Families celebrating Mother's Day on Sunday got a smelly surprise when about 40 dead koi washed ashore near their get-together at Kuli'ou'ou Beach Park.

Alika Malabey, 25, discovered the koi late Sunday afternoon after his younger cousin brought over a decomposing fish carcass.

"It was pretty disturbing," said Malabey, whose family has come to the beach for years and has helped with cleanup efforts there.

"We're already dealing with a lot of invasive species," he said. "This is another thing we don't need in our waters."

The koi appear to have been dumped at the mouth of Kuli'ou'ou Stream and it is unclear whether they were alive or dead when they were released, said Alton Miyasaka, an aquatic biologist from the Department of Land and Natural Resources who investigated the incident at Kuli'ou'ou yesterday.

"If they were alive when they were dumped, they probably didn't last that long," he said. Koi are freshwater fish and would have survived only five or 10 minutes once released in salt water, he said.

The koi ranged in size from 8 to 24 inches and were discovered along a short stretch of beach with three tilapia. All appeared to have come from the same place, Miyasaka said.

The fish were the ornamental kind usually kept in household aquariums or ponds and appeared to have died within the past two days, he said.

Koi can range in price from $50 to hundreds, even thousands, of dollars, which is why the discovery was so unusual, Miyasaka said. He did not know whether there were any koi farms in the area.

The Hawaiian Humane Society had not received any calls about the fish, said Humane Society spokeswoman Kawehi Yim.

The Humane Society has received koi in the past and puts them up for adoption, but Yim said people rarely bring in koi.

"Those are expensive fish," she said. "Most of the time when people are raising them, they're pretty good about taking care of them and giving them to people."

Freshwater fish, even dead ones, can cause damage to saltwater ecosystems if released into the ocean. In this case, there do not appear to be health concerns or long-term effects on the beach area, said DLNR spokeswoman Debbie Ward.

Ward said she has never seen such a large number of koi abandoned in this manner.

"People in Hawai'i value koi," she said. "They're an expensive fish, so it's puzzling as to why this quantity of fish would wind up where they did."