Wharekawa Harbour shellfish and benthic habitat mapping (2010)

TR 2011/33
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Report: TR 2011/33

Author: M Graeme (Natural Solutions - Marine & Terrestrial Ecologists Ltd) and H Giles (Waikato Regional Council)

About this report

Estuaries are highly significant and sensitive ecosystems and have been identified as one of the coastal ecosystems within the Waikato region most at risk from human activities.

Estuaries provide feeding, spawning and nursery habitats for many fish, shellfish and bird species, thereby supporting diverse biological communities. They also provide a buffer between the land and sea interface, influencing coastal erosion and filtering contaminants from the land before they enter the coastal zone. In addition to their important ecological and biogeochemical role, estuaries are also greatly valued by people who use them for cultural, commercial and recreational activities.

Our estuaries are coming under increasing pressure from  population growth and coastal settlement, increased demands for recreational uses (e.g. boating and fishing), development in estuaries (e.g. marine farms and marinas), catchment development (e.g. forestry and agriculture), land clearance and reclamation, excavation and dredging (e.g. for boat ramps and boat channels), introduction of invasive species (e.g. Spartina and saltwater paspalum), and resource extraction (e.g. through fishing), as well as long term climate changes including sea-level rise.

Shellfish are very common in estuaries and along intertidal beaches all around New Zealand. They form a major link between the water column and benthic habitats and are an important food source for many fish and bird species. They are also a popular food source for people.

Shellfish populations are sensitive to habitat changes occurring as a result of human activities, such as sediment accumulation, contaminant enrichment or the development of physical structures. For these reasons assessments of bivalve population trends are often used to underpin ecological health or environmental impact assessments.

Waikato Regional Council has a statutory obligation to protect the region’s natural coastal resources. Because of 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.

Waikato Regional Council has been mapping shellfish beds and habitats in three estuaries: Tairua Harbour, Wharekawa Harbour and Otahu Estuary.

This report presents the results of the Wharekawa Harbour survey.

Read or download the survey report

Wharekawa Harbour shellfish and benthic habitat mapping [PDF, 8.6 MB]

Contents
  Executive summary 1
1 Introduction 3
1.1 Benthic shellfish and habitat mapping project background 3
1.2 Benthic shellfish and habitat mapping project objectives 4
1.3 Shellfish species 4
1.4 Wharekawa Harbour 4
2 Methodology 7
2.1 Sampling sites 7
2.2 Benthic biota 7
2.3 Sediment characteristics 8
2.4 Statistical analysis 9
3 Results 10
3.1 Overview and summary statistics 10
3.2 Sediments 12
3.2.1 Subjective substrate classification 12
3.2.2 Sediment grain size 15
3.2.3 Comparison of subjective categories and grain size 17
3.2.4 Comparison of substrate categories with Redox Potential Discontinuity layer 20
3.3 Bivalves 22
3.3.1 Abundance and spatial distribution 22
3.3.2 Size class distribution 28
3.4 Gastropods 32
3.5 Other biota 32
3.6 Vegetation 37
3.7 Relationship between bivalve density, sediment properties and vegetation 39
3.7.1 Bivalve density at sites with different sediment properties 39
3.7.2 Bivalve density at sites with different vegetation cover 43
4 Summary and discussion 45
4.1 Sediments 45
4.2 Vegetation 46
4.3 Bivalve abundance and distribution 46
4.4 Relationships of bivalve abundance with sediment characteristics and vegetation 47
4.5 Evaluation of habitat mapping and suggestions for improvement of methods 48
5 Conclusions 50
6 Bibliography 51
  Appendix A: Sampling locations 52
  Appendix B: Data 53