Competitive Interactions between California Reef Fishes of the Genus Embiotoca

Embiotoca lateralis and E. jacksoni are very similar morphologically and exhibit the greatest dietary overlap within a guild of microcarnivorous subtidal reef fishes off Santa Barbara, California. Within this zone of sympatry, they segregate by depth: E. lateralis numerically dominates shallow reef areas covered by various algae, from which it picks its prey; E. jacksoni dominates and forages over deeper areas covered by a relatively food—poor "turf" of sessile invertebrates and small algae. E. lateralis rarely fed from turf—covered substrates experimentally translocated from deep water, while E. jacksoni frequently fed from translocated shallow algae. In allopatry, however, each species occupies all reef depths and commonly forages over both food substrates. When E. lateralis was experimentally removed from a reef inhabited by both species, E jacksoni entered shallow areas and foraged over algae. However, removing E. jacksoni from another reef did not affect the distribution of E. lateralis. E. lateralis aggressively dominated E. jacksoni, and E. jacksoni avoided foraging near E. lateralis. E lateralis eventually entered and foraged over deep reef ares only when all shallow algae and E. jacksoni were removed. Sympatric population densities of E. lateralis, here at the southern margin of its geographical range, are much lower than those of allopatric populations elsewhere. Hence, shallow food sources may be sufficiently abundant that marginal E. lateralis populations occupy only this richest end of the bathymetric reef food gradient. This situation may provide a competitive refuge for E. jacksoni in less productive deep reef areas, thus maintaining the coexistence of these species within the same habitat.