About 30 snapper carcasses have been dumped in the Herekawe Stream at New Plymouth's Back Beach.
Taranaki Regional Council compliance manger Bruce Pope said a council officer was unable to establish who was responsible for dumping the fish.
He said the New Plymouth District Council owned the land and would be responsible for cleaning up the mess.
Mr Pope said it was illegal under the Resource Management Act to dump such material into a waterway.
"Dead, rotting fish are classed as a contaminant."
"If a perpetrator is identified in this case, they are most likely to be issued an infringement notice ($750 fine).
"In very serious cases, penalties of up to $300,000 for an individual or $600,000 for a company can apply."
The New Plymouth District Council's manager of enforcement Lloyd Crow said council would remove the fish.
"Dumping fish like that is a very irresponsible act," Mr Crow said.
"If a member of the public had seen the offence being committed we would certainly follow it up."
Egmont Seafoods managing director Keith Mawson was disappointed someone would dump waste in a stream.
He said the number of fish was high for a recreational fisher but he would be surprised if a commercial operator had dumped the waste.
"It's hard to know why someone would do that."
He said the best way of disposing of fish carcasses was to dig a hole in the garden and bury them.
"It's good for the garden, too."
- Taranaki Daily News
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