The Scottish Environment Protection Agency secretly ditched a plan to ban a toxic pesticide contaminating sea lochs and killing wildlife after pressure from the fish farming industry.

Sepa had been intending to prevent salmon farmers from using emamectin to kill sea lice in 2018, but dropped the idea when the Scottish Salmon Producers’ Organisation (SSPO) warned that it would “undermine commercial confidence in the industry”.

The Sunday Herald revealed last month that emamectin and another fish farm pesticide had polluted 45 lochs around Scotland in breach of environmental safety limits. We reported earlier this month that Sepa was planning a “tightening” of the rules for using the pesticide.

But now, in response to a request under freedom of information law, Sepa has released a report that it had been planning to publish in August 2016. The report was suppressed after intense private lobbying by SSPO.

The report highlighted concerns that emamectin could be killing crustaceans such as crabs and lobsters and that current environmental safety assessments could be wrong. Studies had found that the pesticide had spread further from salmon cages than expected and was harming seabed wildlife.

Sepa was planning to impose tighter restrictions on the use of the pesticide for the next two years, the report disclosed. Then it was likely that the ability to use the chemical, marketed as Slice, “will be phased out completely”.

The report stated: “We have informed fish farm operators of Sepa’s position that, unless we see new and compelling evidence to support continued use, the ability to use Slice is likely to be phased out in 2018.”

But this plan was not implemented – and the report not published – after SSPO complained. When shown a draft, the industry group privately cautioned Sepa that publication would trigger “media scrutiny which will seek to undermine the industry’s reputation and will probably damage all of our reputations”.

Instead, earlier this month Sepa published a statement that made no mention of a ban in 2018. “Sepa is reviewing all fish farm licences permitting the use of Slice, tightening conditions for the medicine’s use,” it said.

“This restriction will remain in place while Sepa and the industry carry out further research to either confirm or confound the apparent link between Slice use and possible environmental effects.”

The National Trust for Scotland called on Sepa to act now to prevent further damage to wildlife in protected areas. “I fail to understand how Sepa can justify delaying action to ban this harmful environmental toxin,” said the trust’s senior nature conservation adviser, Dr Richard Luxmoore.

“Many of the existing fish farms lie within special areas of conservation and research by the Scottish Association for Marine Science concludes that the toxins discharged from the farms have already harmed marine wildlife over a large area.”

Guy Linley-Adams, from Salmon and Trout Conservation Scotland, argued that the number of salmon kept in cages should be reduced. “It appears that Sepa has been browbeaten into allowing excessive treatment chemicals to be used, which has damaged the ecology of the sea lochs, particularly wild crustaceans such as crabs and lobsters,” he said.

The original Sepa report was obtained by Don Staniford from the Global Alliance Against Industrial Aquaculture. “Sepa should stop abjectly doing the bidding of the salmon farming industry and ban the use of emamectin immediately,” he said.

Sepa did not deny that the planned ban on emamectin had been dropped. “I am happy for Sepa to be accountable for the outcome of those actions,” said chief executive, Terry A’Hearn.

He added: “Sepa made its own decision in August 2016 not to publish a website article about a scientific report.”

SSPO declined to comment.