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Large areas of cold hypoxic water occur as distinct strata in the eastern tropical Pacific and Atlantic oceans as a result of high productivity initiated by intense nutrient upwelling. Recent studies show that this stra- tum restricts the depth distribution of tropical pelagic marlins, sailfish, and tunas in the eastern tropical Pacific by compressing the acceptable physical habitat into a narrow surface layer. This layer extends downward to a variable boundary defined by a shallow thermocline, often at 25 m, above a barrier of cold hypoxic water. The depth dis- tributions of marlin and sailfish monitored with electronic tags and mean dissolved oxygen (DO) and temperature profiles show that this cold hypoxic environment constitutes a lower habitat boundary in the eastern tropical Pacific, but not in the western North Atlantic, where DO is not limiting. However, hypoxia-based habitat compression has not actually been demonstrated in the eastern tropical Atlantic Ocean, despite this region having similar oceano- graphic features to the eastern tropical Pacific. This paper explores the possibility that habitat compression of tropi- cal pelagic fishes exists in the eastrn tropical Atlantic and examines possible consequences of this phenomenon. We used Atlantic-wide catches of yellowfin tuna (Thunnus albacares) as an example why habitat compression off west Africa could eventually affect the total Atlantic stock. RESUMEN Extensas areas con aguas relativamente frias y bajo contenido de oxigeno (hypoxia) pueden ocurrir en el Pacifico y Atlantico Tropical oriental como resultado de alta productividad biologica en zona de upwelling. Estudios recientes demuestran que estos estratos restringen la distribucion vertical de species pelagicas tropicales como los marlines, pez vela y atunes en el Pacifico tropical oriental al comprimir el habitat de estas especies a una termoclina poco profunda, a menudo solo 25 m, sobre una barrera de agua fria y con bajo contentido de oxigeno. Las distribuciones vertical de marlines y pez vela monitoreadas con marcas eletronicas y perfiles de oxigeno dis- uelto (OD) y temperature muestran que este ambiente relativamente frio y bajo contenido de oxigeno constituye una barrera del habitat en el Pacifico tropical oriental, pero no el Atlantico Norte occidental donde el OD no es limitante. La compresion del habitat basada en la hypoxia no se pudo demostrar en el Atlantico tropical oriental, a pesar de que esta region tiene caracteristicas oceanograficas similares a las del Pacifico tropical oriental. Este docu- mento explora la posibilidad de la existencia de habitats comprimidos para especies de peces tropicales y examina las posibles consecuencias de este fenomeno. Utilizamos colectas de albacore (Thunnus albacores) como ejemplo del efecto de la compresion del habitat en la parte oeste de Africa y su efecto en el stock del Atlantico.

Consequences of Ocean Scale Hypoxia Constrained Habitat for Tropical Pelagic Fishes