July 20 (Bloomberg) -- Fish in French rivers and the Baltic Sea are getting smaller as their habitats warm up, more evidence that climate change is forcing species from bacteria to sheep to adapt to a hotter planet, a new study said.

Average fish size in rivers such as the Loire, France’s largest, declined the past three decades while the geographical range of small fish in the Baltic expanded as herring and sprat got smaller, according to a study published today on the Web site of the Proceedings of the National Academy of Sciences.

Research has shown that animals, including extinct Mediterranean dwarf elephants, shrank in size to adjust to changing climate and geography. Today’s study provides evidence that smaller body size is the third universal ecological response to global warming along with a shift of species to higher latitudes and altitudes and changes in when animals give birth, nest and other seasonal life cycles, the authors said.

“Temperature actually plays a major role in driving changes in the size structure of populations and communities” from bacteria to river and ocean-going fish, wrote Martin Daufresne and colleagues at the Leibniz Institute for Marine Science in Kiel, Germany.

The world’s ocean surface temperature in June rose to 62.6 degrees Fahrenheit (17 degrees Celsius), the warmest since 1880, topping the record set in 2005, the National Oceanic and Atmospheric Administration said on July 17.

Shrinking Sheep

Scientists this month published evidence that sheep on a Scottish island are getting smaller from one generation to the next as temperatures rise because they have to eat less in their early months to survive winters that became shorter and milder.

The results from today’s study confirm three theories about species size and climate. According to “Bergmann’s rule,” animals are smaller in warmer parts of their geographic range while the “James rule” says that within species smaller individuals are found in warmer climates. Finally, the temperature-size rule states that zooplankton, phytoplankton and fish all decrease in size with rising temperature.

The authors said other factors affecting animal size were considered in today’s study, including fishing that results in smaller size as larger members of the species are caught by fishermen.

Not all warm environments result in smaller animals, however. The Titanoboa cerrejonensis, or “titanic boa from Cerrejon,” was a 13-meter (43-foot) snake that existed 60 million years ago and couldn’t have survived unless temperatures averaged 30 to 34 degrees Celsius, (86 to 93 degrees Fahrenheit), scientists wrote in the journal Nature in February.

Its prey in what is present-day northern Colombia included prehistoric crocodiles and giant turtles.

An exploding population of pythons in Florida’s Everglades may face bounty hunters trained to track and kill the snakes, which grow up to 20 feet long, on state land, The Miami Herald reported July 15. The snake’s population may be as high as 100,000 in Everglades National Park, the Herald said.

To contact the reporter on this story: Jeremy van Loon in Berlin at jvanloon@bloomberg.net

Last Updated: July 20, 2009 09:45 EDT