IT will be a vital crossing for thousands of commuting Scots every year.

But now hi-tech equipment is being installed to ensure the construction of the new £1.5 billion Forth Bridge does not deter another great traveller – migrating fish.

Noise levels in the Forth will be monitored during construction of the crossing by special underwater devices.

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If work becomes too loud for travelling salmon and lamprey fish then a warning will be beamed to builders' smart phones and work will be halted.

Salmon moving upstream are crucial to fisheries and rivers that feed off the estuary. As part of measures to lessen the impact of construction work, project managers are putting in place a system that will detect when fish move under the structure.

A series of PAMBuoy devices, costing between £20,000 and £40,000 each, have already been attached to the existing Forth road bridge. They are monitoring noise levels in the firth to make sure fish are not frightened away from protected areas upstream.

The devices were created by the world-renowned Sea Mammal Research Unit at St Andrews University and use hydrophones in the water to monitor noise levels.

They are being leased to the Forth Crossing Bridge Constructors (FCBC) the consortium awarded the main £790 million contract.

Cormac Booth, a scientist working on the project, said: "We have set this device specially to detect noise levels that would affect salmon and lamprey, especially those focused on noise frequencies they hear best.

"No noise levels have exceeded the regulatory thresholds, keeping the estuary open and safe for these fish allowing them to get upstream and enabling the bridge team to continue construction without any fear of breaking environmental laws."

FCBC environmental manager Neil Abraham said he hoped the new Forth crossing would be viewed as a "benchmark for world-class environmental care in large-scale construction projects".

Similar devices have been set up at St Andrews Bay, Arbroath and Stonehaven to measure the activities of sea mammals.

A spokesman for the AA motoring organisation admitted the plans sounded "unusual".

The new Forth road bridge is due for completion by 2016.