New York Times News ServiceSanke, a variety of koi fish, is bred and raised by Mat McCann at Quality Koi Farm in Carneys Point, N.J.

Japanese koi are domesticated carp prized for their size, color, patterns and luster. They are coldblooded, but the people who nurture them in backyard ponds and enter them in competitions are as passionate as any Pekingese owner.

Mat McCann, 40, the British-born manager at Quality Koi Farm, a 43-acre business owned by Joseph Zuritsky, in Carneys Point, N.J., uses Japanese stock and methods to breed international champions. (This ambition is not unlike perfecting American truffles, mozzarella and soccer players.) The farm is open to the public from spring through autumn, and collectors travel from all points to peer into its mud ponds. (The interview was edited and condensed.)

Q: How did you get involved with koi?

A: When I was 7 years old, I started as a hobbyist. My father was going to a koi club meeting, and I was a tagalong.

Q: You just returned from a trip to Japan. Why did you go?

A: This trip was basically to go back to the guy who taught us, our mentor, Toshio Sakai of Isawa Nishikigoi Center, and select from his fish that were born last year. We’re given a unique opportunity to select the best from the best. I only selected a total of 50 fish.

Q: What does it take for a koi to be a champion?

A: Shape is basically the symmetry within the body itself; all fins are present and equal size. The best fish have slender proportions for the first four or five years. Luster is an aspect of the skin quality; it should have a sheen, like silk. In terms of the color quality, it’s a thickness so we almost liken it to how many coats of paint on the fish. We look underneath the scales so, for example, if it’s a red patch and there’s white underneath, it’s the lowest quality.

Q: What’s the typical lifespan of a koi?

A: That greatly depends on the skill of the keeper. Typically, you’re looking at 10 years plus, once you really know what you’re doing. We have fish that are 35 years old. Koi are a hybridized carp, and there are carp in the wild that are 60, 80 years old.

Q: Koi-raising seems to attract a particularly passionate kind of person.

A: It’s been likened to a drug in many respects. It’s not a hobby that you can do halfway and still be successful. A decent-size pond doesn’t come inexpensively, and then each week people like to go out with their kids and buy some koi, and before long they’ve got a selection with names.

Q: What’s the price range from backyard koi to a competition-level fish?

A: We sell koi from $12.50, and most of the backyard-pond level go for less than $100. The very, very best are literally one in a million or beyond. The highest would be $100,000.

Q: Do they have personalities?

A: They do in terms of feeding habits. You can train the carp to hand feed, and they will adjust to your footsteps. They know you’re coming to give them food and they’ll approach you.

Q: Do you get attached to your fish? Do you give them names?

A: All our parent females have names. It’s one way of following the bloodline. Staff frequently get attached to certain fish.

Q: One problem with koi ponds is gas buildup?

A: Bacteria and microorganisms do their thing in the mud, and the other gases present grow to such a point, they take the place of oxygen in the water.

Q: So how do you fix that?

A: That’s one of the jobs students get to do. They probably spend two to three hours a day in a pair of waders walking in the mud. It’s incredibly difficult, especially as it gets hotter. It’s a fantastic weight-loss program. It’s not a job anybody likes but it’s a very important job.

Q: Do the gases smell?

A: Yes. I’ll let your imagination run wild with that one.