The conservation of fish stocks risks being "undermined" because a ban on discards is not being effectively enforced, WWF has warned.
The environmental charity has called for improved monitoring of measures designed to conserve stocks in Scottish and other UK waters.
The number of Scottish fishing boats carrying cameras to monitor their catch, known as remote electronic monitoring, has halved since the introduction of new landing obligations, including a ban on discarding some species.
The figure has dropped from 32 in 2014 to just 15 currently.
North Sea cod and whiting were among the new species added to the ban this year, with the disposal of all fish at sea to be prohibited by 2019.
Helen McLachlan, WWF's fisheries governance programme manager, said: "We have significant concerns about the extremely low levels of monitoring and control of the ban.
"For this policy to work, we need to be confident we know what is happening at sea and how much fish is being removed.
"North Sea cod is slowly recovering and we can't allow this to be undermined by unrecorded and unreported discarding which would compromise management and could lead to overfishing of the stock, undoing the positive steps made to date.
"It's imperative that effective monitoring and control is a priority for management, and we recommend the use of cameras on vessels as the most cost-effective method."
Under a previous incentive programme, boats were given an increased quota for North Sea cod if they agreed to carry cameras.
A Scottish Government spokesman said: "Under the previous North Sea cod scheme, as agreed with the EU and Norway, there was a bespoke quota top-up arrangement available for cod.
"This scheme has come to an end now North Sea cod is subject to the landing obligation, and all cod caught by whitefish vessels must be retained.
"No-one wants to see dead fish being thrown back into the sea - least of all our fishermen.
"Our fleet has already made good progress to reduce the level of discarded fish in Scotland and we are working hard to ensure the ban is implemented in a pragmatic, proportionate and phased way.
"If managed sensibly, the landing obligation will be good for Scotland and help the conservation of fish stocks that offer up dependable and sustainable catches for fishermen.
"Quota incentives will be available for species other than cod in the 2017 scheme, we will set out these details in due course."
Bertie Armstrong, chief executive of the Scottish Fishermen's Federation, said: "No-one hates discarding more than our fishermen, who are making a comprehensive effort to comply with this largely unworkable regulation.
"Fishermen are doing everything possible to adhere to the rules and industry and government are working closely together to develop more selective types of fishing gear that will reduce discarding even further.
"Installing cameras on fishing vessels is no more than a sideshow and the presence or absence of them will not solve the problem. The real issue is getting the rules right and the proper refinements in place."
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