Signs of spring are all around us: Red-winged blackbirds are returning, potholes abound, and street sweeping will soon resume. For me, I know it is spring when colorful fish return to the surface of their pond just off the Linear Park path in Cambridge. These fish are a kind of carp, called koi (short for nishikigoi, as they are called in Japan).
The pond is on the grounds of the Howard Street condominiums, which opened 18 years ago. According to Andrew Grad, the pond was built to temporarily hold stormwater runoff from the property. (The pond has a sandy bottom, so water slowly drains from it.)
Grad and his wife were among the first residents of the new condos. The pond was charming, but mosquitoes laid eggs in the pond, leading to a mosquito invasion in the summer. The condo added 25 goldfish to gobble up the mosquito eggs and larvae, and within two weeks, the mosquitoes were gone.
The following year, an architect moved into the condos. He developed the current layout of the pond interior, which includes a raised center grate and a more constant water level. The pond transitioned from goldfish to koi. The pond is well-shaded, and this is not an accident. Koi need plenty of shade, as they can get sunburned if exposed to the sun too long.
Many people have helped Grad with the koi pond over the years — both people from the condos and from the community. Each year, Grad presents a budget to the condo trustees for the koi pond expenses, including the cost of food, maintenance and repairs. Since there are more than 100 varieties of koi, the fish in this pond display many different patterns and colors.
The origins of koi
Rice farmers originally bred koi northwest of Tokyo in the late 1700s as a food source during snowy winters when they could not get to markets. They kept the fish in irrigation ponds or terraced paddy fields. Farmers noticed color variations in the fish and began selectively breeding them to develop vivid, multicolored fish. Today koi breeding is an art form. Breeders pay careful attention to color, patterns and body shape.
In parts of the world, people hold contests each year to select the best fish. The premier koi show is the All-Japan Koi Show, with thousands of entrants, held in Tokyo each winter. Similar to the National Dog Show in the United States, koi judges choose the best fish of each size and variety. The event culminates with the selection of a grand champion koi.
The most expensive koi ever, a nine-year-old female named S Legend, sold for 203 million yen ($1.8 million) at an auction in October 2018. Collectors wanted this stunning fish because of her exceptional size (3.31 feet), symmetrical body, vibrant orange markings, and immense breeding potential. (A female koi can lay 50,000 eggs per pound of body weight.) While it is not known how much S legend weighed, a three-foot koi often weighs about 35 pounds. But S Legend was unusually stocky, so she likely weighed more. Unfortunately, S Legend died suddenly in 2019.
Most koi live to be about 40. The oldest known koi, named Hanako, is said to have been 226 years old, as determined by a Japanese scientist in 1966, who viewed her scales through a light microscope to count the annual growth rings. Others dispute these findings, however, as counting rings is not an accurate method of determining koi age, and there are no scientific papers documenting the results. So Hanako’s age is probably more legend than fact.
Don’t feed the fish
If you walk by the koi pond in warm weather, the fish swim toward you hoping for food. Don’t feed them — the fish are on a strict diet that changes with water temperature.
In the winter, the koi swim to the warmest part of the pond, near the surface, where an underground sump pump sends a stream of relatively warm 60-degree water. However, when it is extremely cold or windy, the surface water cools faster than the rest of the pond. During these times, the koi settle near a bottom corner of the pond. This past winter was unusually cold, so the fish were often hidden away.
Koi enter a state of torpor when the water temperature falls below 40°F. Grad said koi cannot digest food at this temperature (reached this year in December). So he does not feed the fish again until the water warms above 40 degrees.
Many of the koi have been given names, but Grad says it is difficult to keep track of them, as their coloration changes as they grow older and larger.
The Linear Park path near Trolley Square is closed until December, so you have to choose either a northern or southern detour. If you take the northern detour, you can admire the koi in their ingenious stormwater pond. Of course, this is not the only koi pond in the area; both private residences and public businesses have them. There is even a lovely koi pond inside the greenhouse at the Harvard Business School’s MBA Class of 1959 Chapel (on N. Harvard Street on the Boston side of the river).
Reader photo:
Pamela Avery spotted this Cooper’s hawk in her backyard birdbath in Houston, Texas, in March 2026.
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