Invasive leathery sea squirt found in oyster pot

The oysters in the River Tees have grown a lot this summer

An invasive species has been spotted in an oyster pot being used by a conservation group to monitor river health.

The Tees Rivers Trust had set up an oyster nursery in the Tees estuary to help restore marine habitats and bring back spawning fish to the river.

During the latest pot inspection a leathery sea squirt was spotted and reported before being disposed of.

Programme manager Judy Power said "it was an important reminder to look for invasive species in all habitats".

Ms Power said: "There was one species this month that we were less pleased to see - a leathery sea squirt.

"We have reported it and got rid of it."

Leathery sea squirt, or styela clava, pose a threat to oyster and mussel farming by out-competing shellfish for food and space.

The white ring around the outside of the oyster shows recent growth

However, she said there were positive finds during the inspection including the "amazing growth" of the oysters and other marine life.

"Apart from the oysters, we found six fish, including a scorpionfish and butterfish, the tiniest baby scallop we've ever seen, a little, long clawed porcelain crab which we rarely see and lots of squat lobsters and prawns," Ms Powers said.

Seagrass and oysters were once present in the estuary but were lost due to a combination of over exploitation and deteriorating water quality.

A single oyster can filter 150 litres of water a day through its gills helping to remove contaminants while seagrass can absorb carbon from water and provide small fish with valuable feeding grounds.

Experts believe the two species would form a coastal defence, with oyster reefs and seagrass meadows helping to keep sediment stable and prevent coastal erosion.

In the longer term, conservationists hope to use stones, gravel and broken scallop shells - known as "cultch" - to line the river bed and provide the right habitat for oysters.