Niwa research vessel Ikatere in Napier undertaking seabed mapping of the area using multibeam echosounder in response to Cyclone Gabrielle. Photo / Niwa
Niwa research vessel Ikatere in Napier undertaking seabed mapping of the area using multibeam echosounder in response to Cyclone Gabrielle. Photo / Niwa
Fishermen have already noticed the effects of silt from Cyclone Gabrielle on the fish population.
For the first time, National Institute of Water and Atmospheric Research (Niwa) scientists were able to compare the seabed before and aftera cyclone in New Zealand with recent mapping off Hawke’s Bay’s coast.
From April18-29 the research vessel Ikatere mapped about 200 square kilometres of seabed off the coast of Hawke’s Bay using a multibeam echosounder from 15 metres deep.
The device provides images of the seabed shape and identifies seafloor characteristics, important for observing how sediment that came out of the rivers during the cyclone has affected the ocean floor.
Previous monitoringwas carried out to help Niwa and the Hawke’s Bay Regional Council identify which areas were important for biodiversity habitat, and to improve ongoing reporting on the state of the marine environment.
Niwa marine geologist Dr Joshu Mountjoy said a fair amount of work had been done around near-coastal impacts after storms but this was the first time they have had a detailed data set from before the actual event.
“It is quite unusual to have that much data and it has been a great partnership with Hawke’s Bay Regional Council collecting the datasets pre-cyclone,” Mountjoy said.
”We are going back to map places we have mapped before so we will be able to make that comparison, but that is going to take a bit of time.”
He said it would take a while to analyse the 13 days’ worth of data collected, but there were early positive signs that sediment wasn’t covering everything on the seafloor because the hard substrate that was present in the original mapping was still visible in the new data.
“For example, at Wairoa and Pania Reef we are still seeing reef structure and hard substrates at the seabed so everything is not totally covered in sediment which is a really good start,” he said.
“The details of that and how it changes as you come into shallower water is something we are going to have to look at pretty closely until we can get a clear picture.”
Mountjoy said they could see where silt had accumulated, but they still had to confirm it was related to Cyclone Gabrielle and that it had changed since before the cyclone.
“We want to understand if there is a significant amount of sediment accumulated over the harbour substrates, especially where these are key habitats that have been identified by Hawke’s Bay Regional Council.
“Some of these areas might be nurseries for fish species and obviously if there are mussel beds there and they become smothered in sediment then that is going to have a big impact,” he said.
Hawke's Bay was unique because it haddata from before the cyclone to compare new images of the seabed to, according to Niwa marine geologist Dr Joshu Mountjoy. Photo / Niwa
Fisherman Karl Warr, of Better Fishing, normally seeks flounder and gurnard in waters less than 50 metres deep and he has already noticed changes in the population since the cyclone.
He said the fish had likely moved into deeper waters after being disturbed by silt and sediment washed out during the cyclone.
