Kākahi Monitoring Report 2019

Käkahi Monitoring Report Käkahi monitoring for the Wairarapa Moana Wetlands Project August 2019

www.waiwetlands.org.nz

Introduction Nau mai haere mai, welcome to the fifth annual report for the Wairarapa Moana käkahi monitoring programme. This year was our third survey at the Lake Domain Reserve site (on the northern shore). The other two surveys were completed at the Lake Shore Scenic Reserve site (on the western shore).

This programme is the first of its type in New Zealand. It gives an important insight into the development and ecological impacts on käkahi - our threatened native freshwater mussels.

Zone markers for collections

Approximate survey zones Monitoring site Lake Wairarapa

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Methods The 2019 monitoring took place on 10 February 2019, and involved a group of 31 community volunteers of all ages. A 400 metre stretch of shoreline was sectioned off into 50 metre intervals to create 8 replicate survey zones. After we recorded our measurements, the käkahi were returned to the zone from which they were collected. Weather conditions were good, with clear skies, warm water, and moderate winds.

Looking for käkahi If you have yet to meet one of these special native critters, the warm water temperatures of summer are the perfect time for a käkahi hunt. Wander out in the shallows without your shoes on at Lake Domain or Wairarapa Lake Shore Scenic Reserve and feel around on the bottom with your hands and feet - you may just find some käkahi! Be careful when you hold them (they’re delicate), and be sure to place them safely back in the water once you’ve had a look.

Collecting the käkahi

Scoring shell erosion

Each of the 8 replicate zones was surveyed by a team of 3-5 people. In order to standardise collection effort, each team estimated their effort in terms of adult equivalents (for example, 2 young children working as collectors would be counted as 1 adult). Other team members worked as timekeepers. A maximum of 50 käkahi were collected from each replicate zone to minimise disturbance. Each group of collectors waded, feeling through the substrate (of mud, sand and gravel) for käkahi with their feet and hands.

Each käkahi collected was also scored according to the amount of erosion present on the shell. If no or very little erosion was present on the shell then it was scored as ‘one’. If most of the top layer was eroded it was scored as ‘four’ (with intermediate scores of two and three). Recording shell erosion was a simple addition to the other parameters we collected and has the potential to provide information in the future regarding environmental changes such as wave action, substrate composition and water chemistry, as well as highlight any differences between the species.

Measuring the käkahi population density

The monitoring programme

Käkahi were collected either for 30 minutes or until 50 animals had been collected, whichever occurred first. This allowed us to standardise our results per unit time1. If 50 käkahi were collected in less than 30 minutes, we recorded the amount of time it took to collect them. For example, if it took 15 minutes to collect 50, then it was assumed that 200 would be collected in an hour.

Dividing this by two adult equivalent collectors would give an abundance measure of 100 käkahi per person, per hour.

The site will be surveyed in a similar fashion every two years, alternating with the western lake shore site at Lake Shore Scenic Reserve. Our methods can accommodate around 30 people (of all ages!). Each year we get new people joining in, as well as repeat attendance from people who have been with the programme from the beginning and worked on most if not all surveys. It’s a great mix, as the latter uses their experience to help the former.

Recording species type Käkahi were identified by species, as either the ‘common’ käkahi (Echyridella menziesii) or the ‘Auckland’ käkahi (E. aucklandica).

As well as allowing us to keep track of both species separately, this will allow us to monitor species ratios within the population.

Measuring shell length Shell lengths were measured to the nearest millimetre using Vernier callipers. The size of käkahi is related to their age, although the relationship between shell length and age varies with location and species and further work is needed to quantify the relationship between shell length and age at this site. We can however, assume that similar sized käkahi at the same site are ageing at similar rates, so by tracking size, we can track the rate at which the population is ageing.

From left to right: ‘common’ käkahi (Echyridella menziesii), ‘Auckland’ käkahi (E. aucklandica).

Ecological data is commonly standardised per unit area in order to be compared to future data and/or data from other areas. Käkahi in lake Wairarapa are too sparse and patchily distributed for quadrats (for example) to return useful data, and large areas would be needed in order to collect enough käkahi to draw valid conclusions. Because achieving complete coverage of large areas would necessitate spending long periods of time in cold water, this option presents a health and safety issue. For these reasons the use of time as a quantifying unit was considered the most suitable option (this method is also used elsewhere for käkahi surveying).

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The three musketeers There are three species of käkahi in New Zealand.

Echyridella menziesii is distributed throughout New Zealand, E. aucklandica has been found in Auckland, Northland, the lower North Island and the very bottom of the South Island. E. onekaka is known only from sites in the north-west of the South Island. E. menziesii and E. aucklandica are found in Lake Wairarapa. All three species are only found in New Zealand and all are classified as either “Threatened” or “Declining”.

