Climate change in our backyard: warming of Indian Ocean threatens fish catch

By Dinesh C Sharma Even as the world tries to make sense of decisions taken at the recent climate change conference in Paris, scientists have come up with fresh evidence of how global warming is beginning to nibble the food chain right in India’s neighbourhood – the Indian Ocean. Oceans play a critical role in both short and long term weather and climatic patterns. Nearly 90 percent of extra heat generated due to emission of greenhouse gases from the landmass is absorbed by oceans, warming them up. Indian Ocean, considered one of the most productive seas, has seen warming greater than other oceans. The warming in Indian Ocean during the past century has been estimated up to 1.2 degree C, which is very large compared to a global surface warming of up to 0.8 degree C during the same period. Now an international study led by scientists from Pune-based Indian Institute of Tropical Meteorology (IITM) has shown that the warming of Indian Ocean is affecting productivity of its marine eco system. Simply put, the food web necessary for fish production in the seas is getting affected due to the warming. This, in turn, is resulting in dwindling fish catch rates in the Indian Ocean. [caption id=“attachment_2554584” align=“alignleft” width=“380”]  Representational image. Image courtesy: Reuters[/caption] The decline in phytoplankton- microscopic plants that are part of the aquatic food web - is significant. Phytoplankton contains chlorophyll and provides food for a range of sea creatures including fish. The rapid warming in the Indian Ocean has played an important role in reducing the phytoplankton up to 20 percent during the past six decades, the study has reported. Analysis of satellite data, in conjunction with climate models based on past climatic data, shows that the decline is up to 30 percent in the western Indian Ocean during last 16 years. Observational field data from ocean profiling floats deployed in the Arabian Sea was used to validate satellite data. Warmer ocean surface temperatures result in less dense water on the surface and denser water in the subsurface, which is known as stratification. This, scientists say, inhibits vertical mixing of nutrient-rich subsurface waters with surface waters. Vertical mixing is necessary to bring nutrients into upper layers of the ocean where sufficient light is available for photosynthesis. Besides sustaining the marine food web, phytoplanktons absorb solar radiation and modulate the upper ocean heat flux. Thus, they have a major role in influencing various climate processes including the carbon cycle. sea surface temp [caption id=“attachment_2554588” align=“aligncenter” width=“453”]  The graph shows a decline in levels of chlorophyl together with rising sea surface temperature (SST).[/caption] “Such a decline in the marine phytoplankton may cascade through the food chain, turning this biologically productive region into an ecological desert. It can potentially impact food security in the Indian Ocean rim countries and also the global fisheries market,” warns Roxy Mathew Koll, scientist at the Centre for Climate Change Research of IITM, who led the study. The research findings appeared in scientific journal Geophysical Research Letters on Monday. The abundance of tuna and other fishes is directly linked with phytoplankton availability and spread. The Food and Agriculture Organisation (FAO) data shows that the Indian Ocean accounts for 20 percent of total tuna catch, especially the economically valuable bigeye tuna. Available data from the Indian Ocean Tuna Commission also shows that the tuna catch rates in the Indian Ocean have declined by 50 to 90 percent during the past five decades. Much of this decline has to do with increased industrial fisheries, but reduced availability of phytoplankton is a major player, Koll explained.  “The Indian Ocean appears to be a great challenge not only for humans, since the monsoon is weakening as the sea is warming, but also for marine resources since the bottom of their food chain too is weakening,” pointed out Raghu Murtugudde of University of Maryland and a co-author of the study. Further fish catch decline in the future could also have a geopolitical and security fallout, as a collapse of fisheries in the region could only exacerbate the regional instabilities. Besides IITM, scientists from National Institute of Oceanography (Hyderabad), Indian National Centre for Ocean Information Services, National Centre for Antarctic and Ocean Research (both Goa), University of Maryland, University of Cape Town and LOCEAN-IPSL (Paris) participated in the study. An earlier study by IITM had shown that warming in the Indian Ocean is resulting in weakening of the Indian monsoon. The land-sea surface temperature difference over South Asia has been decreasing due to rapid warming in the Indian Ocean and a relatively subdued warming – and even cooling in some parts - over the subcontinent. This enhanced warming of the ocean reduces land-sea temperature difference, dampening monsoon circulation and affecting rainfall. Key findings: • Reduction of up to 20% in marine phytoplankton in the Indian Ocean during the past six decades • Reduction in marine productivity is due to rapid warming in the Indian Ocean • Future climate projections indicate further warming and subsequent reduction in marine productivity