 The gulf of Corinth, a small, deep basin in the central Mediterranean, hosts high densities of mesopelagic fish, which dominate the pelagic ecosystem and serve as a primary food source for local dolphin populations. Three seasonal acoustic cruises reveal distinct ecotypes and vertical distribution patterns of deep scattering layers, DSLs, that correspond to specific species or groups. The study found that Morollicus mulele forms shoals along the shelf break, Ugeopelicus hemigemnus dominates a non-migrant thin DSL at 150 to 280 m, and myctophids make up a thick, partially migratory DSL at 250 to 600 m. The study also identified species-specific and size-related patterns in the vertical distribution of fish during both daytime and nighttime. This article was authored by Z. Coppolones, a siopartis, amesias, and others.