The number of seals in the Tees estuary is declining despite more seal pups being born.
Experts are concerned about the "missing" seals
This year six pups were born, which is one more than last year, but overall the numbers are down on previous years.
An organised programme of monitoring is being set up to find out whether the population is in decline in the area, or whether seals are moving elsewhere.
People are being asked to look out for seals in the area and to pass on their information.
Jonathan Gibson, from Industry Nature Conservation Association (Inca), said: "In the last few years, there was a steady increase.
"This year and last year we found that the numbers were down slightly overall on the years before but one good point is this year we have had six pups, which equals our best haul of pups, and is one more than last year.
"We have found that there are around 10 or so seals less than a couple of years ago."
Seals came back to Seal Sands in the mid 1980s and Inca has been monitoring them since 1989, when Tees Seals Research Programme was started. The seals started breeding in 1994.
Mr Gibson said with all seal colonies, there were fluctuations in numbers and the numbers of missing seals in the Tees estuary were relatively small.
On Tuesday, from 1200 BST until 1300 BST, the group is running a co-ordinated seal watch to try and find the seals.