Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
The sediments and biota of the Estuary of the Heathcote and Avon Rivers/Ihutai and tidal reaches of the Avon/Ōtākaro and Heathcote/Ōpawaho rivers Summary report on data collected in 2014
R15/131 ISBN 978-0-908316-70-0 (print) 978-0-908316-71-7 (web)
Report prepared by Lesley Bolton-Ritchie
June 2015
Data collected by EOS Ecology, for Christchurch City Council
R15/131 ISBN 978-0-908316-70-0 (print) 978-0-908316-71-7 (web)
PO Box 345 Christchurch 8140 Phone (03) 365 3828 Fax (03) 365 3194
75 Church Street PO Box 550 Timaru 7940 Phone (03) 687 7800 Fax (03) 687 7808
Website: www.ecan.govt.nz Customer Services Phone 0800 324 636
Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Table of contents
1 Introduction ...... 1
2 Sampling ...... 2 2.1 Sampling sites ...... 2 2.2 Collection of samples ...... 3 2.2.1 Sediments ...... 3 2.2.2 Biota ...... 3
3 Results ...... 4
4 References ...... 17
Environment Canterbury Technical Report i Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
ii Environment Canterbury Technical Report Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
1 Introduction
Healthy intertidal estuarine sediment supports a diversity of animals, including anemones, snails, shellfish, worms, crabs and hoppers. The presence and survival of these animals (biota) depends on the quality and salinity of the water1 and the quality and grain size distribution of the sediment. The diversity and abundance of the animals provides food for fish and birds while shellfish, such as cockles, are a valued food item for many people. In addition these sediment-living animals, by their normal actions of feeding and burrowing, keep the sediments well oxygenated and healthy.
The quality of sediment has the potential to be impacted by: • soil that runs off the land and into the rivers and estuary; • excessive amounts of organic matter; • the quality of the overlying water including high nutrient concentrations or low oxygen concentrations; • contaminants such as metals, pesticides and herbicides that enter the rivers and the estuary in stormwater and other legal and illegal discharges.
The features of the sediment that influence the types and abundance of the animals that live on or in it are: • the size of the sediment grains that make up the sediment; • the amount of organic matter in the sediment; • the concentrations of potentially toxic contaminants.
This annual report is a summary of the sediment and biota data for each site sampled in 2014. There are currently no national indicators that can be applied to the data to describe the relative state of the sediments and biota, for example, good health, poor health, very poor health. Rather, information of the present state is provided as sediment grain size distribution and summary data on the animals living in the sediment and the animals and obvious plant species on the surface of the sediment.
1 The water within the estuary is primarily a mix of the freshwater flowing out of the Avon/ Ōtākaro and Heathcote/ Ōpāwaho rivers and the sea water from Pegasus Bay that flows into the estuary as the tide rises. The annual water quality summary reports for 2014 can be found at www.ecan.govt.nz search:Ihutai
Environment Canterbury Technical Report 1 Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
2 Sampling
2.1 Sampling sites The sites sampled in 2014 are shown in Figure 2-1.
Figure 2-1: The sampling sites within the Estuary of the Heathcote and Avon Rivers/Ihutai and in the tidal reaches of the Avon/Ōtākaro and Heathcote/Ōpāwaho rivers
2 Environment Canterbury Technical Report Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
2.2 Collection of samples The methods used follow the national protocol for monitoring estuarine environments (Robertson et al., 2002). The GPS co-ordinates of each site were recorded. All sites were sampled in March 2014. The sites were sampled by EOS Ecology on contract to the Christchurch City Council.
2.2.1 Sediments The top 20 mm of sediment was collected. Five composite sediment samples were analysed for sediment grain size distribution
2.2.2 Biota Plants and animals on the surface of the sediment (epifauna) Fifteen, 50 cm x 50 cm (0.25 m2) quadrats were sampled. The number of each different type of animal on the surface of the sediment and the number of crab burrows in each quadrat was counted and recorded. The percentage of the surface covered by seaweeds and seagrass was determined using a grid overlying the quadrat.
