Niwa researchers have been out on the Hauraki Gulf to find out more about snapper nurseries, mapping the sea floor habitats where the young fish live to learn more about what they need to stay healthy and make it to adulthood.

Snapper was a keystone species in the Hauraki Gulf, which meant their numbers and size played an important role in how the coastal marine environment worked, fisheries scientist Dr Mark Morrison said.

He was now investigating whether the ability of marine ecosystems to support fish nurseries had been greatly reduced by human impact, creating habitat "bottlenecks" that prevented enough young fish becoming adults.

Dr Morrison is leading a five-year MBIE Endeavour Fund research programme that he hopes will ultimately result in new options to assist in the future management of snapper in the Hauraki Gulf and off Northland's east coast, as well as juvenile blue cod in the Marlborough Sounds.

Advertisement

The first part of the programme identified the locations of juvenile snapper (up to a year old and less than 9cm) in previously unknown Hauraki Gulf nurseries, then examined both the Gulf and East Northland nurseries in finer detail to see what makes a good nursery.

Now Foundation North (via its GIFT — Gulf Innovation Fund Together), Niwa, the Auckland Council and Waikato Regional Council are funding habitat-mapping of key nurseries using multibeam echo sounder technology on board the Niwa research vessel Ikatere.

Multibeam echo sounders send out a fan of acoustic beams, or sound waves, directed downwards from the bottom of a boat. The beams reflect off the seafloor, enabling it to be mapped in extraordinary detail, showing both habitat types (eg. reefs, mud, sand, gravel, horse mussel beds) and the wider seafloor landscapes in which they occur.

Dr Morrison said large subtidal seagrass meadows had disappeared from the Gulf by the 1930s, but were still in abundant in several large East Northland estuaries. Juvenile snapper could be more than 100 times more abundant there than in other, non-seagrass nurseries.

Meanwhile, counting daily growth rings on juvenile snapper ear bones by BSc (Hons) student Christine Stewart, co-supervised by Dr Richard Taylor, University of Auckland, had revealed that juvenile snapper grew significantly faster and were heavier in subtidal seagrass.

The source of those results included juvenile snapper from the subtidal seagrass meadows of Whangarei Harbour, which were lost in the 1960s, but finally began to recover 50 years later and were now extensive, albeit not yet reaching historical levels.

"There has been a huge change in the coastal habitats and environment of the Gulf," Dr Morrison said.

"We would like to see it go back to what it once was as much as is possible, while acknowledging that a large human population now lives right on the Gulf. Observations such as the recovery of the subtidal seagrass meadows of Whangarei Harbour give us hope that pragmatic restoration is possible."

Once the multibeam mapping was completed, new habitat maps would be developed and made available to the public.

Foundation North CEO Jennifer Gill said depleted fish stocks were a key environmental issue facing the Hauraki Gulf.

"This study will weave the findings from innovative technology and local knowledge to create maps of coastal habitats that are easy to understand. Having this scientific knowledge made accessible to communities will enable them to participate in discussions to ensure the survival of snapper in the Hauraki Gulf."

Dr Morrison added that next year a team would undertake a tagging programme across parts of the mapped seafloor habitats, so researchers could determine how many fish were surviving, how fast they were growing and how far they moved.

Tagging would help define which habitats were most valuable for juvenile snapper, and where new management approaches might be needed.

A computer simulation model would also be developed to bring all the scientific research evidence together to enable decision-makers to try out different management scenarios in a virtual "what if" situation, to establish how environmental degradation reduced the value of juvenile fish habitats and what could be done to reverse that decline.