A ban on discarding unwanted fish into the sea will be difficult to comply with, researchers have claimed, the BBC reports.
Boats having to land unwanted fish caught in their nets started being phased in at the start of the year, under European laws.
Throwing dead fish back into the sea had increased due to strict EU quotas on which fish could be landed in a bid to conserve stocks.
The University of Strathclyde said it would pose the industry problems.
The ban is part of a new deal on fishing quotas and industry rules that was reached after negotiations in Brussels.
The ban came into force in January for species such as herring and mackerel, and will take effect a year later for cod and haddock.
The Strathclyde study concluded reducing waste and improving fish stocks was uncertain.
Prof Mike Heath, who led the research, said: "The campaign to change the CFP (Common Fisheries Policy) and get the discard ban onto the statute book was based on the assertion that half of all fish caught are discarded, and that EU quota restrictions are to blame.
"But our comprehensive assessment of discarding in the North Sea shows that's not the case.
"The wasteful practice of over-quota discarding has to be stopped but our study shows that the fundamental reason for the increasing proportion of catch being discarded is that the stocks have become more dominated by undersized fish.
"This is a legacy of overfishing in the past."
Dr Robin Cook, who also took part in the research, said: "The discard ban will force skippers to use up the storage capacity aboard their vessels bringing ashore fish for which there is no significant market.
"This will have a detrimental effect on the profitability of the fishing industry, with no real benefit to the stocks. Further improvements in the performance of fishing gears are needed to overcome this, so that small fish are no longer caught."
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