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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., Auxissp.) were predominantly found in the surface oxic level, whereas coastal‐demersal species (e.g., Bregmaceros bathymaster, Ophidionspp.) were prevalent in the hypoxic level above the core. Meso‐bathypelagic species (e.g., Diogenichthys laternatus, Cyclothonespp.) 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.

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 bathymasterlarvae 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 panamenseas the indicator species. During the warm El Niño event of 2015–2016, a tropical larval fish habitat (Auxisspp.) 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. Dissolved oxygen in the ocean is crucial for the biological cycles of all organisms. There are vast zones in the ocean where the oxygen has dropped to levels that only a few, very well adapted, organisms can live. Dissolved oxygen measurements in the past two decades in the Mexican Pacific have shown that the concentrations have decreased, and water layers with nearly no oxygen are now closer to the surface, potentially impacting larval fish species that are not adapted to extremely low oxygen concentrations. Nevertheless, there is a set of fish larvae that is well adapted to low dissolved oxygen concentrations, principally small forage fish, which have a permanent presence in the poorly oxygenated waters, while some larvae of species with fisheries interest, like small tunids, only live in the well‐oxygenated surface layer and are more abundant in warm years. The loss of oxygen in the Mexican Pacific has not had an effect on the small forage fish larvae but can affect those larvae of fish of commercial interest, with less adaptation to low oxygen concentrations. Deoxygenation processes occur in the northern limit of OMZ off MexicoThinning of the hypoxic layer affects its inhabitants in different waysFish larvae of some species show resilience to changes of dissolved oxygen

A simplified analytical model was used to solve the salinity balance equation and study the relative contributions of freshwater flux and salt advection to the salinity tendency between the Gulf of Tehuantepec and Cabo Corrientes (14–22°N). The model used climatological mixed‐layer salinity, satellite‐derived geostrophic velocity and evaporation minus precipitation data in a four‐domain scheme. The mean circulation derived from the model largely coincides with previous reports and suggests that, in the steady state, the Mexican Coastal Current is decoupled from the Tehuantepec region. The analysis of the time‐dependent variability showed that the salt advection can be largely explained by the annual variations of the alongshore velocity (corresponding with the Mexican Coastal Current) and an average salinity. At the seasonal scale, the salt advection and the freshwater flux have similar contributions to the resulting salinity tendency. Near Tehuantepec, due to a local salinity regime, the analytical model does not represent well the observations, while in Manzanillo and Zihuatanejo the agreement between the model and the observations is good in both amplitude and phase. The accuracy of the model in representing the observations is improved in the first third of the year by inclusion of the semiannual scale and the nonlinear processes in the equation of salt advection. Salinity balance and its related processes are explained with a simple two‐dimensional modelThe area's salinity decreases between June and October as a result of net precipitation and poleward advection of fresh tropical watersMexican Coastal Current is the principal agent for salt advection between Tehuantepec and Cabo Corrientes

The analysis of primary productivity (PP) is essential for understanding the dynamics of marine ecosystems such as the Gulf of California Large Marine Ecosystem (GC-LME). This ecosystem is highly productive and supports an intense fishing activity (deep-sea, artisanal and sport fishing). The main objective of this investigation was to describe the spatial and temporal variability of phytoplankton primary productivity within the GC-LME based on 14 regions. A primary productivity database was built using 524 surface 14C productivity measurements from 1960 to 2016. This information was supplemented with productivity calculations from satellite models. The analysis of the statistical descriptors of in-situmeasurements revealed that primary productivity spatial distribution displayed a latitudinal gradient with increasing values to the north. In the warm season, the highest with 363.90mgCm−3D−1), Archipelago Region (Region VII 181.74mgCm−3D−1) and Canal de Ballenas (Region V 152.99mgCm−3D−1). The lowest surface values were reported in the southern regions (IX, XI and XIII), with average values of 19.42, 29.22 and 18.15mgCm−3d−1. This is the deepest portion of the GC-LME that has been described as a complex area with a thermohaline structure related to the confluence of three different currents — North Equatorial Current, California Current and GC-LME water — that produce local fronts, eddies and intrusions, that affect productivity. In general, the temporal comparison showed that surface PP averages were higher in the warm season than in the cold season. In the GC-LME nutrient availability is high all year round, so these higher PP values are explained because in the warm season phytoplankton has more day-light hours.

