Marine mammals thriving in Thames

SNIPPETS

Ten years of public sightings show that large marine mammals are regularly found in the River Thames. The Zoological Society of London (ZSL) has received records of 2,732 animals over that period. Seals were the most common animals seen, with many spotted around London’s Canary Wharf, probably because many people spot them from its skyscrapers.

In addition, the public reported 444 porpoises and dolphins on the river, and 49 whales. Joanna Barker, ZSL’s European conservation projects manager, said, “Many people looking into the Thames see a murky, dirty environment. But, actually, beneath the waves, it is full of life. We have a huge range of fish and invertebrates, and also top predators.”

Just 50 years ago, the Thames was so polluted, it was declared “biologically extinct”, too dirty for anything to survive there. But the public sightings confirm that the River is springing back to life. And many animals are venturing further into the English capital’s waterway.

Seals were seen as far upstream as Teddington and Hampton Court Palace, in south west London. And dolphins and porpoises were spotted at Teddington Lock, with large pods spotted close to Kew Gardens and Deptford. A whale even visited central London in 2006, but the bottle-nose did not survive. Other, healthier whales have been seen around Gravesend in Kent. “The fact we get so many sightings in central London suggests the fish stocks are moving in to support these marine predators,” said Joanna.

In addition to the public’s reports, the team at ZSL has also been conducting detailed seal surveys along the greater Thames Estuary. For the last three years, they have used planes and boats to count the number of seals along the River. The scientists estimate that there are about 670 harbour seals along the Estuary. The number of grey seals is not known; however they appear to be doing well in this stretch of River.

“We do think this area is really important,” said Joanna. “It’s quite sheltered compared with the North Sea, and there is a whole different range of environments and habitats for the marine mammals to use. So we think that London and the Thames Estuary is an important environment for these species. And we are keen to get more sightings year on year, and to build up a better picture of the places that marine mammals are using.” The public are being asked to send their marine mammal sightings to ZSL.

Rebecca Morelle

Consistency in predator-prey pattern

Predators may depend on prey for food, but an increase in the number of prey animals does not bring a proportional increase in the number of predators, a new study has found.

The analysis, which appears in the journal Science, relies on data from more than 1,000 studies of animals in more than 1,500 locations worldwide.

“Looking at this very large scale suggested a level of organisation that was previously not recognised,” said Ian A Hatton, a biologist at McGill University.

Ian and his colleagues first set out to compare carnivores and herbivores in different ecosystems in Africa, like the Serengeti and the Kalahari. The researchers found a consistent pattern: Numbers of predators do not increase as rapidly as prey. The differences between the two populations are formulaic and predictable. Surprised by the finding, the scientists went on to look at ecosystems in other parts of the world.

In crowded settings, prey produce fewer offspring because of resource constraints. And because predators tend to target the young and the old, they have less to eat. The pattern could help researchers monitor ecosystems and endangered species. “If we went to India, for example, it could help figure out how many tigers there should be,” Ian said.