Spotlight

Doctor Eel

More than 40 years of research on eels has earned recently retired Victoria University zoologist Peter Castle the prestigious degree of Doctor of Science.

Associate Professor Castle already holds the title of Doctor, having gained a PhD in zoology about 40 years ago -- with a study on eels. The degree of Doctor of Science recognises a large body of top-quality research on a particular subject.

One of the reasons he became fascinated with eels, Castle says, is the puzzles they present. The young are usually quite different from adults, so it is very difficult to work out which species young eels belong to, although new DNA techniques are helping to unravel some of these mysteries.

Most of his work has been on marine eels, which covers a great variety of fishes up to two metres long living anywhere from the sea surface to the ocean floor. More than 700 species of marine eels have been found in the Indo-Pacific area. He has identified four new genera and more than 20 species and subspecies of marine eels.

However the two species of New Zealand freshwater eels -- one of them unique to this country -- have presented a particularly intriguing puzzle.

When they near the end of their life cycle they swim out into the ocean to breed, and tiny young "glass eels" are borne back to New Zealand on currents, but for many years no-one knew what happened in between.

In 1995 Castle joined an international team of scientists on the University of Tokyo vessel Hakuho Maru in the western Pacific. They searched for young eels of several species by trawling nets with very fine mesh -- about one millimetre -- near the surface of the Pacific. The smaller the eels they were catching, the nearer they knew they were getting to the spawning area.

"The smallest ones captured were less than 20mm long, which means they were not much more that two weeks old," Castle says. "Given the known speed and direction of the current, which flows strongly east to west at the latitude where we collected the specimens, they must have spawned just a few hundred kilometres further to the east."

This indicates that the spawning grounds for the South Pacific species lie between 10 and 15 degrees south of the Equator, between Samoa and Fiji but more towards Samoa -- a long way from New Zealand.

Born in Timaru, Castle studied at Victoria University, gained his PhD there, and spent all his academic career at the university with the exception of three years in South Africa. His interest in eels began when his PhD supervisor suggested that he study some unidentified eels caught in Cook Strait, and his research on the subject has since taken him around the world.

He has studied collections of eels at museums, universities and other institutions to help him identify specimens he finds.

"I like to think of it as a detective story," he says.

Castle pioneered the linking of eels' larval stage with adults.

"The two had been regarded as separate; the classification of the adults was different from that of the young. I became experienced in looking at adults and young eels and making the connections. My main fascination has been trying to match the early life stages of eels with mature eels."

One reason why this has always been difficult is that it has not usually been possible to keep the juveniles and see what they grow into.

"They're delicate creatures, and we don't know what they eat," Castle says.

Castle's presentation for his DSc included more than 60 scholarly publications, all of them illustrated by himself. Good illustration calls for considerable artistic skill as well as scientific expertise.

"I've always enjoyed the illustration. It's something you can't do by computer scanning -- you've got to look at the specimen and come to an understanding of the way an animal is likely to be in life."

With Hawaiian colleague Jack Randall he has just submitted an article about a peculiar group species known as "garden eels", found in the tropics.

"They live on the ocean floor, half buried in the sand, standing up vertically -- a group of them looks like a cluster of plants. If they're approached, they disappear into their holes."

Freshwater eels -- especially when smoked -- are popular food among Maori in New Zealand, and also in some other countries such as Japan, France, Germany and Denmark. Castle is quite partial to them himself, and suggests that the food factor is why more attention has been paid to freshwater eels than to marine ones.

Many eel mysteries -- such as how they navigate to their spawning grounds -- remain to be solved, and Castle hopes to continue some aspects of his research now he has retired, possibly in association with Te Papa.

"These sorts of studies never end," he says.