The Department of Conservation (Doc) said there were now few, if any, of the large old plants left in the area.
New Zealand has just left a rare triple-dip La Nina, which pushed up sea temperatures for three summers.
Doc West Coast marine reserves ranger Don Neale said Niwa was completing a report on the drone surveys it carried out in the Punakaiki Marine Reserve in 2020 and 2023 to compare changes.
Significant changes identified so far included mussels and bull kelp.
"It appears that the last known ‘old growth’ rimurapa [bull kelp] bed in the Punakaiki Marine Reserve was badly affected by last summer’s heatwave," Mr Neale said.
While there was still some in the reserve, and in areas off the Coast Road outside the reserve, there were few, if any, dense beds of large old plants left.
Doc was looking to expand the photographic monitoring of bull kelp to include some sites in South Westland using aerial photos taken in 2009.
Meanwhile, last month Doc had an interesting report of a beach-cast dead turtle on a Greymouth beach, via a photo posted on Facebook.
"Unfortunately, we were unable to find or recover the animal. It has been identified as an olive ridley turtle."
This was the first stranding of an olive ridley recorded on the West Coast, though they were the most commonly stranded turtle species in New Zealand, Mr Neale said.
"There are only six historic records of turtles on the West Coast."
The different weather may also have hit New Zealand fur seal populations
Mr Neale said from the fur seal pup population count last January it appeared there were significant declines at all three West Coast colonies being monitored — Wekakura Point in northern Buller, Cape Foulwind and Taumaka Island off Haast.
"We’ve monitored these colonies every year or two since 1991. The causes of decline are most likely related to climate and related food-prey species changes and/or fisheries by-catch."
— Greymouth Star