The three‐dimensional distribution of the paralarvae of the complex Sthenoteuthis oualaniensis‐Dosidicus gigas(Cephalopoda: Ommastrephidae) was analyzed at the northern limit of the shallow oxygen minimum zone in the Eastern Tropical Pacific in April 2012. The upper limit of the oxygen minimum water (∼44 µmol/kg or 1 mL/L) rises from ∼100 m depth in the entrance of the Gulf of California to ∼20 m depth off Cabo Corrientes. Most of the paralarvae of this complex, dominated by D. gigas, were concentrated in the Gulf entrance, between the thermocline (∼20 to ∼50 m depth) and the sea surface, in the warmest (>19°C) oxygenated (>176 µmol/kg) layer. The highest abundance of paralarvae was detected in an anticyclonic eddy (∼120 km diameter and >500 m deep), which contained lower‐salinity water (<35 g/kg), consistent with formation in the California Current. Lower paralarvae abundance was recorded further south off Cabo Corrientes, where hypoxic layers were elevated as water shoaled nearshore. Almost no paralarvae were found in the north of the study area beyond the strong salinity front (∼34.8–35.4 g/kg) that bounded the anticyclone. These results showed an affinity of the paralarvae for lower‐salinity, oxygenated water, illustrated by the influence of the mesoscale anticyclonic eddy and the salinity front in their distribution. Based on this study, it can be concluded that the expansion of the depth range of hypoxic water observed in the Eastern Tropical Pacific may be increasing environmental stress on the paralarvae by vertically restricting their habitat, and so affecting their survival. Dosidicus gigasparalarvae of E. Trop. Pacific live above shallow hypoxic layerMesoscale eddies and saline fronts affect paralarvae distributionA vertically expanding hypoxic zone may reduce the paralarvae habitat

On the basis of five oceanographic cruises carried out in the Eastern Tropical Pacific off Mexico, relationships between the larval fish habitats (areas inhabited by larval fish assemblages) and the environmental circulation scales (mesoscale, seasonal, and interannual) were examined. Analysis of in situ data over a grid of hydrographic stations and oblique zooplankton hauls with bongo net (505 µm) was combined with orthogonal robust functions decomposition applied to altimetry anomalies obtained from satellite. During both cool (March and June) and warm (August and November) periods, Bray‐Curtis dissimilarity Index defined three recurrent larval fish habitats which varied in species composition and extent as a function of the environmental scales. The variability of the Tropical larval fish habitat (characterized by high species richness, and dominated by Vinciguerria lucetia, Diogenichthys laternatus, and Diaphus pacificus) was associated with the seasonal changes. The Transitional‐California Current larval fish habitat (dominated by V. lucetiaand D. laternatus, with lower mean abundance and lower species richness than in the Tropical habitat) and Coastal‐and‐Upwelling larval fish habitat (dominated by Bregmaceros bathymaster) was associated mainly with mesoscale activity induced by eddies and with coastal upwelling. During February 2010, the Tropical larval fish habitat predominated offshore and the Transitional‐California Current larval fish habitat was not present, which we attribute to the effect of El Niño conditions. Thus, the mesoscale, seasonal, and interannual environmental scales affect the composition and extension of larval fish habitats. Variability of larval fish habitats in the Eastern Tropical PacificWater masses and larval fish habitats in the Eastern Tropical PacificENSO influences in the larval fish habitats in the Eastern Tropical Pacific