Crab burrows Crab burrows are common in the estuary. Burrows are easily counted but whether a crab lives in the burrow is not known. The type of crab in the burrow is also not known. The two common crabs that burrow in the estuary are the stalk-eyed mud crab (Hemiplax hirtipes) and the mud crab (Austrohelice crassa).
Seaweed cover Green sea lettuce (Ulva sp.) and the red seaweed Gracilaria chilensis occur in the estuary. Where these seaweeds occur they cover the mudflat, so in a quadrat it is the percentage of the mudflat covered by seaweed that is used as a measure of seaweed abundance.
Animals living in the sediment (infauna) Fifteen 130 mm diameter x 150 mm deep cores were sampled. Each sample was sieved through a 0.5 mm screen and the material retained on the screen stored in alcohol for analysis. The number of each different type of animal in each core was counted and recorded.
Size distribution of some common animals The height of all mudflat snails present in the quadrats was measured. The length of all cockles in each quadrat (dug to a depth of 120 mm) was measured.
Environment Canterbury Technical Report 3 Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Mudflat snail (Amphibola crenata) Mudflat snails live on the sediment surface. They feed on the micro-organisms and organic matter in the sediment. As they feed they leave a string-like trail of waste. They lay their eggs into a tyre-like rim of mud. Mudflat snails can tolerate a wide range of salinity and temperature and exposure to air for considerable periods of time.
Cockles (Austrovenus stutchburyi) Cockles live just below the sediment surface. The size classes described in the results are: Recruits 0 – 5 mm Juveniles 5 – 20 mm Reproductive adults >20 mm Edible size >35 mm
3 Results The results are presented by site.
Plover Street
Sediment grain size distribution
Sample % clay % silt % sand
% sand 1 1.2 9.8 89.0
2 3.3 19.2 77.5 % silt 3 2.1 13.6 84.3 % clay 4 2.2 14.2 83.6 5 2.3 13.4 84.3
This graph shows the average percent of each grain size group. There is some variability in the grain size composition between samples. The sediment at this site is muddy (mud=silt+clay) sand.
4 Environment Canterbury Technical Report Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Invertebrate animals present at this site
Scientific name Common name (710 individuals measured) Amphibola crenata mudflat snail COCKLES Arthritica sp. small bivalve 0% recruits
Austrovenus stutchburyi cockle 25% juveniles Cominella glandiformis mudflat whelk snails and Cyclomactra ovata fragile-shell bivalve 75% reproductive adults shellfish Diloma subrostrata black topshell 0.3% edible size Macomona liliana wedge shell Micrelenchus tenebrosus topshell Size distribution of cockles 300 Notoacmea elongata small limpet 37% Potamopyrgus sp. small black snail 250 Aglaophamus macroura Aonides sp. 200 26% Boccardia sp. 21% Capitella capitata 150 Capitellidea Glycera americana 100 12% Hemipodia simplex cockles of Number Heteromastus filiformis 50 4% worms Manayunk ia sp. 0% Nicon aestuariensis 0 5 10 15 20 25 30 35 40 Orbinia papillosa Shell length (mm) Perinereis brevicirris Prionospio auck landica Scolecolepides benhami Scolelepis sp. Scoloplos cylindrifer
Oligochaeta Amphipoda hoppers There were more animal taxa living in the Elminius modestus barnacle sediment at this site than at other sites. Halicarcinus sp. pill-box crab Worms are the most abundant animals with crustacea Helice crassa mud crab the worm Aonides sp, dominating the animal Hemigrapsus crenulatus hairy-handed crab community living in the sediment. Hemiplax hirtipes stalk-eyed mud crab Anthopleura aureoradiata anemone other Nemertea ribbon worm
Summary of the presence and abundance of animals living in/on the sediment (core samples)
