A member of the public reported on Monday that more than 3,000 fish, mainly roach, were dead at Queen Elizabeth II Jubilee Country Park lake at Ashington.
The Environment Agency has carried out analysis of the fish and evidence suggests they may have been dead for up to two weeks.
It has also taken water samples from the lake which have been sent for analysis, with the results expected next week. At this stage the cause is unknown.
The lake is owned by Northumberland County Council and it has been informed of the incident.
Graham Siddle, land and water team leader at the Environment Agency in the North East, said: "We’re carrying out an investigation to find out why this has happened. It would appear the fish have been dead for some time. We received the report on Monday, possibly because this is when they first appeared on the surface of the lake.
"The park is very popular with local people and we’d like to hear from anyone who saw distressed or dead fish in the lake prior to last weekend.
"It’s also important that people report incidents like this to us as soon as possible so we can investigate. Anyone who sees dead or distressed fish in our rivers and lakes should contact our incident hotline on 0800 807060."
A county council spokesman said: "The county council is concerned over the incident, at what is a popular beauty spot and wildlife haven, and is working with the Environment Agency to try and identify the cause."