Oxygen minimum zones (hypoxic zones), reported for over 400 geographical regions, is a key stressor of coastal ecosystems worldwide. Not surprisingly, hypoxia has become a serious environmental challenge to fishery management. As one of the most prominent oxygen depletions of the ocean, the Arabian Sea oxygen minimum zone extends from the central part of the basin to the coasts of Oman, Iran, Pakistan and India and makes fisheries vulnerable to the shelf hypoxia. Shelf oxygen depletion causes numerous fish kill incidents along the Omani coast. Nevertheless, temporal variability of the shelf hypoxia and its impact on fish catches are still poorly investigated.
Dr Sergey Piontkovski (from the Department of Marine Sciences & Fisheries at Sultan Qaboos University) and his colleagues have accomplished the analysis of data from 53 International oceanographic expeditions carried out in the western Arabian Sea during the past 50 years. This analysis was based on about 30,000 vertical profiles of temperature, salinity, water density and over 2,000 vertical profiles of dissolved oxygen recorded in the depth range from the surface to 300m. Scientists have found out that the thermal stratification of the water column increased and the oxygen minimum zone shoaled from about 150m in the 1960s to 80m in the 2000s. The discovered phenomenon is a warning sign pointing at the fact that the habitat of pelagic fishes is becoming thinner and pushed up to the surface. Perhaps, these oxygen changes were underlain by and related to long-term changes in the intensity of monsoonal winds and thermal structure of the water column.
For example, the summer temperature increase in the western Arabian Sea over the past 50 years exceeded the mean rate reported for tropical latitudes of the world ocean. In being land locked, the Arabian Sea beats world records. The most pronounced changes have been taking place in the upper 30m layer. This means that the thermal stratification of the water column has developed. This could have affected the shoaling of the oxygen minimum. Plus, warming of the water column decreases the solubility of the oxygen coming from the atmosphere.
Large pelagic fishes are active swimmers which require a lot of oxygen for their metabolism, especially in warm tropical waters. The dissolved oxygen concentration below 2 millilitres per litre induces symptoms of stress for many tropical pelagic fishes; therefore, this concentration is believed to be the hypoxic threshold. If this threshold slowly moves up in the water column (over years), fish populations become compressed in the upper layer, which makes them more exposed to fishery.
A vulnerable question to be asked is what will happen to the pelagic ecosystem of Omani shelf in the nearest future, if the current depth of critical oxygen concentration is about 80m and the oxygen minimum zone is still moving up to the surface? The “uncomfortable shelves” are used to be abandoned, by large pelagic fishes. In the case of Omani shelf this means that declining landings of large pelagic fishes (catches per unit of effort) could have an environmental origin, along with over-fishing.
As far as the upward motion of the critical oxygen concentration is concerned, the trend seems to be dangerous indeed, but it is quite possible that shoaling of the critical concentration depth would be switched to and replaced by a subsequent deepening. In other words, let’s hope that we are dealing with a long-term fluctuation of water mass properties; the event which is quite common in oceanography. Long-term fluctuations of physical, chemical, and biological characteristics of the ocean are climate driven phenomena. In the Arabian Sea, they are mediated by a basin scale and global scale atmospheric anomalies, like the Siberian High atmospheric pressure system, the Indian Ocean Dipole, and the El-Ninõ Southern Oscillation. In turn, the cyclicity of atmospheric processes (like monsoonal winds) could be mediated by extra-terrestrial forces; for instance, lunar tidal actions which have the periodicity of 56, 95, 125 and 1470 years. These cycles were detected within the thickness of sediment core layers extracted from the Oxygen Minimum Zone of the Pakistan shelf. However, fluctuations of physical, chemical and biological characteristics of the Arabian Sea on the scale of tens to hundreds of years are poorly understood processes so far.