Albatrosses sniff out squid
1 October 2010, by Tamera Jones
The grey-headed albatross's favourite food is a type of squid that normally lives deep in the Southern Ocean. So, how do the birds find them? It turns out that albatrosses probably track squid down by smelling the oily slicks that float to the surface when they feed.
But the squid don't just randomly come up for food. The study found that they tend to come to the surface in a region of the Southern Ocean called the Antarctic Polar Front. This is where warmer waters from the Pacific, Atlantic and Indian Oceans meet the cool seas surrounding Antarctica, generating the windiest and stormiest waters in the world.
These waters are so rich in nutrients that they're a magnet for all kinds of marine life, including the oil-rich fish which the squid prey on. 'The squid probably release an oily slick as they feed on fish which are rich in oil,' says Professor Paul Rodhouse from the British Antarctic Survey and lead author of the study.
From extensive research on Bird Island in South Georgia, scientists knew that grey-headed albatrosses (Thalassarche chrysostoma) feed mainly on squid. They knew this because the birds can't digest a part of the squid called the beak, which is made from a similar material to our fingernails. The squid use their beaks to break fish down before they swallow them.
'When young grey-headed albatrosses are ready to fledge the nest, they regurgitate all the beaks that have collected in their stomachs,' explains Rodhouse. 'If you sift through this, you can identify which species of squid the albatross ate.'
'The squid probably release an oily slick as they feed on fish which are rich in oil.'
Professor Paul Rodhouse, the British Antarctic Survey
By doing this, Rodhouse managed to figure out which squid the albatrosses had eaten while studying them on Bird Island. Except for one beak, which he couldn't identify.
It's all very well knowing albatrosses eat squid, but researchers have struggled to understand how they find them. Not just that, but there was scant evidence that enough squid come to the surface to be a reliable food source.
Some scientists suggested albatrosses might be preying only on squid that have light-emitting organs, because they'd be easier to track down at night. Other researchers thought the birds could be going for either older squid, which die after spawning and float at the surface, or those vomited up by sperm whales.
Rodhouse and his colleague Peter Boyle, from the University of Aberdeen, got the opportunity they were looking for to test their ideas while onboard the RRS James Clark Ross back in the 90s. They attached tiny satellite transmitters to three albatrosses on Bird Island. One stopped working, but the other two showed that the birds were flying up to 250 miles to the Antarctic Polar Frontal zone.
When Rodhouse and Boyle overlaid the two birds' tracks with satellite data on sea surface temperature, they realised the birds concentrated their feeding around the Antarctic Polar Front. So, they decided to trawl in this area to see exactly what the albatrosses were feeding on.
While the seven trawls they did all yielded squid, the most common species they found was Martialia hyadesi – the exact squid Rodhouse had trouble identifying on Bird Island.
Far from being old squid that might have been floating at the surface, they found that the squid were fairly young, so could only have been living in the depths of the ocean and coming up to the surface to feed.
'Martialia hyadesi doesn't have any light-emitting organs either, so that's not how the albatrosses were finding them,' says Rodhouse.
It looks like this squid comes up to the sea surface in low light conditions to feed when it thinks it's safe, and when it's close enough to the surface, the albatrosses plunge into the water to grab them. 'We know albatrosses can dive to a depth of about six metres, and with this extra information, it all seems to makes sense,' Rodhouse adds.
The research was published by the ICES Journal of Marine Science.
Paul G. Rodhouse, Peter R. Boyle, Large aggregations of pelagic squid near the ocean surface at the Antarctic Polar Front, and their capture by grey-headed albatrosses, ICES J. Mar. Sci., (2010) 67 (7): 1432-1435. doi: 10.1093/icesjms/fsq069 First published online: June 7, 2010
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