Why can fatty tuna on slightly warm sushi rice feel more buttery than the same fish served ice-cold?

Answer: Softens the fish fat

Softens the fish fat ✓ — Right. Fatty tuna and salmon belly are prized partly because their fats soften at temperatures far below a steak's searing heat. Sushi University lists chutoro and otoro around 23-26 C, and a specialist fish retailer notes that warm nigiri rice accelerates otoro fat melt. The effect is physical, not poetic: temperature changes how quickly fat coats the tongue.

Makes fish taste sweeter — No. Warmth can change flavor release, but it does not create sweetness in the fish by itself. Good tuna may taste sweet because of quality, fat, aging, amino acids, and handling, while the warm-rice effect here is mainly mouthfeel. If the same fish feels more buttery on nigiri than ice-cold, fat softening is the cleaner mechanism.

Adds buttery rice aroma — No. Rice aroma can support the bite, but it does not turn buttery on command. The cited fish-temperature sources point to fat melting and mouthfeel in salmon toro, chutoro, and otoro. Warm shari is a heat partner for the fish, not a new source of butter flavor.