What does "Fish kills" mean? What are the mechanisms behind Fish kills and what are the impacts? How does this relate to the climate change research in the Arabian sea?"Fish kills" is a term used to describe an event in which a large amount of marine life dies off over a period of time due to natural or unnatural causes. This event is usually restricted to coastal and estuarine ecosystems and can be caused by any combination changes in biological, chemical or physical processes(1).
[Image source: Google earth, fish kill events gathered from various articles from over the past decade or so]
Phytoplankton growth is a pivotal part of any marine ecosystem because algae form the base of the food chain. Algal blooms depend mainly on sunlight and the availability of nutrients like phosphorus (p) and nitrogen (n). Naturally, phytoplankton blooms occur as a result of upwelling events. Upwelling is a process by which cold nutrient rich water is brought to the surface from the deep as a result of wind moving across the ocean. With more nutrients available in the photic zone (the area in which there is sufficient sunlight present that is needed for photosynthesis) phytoplankton growth accelerates. One cause of fish kill events is when high nutrient levels cause massive phytoplankton blooms which result in the depletion of the dissolved oxygen in the water (eutrophication). When phytoplankton use up all the excess nutrients, they die and fall from the photic zone. As the dead phytoplankton sinks, it is broken down by bacteria which consume oxygen. Lack of oxygen strains marine biota by slowing metabolism, reproduction and movement speed; all of these affects make fish vulnerable to predators and fisherman. Cheryl Lyn Dybas (2005) states that under fully oxygenated conditions, all life f
unctions normally. Under low oxygen or hypoxic conditions fish begin to experience difficulty breathing when the oxygen concentration is about 5ppm. When the oxygen concentration reaches about 3ppm, sharks leave the area and when the oxygen concentration reaches 2ppm fish leave the area. Below 1.5ppm all life that do not or cannot leave the area, die (2)(3). Areas devoid of marine life are referred to as dead zones. The image on the left illustrates how dead zones are formed. 
[Image source: How dead zones are formed (EPA 2007)]In addition to upwelling, excess nutrients inputs come from:
- Agricultural runoff,
- Atmospheric deposition
- Erosion
- Runoff from developed land (i.e. lawn fertilizer, sewage etc)
Scientists have found that because the Arabian Sea has naturally high productivity (conversion of solar energy, carbon dioxide and water into forms available for use in the food web), that it has one of the thickest zones of oxygen depleted water (below 100m depth). The Arabian Sea area is heavily influenced by its monsoon seasons which cause upwelling events which make up a large portion of the nutrient influx for the area. Other nutrient sources are dust storms from the Arabian peninsula, and environmental pollution (agricultural, ballast release, chemical residue from the war zones etc...)
(In my research on fish kills I have come up with a question. Knowing that the Arabian Sea is naturally lower in oxygen concentration, what is the natural average concentration of dissolved oxygen in the Arabian sea?)
For example, environmental pollution has had a huge impact on the Shatt-Al-Arab river estuary which empties into the Kuwait Bay. For more information visit: http://www.unep.org/geo/geo3/english/480.htm
Climate change is an interconnected event based on long term change of regional weather which is thought to be caused by an increase in atmospheric carbon. It is thought that climate change is causing global warming. In the Arabian Sea, it has been recently suggested that climate change is also causing intensified monsoon winds and in turn, intensified upwellings. With intensified upwellings, there have been larger algal blooms as a result (4). This is a possible hypothesis for why there has been such of an increase in fish kill events over the past few decades.
Knowing that there is an increase in upwelling and productivity in the Arabian Sea, there is also an increase in upwelling of anoxic water which has been known to cause fish kills. In late August of 2000, a fish kill event occurred off the northern coast of Oman in the Persian gulf. Although the media attributed the deaths to a
release of ballast water from a US tanker, Dr. John M. Morrison, a professor of Oceanography at North Carolina State University, found that the event was due to upwelling. Satellite imaging captured the upwelling event which can be seen to the right. In the top image, the sea surface temperatures are normal as evidence by the pink/orange colors, however the bottom image was taken 10 days later there is significantly more blue/purple coloring which means that cold water was upwelled. Dead zone-fish kill events are usually preceded by large catches of fish and then discoloration of surface water because of the algal bloom. This notion originally led Dr. Morrison to believe the upwelling was nutrient rich. However after testing, it was determined that pockets of oxygen depleted water was upwelled due to an unusual and strong easterly wind. Fisherman experienced a bountiful catch because all fish in the area were compacted in the small pockets of oxygen rich waters and therefore easier to catch. For more information about fish kills off the coast of Oman, visit these websites: http://earthobservatory.nasa.gov/Study/oman/, http://news.bbc.co.uk/2/hi/world/monitoring/media_reports/908134.stm.
The mechanics of a pathogen fish kill are much more simple. A pathogen is introduced into the water and fish become effected. However due to climate change we are dealing with an intensified scale of infection. With even slight increases in temperature and pH of the water it becomes more difficult for marine life to function normally. It effects body temperature, metabolism and reproduction. This makes marine biota more susceptible to pathogens of any kind.
From July to December 2007, in Maldives there were many fish kill events. Researchers suggest that these events were caused by a number of reasons including exposure of the O. nigers to harmful algal blooms and the prescence of Streptococcus sp. which is a species of bactera known to be associated with fish kills. This bacteria was found in the spleen of the dead fish. There was also a similar report in the 1980s from off the coast of Sri lanka in a case where O. nigers were killed (1). However according the the author of "A compilation of reported fish kills in Maldives," the study was not conclusive and that more testing and funding is needed.
Socially, cultures with fishery based economies must be largely affected by fish kill events. The occurance of a fish kill event must lead to a lack of food for fisherman's family, a lack of product to export for income and on top of that increased prices due to demand. What I want to know is how these events affect specific locations like a large coastal country like Oman and Kuwait whose economies are not based solely on fisheries verses an island nation like the Republic of Maldives where fishing is key?
1. Naeem, S. and S.A. Sattar, A compilation of reported fish kills in Maldives. 2007, Marine Research Centre Ministry of Fisheries, Agricultrue and Marine Resources. In the Oman and the Arabian sea
2. Dybas, C. L. (2005). "Dead Zones Spreading in World Oceans." BioScience 55(7): 552-557.
3. Armbrecht, C. (2007). "Dead zones"
4. Goes, J., et al. Eurasian Warming, Snow melt and phytoplankton blooms in the Arabian Sea. 2007 [cited 2008 25 March 2008]; Available from: www.bigelow.org/climatechange/.
4. Goes, J., et al. Eurasian Warming, Snow melt and phytoplankton blooms in the Arabian Sea. 2007 [cited 2008 25 March 2008]; Available from: www.bigelow.org/climatechange/.
