The new statutory ban came into force as the 2015 season got under way on Monday.
It means that even if a salmon dies, it must be left in the water and not retained.
And it supercedes the voluntary 100% catch and release policy which the River Tweed Commissioners and beat proprietors have already invoked in a bid to preserve spring running salmon.
However that voluntary stricture, although not supported by law, will still apply to all salmon taken between April 1 and June 30. But sea trout may be killed and retained.
The issue of diminishing stocks on the world-famous river continues to be a burning issue within the angling fraternity, especially as catches fell alarmingly last year when around 4,000 salmon were taken compared to an annual average of 11,000 over the previous five years.
But while pessimistic voices are forecasting extinction, Peter Straker-Smith, owner of the Carham beat between Kelso and Coldstream, believes predictions of the species’ demise are, to say the least, premature.
Writing on the online platform Tweedbeats, Mr Straker-Smith acknowledges that only 130 fish were taken on his beat last year against a five year average of 363.
But catch records at Carham indicate such slumps – followed by recoveries - have occurred before, notably between 1967 and 1979 when an average of 70 fish were taken due to the collapse of the spring fishery associated with the disease ulcerative dermal necrosis (UDN).
As to what went wrong last year, Mr Straker-Smith states: “We do not know and probably never will.
“Similar effects were suffered across the north Atlantic which rules out local causes and any effects of man’s predation of returning salmon, including by the perennial bogeymen: drift and coastal nets and pelagic trawlers.
“Though clearly important to the welfare of salmon, they cannot be blamed for 2014 being so much worse than previous years across such a wide area. The fish that did return to Tweed were of decent size and in good condition which does not indicate they were particularly short of food [at sea].
“We will not know until many years have passed if this is a long-term problem, but in the short term, we have every reason to believe there have been quite enough spawning fish in the system in 2014 to fully populate the burns with fry.
“So far the winter has not produced dramatic spates to wash out the redds so there is no cause for panic.
“We know dramatic falls in catches happen from time to time… but, to date, all such falls have been followed sooner or later by recovery.”