Anoxic waters constrain the vertical distribution of fish developmental stages in an oxygen minimum zone

Abstract

In the Eastern Tropical North Pacific Oxygen Minimum Zone (ETNP-OMZ), fish larvae undergo development amidst highly variable dissolved oxygen environments. As OMZs expand, understanding the implications of low-oxygen environments on fish development becomes increasingly relevant for fisheries management and ecosystem modeling. Using horizontal zooplankton tows to track five oxygen levels (oxic [200 μmol/kg], hypoxic [100 μmol/kg] suboxic [10 μmol/kg], anoxic [<1 μmol/kg], and deep [10 μmol/kg at ~ 1000 m depth]), this study analyzed the three-dimensional distribution of fish larvae and adults across the ETNP-OMZ. Results revealed a wide midwater anoxic core, extending from Costa Rica to Baja California, that was almost devoid of fish larvae (< 1 larvae/1000 m³). Early larval stages primarily inhabited the oxic and hypoxic levels above the core, while postflexion and transformation stages occurred across a wider oxygen gradient, including the deep level below the anoxic core. Epipelagic species (e.g., Auxis sp.) were predominantly found in the surface oxic level, whereas coastal-demersal species (e.g., Bregmaceros bathymaster, Ophidion spp.) were prevalent in the hypoxic level above the core. Meso-bathypelagic species (e.g., Diogenichthys laternatus, Cyclothone spp.) were present throughout the study area, including below the anoxic core as transformation larvae and juveniles. These findings indicate that a vertical expansion of anoxic waters in OMZs could further constrain the habitat of epipelagic species, while also affecting the ontogenic vertical movements of meso-bathypelagic species.