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

Palabras clave: Fish larvae; shallow Oxygen Minimum Zones; Eastern Tropical North Pacific; deoxygenation

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