number of taxa per core number of individuals per square metre snails and snails and Species Species worms crustacea other worms crustacea other shellfish shellfish evenness diversity Minimum 4 5 1 0 1053 2256 75 0 0.33 0.87 Average 5.7 7.8 2.0 0.7 2741.9 12286 391 70.2 0.57 1.59
Maximum 7 11 4 2 6466 28271 902 301 0.90 2.56
Environment Canterbury Technical Report 5 Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Proportion of each type of animal living in/on the sediment (core samples) blue colours are snails and shellfish red colours are worms grey colours are crustacea green colour is an anemone
Aonides sp. Micrelenchus tenebrosus Scoloplos cylindrifer Capitella capitata cockles Heteromastus filiformis stalk-eyed mud crab Manayunkia sp. mudflat whelk small limpet black topshell Prionospio aucklandica wedge shell small bivalve hoppers Scolecolepides benhami Nicon aestuariensis five worm taxa Anemone pill-box crab Glycera americana ribbon worm hairy-handed crab Orbinia papillosa Perinereis brevicirris Scolelepis sp. two crustacea taxa three snails and shellfish taxa
Summary of the abundance of the animals living on the sediment surface (quadrat samples)
Anemone crab burrows mudflat snail mudflat whelk black topshell topshell small limpet number per square metre minimum 0 0 0 0 48 8 0
average 3 6.7 9.7 6.7 115 66.7 35.3
maximum 20 32 28 24 244 216 84
Size distribution of mudflat snails 40 Sea lettuce Seagrass
30 % cover 97% minimum 0 0
mean 1.6 23.7 20 Maximum 8.2 100 No. quadrats in
Number of individuals 10 w hich present 5 8
3% 0 5 10 15 20 25 30 35 Shell height (mm)
6 Environment Canterbury Technical Report Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Pleasant Point jetty
Sediment grain size distribution
Sample % clay % silt % sand % sand 1 1.0 7.7 91.2 % silt 2 0.9 7.3 91.8
% clay 3 1.8 12.2 86.0 4 0.2 6.8 93.0
5 0.3 6.9 92.9
The graph shows the average percent of each grain size group. There is a small variability in the grain size composition between samples. The sediment at this site is muddy (mud=silt+clay) sand.
Invertebrate animals present at this site COCKLES (48 individuals measured) Scientific name Common name 0% recruits Amphibola crenata mudlfat snail 6% juveniles Arthritica sp. small bivalve Austrovenus stutchburyi cockle snails and 94% reproductive adults Cominella glandiformis mudflat whelk shellfish 4% edible size Cyclomactra ovata fragile-shell bivalve
Notoacmea elongata small limpet
Potamopyrgus sp. small black snail Size distribution of cockles Aglaophamus macroura 25 Aonides sp. 44% Boccardia sp. 20 Capitella capitata Capitellidea 15 worms Manayunk ia sp. Nereis cricognatha 23% 23% Nicon aestuariensis 10
Scolecolepides benhami cockles of Number Scolelepis sp. 5 Scoloplos cylindrifer 4% 4% 2% Amphipoda hoppers 0 Flabellifera lice 5 10 15 20 25 30 35 40 crustacea Shell length (mm) Halicarcinus sp. pill-box crab Hemiplax hirtipes stalk-eyed mud crab other Nemertea ribbon worm Worms are the most abundant animals living in
the sediment at this site. Four worm taxa make
up 75% of the individuals. There are more
snails and shellfish taxa and individuals than
crustacea taxa and individuals
Environment Canterbury Technical Report 7 Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Summary of the presence and abundance of animals living in/on the sediment (core samples)
number of taxa per core number of individuals per square metre
snails and snails and Species Species worms crustacea other worms crustacea other shellfish shellfish evenness diversity
Minimum 1 2 0 0 75 451 0 0 0.48 1.01
Average 2.9 4.9 1.2 0.4 441.1 3012.6 125.3 40.1 0.80 1.74 Maximum 5 9 2 1 1053 10376 526 150 0.98 2.51
Proportion of each type of animal living in the sediment (core samples) blue colours are snails and shellfish red colours are worms grey colours are crustacea
Scolelepis sp. Manayunkia sp.
