Otahu Estuary shellfish and benthic habitat mapping (2009)

TR 2011/36
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Report: TR 2011/36

Author: N Singleton, H Giles, M Graeme, P Ross

About this report

Estuaries are among the most productive, diverse and ecologically important coastal environments in the Waikato region.

They support diverse ecological communities, and are spawning and nursery areas for many species of fish. They also function as a natural buffer between the land and sea, filtering sediment and contaminants from catchment water before it enters the coastal zone. In addition to these important ecological functions, estuaries are greatly valued by humans for cultural and recreational activities.

Estuaries are heavily used coastal areas within the Waikato region and are under increased pressure because of population growth, increased development with ensuing run off of nutrients and sediments, and coastal developments such as marinas and marine farms that use estuarine space. These uses can cause changes to water quality, increased levels of sedimentation and other contaminants, mechanical disturbance and shellfish harvesting.

Within estuaries, shellfish provide important ecosystem services. These include food for birds, fish and other estuarine animals and the filtration of water which improves water clarity and removes sediments and nutrients from the water column. Another example is the mixing of sediments by shellfish which facilitates nutrient regeneration and oxygen supply.

While shellfish are more resilient to environmental change than many other organisms living in our estuaries, they are sensitive to changes in habitat that result from many of the pressures facing estuaries. For example, they are at risk of smothering by sediments, displacement by activities such as dredging and other types of habitat modification, and habitat and water quality changes resulting from increases in nutrient run off from land.

Waikato Regional Council has a statutory obligation to protect the region’s natural coastal resources. Due to their cultural and ecological importance, the protection of shellfish beds is a priority.

In order to protect shellfish beds, or detect any changes to them arising from human activity, it is essential to know their extent (i.e. to map where they are found) and how large and dense the beds are. Based on mapping surveys conducted by the Department of Conservation, Waikato Regional Council has mapped shellfish beds and habitats in three estuaries: Otahu Estuary, Wharekawa Harbour and Tairua Harbour.

This report presents the results of the Otahu Estuary survey.

Read or download the survey report

Otahu Estuary shellfish and benthic habitat mapping (2009) [PDF, 3.9 MB]

Contents
  Executive summary iv 
1 Introduction 1 
1.1 Benthic shellfish and habitat mapping project background  1 
1.2 Benthic shellfish and habitat mapping project objectives  2 
1.3 Shellfish species 2 
 1.4 Otahu Estuary 2 
2 Methodology  5 
2.1 Sampling sites  5 
2.2 Sediment characteristics  5 
2.3 Statistical analysis  6 
2.4 Benthic biota  7 
3 Results 7
3.1 Overview 7
3.2 Sediments 8
3.2.1 Subjective substrate classification 8
3.2.2 Comparisons between substrate categories and grain size 8
3.2.3 Comparison of subjective substrate categories with Redox Potential Discontinuity (RPD) layer depth 10
3.3 Bivalves 12
3.3.1 Abundance and spatial distribution 12
3.3.2 Size class distribution 12
3.4 Gastropods 20
3.5 Vegetation 20
3.6 Relationship between bivalve density and sediment properties 22
4 Summary and discussion 23
4.1 Sediments 23
4.2 Bivalves 23
  References 25
  Appendix A: Map of sampling site locations 26
  Appendix B: Sampling site characteristics 27
  Appendix C: Shellfish abundance 29
  Appendix D: Variability of bivalve abundance 31