Larval Fish Habitats and Deoxygenation in the Northern Limit of the Oxygen Minimum Zone off Mexico

The present state of deoxygenation in the northern limits of the shallow oxygen minimum zone off Mexico is examined in order to detect its effects on larval fish habitats and consider the sensitivity of fish larvae to decreased dissolved oxygen. A series of cruises between 2000 and 2017 indicated a significant vertical expansion of low oxygen waters. The upper limit of suboxic conditions (<4.4 μmol/kg) has risen ~100 m at 19.5°N off Cabo Corrientes and ~50 m at 25°N in the mouth of the Gulf of California. The larval habitat distribution was related to the geographic variability of dissolved oxygen and water masses between these two latitudes. One recurrent larval habitat, with Bregmaceros bathymaster larvae as the indicator species, extended throughout the water column off Cabo Corrientes from Subtropical Subsurface Water (suboxic conditions) to the surface (220 μmol/kg). The second recurrent habitat was located between the oxycline (>44 μmol/kg) and the surface in association with the Gulf of California Water, with Benthosema panamense as the indicator species. During the warm El Niño event of 2015–2016, a tropical larval fish habitat (Auxis spp.) associated with Tropical Surface Water appeared to change the larval habitat distributions. These results indicate that some species are resilient to changes of dissolved oxygen and temperature generated by El Niño events and by continuing deoxygenation, although other species with more limited environmental windows could be affected by deoxygenation, probably leading to a change of the pelagic ecosystem over time.