Science shows when things turned bad for the Hauraki Gulf’s snapper
For centuries, the snapper relied on low-salinity estuarine nurseries that provided crucial and stable habitats for juvenile development, but that all changed following the industrial revolution in the 19th century.
“Deforestation, agricultural expansion, and urban development led to increased sedimentation and nutrient loading in estuaries, which, in turn, caused habitat loss, reduced water quality, and heightened turbidity,” explained study author Dr Julian Lilkendey, of the Auckland University of Technology (AUT).
These changes – worsened by over-fishing and new pest species – made estuaries less suitable as nurseries for snapper, pushing them into other habitats.
“Together, these human-induced stressors did not only displace snapper but also reshaped the entire coastal ecosystem, affecting biodiversity, food webs, and ecological resilience.”
To re-piece the picture, Lilkendey and fellow researchers from AUT, the Auckland War Memorial Museum, and Otago University turned to otoliths, or fish ear bones, which are formed layer by layer as fish grow.
“The chemical composition of otoliths acts like a diary, recording the environments fish experienced throughout their lives,” Lilkendey said.
“This allowed us to trace how snapper habitat-use has shifted over time.”
The team also used radiocarbon dating methods on charcoal and other organic materials found in Māori middens – ancient trash heaps – to establish a timeline for environmental changes.
“The midden contexts, together with historical fishery surveys and modern recreational fisher samples, allowed us to connect chemical profiles from otoliths to specific time periods,” Lilkendey said.
“This approach enabled us to pinpoint exactly when snapper movement patterns shifted in response to environmental pressures.
“It’s kind of a sad story, really – despite this degradation, the Hauraki Gulf paradoxically still supports a significant snapper fishery, suggesting untapped carrying capacity and the big potential for restoration.”
Project lead and co-author, AUT marine biologist associate professor Armagan Sabetian, expected the new findings would have important implications for such efforts.
“Understanding how snapper historically used their habitats and how this is encoded in their otoliths enables us to set meaningful targets for recovery,” he said.
“By aligning snapper movement behaviour with pre-industrial baselines, we can be confident that the environmental conditions necessary to support these behaviours are being restored.”
The findings come as the Government moves ahead with plans to introduce 19 new marine protection areas in the Gulf, expanding its network of reserves and other protected zones.
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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