Changes to Austrovenus stutchburyi growth rate since early human settlement in New Zealand: an indication of the extent of human impact on estuarine health

Abstract

When humans first arrived in New Zealand around 1250AD they started making changes to the environment to make it more habitable. Coastal marine ecosystems such as estuaries are an important link between the land and the sea and are sensitive to changes in environmental conditions such as sediment load and fluxes in nutrients. The New Zealand cockle (Austrovenus stutchburyi; tuaki, tuangi), is a filter-feeding bivalve species that is common in estuaries throughout New Zealand. The growth rate of A. stutchburyi is recorded as growth bands in the shell and is affected by many factors, including nutrient concentrations and sediment load within the water column. A. stutchburyi is consequently well-suited as an indicator species for studying temporal changes in environmental conditions. A. stutchburyi was an important food source for early Maori and shells are abundant in middens nationwide. In this study, the growth rates of modern and archaeological A. stutchburyi shells were measured to determine how the growth rate has changed through time. By using cockle growth rate as an indicator of estuarine health, we can determine the impact that anthropogenic changes have been having on the health of estuarine environments since the early arrival of humans in New Zealand.

This research analysed growth parameters of A. stutchburyi shells from midden sites across a range of environments throughout New Zealand and compared them to modern shells collected from the same localities. Thin sections of shells were prepared, and the width of summer, winter, and annual growth bands were measured between years five and twelve for each shell to determine an average growth rate. Growth rates of shells from different time periods ranging from the 1300’s AD to present were studied.

This study found that there was an overall trend of declining growth rate in shells over time, with no sites showing an increase in growth rate. Shells from sites with the most highly modified catchment areas showed the greatest change in growth rate over time (up to a 50% reduction in growth per year), and shells from sites with the least modified catchment areas did not show significant changes in growth rate over time.

A. stutchburyi is a culturally important species and is recreationally and commercially harvested within New Zealand. The information gained in this study provides a baseline for the health of estuaries, as indicated by growth rate of cockles, prior to major anthropogenic impacts to the surrounding environment. These baselines can be used to inform future management of A. stutchburyi stocks and to aid in the conservation and restoration of estuarine areas.