Coral reefs in the far north of the Great Barrier Reef are showing lasting effects from the mass bleaching of 2016 and 2017 and in some cases their health has declined further, according to fresh surveys by the Australian Institute of Marine Science.

Preliminary results of surveys by Aims scientists in January show several reefs have not recovered from the back-to-back bleaching, although the agency said some reefs they surveyed were in good condition.

Researchers undertook a 25-day expedition to remote far northern reefs, at a cost of $1.4m, partly funded by the Great Barrier Reef Foundation from money it was awarded in a record grant by the government last year.

Some of the reefs examined had not been surveyed before.

In the Cape Grenville sector, six reefs were surveyed where the agency had collected data in 2017, after the 2016 bleaching. Of these, hard coral cover had declined further at five reefs, while one was unchanged.

Three other previously unsurveyed reefs had “low, moderate and high coral cover”, the report says.

Across the site scientists found widespread coral bleaching “but at low levels”, and there was also fresh damage to reefs attributed to destruction from tropical cyclone Penny this summer.

In the Princess Charlotte sector, Aims returned to five reefs studied in 2017, of which two had deteriorated further and three were unchanged.

The scientists said in their preliminary report that coral bleaching was widespread at low levels across this section of reefs, but some areas showed “significant coral bleaching”. These reefs also appeared to have been damaged by tropical cyclone Penny.

Mike Emslie, a marine ecologist at Aims who was one of the scientists on the expedition, said “some reefs that were severely bleached in 2016 are still in bad shape”.

“However, other reefs that escaped the worst of the bleaching still have reasonably healthy amounts of coral.”

Emslie said the team’s preliminary results showed low to moderate numbers of young corals, which indicated that the effects of bleaching were still being felt.

“Fish numbers and diversity were high on most reefs. However, this was the first survey of reef fish communities undertaken by Aims in the far north and we cannot speculate on whether fish communities have been altered by the coral bleaching,” he said.

The coral scientist Terry Hughes has also worked with research teams surveying parts of the far northern Great Barrier Reef.

He said the preliminary results of the study, and the lack of observed improvement in previously surveyed reefs, showed how difficult it was for corals to rebound after mass bleaching.

“That doesn’t surprise me because it takes 10 years for coral to rebound,” Hughes said.

“There’s been some reports of magical recovery, but the ecological memory of the bleaching will be long-lasting.

“The big unknown is when it will bleach again, but it will almost certainly be before those reefs have time to recover.”