Capitella capitata
Scolecolepides benhami Nicon aestuariensis small bivalve mudflat snail Potamopyrgus sp. stalk-eyed crab cockles hoppers
Aonides sp.
ribbon worm Scoloplos cylindrifer mudflat whelk three worm taxa pill-box crab fragile-shell bivalve Aglaophamus macroura
lice
small limpet
Summary of the abundance of the animals living on the sediment surface (quadrat samples)
Size distribution of mudflat snails 200
91% crab mudflat black small 150 burrows snail topshell limpet
number of individuals per square metre 100 minimum 0 0 0 0 average 8.3 49.9 1.6 15.5 maximum 24 76 8 40 individuals of Number 50
4% 3% 1% 1% 0 5 10 15 20 25 30 35 Shell height (mm)
8 Environment Canterbury Technical Report Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Avon River/Ōtākaro mouth
Sediment grain size distribution
Sample % clay % silt % sand
1 8.4 68.1 23.5 % sand 2 6.4 48.6 44.9 % silt 3 4.7 36.9 58.4 % clay 4 5.3 40.4 54.3 5 4.7 36.9 58.4
This graph shows the average percent of each grain size group. There is considerable variability in grain size composition between samples. The sediment in two samples is sandy mud (mud = clay
+silt). The sediment in three samples is muddy sand.
Invertebrate animals present at this site
Scientific name Common name Amphibola crenata mudflat snail Arthritica sp. small bivalve snails and Austrovenis stutchburyi cockles shellfish Cyclomactra ovata fragile-shell bivalve Potamopyrgus sp. small black snail Aglaophamus macroura Capitella capitata Heteromastus filiformis worms Manayunk ia sp. Nicon aestuariensis Scolecolepides benhami Amphipoda hoppers Flabellifera lice Munnidae crustacea Halicarcinus sp. pill-box crab Hemigrapsus crenulatus hairy-handed crab Hemiplax hirtipes stalk-eyed mud crab
Summary of the presence and abundance of animals living in/on the sediment (core samples)
number of taxa per core number of individuals per square metre Species Species snails and worms crustacea snails and worms crustacea shellfish shellfish evenness diversity Minimum 2 2 2.9 8873 1203 602 0.23 0.58
Average 2.7 2.3 2 30367 1980 6511 0.53 1.06
Maximum 4 3 6 121808 3008 26918 0.74 1.43
Environment Canterbury Technical Report 9 Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
The difference between the minimum and maximum the number of individuals of snails and shellfish
and crustacea indicates that the abundance of species (notably the small black snail, the small
bivalve and hoppers) is patchy. The high number of these animals in some samples did result in a
low species evenness and low species diversity in these samples.
Proportion of each type of animal living in the sediment (core samples) blue colours are snails and shellfish red colours are worms grey colours are crustacea
small black snail
hoppers small bivalve
Nicon aestuariensis Scolecolepides benhami
stalk-eyed mud crab
lice
hairy-handed crab
soft shell bivalve
pill-box crab
mud snail
4 worm taxa
Munnidae
Summary of the abundance of the animals living on the sediment surface (quadrats)
Crab burrows mudflat snail small black snail There is no graph of the size number of individuals per square metre distribution of mudflat snails Minimum 16 0 0 because there were not enough Average 42.1 3.7 191 snails present in the 0.25 m2 Maximum 64 12 400 quadrats.
Gracilaria (red seaweed) Two cockles were found within % cover the quadrats.
minimum 0
mean 2 Maximum 14.3 No. quadrats in 7 w hich present
10 Environment Canterbury Technical Report Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Discharge point
Sediment grain size distribution
Sample % clay % silt % sand
1 0.9 14.3 84.8 % sand 2 0.7 14.0 85.3
% silt 3 1.9 21.6 76.5 % clay 4 1.0 13.3 85.7
5 0.1 8.0 91.9
This graph shows the average percent of each grain size group. There is some variability in the grain size composition between samples. The sediment at this site is muddy (mud=silt+clay) sand.
Invertebrate animals present at this site
Group Scientific name Common name Amphibola crenata mudflat snail Arthritica sp. small bivalve COCKLES (49 individuals measured) Austrovenus stutchburyi cockle Snails and Cominella glandiformis mudflat whelk 2% – recruits shellfish Cyclomactra ovata fragile-shell bivalve 24% – juveniles Micrelenchus tenebrosus topshell 72% - reproductive adults Notoacmea elongata small limpet 2% - edible size Potamopyrgus sp. small black snail Aonides sp.
