The fishing industry will have difficulty complying with new EU Common Fisheries Policy (CFP) rules banning the throwing away of unwanted fish, according to research at the University of Strathclyde.
The aim of the regulations, which which began to come into force in January 2015, is to reduce waste and improve fish stocks - but the Strathclyde study concluded that this outcome was uncertain.
It found that, over time, quantities of fish discarded have declined since the early 1980s - because overall catches have decreased. However, the proportion of catch which is discarded has increased from around 25% to 35%, because catches have become increasingly dominated by small fish.
Professor Mike Heath, of Strathclyde's Department of Mathematics and Statistics, led the research. He said: "The campaign to change the CFP 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."
Quotas are annual limits, set by the EU, on the quantities of each species of fish which can be landed for sale. They are set for conservation reasons, to reduce overfishing.
In addition, each of the main commercially-important species has a minimum landing size, designed to discourage fishermen from catching immature fish that have not had a chance to spawn. It is illegal to retain undersize fish aboard a fishing vessel, and in any case there is little or no market for such small fish.
Dr Robin Cook, of the Department of Mathematics and Statistics, who also took part in the research, said: "Our study shows that, in the last 10 years, the proportion of whitefish catch in the North Sea which has been discarded is much less than half. In addition, most of the discarded fish were undersize, not over-quota. Overall, quota restrictions have not been the main factor behind discarding.
"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."
The dominance of small fish is partly due to overfishing, which has shifted the range of fish in the seas towards smaller sizes and to smaller species such as plaice and dab. These two flatfish now comprise 50% of all fish discarded.
Policy changes in the last few years have dramatically reduced the extent of overfishing, but it will take time for this to be reflected in the stocks. Meantime, although there have been great improvements in the performance of fishing gears, it remains difficult for trawlers to avoid catching quantities of small fish.
The study has been published in the journal PLOS ONE.