Scientists on Hawke’s Bay: Cyclone Gabrielle-related damage to seabed could hit fish population

Scientists have discovered “small declines” in seabed organisms such as sponges and kelp in Hawke’s Bay following Cyclone Gabrielle.

While that sounds insignificant, scientists say it could lead to reduced fish levels in Hawke Bay in years to come due to juvenile fish having less shelter from predators.

A fishing advocate says he certainly hopes the findings do not translate to a dip in fish levels in the future, and the summer season just gone provided great fishing in the region.

National Institute of Water and Atmospheric Research (Niwa) scientists conducted surveys of the seabed in Hawke’s Bay and Tairāwhiti/Gisborne in April, June and October last year on behalf of Fisheries NZ, using underwater cameras.

Those surveys looked specifically at how sediment and silt impacted the marine environment after being washed into the ocean during the floods of February 2023.

A report was completed in March and released to media this week.

“The seafloor model showed small declines in structure-forming organisms such as sponges [and kelp] for Hawke’s Bay following Cyclone Gabrielle,” the report read.

“These declines were not substantial, most likely because the region is already impacted by decades of fishing and increased sedimentation.”

However, the report stated the decline could have a flow-on effect for the fish population in future years.

Niwa scientist Dr Daniel Leduc, who managed the project, said that was because kelp and sponges provide habitats for juvenile fish and protection from predators.

Sediment build-up on the seafloor in a marine area known as Wairoa Hard. Photo / Niwa

“[Juvenile fish] are heavily reliant on the habitats that are provided by kelp and sponges.

“Therefore, if [kelp and sponges] are heavily impacted as a result of the cyclone, that means down the line in a few years’ time, when the juveniles have grown into adult [fish], we will see a decline in the adult population in the fisheries.”

Hawke’s Bay Sports Fishing Club life member Wayne Bicknell, who sits on regional and national recreational fishing boards, said he hoped the findings did not translate to a dip in the fish population.

“We are definitely hoping that is not the case, but it really is a wait-and-see [situation].”

He said there was no fishing straight after the cyclone in Hawke Bay, but the most recent summer had made for a great fishing season.

“In the Springs Box fishing area [in Hawke Bay], it has been the best fishing we have had in a long time.”

Niwa scientists conduct surveys of the seabed last year. Photo / Niwa

He said the fishing club would continue to keep a close eye on species within the fish population, such as snapper, with its ongoing surveys.

Sediment build-up not a new issue

The report stated sediment build-up in Hawke Bay is an ongoing issue.

“Sediments have accumulated along the coast and shelf of Hawke’s Bay and Gisborne at an increasing rate for decades due to deforestation and land use change,” the report read.

“Addressing the long-term issue of sedimentation in marine ecosystems as well as the impact of extreme weather events will require addressing the factors that have made New Zealand’s catchments more prone to erosion.”

During its earliest survey in April, scientists discovered significant sediment build-up of about 1m in thickness at Pania Reef, Tangoio Reef and the Clive outfall area in Hawke Bay.

Those spots were not surveyed again in June and October.

Sponges and sea life on the seabed observed as part of the survey. Photo / Niwa

Leduc said a restraint in the research was there was no survey conducted before the cyclone, which meant they could not compare the data.

“[We] found that of the 36 locations we surveyed with an underwater camera, 11 showed signs of sediment impact,” Leduc said.

“However, because there aren’t comparable pre-cyclone camera observations, we weren’t able to directly link these to the cyclone.”

Satellite imagery showed sediment floating close to the ocean surface for about three months after the cyclone near the coast, before returning to normal.

Gary Hamilton-Irvine is a Hawke’s Bay-based reporter who covers a range of news topics including business, councils, breaking news and cyclone recovery. He formerly worked at News Corp Australia.