Results – the 2019 survey Käkahi abundance and species composition A total of 343 käkahi were collected during the count (Table 1). Of these, 328 (96%) were the ‘common’ käkahi and 15 (4%) were the ‘Auckland’ species. Recorded abundances ranged from 16 per person per hour, (in zone 2), to 245 per person per hour (in zone 7), with an overall average of 129 ± 332 per person per hour.

Table 1. Numbers of käkahi collected from the northern shore monitoring site in 2019.

Replicate zone

No. of ‘common’ found

No. of ‘Auckland’ found

Collection time (minutes)

Number of adult equivalent collectors

Total Käkahi (number of käkahi collected per person, per hour)

1 2 3 4

44 24 23 45

1 1 0 5

25 30 3 20

2 3 2 4

53 16 230 34

5

46

4

10

2

138

6 7 8

47 49 50

3 1 0

9 6 7

3 2 2

105 245 214

Totals:

328

15

110

20

–

Shell length As in previous years, the size distribution of käkahi collected during this survey was strongly unimodal, with few small animals.

Therefore, we can assume that the age distribution is similarly unimodal, with very few juveniles present. Overall, a very small range of shell lengths was recorded, with most käkahi falling within a narrow range representing older adults (Table 2, Fig. 1). The ‘common’ käkahi ranged in length from 37–79 mm, and the ‘Auckland’ species from 56–91 mm. Average lengths were 61 ± 0.3 mm and 67 ± 2.4 mm respectively. Nearly all (96%) of the ‘common käkahi’ found were between 50 and 70 mm, whereas most (93%) of the ‘Auckland’ käkahi were longer—between 60 and 80 mm. One käkahi which fit the description of ‘juvenile’ (i.e. < 38 mm3) was collected (a 37 mm E. menziesii).

Table 2. Shell length of käkahi collected from the northern shore monitoring site in 2019.

Size class (mm)

Number of käkahi ‘common’

‘Auckland’

0-10

0

0

11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 91-100

0 0 2 7 136 178 5 0 0

0 0 0 0 3 7 4 0 1

Total:

328

15

Older than you?

Käkahi live for quite a long time, some are thought to be over 50 years old.

Mean ± 1 SE

2

Shell length less than 38 mm (or less than approximately 5 years old) has been used in the past to represent juvenile käkahi e.g. James MR (1985).

Distribution, biomass, and production of the fresh-water mussel, Hyridella-menziesi (Gray) in Lake Taupo, New Zealand. Freshwater Biology 15: 307–314.

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Frequency

Frequency

Figure 2. Histograms showing length distributions of (left) ‘common’, and (right) ‘Auckland’ käkahi surveyed during the Wairarapa Moana 2019 käkahi count.

Length class (mm)

Length class (mm)

Shell erosion A range of erosion conditions was recorded from the shells of käkahi collected, with ‘common’ käkahi shells being generally more eroded (Fig. 2). For this species, 58 scored I (18%; least erosion), and 29 scored 4 (9%; most erosion), with all the rest (73%) scoring either 2 or 3 (intermediate erosion). Käkahi shells from the ‘Auckland’ species, on the other hand, all scored either 1, 2 or 3, with the majority (53%) scoring 1.

Frequency

Frequency

Figure 3. Histograms showing erosion condition of (left) ‘common’, and (right) ‘Auckland’ käkahi surveyed at the northern shore site in 2019.

Erosion score

Erosion score

Comparisons with previous years Käkahi abundance and species composition Five replicate zones were searched during 2015, 10 during 2017, and 8 in 2019. Higher käkahi abundances were recorded in 2017, compared with 2015 and 2019 (Table 3), which is likely attributable to the exceptionally clear water conditions experienced during the 2017 survey, which made collection generally faster. The lower abundance during 2015 is also likely due to the fact that only 5 replicates were searched, and sites 1–5 have lower amounts of käkahi present that sites 6–10 (closer to the Tauherenikau delta).

Overall, the observed abundance variation is in line with what would be expected given variable lake conditions.

Table 3. Käkahi abundance at the northern shore monitoring site during monitoring in 2015–2019.

Käkahi density (number of käkahi collected per person, per hour) Replicate zone

2015

2017

2019

E. menziesii

E. aucklandica

E. menziesii

E. aucklandica

E. menziesii

E. aucklandica

1

33

0.05

19

0.03

53

0.02

2

32

0.05

39

0.07

16

0.01

3

21

0.12

86

0.13

230

0.00

4

86

0.23

147

0.05

34

0.06

5

111

0.08

470

0.50

138

0.20

6

–

–

157

0.17

104

0.11

7

–

–

80

0.06

245

0.08

8

–

–

250

0.00

215

0.00

9

–

–

500

0.00

–

–

10

–

–

59

0.11

–

–

Average

57

0.11

181

0.11

129

0.06

– Zone not searched

E. aucklandica comprised 8% of käkahi present in 2015, 5% in 2017, and 4% in 2019 (Fig. 3). While this could be interpreted as a relative decline, overall collection numbers of this species are low, and the differences between years are not statistically significant4.