Boccardia sp. Size distribution of cockles 25 Capitella capitata
Glycera americana Heteromastus filiformis 20 37% Manayunk ia sp. 35%
worms Nicon aestuariensis 15 Perinereis brevicirris
Prionospio sp. 10 Scolecolepides benhami 16% Scolelepis sp. cockles of Number 5 Scoloplos cylindrifer 6% Oligochaeta 2% 2% 2% 0 Amphipoda hoppers 5 10 15 20 25 30 35 40 Halicarcinus sp. pill-box crab crustacea Shell length (mm) Hemigrapsus crenulatus hairy-handed crab Hemiplax hirtipes stalk-eyed mud crab other Nemertea ribbon worm
The abundance of the small bivalve (the most abundant taxa at this site) is patchy as is the abundance of the worm Scolecolepides benhami. The high number of these animals in some samples did result in a low species evenness and low species diversity in these samples.
Environment Canterbury Technical Report 11 Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Summary of the presence and abundance of animals living in/on the sediment (core samples)
number of taxa per core number of individuals per square metre snails and snails and Species Species worms crustacea other worms crustacea other shellfish shellfish evenness diversity Minimum 1 2 0 0 226 677 0 0 0.47 0.70 Average 2.9 4.5 1.1 0.1 3343.4 2611.6 240.6 5.0 0.63 1.32
Maximum 4 8 3 1 8647 4511 677 75 0.81 1.87
Proportion of each type of animal living in the sediment (core samples) blue colours are snails and shellfish red colours are worms grey colours are crustacea
small bivalve Scolecolepides benhami Capitella capitata Nicon aestuariensis stalk-eyed mud crab mudflat snail six worm taxa cockles Scoloplos cylindrifer Boccardia sp. mudflat whelk Micrelenchus tenebrosus Manayunkia sp. two mollusc taxa hoppers Potamopyrgus sp. pill-box crab Aonides sp. hairy-handed crab ribbon worm
Summary of the abundance of the animals living on the sediment surface (quadrat samples)
Size distribution of mudflat snails 200 crab mudflat mudflat topshell small limpet burrows snail whelk 56% number of individuals per square metre 150
minimum 0 40 0 0 0 average 15.5 75.7 1.9 7.2 14.9 maximum 56 148 12 36 44 100 28%
Number of individuals 50 12% 4% 0% 0 5 10 15 20 25 30 35 Shell height (mm)
12 Environment Canterbury Technical Report Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Humphreys Drive
Sediment grain size distribution
Sample % clay % silt % sand % sand 1 2.4 26.3 71.2 % silt 2 3.3 31.9 64.8
% clay 3 3.0 29.1 67.9
4 2.5 28.2 69.2 5 3.4 32.8 63.7
This graph shows the average percent of each grain size group. There is some variability in the grain size composition between samples. The sediment at this site is muddy (mud=silt+clay) sand.
Invertebrate animals present at this site
Scientific name Common name Amphibola crenata mudflat snail snails and Arthritica sp. small bivalve shellfish Cyclomactra ovata fragile-shell bivalve Potamopyrgus sp. small black snail Aonides sp. Capitella capitata Manayunk ia sp. Nicon aestuariensis worms Perinereis brevicirris Scolecolepides benhami Scolelepis sp. Oligochaeta Amphipoda hoppers crustacea Helice crassa mud crab Hemiplax hirtipes stalk-eyed mud crab other Nemertea ribbon worm
Summary of the presence and abundance of animals living in the sediment (core samples)
number of taxa per core number of individuals per square metre
snails and snails and Species Species worms crustacea other worms crustacea other shellfish shellfish evenness diversity
Minimum 1 2 1 0 226 376 75 0 0.18 0.36 Average 2 3.2 1.7 0.1 2596.6 1564 2551.4 10 0.69 1.33
Maximum 3 5 3 1 7820 2707 34512 75 0.95 1.82
Environment Canterbury Technical Report 13 Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
The difference between the minimum and maximum the number of individuals of snails and shellfish and crustacea indicates that the abundance of two taxa (notably, the small bivalve and hoppers) is patchy. The high number of these animals in some samples did result in a low species evenness and low species diversity in these samples.
