Three-dimensional distribution of larval fish habitats was analyzed, from the upper limit of the shallow
oxygen minimum zone (0.2 mL/L) to the sea surface, in the eastern tropical Pacific Ocean off Mexico in
February 2010. The upper limit rises from 250 m depth in the entrance of the Gulf of California to
80 m depth off Cabo Corrientes. Three larval fish habitats were defined statistically: (i) a Gulf of
California habitat dominated by Anchoa spp. larvae (epipelagic species), constrained to the oxygenated
surface layer (43.5 mL/L) in and above the thermocline (60 m depth), and separated by a salinity front
from the Tropical Pacific habitat; (ii) a Tropical Pacific habitat, dominated by Vinciguerria lucetia larvae
(mesopelagic species), located throughout the sampled water column, but with the highest abundance
in the oxygenated upper layer above the thermocline; (iii) an Oxygen Minimum habitat defined mostly
below the thermocline in hypoxic (o1 mL/L; 70 m depth) and anoxic (o0.2 mL/L; 80 m depth)
water off Cabo Corrientes. This subsurface hypoxic habitat had the highest species richness and larval
abundance, with dominance of Bregmaceros bathymaster, an endemic neritic pelagic species; which was
an unexpected result. This may be associated with the shoaling of the upper limit of the shallow oxygen
minimum zone near the coast, a result of the strong costal upwelling detected by the Bakun Index. In
this region of strong and semi-continuous coastal upwelling in the eastern tropical Pacific off Mexico, the
shallow hypoxic water does not have dramatic effects on the total larval fish abundance but appears to
affect species composition