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.

24Vertical distribution of the cephalopod paralarvae was investigated in relation to a system 25 of two cyclonic and three anticyclonic eddies in the southern Gulf of California and a front 26 in the adjacent Pacific Ocean. Results showed that the preferential habitat for the 27 Sthenoteuthis oualaniensis -Dosidicus gigas "SD-complex" in both regions was the 28 oxygenated surface mixed layer and the thermocline. The highest abundances occurred in 29 of one of the anticyclonic eddies and a frontal zone, which are convergent structures. 30 Enoploteuthid and Pyroteuthid paralarvae both displayed their highest abundances in the 31 thermocline. Pyroteuthids dominated in the cyclonic eddy whereas Enoploteuthidae were 32 less evident in the eddy system. Pyroteuthids were observed on the western (California 33 Current) side of the frontal zone, and Enoploteuthids on its eastern (Gulf of California) 34 side. The octopods and the complex of Ommastrephes-Eucleoteuthis-Hyaloteuthis 35 paralarvae were present below the thermocline. Both groups had a scarce presence in the 36 eddy system and high abundance near the frontal zone. The octopods abounded on the 37 eastern side in association with the low dissolved oxygen concentrations (< 44 µmol kg -1 ) 38 of Subtropical-Subsurface Water; the complex on the western front side was immersed in 39California Current Water. It may be concluded that the spawning and early stages of 40 development of these cephalopod groups are associated with particular mesoscale structures 41 of the water masses. For example, the "SD complex" inhabits the surface water masses, 42 preferentially in convergence zones generated by mesoscale activity. 43 44

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 riseñ 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.Plain Language Summary 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.

No abstract

No abstract