Climate change, ecological hypoxia and fish kill

People have expressed deep concern recently about fish kill in large water bodies in Bangladesh and eager to know about its source. The term 'fish kill' refers to a localized die-off of fish populations which may also be associated with more generalized mortality of aquatic life.
Some media reports suggest that the possible reason for the death of fish, ducks and other aquatic life is due to uranium in the floodwater. Local expert opined that the toxicity level of the water got worse due to rotting of the paddy. They emphasized three major components in various haors including the percentage of oxygen dissolved in the water, toxic ammonia and pH level of the water. But problem of identifying the cause of such environmental stress is -- how do we know, what we know? And how do we know, what we don't know?
Obviously low levels of dissolved oxygen (DO), also known as ecological hypoxia is a primary indicator of water quality, and as such, it is also directly related to the health of that water's ecological community. In fact, it is most often the number one reason why fish kills occur.
Unlike the Earth's atmosphere (which is roughly 20 per cent oxygen), water contains only a fraction of a per cent of oxygen that is dissolved. This oxygen is needed in order to sustain aquatic life and deliver nutrients to aquatic flora, but insufficient oxygen saturation can be deadly to a body of water's fish.
There are few reasons why dissolved oxygen can be reduced in a body of water. Along with atmospheric pressure and salinity, temperature is a main factor in the amount of DO that water can be saturated with. Rising water temperature will lead to less oxygen within that water. As a result, all organisms that depend on oxygen to survive (from fish to algae and bacteria) will compete over this diminishing resource.
Regardless of water temperature, fish also compete with plants for sunlight, oxygen, and natural resources. Thus, the state of a lake's plant life will also have an effect on DO levels. Invasive plants can lead to oxygen depletion, as will excess algae caused by nitrogen and or phosphorus.
In general, it may be worth installing an aeration system that will at least maintain consistent DO levels, if not improve them over time. If the root of the problem is algae overgrowth vis-a-vis phosphorus enrichment, then the solution will likely be more effective waste management, irrigation and or storm runoff.
There are other reasons when hypoxic condition can developed such as thermal pollution. A common cause of thermal pollution is the use of water as a coolant by power plants. When water used as a coolant is returned to the natural environment at a higher temperature, the change in temperature decreases oxygen supply and affects ecosystem composition.
Fish and other organisms adapted to particular temperature range can be killed by an abrupt change in water temperature. One has to keep eye on such event if Rampal power plant goes ahead.
Fish kills may also occur due to the presence of disease, agricultural and sewage runoff, oil or hazardous waste spills, hydraulic fracturing wastewater, sea-quakes, inappropriate re-stocking of fish, poaching with chemicals, underwater explosions, and other catastrophic events that upset a normally stable aquatic population.
I can cite three cases of fish kill in United States and United Kingdom. In 2015 the New York state department of environmental conservation investigated an incident of fish kill and they have found that the bunker fish, also known as Atlantic menhaden, were most likely chased into the canal by other predatory fish. The canal was closed, so fish was trapped and the trapped fish died due to a lack of dissolved oxygen and the high density of fish.
In 2014 thousands of fish have been killed in the Norfolk and Suffolk areas in United Kingdom after high storm tides pushed saltwater into the freshwater river system. The change in conditions from fresh to salt water was very rapid and once the fish sense the water has become too salty it's often too late for them to escape. However the Environment Agency saved hundred thousand fish by relocating them quickly to freshwater.
In 2013 hot weather and depletion of dissolved oxygen in water caused a major fish kill in a Norfolk river. Environment Agency officers monitor oxygen levels in the River Delph noticed a dip that posed a significant threat to fish. Aerating equipment was being used to dose the water daily with hydrogen peroxide to increase oxygen levels. The agency has asked river users to look out for signs of fish in distress, either gasping at the surface or swimming abnormally.
Bangladesh is one of the world's largest deltas. The country is literally crisscrossed by approximately 250 large and small rivers. The flood plains of these rivers and the natural depressions such as Haor, Baor and Beel go under water during the monsoon and create huge open water fish habitat.
Along the coast in the south, an estimated 2.5 million hectare of low lying lands are subject to tidal inundation. Much of these areas provide temporary nursery and grazing grounds for different fish shrimp species.
However they are vulnerable to climate change. Two-thirds of the country is less than 5 metres above sea level, making it one of the most flood prone countries in the world. Rainstorms in the Indian hills contribute to severe raise of water levels in rivers and cause widespread flooding in addition to local monsoon rains.
Future climate change seems likely to increase the destructive power of monsoon floods in Bangladesh. There is a clear consensus that temperatures in the Ganges-Brahmaputra-Meghna basin would rise by 1 to 3�C by 2050 along with increases of about 20 per cent in precipitation. Several studies forecast that just a 1�C increase in temperature and 5 per cent rise in precipitation would result in an increase of 20 per cent in the area flooded in Bangladesh.
The Haor Basin located in the north-east of the country is at particular risk from climate change. It is a tectonic depression, and is being pushed down as the Indian plate collides with the Eurasian plate. The Sylhet sub-basin is estimated to be sinking at 2.1 cm per year because of down-thrusting under the Shillong geological fault line.
However some suggest that the rate of sinking is much less at 2-4 mm per year. This sinking will mean that annual flooding will become more extensive. This will be exacerbated by sea level rise. However, the exact effect combined with tectonic sinking has not yet been determined.
In terms of biodiversity status, International Union for Conservation of Nature (IUCN) report published in 2015 suggest that in comparison with 2000 the total threatened fin fish species has increased by 10 species and the 13 endangered and vulnerable crustaceans  species has been declared in recent year.
In addition to this the aquatic resources degradation is clearly evident due to conversion of wetland to agricultural land, road or housing and also due to the use of huge pesticide either accidental seepage of pesticide from agricultural land or just excessive use.
Fisheries Statistical Report of Bangladesh (2014-2015) suggests that the natural spawn from river area is less than 1 per cent of the country demand. Resource depletion from various manmade and climate problems resulted in low fish spawn production and fluctuations every year. IUCN report suggests that rainfall, drought, temperature, water depth and storms or surges are some important climatic factors that are influencing fish and fisheries of Bangladesh
Considering the existing depletion, degradation, biodiversity loss and abrupt climatic condition if fish resources are not monitored on a daily basis, then it would be difficult to deter such fish kill event. Aerating the water and regularly testing the water quality will both serve to ensure that fish kills are avoided as much as possible.
In addition, it is imperative that farmers properly irrigate their crops and control their runoff, that both private and municipal facilities dispose of waste appropriately, and that water pollution is avoided at all costs.
Dr Kanan Purkayastha is an Environmental Scientist and a Fellow of the Institution of Environmental Sciences in UK