How ocean warming will kill fish, make them smaller and potentially toxic

No matter how near or far you live from the coast, you’ll be affected by it: ocean warming may become one of the biggest threats to ecosystems and food security.

Ocean warming, driven by increasing carbon emissions and rising temperatures, may become one of the biggest challenges facing humanity and threatening the Earth’s life systems, affecting even those living far from oceanic coasts. Already impacting people, fish stocks and crop yields, it may lead to more extreme weather events and increased risk from water-borne diseases including cholera. Fuelling global warming, it would put the livelihood of agrarian and fishing communities in the Indian and Pacific Ocean regions at stake, cautions the International Union for Conservation of Nature (IUCN) in a report

Hurricane Kilo, Hurricane Ignacio and Hurricane Jimena churn in the Pacific Ocean on the 31st of August 2015 © NOAA/Getty Images

Affecting crop yields

“There is likely to be an increase in mean global ocean temperature of 1-4 degrees Celsius by 2100,” reads the IUCN report. “The greatest ocean warming overall is occurring in the Southern Hemisphere and is contributing to the subsurface melting of Antarctic ice shelves”.

In addition, subsurface heat in the Pacific and Indian Oceans at a depth of 100 to 500 metres has profound implications for the Earth’s future warming. In a strong El Niño year this subsurface heat is suddenly transferred to the surface layer and released into the atmosphere. If frequent and continued for a number of years, it further contributes to global warming.

With oceans warming over the course of this century, El Niño events are expected to double in frequency and become more intense according to projections, therefore affecting Pacific and Indian Ocean monsoon systems. This will badly impact crop yields and, “may affect most Asian countries including India where agriculture is primarily monsoon rain-fed,” says Doctor Rahas Bihari Panda of the Department of Environment Science at India’s North Odisha University.

Such a situation in the food-producing Asian continent may lead to severe food insecurity and hunger across the globe.

Reducing fish mass

Already influencing ecosystems from polar to tropical regions, ocean warming is driving entire groups of marine species such as plankton, jellyfish, turtles and seabirds up to 10 degrees of latitude north and south of the Equator to move towards the poles. It has also led to the expansion of low oxygen areas because warmer water holds less dissolved oxygen.

Under the combined influence of rising sea temperatures and less dissolved oxygen, maximum fish size across all seas is predicted to decline by 10 per cent, says Karin Limburg, Professor at State University of New York’s College of Environmental Science and Forestry. Such a reduction in fish mass may strongly impact fisheries, exacerbating the already stressed situation in those parts of the world that depend on them for protein, Limburg adds.

ocean warming
A small catch of fish caught in the nets of fishermen in the Indian Ocean explains the livelihood threats to the communities © Andrew Holbrooke/Corbis

Declining fish production

The oceanic phenomenon has already reduced the abundance of fish species in East Africa and the Western Indian Ocean by killing parts of the coral reefs they depend on. On top of the fact that tuna catch rates in the Indian Ocean have declined by 50-90 per cent over the past five decades, reduced phytoplankton may become an additional stress factor leading to a decline in the region’s fisheries, say scientists at the Indian Institute of Tropical Meteorology on the basis of a study of the area.

Harvests from marine fisheries in Southeast Asia are expected to fall by a rate between 10 and 30 per cent by 2050, compared to the period between 1970 and 2000, according to the IUCN report. This would affect the livelihood of developing countries’ fishing communities, also making it difficult to meet the global demand deriving especially from areas such as Japan, the USA and Europe.

Making fish inedible

Warmer water also leads to harmful algae blooms that cause neurological diseases such as ciguatera in humansAs fish consume toxic marine algae in the sea they produce toxins in their tissues and become carriers of the poison. So, of the fish caught for human consumption, part of it may not be safe.

In a recent outbreak, over 100 people in India fell sick from a ciguatera infection. With an annual 50,000 cases around the world, nearly 400 million living in the Caribbean basin, Pacific Ocean and Indian Ocean regions are potentially at risk, according to The Louis Malardé Institute

Fishers can catch up to 44 different species of fish in the Bay of Bengal now. Will it be the same if ocean warming continues at the present rate? © Antoine Gyori/Corbis

Need for urgent action

Ocean warming therefore poses one of the biggest threats to food security and the nutritional ambitions set in the United Nations’ Sustainable Development Goals (SDGs), a set of global goals defining the international agenda moving towards 2030: in particular Goal 2 aims to end hunger, achieve food security and improved nutrition, and promote sustainable agriculture.

Hence, it has become essential that urgent attention is turned towards ocean warming in order to save ecosystems, marine resources and fisheries. Only strong measures, including cutting greenhouse gas emissions rapidly and substantially, can contribute to checking the trend threatening all life on Earth.

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