Percentage of total kakahi collected

Figure 4. Species composition of käkahi collected at the northern shore monitoring site during 2015–2019.

E.menziesii E.aucklandica

F (5, 10, 8) = 0.53, P = 0.60

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Shell length â&#x20AC;&#x2DC;Commonâ&#x20AC;&#x2122; käkahi shell length was very uniform and the population average increased by 1mm during the 2015-2019 period (Table 4; Fig. 4) â&#x20AC;&#x201C; a difference which was statistically significant5. â&#x20AC;&#x2DC;Aucklandâ&#x20AC;&#x2122; käkahi shell length was more variable and showed no significant trend6.

Table 4. Käkahi length at the northern shore monitoring site during monitoring in 2015â&#x20AC;&#x201C;2017.

â&#x20AC;&#x2DC;commonâ&#x20AC;&#x2122; käkahi

â&#x20AC;&#x2DC;Aucklandâ&#x20AC;&#x2122; käkahi

2015

2017

2019

2015

2017

2019

Mean length (mm)

60 Âą 0.3

60 Âą 0.2

61 Âą 0.3

68 Âą 1.9

70 Âą 1.7

67 Âą 2.4

Length range (mm)

37â&#x20AC;&#x201C;70

40â&#x20AC;&#x201C;84

37-79

57â&#x20AC;&#x201C;80

48â&#x20AC;&#x201C;84

56â&#x20AC;&#x201C;91

Mean shell length (mm)

Mean shell length (mm)

Figure 5. Mean shell length of (left) common and (right) Auckland käkahi collected at the northern shore monitoring site during 2015â&#x20AC;&#x201C;2019.

Shell erosion Representation across the shell erosion classes was similar and within expected levels of variation for each species over 2015-2019 (Table 5; Fig. 6).

â&#x20AC;&#x2DC;commonâ&#x20AC;&#x2122; käkahi

â&#x20AC;&#x2DC;Aucklandâ&#x20AC;&#x2122; käkahi

Percentage in erosion class:

2015

2017

2019

2015

2017

2019

1

36

15

18

80

54

53

2

36

40

43

13

46

33

3

22

31

30

7

0

13

4

6

14

9

0

0

0

Percentage of kakahi collected

Percentage of kakahi collected

Figure 6. Representativeness in shell erosion classes for (left) â&#x20AC;&#x2DC;commonâ&#x20AC;&#x2122; and (right) â&#x20AC;&#x2DC;Aucklandâ&#x20AC;&#x2122; käkahi collected at the northern shore monitoring site during 2015â&#x20AC;&#x201C;2019.

F (177, 441, 328) =6.04, P = 0.002

5

6

F (15, 23, 15) = 0.35, P = 0.71

Conclusion As in previous years, the size of the käkahi collected at the northern shore site in 2019 indicated that the population is comprised mostly of adults. E. menziesii is much more common than E. aucklandica, with the former typically comprising +90% of käkahi present. E. aucklandica grows longer than E. menziesii, and typically has lower levels of shell erosion.

No differences were evident between monitoring years in terms of shell erosion, but mean shell length measurements over 2015–2019 indicate that the population average age is increasing i.e. insufficient recruitment is occurring to sustain the population. The same trend was indicated at the western shore site over 2016-2018. However, given the natural variation inherent in population data, it is necessary for multiple years’ worth of data to be collected before a clear trend can be definitively identified. The next monitoring event for the northern shore site will be in 2021.

A special lifecycle Marine mussels release eggs and sperm for external fertilisation in the water, but a mother käkahi keeps her eggs inside her shell to develop. She takes in sperm through her siphon and her eggs are then fertilised internally and kept in her brood chamber until they develop into larvae called glochidia.

Meet the scientist: Amber McEwan is an ecologist and the principal scientist at Riverscapes Freshwater Ecology (www.riverscapes.co.nz). She has many years of experience working with (and admiring!) käkahi, and is currently researching ways to successfully translocate them as one means of saving populations.

Amber is also an enthusiastic science communicator and freshwater advocate. She has written five children’s books, designed a card game, and has written many articles about freshwater issues published in various media. She also manages Riverscapes Kids which offers educational resources that are free to download (www.riverscapeskids.co.nz).

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S W E TLAND

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RAPA

www.waiwetlands.org.nz

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biodiversity@gw.govt.nz

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If you'd like to get involved in future surveys please get in touch:

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