Warmer world may mean fewer fish

Posted: 23 February 2008

Climate change is emerging as the latest threat to the world's dwindling fish stocks. A new United Nations report says that at least three quarters of the planet's key fishing grounds may become seriously impacted by changes in circulation as a result of the ocean's natural pumping systems fading and falling.

World ocean thermohaline circulation.
World ocean thermohaline circulation.
World ocean thermohaline circulation. The global conveyor belt thermohaline circulation is driven primarily by the formation and sinking of deep water (from around 1500m to the Antarctic bottom water overlying the bottom of the ocean) in the Norwegian Sea. When the strength of the haline forcing increases due to excess precipitation, runoff, or ice melt the conveyor belt will weaken or even shut down. The variability in the strength of the conveyor belt will lead to climate change in Europe and it could also influence other areas of the global ocean. Click on image to enlarge
These natural pumps, dotted at sites across the world including the Arctic and the Mediterranean, bring nutrients to fisheries and keep them healthy by flushing out wastes and pollution.

Scientists explain, for example, that a quantity of water equal to all the river discharge from all rivers flowing into the Mediterranean is transported every two months from the Gulf of Lions to the deep Western Mediterranean via the Cap de Creyus canyon.

It has a critical impact on the population of the heavily harvested crevette rouge or deep sea shrimp, by bringing food that in turn triggers a sharp increase in young shrimp resulting in plentiful catches three to five years after the 'cascading' event. "Imagine what will happen if climate change slows down or stops these natural food transport and 'flushing' effects in waters that are often already polluted, heavily fished, damaged and stressed", said Dr Christian Nellemann, who headed the rapid response team that compiled the report."We are gambling with our food supply".

The impacts of rising emissions on the marine world are unlikely to end there. Higher sea surface temperatures over the coming decades threaten to bleach and kill up to 80 per cent of the globe's coral reefs - major tourist attractions, natural sea defences and also nurseries for fish.

Estimated contributions to sea-level rise (1993-2003).
Estimated contributions to sea-level rise (1993-2003).
Estimated contributions to sea-level rise (1993-2003). The two main reasons for sea-level rise are thermal expansion of ocean waters as they warm, and increase in the ocean mass, principally from land-based sources of ice (glaciers and ice caps, and the ice sheets of Greenland and Antarctica). Global warming from increasing greenhouse gas concentrations is a significant driver of both contributions to sea-level rise. Click on image to enlarge
Meanwhile there is growing concern that carbon dioxide emissions will increase the acidity of seas and oceans. This in turn may impact calcium and shell-forming marine life including corals but also tiny ones such as planktonic organisms at the base of the food chain.

Dead water

The findings come in a new report entitled In Dead Water which has for the first time mapped the multiple impacts of pollution, alien infestations, over-exploitation and climate change on the seas and oceans.

"The worst concentration of cumulative impacts of climate change with existing pressures of over-harvesting, bottom trawling, invasive species infestations, coastal development and pollution appear to be concentrated in 10-15 per cent of the oceans," says the report.

This 10-15 per cent of the oceans is far higher than had previously been supposed and is "concurrent with today's most important fishing grounds" including the estimated 7.5 per cent deemed to be the most economically valuable fishing areas of the world.

Half the world's catch is caught along Continental shelves in an area of less than 7.5 per cent of the globe's seas and oceans. And an area of 10-15 per cent of the world's seas and oceans cover most of the commercial fishing grounds.

Snake pipefish

The report, the work of UN Environment Programme (UNEP) scientists in collaboration with universities and institutes in Europe and the United States, was launched yesterday during UNEP's Global Environment Forum taking place in Monaco.

Achim Steiner, UNEP Executive Director, said "Climate change threatens coastal infrastructure, food and water supplies and the health of people across the world. It is clear from this report and others that it will add significantly to pressures on fish stocks. This is as much a development and economic issue as it is an environmental one. Millions of people including many in developing countries derive their livelihoods from fishing while around 2.6 billion people get their protein from seafood."

The report comes in wake of findings issued last week by a team led by the National Center for Ecological Analysis and Synthesis which estimates that over 40 per cent of the world's oceans have been heavily impacted by humans and that only four per cent remain relatively pristine.

It also comes amid concern that sea bird chicks in the North Sea may be being choked after being fed on a diet of snake pipefish - a very bony species. Over the past five years snake pipefish numbers have boomed a meeting of the Zoological Society in London was told last week. Changes in ocean currents bringing the fish into North Sea waters, might bve one reason for their sharp increase in numbers, the experts suggest.

The report 'In dead Water: Merging of climate change with pollution, over-harvest, and infestations in the world's fishing grounds' can be accessed here or at www.unep.org