Looking at undersea landscapes from the air – finding fish nursery “hotspots” in the Kaipara Harbour

NIWA is sampling subtidal seagrass meadows, and other habitats, in the southern Kaipara Harbour, from February to March this year. This week, the scientists have been looking at the ‘hottest spots’ for juvenile snapper.

Dr Mark Morrison, Marine Ecologist, NIWA Auckland, says, “The Kaipara is the major harbour for west coast juvenile snapper habitats, so it is a really key place, and we know it’s under stress from sedimentation and other land-based impacts that degrade key habitats.”

NIWA scientists recently mapped the shallow water habitats of the southern Kaipara Harbour using aerial photography, to produce very detailed habitat maps.

Aerial photography can map habitats down to 3–4 metres water depth, depending on water clarity. It shows critical seagrass habitats and how they are distributed.

It also very clearly maps patches of Asian date mussels, an invasive species that has formed large beds in the Kaipara.

These aerial images are then turned into habitat maps, which combined with fish sampling ‘on the ground’, allow us to map where the critical areas for fish nurseries are, and to then monitor their health over time through cost-effective remote sensing.

“In the past, we have done broad scale fish studies across many harbours, but this study will provide very detailed information on New Zealand’s largest harbour, linking fish to fish habitats identified by remote sensing,” says Dr Morrison.

The submerged areas of subtidal seagrass meadows, in northern New Zealand, can support high numbers of juvenile snapper, trevally, parore, spotties, piper, pipefish, and other species.

Dr Morrison says, “We also know that the Kaipara Harbour, and in particular the ‘living’ habitats within it, such as subtidal seagrass meadows and horse mussel beds, provides most of the juvenile snapper recruits to the west coast North Island snapper populations, and the associated fishery. It may also be significant for other fish, such as trevally.”

Seagrass habitats are under threat in New Zealand, with substantial areas having been lost in the past, along with the juvenile fish that they produced.

This study will examine what components of these habitats are most important to small fish, through sampling across seagrass habitat landscapes and quantifying where fish densities and number of species are highest.

The scientists will collect fish using small, hand-hauled, fine-mesh beach seines, pulled up onto a specially designed ramp deployed from an oyster barge. They will sample 80 to 100 sites spread across 300 square kilometres in the southern Kaipara Harbour.

“Sampling across different habitat types and environmental conditions will allow us to quantify what fish are associated with each,” says Dr Morrison.

“The research will then help us identify which factors are most critical, so that the most important habitats can be identified, mapped, and protected.”

The aerial photography was funded by the Ministry for Primary Industries (MPI), because these habitats are important for fish stock and ecosystem based fisheries management and marine spatial planning. The fish sampling research is funded by the Ministry of Business, Innovation and Employment (MBIE). 

NZ Aerial Mapping was contracted to provide the aerial photography used in this research. 

An intertidal bed with different seagrass density patches (tidal creek 15 to 20 m across), the bed is 280 m wide. The bright red area at the image bottom is farm paddocks, adjacent to this is a tidal creek and associated mangrove forest. [NIWA]
A subtidal bed showing rosette seagrass patches associated with higher wave energies (larger patches 15–20 m across). This is a valuable juvenile snapper nursery habitat. [NIWA]
The area covered by the aerial photography to date (c 180 km2). The area to the east and south has also recently been mapped. [NIWA]