Proportion of each type of animal living in/on the sediment (core samples) blue colours are snails and shellfish red colours are worms grey colours are crustacea
hoppers small bivalve Scolecolepides benhami Nicon aestuariensis Potamopyrgus sp. stalk-eyed crab Capitella capitata mudflat snail mud crab Scolelepis sp. Manayunkia sp. three worm taxa ribbon worm soft-shell bivalve
Summary of the abundance of the animals living on the sediment surface (quadrat samples)
Size distribution of mudflat snails 35
30 44% crab burrows mudflat snail 25 Number per square metre 31% minimum 0 0 20 average 89.9 17.1 15 maximum 412 52 16% 10
Number of individuals
5 6% 3% 0 5 10 15 20 25 30 35 Shell height (mm)
14 Environment Canterbury Technical Report Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
Heathcote River/ mouth
Sediment grain size distribution
Sample % clay % silt % sand % sand 1 8.0 52.5 39.5
2 4.5 30.5 65.0 % Silt 3 3.0 23.3 73.7 % clay 4 3.7 25.6 70.7
5 4.4 30.9 64.7
This graph shows the average percent of each grain size group. There is considerable variability in grain size composition between samples. The sediment in one sample is sandy mud (mud = clay +silt). The sediment in four samples is muddy sand.
Invertebrate animals present at this site
Group Scientific name Common name Amphibola crenata mudfalt snail snails and Arthritica sp. small bivalve shellfish Austrovenus stutchburyi cockle Potamopyrgus sp. small black snail Aonides sp. Capitella capitata Capitellidea Manayunk ia sp. worms Nicon aestuariensis Scolecolepides benhami Scolelepis sp. Oligochaeta Amphipoda hoppers crustacea Helice crassa mud crab Edwardsia leucomelos anemone other Dolichopodidae fly larva Nemertea ribbon worm
Summary of the presence and abundance of animals living in/on the sediment (core samples)
number of taxa per core number of individuals per square metre Species Species snails and worms crustacea other snails and worms crustacea other shellfish shellfish evenness diversity
Minimum 0 1 0 0 0 150 0 0 0 0
Average 1.3 2.7 1.5 0.2 190 682 647 20 0.85 1.36 Maximum 677 2707 4737 150 4 7 2 1 0.98 2.03
Environment Canterbury Technical Report 15 Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
There were a low number of individuals living in the sediment at this site compared to other sites. In one sample there was only one taxa present which resulted in a 0 score for species evenness and diversity. Even though there were a low number of individuals present, the individuals present in any one core were from a number of taxa which reflects in the high average and maximum species evenness and the high maximum species diversity values. The pie graph below shows the spread of individuals across the taxa.
Proportion of each type of animal living in the sediment (core samples) blue colours are snails and shellfish red colours are worms grey colours are crustacean green colour is an anemone
hoppers Oligochaeta mud crab Capitella capitata Nicon aestuariensis small bivalve Scolecolepides benhami Potamopyrgus sp. mudflat snail Manayunkia sp. Capitellidea Scolelepis sp. Aonides sp. anemone cockles fly larva ribbon worm
Summary of the abundance of the animals living on the sediment surface (quadrat samples)
Size distribution of mudflat snails 80 Crab burrows mudflat snail number of individuals per square 60 metre 42% Minimum 12 0 35% Average 141 35 40 Maximum 304 96
Number of individuals 20 11% 6% 5% 1% 0 5 10 15 20 25 30 35 Shell height (mm)
16 Environment Canterbury Technical Report Healthy Estuary and Rivers of the City Water quality and ecosystem health monitoring programme of Ihutai
4 References
Robertson, B., Gillespie, P., Asher, R., Frisk, S., Keeley, N., Hopkins, G., Thompson, S and Tuckey, B. 2002. Estuarine environmental assessment and monitoring: A national protocol. Part A. Development, Part B. Appendices, and Part C. Application. Prepared for supporting Councils and the Ministry for the Environment, Sustainable Management Fund Contract No 5096. Part A. 93p. Part B 159p. Part C. 40p plus field sheets.
Environment Canterbury Technical Report 17