spanish is atun. According to Merriam-Webster, the etymology is:
In today’s Spanish (at least, according to all dictionaries I have available), tuna refers only to the fruit (although there are some slang usages), not the fish, which is atún. The fruit’s name is derived from Taino, and entered Spanish about 1555. But in English, it is much more commonly called prickly pear. Merriam-Webster gives the date of prickly pear as 1612.
Imagine if we named all food that way. “Chicken bird salad.” “Beef quadruped burger.” “Lobster crustacean bisque.” “Peanut legume butter sandwich.” etc.
Etymologically, the names tuna, tunny, Thunnus, thynnus, etc. may be derived from a word in a Semitic language, possibly Phoenician, that means some sort of sea monster. Compare the similar Hebrew word for ‘giant fish’, tanin. It can also refer to a giant snake, and the related Arabic word tinnin means ‘dragon’. The initial a- in Spanish atun comes from the Arabic definite article al- which is assimilated to the following initial t-, so Arabic al-tun is pronounced “at-tun,” and Spanish reduces the doubled consonant sounds to single consonants.
Well, as I think I may have mentioned before, the Germans say “Thunfisch”, while the Swedes say “tonfisk”, the Icelanders say “túnfiskur”, and the Danes and Norwegians say “tunfisk”.
Moreover, the rest of the English-speaking world used to say “tunny fish”, and they still do sometimes, so you needn’t get up on your high horse-mackerel about the “redundancy” of the US name.
In Thai, the word “fish” is appended onto every type of fish, only at the beginning, not the end. The word is pla, and “tuna fish” is pla tuna (they use the English word “tuna”). But pla is stuck on every type of fish – pla-this and pla-that.
The same with all birds (nok). It’s not a parrot, but a “parrot bird” (nok kaew). (That actually translates literally to “glass bird.”)
about the “redundancy” of the US name.