The tuna fish (a corruption of 'tuner fish') was used before the invention of tuning forks as a natural source of reference frequencies to tune early musical instruments such as the lyre, harp, and the harpsichord. The spines of the tunas' dorsal fin correspond to the Baritone range on a piano keyboard; F sharp minor to middle-income C. When a spine is plucked, the tuna contracts its anal sphincter muscles at a uniform rate corresponding to the distance between the spine and the anus, emitting a clear, accurate tone, along with a stringy poo. This tends to annoy the tuna, which is why they always look so surprised.
The first reference to using a tuner fish is found in a diary entry for 3rd March 1066, by Samuel Peypes (1066-1812), who wrote;
"And so offe to Moiras Wet Fish Emporium at Cheapside to procure a tuning-fish that Mrs Peypes may adjust the pianoforte before this evenings dinner party, for it doth sound lyke a cat-i'the-mangle"
In 1981 experimental French Musician Jean Missile-Jar attempted to play a tuna, live, at a concert at a food-processing warehouse on the outskirts of Rotterdam. During the performance, some of Missile-Jar's trademark laser-beams hit the fish and the tuna began to dehydrate causing the pitch to vary wildly. In an attempt to re-hydrate the tuna, roadies poured mayonnaise over it, as there were thousands of boxes of Hellmans stacked in the warehouse waiting to be delivered to supermarkets. Unfortunately the tuna expired and after the concert roadies ate the tuna using slices of bread to keep the slimy mayonnaise off their fingers, inadvertently inventing the tuna-mayo sandwich.