Influence of the upwelling of the oxygen minimum layer on the distribution of fish mass mortalities

Upwelling occurs mostly in the eastern parts of the oceans along the coast and at the boundaries of currents. The primary reason for the upwelling is to be found in the relatively steady winds in the tropics and sub tropics blowing along the coast and/or in the wind divergences of the equatorial regions. A secondary reason for the upwelling may be the localtopography of the coast and the continental shelf in relation to permanent ocean currents. These secondary reasons account usually for the upwellings in the western part of the oceans.

The upwelling phenomena of water with low oxygen content can be devided into two groups :

1. upwelling in presence of a (tropical) sub-surface oxygen minimum layer;
2. upwelling of bottom waters from which the oxygen has been consumed by organics sediments

The distinct oxygen minimum layer is observed in tropical and subtropical parts of the oceans. This layer is usually better developed than in the western  parts of the oceans. The most pronounced oxygen minimum layer is found in the Arabian Sea, somewhat less pronounced in the Gulf of Guinea. In normal conditions this oxygen minimum layer is at 100 to 150 m. The upper boundary of the layer is relatively sharp. In the core of this layer in Arabian Sea there is no oxygen but hydrogen suphide; in the Gulf of Guinea the corresponding oxygen contentis about 0-5 ml.l. or some 10 per cent from saturation. The pacific oxygen minimum layer, appearing at greater depths, is relatively little developed. Although the oxygen minimum layer has been describe as the layer of maximum consumption by some and as the layer of minimum replenishment by others, it cannot be said thet either of the explanations is absolutely correct. since both factors probably are active in developing this more or less permanent oxygen minimum layer.

Consumption of oxygen close to bottom occurs if the sediment contain much organic matter. this usually the case over shallow shelves whewe upwelling normally occurs, causing high production of organic matter in the waters above. Oxygen can also be exhausted from more stagnant waters in isolated, poorly ventilated basins of shelf.

Upwelling in presence of a tropical sub-surface oxygen minimum layer. When the oxygen minimum layer starts to rise along the continental shelf owing to seasonal start of the upwelling, the aimals, including fish, migrate in front of it into shallow water or rise into the surface layer. There is a zoe in shallow water where mixing by tidal currents and wave keeps the water aerated. Usually a high concentration of demersal fish is found at the seaward boundary of this zone. This boundary is not static but fluctuates back and forth with the tides and with the intensity of upwelling, varyng as a function of the winds. Seawards of the upwelling area in the surface waters is observed a high basic organic production. A little further offshore pelagic fish gather and feed on zooplankton populations which develop if and when the upwelling has lasted about a month or more.