The fisheries protection agency aims to help conserve fish stocks
Two new fisheries protection aircraft have been out of action for six weeks, sparking warnings from an MEP of "pirate fishing" in Scottish waters.
The Scottish Government said delays in completing the necessary documentation had prevented the planes returning to the skies.
Conservative MEP Struan Stevenson said Scotland had been left vulnerable to illegal fishing by foreign crews.
The aircraft are expected to return to service on 7 September.
The protection agency, established in 1991, enforces UK, European Union and international fisheries laws and regulations in the seas around Scotland and in Scottish ports.
It owns two Reims Cessna Caravan II F-406 aircraft contracted to Highland Airways and based at Inverness Airport.
The agency, which took delivery of the planes in February, also has four ships.
Mr Stevenson said the air patrols were vital to monitoring Scottish waters.
He said: "They have been out of action for a couple of months now and during that time there could have been Russian trawlers, Korean trawlers, or trawlers from any of the nations that are involved often in illegal pirate fishing."
The Scottish Government said the surveillance aircraft entered service with certain items of non-critical operational equipment not fully commissioned as a result of the late availability of some parts.
A spokesman said the planes had returned to Reims Aviation in France for routine scheduled post-delivery warranty checks with the final fitting of the equipment being completed at the same time.
He added: "There have been some delays in completing the necessary documentation associated with the fitting and commissioning of the equipment but this process is now almost complete and it is estimated that both aircraft will return to service on 7 September.
"Normal surveillance of fishing activity has continued using the satellite based Vessel Monitoring System and the tasking of Fishery Protection Vessel using VMS derived information has been adjusted when necessary to compensate for this temporary lack of aerial surveillance capability."