A new study shows shellfish diversity in the Mahau Sound is at its lowest point in history.
The findings are from a Niwa study commissioned by the Marlborough District Council using the latest scientific techniques.
It found the effects of increased soil erosion, sedimentation and harvesting have profoundly affected the subtidal environment in the Mahau Sound, which is in the Pelorus Sound/Te Hoiere in Marlborough.
Marlborough District Council's coastal scientist Oliver Wade said around 70 percent of material in the Mahau Sound was legacy sediment from deforestation, mining and burning. It had been present for some time and continued to move around.
"Legacy sediment from deforestation, mining and burning during the mid to late 1800s has accumulated as flood plain deposits and throughout Te Hoiere/Pelorus Sound. This historical catchment disturbance and land use activity brought a 10-fold increase in sediment accumulation rates, relative to previous centuries."
It presents a challenge as the marine environment continues to suffer from past activities like gold mining, native forest clearance and pastoral farming.
"This suggests that despite our best efforts to improve land management, the marine environment will take longer to recover as this sediment continues to have an impact well into the future."
The remaining 30 percent is contemporary erosion.
Sources of sediment by land use deposited at confluences and the contributions (percent) from major tributaries in the Pelorus-Rai and Kaituna catchments. The percentage soil proportions in the centre of each pie chart show the contribution of that subcatchment to sediment deposition in the river system (ie 100 percent at the catchment outlet). Data collected in April 2016 and May 2018. Photo: Supplied / Marlborough District Council
"Subsoils and streambank erosion make up the largest proportion, with smaller proportions attributed to erosion from land associated with primary industry and native forest. These proportions vary according to location."
Wade said the study improved understanding of the sources of sediment that impacted the marine environment and how they had changed over time.
It showed integrated catchment and marine management was needed to stop further degradation.
Wade said the Te Hoiere/Pelorus Restoration Project provided a platform for the community to come together and engage in the process.
The project was launched last year and is a large-scale initiative to restore the catchment by improving freshwater and land resources from the mountains to the sea.
It is being led by Kotahitanga mō te Taiao, an alliance between several councils, iwi and the Department of Conservation in the top of the South Island.
In October, it received a $7.5 million boost from the Jobs for Nature programme to accelerate river restoration work, planting, weed control, animal pest control and habitat enhancement for native species such as pekapeka/bats, mioweka/banded rail and shortjaw kōkopu.
A nursery will also be established on Ngāti Kuia land to propagate and grow eco-sourced natives to be planted as part of the project.
It will create jobs for up to 79 people over four years.
