Physiology of Finfish and Shellfish (2+1)

4.4.6. The oxygen minimum layer

Most of the world’s oceans contain, at intermediate depth, zoned where the oxygen content is very low. There is a large such area in the northeastern tropical Pacific, which is known as the oxygen minimum layer. Off the coast of central and southern Mexico the layer extends from approximately 100 to 900 m in depth, and the dissolved oxygen content is generally less than 0.1 cm3 per liter sea water, with relatively little variation throughout the year. Nevertheless, a surprisingly abundant population of animals lives in the oxygen-poor water.

Some of the animals, particularly the fish, make daily migrations to the surface, ascending at dusk and returning to depth before sunrise, in vertical movements that amount to 300 m or more. Paradoxically, the fish present in the low-oxygen region often have gas-filled swim bladders that contain a high percentage of oxygen.

During the day, when the fish are at depth, it seems likely that they must obtain energy by anaerobic pathways. Their blood has a low oxygen capacity and their hemoglobin such a strikingly low affinity for oxygen that no more than 1% of the hemoglobin would be oxygenated while the fish are at depth (Douglas et al., 1976).