Monday, 8 October 2012

Warm the sea to shrink the fish?

Climate change will modulate physical and chemical properties of the ocean; changes of  temperature and acidity are the best understood. Evidently, temperature sensitive properties such a oxygen content of seawater will change too. Its change is going hand in hand with the change of seawater temperatures.  In turn changes of seawater temperature and oxygen content will directly affect marine animals, such as fish and other vertebrates. To recall the obvious, these animals are breathing water to gain the oxygen they need, they are water-breathers.  

Shark - from Animal Corner
The body size of aquatic water-breathers is strongly effected by temperature and oxygen content of the seawater. The maximum body weight of fish is limited by internal physiological balances. The energy demand for swimming, growing living has to be balanced by its supply through "breathing water". The ultimate key factor for energy supply is the amount of oxygen that the animal can breath from water passing through its gulls.  Some fish, sharks have to keep swimming with open mouth to drive a sufficient amount of water through their gulls to survive; their fate: swim or die.

For most fish their fate is a bit less dramatic. If the supply of oxygen is just sufficient to balance consumption for swimming and living, then the fish stops growing. In the sea oxygen limitation is one of the most fundamental limitation to growth of fish and other vertebrates.  Thus it is not for fun, that big marine animals as dolphins, whales or seals breath air.

Facing off their environmental conditions many big fish evolved to prefer cold waters and to exploit its marginally increased oxygen content.  As warmer the seawater is as lesser oxygen it can carry.  Cold bottom waters in which many fish find their preferred habitat have temperatures below 10 degree centigrade and relatively high oxygen content (some mmoles per cubic meter).

World Ocean Circulation Experiment data - oxygen distribution in North Atlantic -  from
Overall the ocean is projected to become a little warmer and less oxygenated under global warming conditions. The bottom water temperatures in the oceans are projected to increase by a tenth of a degree over half a century, and oxygen concentration is projected to decrease by some mmoles per cubic meter over the same period. This is little and fish can, happily, swim away - at least in many cases. Cold water fish move towards the poles and abundance of their populations shrinks if they cannot escape.

Thus the most prominent biological responses to higher temperatures or lower oxygen content are changes of geographical  distribution of fish and their productivity. Simple thermal tolerance being one of the known factors limiting the geographical distribution of species in sea. Take the example of Cod in the North Sea.

Adult Cod - Photo: August Linnman

Cod is preferring cold and thus oxygen rich waters. Increasing water temperatures of the North Sea during the last decades limited the area in which Cod is found, for many reasons: "Because global warming is making the sea warmer the fish are moving further north to look for colder waters, it has been revealed. Over the last 40 years the North Sea's temperature has increased by one degree centigrade, which has proved enough to prompt cod to seek alternative habitats. The warming has also changed the plankton distribution and that has hastened the departure of the cod." (The Telegraph 8th October 2012).

Beyond simple preference for colder water marine fish react also to astonishingly small changes of oxygen content. Both theory and observations support the assumption that warming and reduced oxygen will reduce also the body size of marine fishes.  That is well known, but surprisingly strong is the response.

The expected change of oxygen content of seawater because of global warming is small. The resulting changes in maximum body size of fish were found to be about 20%, what is a lot showing how effective the fish adapted to use that limited resource, oxygen.

Marine biologist [1] were examining the integrated biological responses of over 600 species of marine fishes [*] to build a general picture. They fund when looking at the global scale, that tropical and intermediate latitudinal areas will be more heavily impacted, than sub-polar areas. Average reduction of body size being more than 20%, than sub-polar areas.

From Greenpeace - Cod in Crisis
The marine biologist found changes of geographical distribution of fish, reduced abundance and shrunken body size. In a give habitat, about half of this shrinkage of maximum body size is to changes in physiology of the fish living there. The marine biologist found also that species composition changes. Species with the potential to grow big are replaced by different species of lesser maximum body size.

Evidently, all these changes are important for fishery and thus for (our) food. Their understanding provide an additional  insight to the aggregated impact of climate change on marine ecosystems, that our adaptation to global change has to consider beyond (stupid) overfishing.

[*] The researchers use a global model that has an explicit representation of ecophysiology, dispersal, distribution, and population dynamics of 600 species. They show that assemblage-averaged maximum body weight is expected to shrink by 14–24% globally from 2000 to 2050 under a high-emission scenario for global warming.

[1] Shrinking of fishes exacerbates impacts of global ocean changes on marine ecosystems; William W. L. Cheung, Jorge L. Sarmiento, John Dunne, Thomas L. Frölicher, Vicky W. Y. Lam, M. L. Deng Palomares, Reg Watson & Daniel Pauly; Nature Climate Change, 30 the September 2012 (online publication)


  1. Hi, I see you are using my cod picture! You are welcome to do so but please use the correct attribution. The picture is taken by me, under my copyright, and published to wikimedia commons under a CC-BY SA license.
    Please see

    /August Linnman

    1. Hello August - the reference is modified. Thanks for helping & patience. A great photo! - regards, Martin