The sight of valuable fish being thrown back into the sea, mostly to die, has been a public relations headache for the fishing industry and driven calls for a discard ban by celebrity chefs and environmentalists.

European fishing boats have long discarded fish before entering port to ensure they meet quotas, by some estimates up to a quarter of their catch, in a wasteful practice that damages the stocks the limits were meant to protect.

A simple ban, as agreed by the EU last month, would seem the obvious answer but implementation is fraught, with fishermen wanting help to meet the extra costs while green groups say exemptions make it near unworkable.

After the European Parliament last month voted through a complete ban from next year, current EU chair Ireland thrashed out an accord based on the principle that in future all fish caught must be landed.

So far so good -- but if there are no discards, that means more fish on the dockside and a host of new problems on quotas, and catch and fisheries management.

Major powers Spain and France sought leeway to set one fish species quota against another, or to swap entitlements between boats, fisheries or even other countries.

Most important, they won exemptions to the discard ban.

In the first two years of the new policy, which Ireland hopes to finalise with the Parliament and EU leaders by June, fishermen will have the right to discard up to 9.0 percent of their catch, falling to 8.0 percent for the next two years and then finally to 7.0 percent.

Environmentalists say these provisions undermine the proposal.

"All the exemptions make it very difficult to guarantee that they are implementing the discard ban," said Javier Lopez of the Oceana group. "This is our concern -- it is an improvement but it is very difficult to control."

Saskia Richartz, Greenpeace EU fisheries policy director, said that if there were a total discard ban, then "if someone throws something overboard, then clearly they are doing something illegal," making it easier to police.

The discard exemptions mean "it is not a ban, it is a restriction," Richartz said.

In contrast, fishermen's groups, having previously attacked the "simplistic and populist approach of a total ban," welcomed the accord.

Europeche and Copa-Cogeca noted "the important step" taken with "the establishment of a gradual timetable for the progressive implementation of a discard ban and the introduction of management tools to help the fishing industry adapt to the obligation to land all catches."

But "there are still many outstanding issues to be resolved," they said in a joint statement, "particularly for the practical implementation of a discard ban ... and its consequences."

It was essential, they added, that the authorities "minimise the socio-economic impact of a discard ban on fishing communities by accompanying targeted measures and investments" to help them adjust to the new regime.

An EU source said it was "clear that a zero ban would not be practicable or possible to implement so there has to be some way of allowing for that, particularly if you want to secure" industry support.

Lopez said he understood the problems the fishermen face but "their main concern is the cost ... who is going to pay" for the extra work involved in handling the larger catches, or adapting fishing gear to reduce the problem.

A discard ban is important "not because it means less fish are killed but because the economic cost ... will incentivise fishermen" to minimise the bycatch, Richartz said.

The discard ban is a small but headline-grabbing part of a wider overhaul of the EU's Common Fisheries Policy which is intended to put the industry on a sustainable basis after years of overfishing.

In this respect, Parliament's approach is simpler -- it will mean more fish in the sea from the start and so help stocks recover faster, while the Irish-negotiated accord means less, Richartz said.

For example, quota flexibility means that one year fishermen may take 10 percent over the agreed limit. The following year, that species is being fished again but from 10 percent below what it should be to make up for what has already been taken, she argued.

"We are always borrowing against the future," Richartz said. "You have less and less reproducing stock so that we end up in a downward spiral."