Strange fish -- A study reveals the decline in diversity in the oceans

Strange fish -- A map of the oceans’ fish reveals a pattern and a decline in diversity over the past half century<.h3>http://www.economist.com/science/displayStory.cfm?story_id=4223611

"ON THE land, animals and plants are not distributed evenly. At the planetary scale, the most striking feature of terrestrial diversity is its gradient across latitudes—many more species are found in equatorial regions than in latitudes north and south of the equator. Terrestrial ecologists, though, have had a head start over their marine colleagues. In the ocean, global patterns have been something of a mystery. This, though, is changing and, as it does, it is revealing that marine diversity is shrinking fast.

This week, Boris Worm, a marine biologist at Dalhousie University in Canada, and colleagues at other institutions, published a map of tuna and billfish diversity in the world’s open oceans. The map reveals that the greatest diversity of these predators is not at the equator but at intermediate latitudes—between 15? and 30? north and south. The map was produced using Japanese longline fishing records. Longlines—baited lines up to 100km long—are the most widely used fishing gear in the world’s open oceans. Diversity “hot spots” are also found off the American and Australian subtropical east coasts, south of the Hawaiian island chain, east of Sri Lanka and in the south-eastern Pacific.

This pattern of diversity is likely to hold for many other marine species as well. The researchers were able to show that their Japanese data was in strong agreement with records from American and Australian management agencies. While these covered a fraction of the geographic range, they included a range of species including sharks, rays, whales, dolphins, turtles and large seabirds. Zooplankton, the tiny ocean animals that act as a food source for many marine animals, are also known to follow a similar pattern of diversity. This all suggests that the map, published in the journal Science, is indicative of patterns in a wide range of ocean species.


The most likely explanation for the pattern is a combination of sea surface temperature—warm but not too warm—and the levels of dissolved oxygen in the water. Between the temperatures of 5?C and 25?C, higher temperatures mean that more species will be found. But above 27?C, this trend is reversed and, the hotter the water, the fewer the species.

..."