The otolith is an archive of the growth, environmental and ecological history of that fish. Its size and shape can be used to determine the age of the fish. The chemicals in each layer within the otolith change depending on surrounding factors such as water and provide a signature to mark time and place.
Back in 2003, researchers from NIWA applied this method to study snapper – one of Aotearoa New Zealand’s largest and most valuable commercial coastal fisheries. Snapper have been heavily fished over time and there were knowledge gaps around the significance of snapper nurseries on the west coast of the North Island, which needed to be filled in order to better manage this fishery.
The researchers used otolith microchemistry to determine whether estuarine nursey grounds for snapper generated unique ‘chemical fingerprints’ and whether these could be used to match adult snapper in the open sea to their nursery ground in a west coast harbour.
Baby snapper. Image credit: Crispin Middleton/NIWA.
Juvenile snapper were taken from seven estuaries along the west coast of the North Island that are known to be nursery grounds for snapper. They measured eight different chemical elements in the otolith of each fish and were able to find a robust ‘chemical fingerprint’ that could distinguish Kaipara, Manukau and Whangapae harbours uniquely, and group Hokianga, Whāingaroa/Raglan, Aotea and Kawhia harbours together.
Because it takes snappers around 3-4 years to reach maturity and swim away from the nursery ground out into the open sea, the researchers waited four years before undertaking the second part of the study.
In part two, they collected many adult snapper from commercial catches from four zones over 700 km of coastline from Ninety Mile Beach in the far north down to Mana Island in Pōneke Wellington. The researchers aged the fish by analysing the otolith structure to find four-year-old snapper, as it is possible that changes in the environment could mean the signatures from each nursery changed each year. They selected 140 snapper with 20-30 snapper from each zone.
Comparing the chemical fingerprint of these snapper to the harbour-specific patterns established earlier in the project showed that 98% of the adult snapper were originally juveniles from Kaipara Harbour.
The implications of these findings are that the Kaipara Harbour appears to sustain most of the adult coastal snapper populations on the west coast of the North Island. Environmental damage or habitat changes in that harbour could have widespread negative impacts on snapper populations the length of the North Island. Fisheries management for snapper therefore needs to look beyond fishing limits to also consider ways to protect the high quality nursery habitat that exists in the Kaipara Harbour.
The Kaipara Harbour is known to be heavily impacted by land-based activities, including accelerated sedimentation due to changing land use (discussed in the section ‘Land-based activities impact coastal fisheries’).[1] There is increasing recognition of the importance of remediating and protecting the Kaipara Harbour, with the government announcing in July 2020 that the Kaipara Moana Remediation Programme would get $100 million towards these efforts.
Microchemical analysis has uncovered details about snapper habitat use on the west coast of the North Island that shows clearly that ongoing and future efforts to manage this fishery and ensure sustainable stocks for years to come need to focus on keeping the Kaipara Harbour healthy. This is relevant to the range of estuaries and harbours that are nursery grounds for our fisheries.
The implications of these findings are that the Kaipara Harbour appears to sustain most of the adult coastal snapper populations on the west coast of the North Island. Environmental damage or habitat changes in that harbour could have widespread negative impacts on snapper populations the length of the North Island. Fisheries management for snapper therefore needs to look beyond fishing limits to also consider ways to protect the high quality nursery habitat that exists in the Kaipara Harbour.
The implications of these findings are that the Kaipara Harbour appears to sustain most of the adult coastal snapper populations on the west coast of the North Island. Environmental damage or habitat changes in that harbour could have widespread negative impacts on snapper populations the length of the North Island. Fisheries management for snapper therefore needs to look beyond fishing limits to also consider ways to protect the high quality nursery habitat that exists in the Kaipara Harbour.
References and footnotes
[1] Morrison, M. A. et al. (2009) A review of land-based effects on coastal fisheries and supporting biodiversity in New Zealand, New Zealand aquatic environment and biodiversity Report No. 37, p. 100.