MYSTERY surrounds a drop in the number of seals at a North colony.

It is the first time in 14 years, since recording began, the number of seals on the Seal Sands mudflats at Teesmouth has declined.

The colony made history when, in the 1960s, it became the first in Europe to re- establish itself on an industrial site, having been driven away by industrial development and human disturbance.

Their return after more than 100 years was followed by a steady growth and, by last year, the colony had reached 71 harbour seals, up from 23 in 1989, and 30 grey seals, up from 18 in 1989.

However, this year there were 58 harbour seals, down 13, and 26 grey seals, down four.

The number of pups is also slightly down. Last year six harbour pups were born, but this year there were only five.

Grey seals do not breed in the area because the habitat is unsuitable for the species.

Last year, there were fears of an outbreak of phocine distemper virus, which affects the seal's immune systems making them susceptible to illnesses such as pneumonia. However no cases have been found on Teesside and it is unclear what has been causing the decline.

Geoff Barber, from the Industry Nature Conservation Association (Inca) which monitors the colony, said: "This is a relatively small decline but we do need to find out the causes.

"We are not unduly concerned at this stage, but clearly if numbers are down again next year, then that is an indication something worrying is happening to the colony."

It is thought the colony could be a similar size but that it has been expanding its range, with some animals moving to Newport Bridge, at Middlesbrough, and to Greatham Creek, near Hartlepool.

Another cause may be the changes to the distribution of fish, caused by the recent long, hot summer.