Inside the maze of fishmongers and sushi shops of Tokyo's Tsukiji district, buyers and tourists are assailed by more than just the smell of fresh seafood. There's also the incessant buzzing.

Starting before dawn, the world's biggest fish market resounds with the noise of scooters zipping away with deliveries for high-end restaurants in nearby Ginza. It's a racket particular to the tiny 50cc engine that powers them — the same type of engine that Honda Motor Co. was founded on, and that helped propel the company into the biggest motorcycle maker in the world.

Tsukiji depends on these easily maneuverable scooters, yet their existence is threatened by stricter emissions regulations set for 2020. Honda, Yamaha Motor Co. and Suzuki Motor Corp. are retiring a host of 50cc models this year and warning they may phase them out altogether as the spiraling costs of compliance make them an unprofitable part of Japan's estimated ¥160 billion motorcycle market.