65202 Appendix Three The Cawthron Omakiwi Cove suction dredging trial Interim progress report for Phase II.pdf

1. Introduction
Exotic Caulerpa (Caulerpa brachypus and Caulerpa parvifolia) was identified in Te Rāwhiti Inlet,
Bay of Islands, in May 2023. An incursion response for Northland was initiated, the focus of
which was development of methods and technologies to remove exotic Caulerpa biomass.
Omakiwi Cove was selected as the study site for this trial as dense meadows of exotic Caulerpa
are present there. A mechanical suction dredge was developed by Johnson Bros Ltd. (JBL) and
deployed in Omakiwi Cove in February 2024, where it remained until April 2024. During this time
(‘Phase I’) the dredging equipment was tested, and repairs and improvements were made.
Concurrently, scientific research was undertaken to document and evaluate the efficacy and
suitability of suction dredging as a removal technology. All dredging and monitoring activities
undertaken for Phase I are reported fully in Davidson et al. (2024).
By the end of Phase I (9 April) almost 2,000 m2 had been dredged and high levels of removal from
treated areas had been achieved. However, hurdles to suction dredging as an effective removal
technology remained; long stand down periods were required to de-water and securely dispose
of all dredged material and, despite significant rates of removal in dredged areas, some exotic
Caulerpa remained. Intentions for the next phase of the trial (‘Phase II’) include installation and
assessment of a system to separate dredged material, and identification of an effective and
efficient method for removing remnant exotic Caulerpa. Scientific monitoring will also continue,
to support and monitor dredge activity.
The intention of this document is to provide a brief interim report of work undertaken to date as
part of Phase II of the suction dredge trial (29 April to 16 June), and to note some initial
observations and considerations that have arisen during this time. It is not the intention of this
document to discuss results or findings of Phase II; these will be detailed in a full report written
upon completion of Phase II.

2. Mechanical suction dredging
2.1. Modifications made for, and during, Phase II
Installation of trommels
The barge underwent a full biosecurity inspection following completion of Phase I (9 April), then
returned to Ōpua Marina where a trommel system was installed onto the deck of the barge (Fig.
1A). This system separates dredged material, enabling sediment to be returned to the seafloor
(Fig. 1B) and exotic Caulerpa to be retained onboard (Fig. 1C). This modification negates the
time-consuming requirement to collect and dispose of all dredged material which, during Phase
2

I, necessitated long stand-down periods while the collection bags de-watered and the barge
was grounded to offload dredged material. This work was completed by 26 April and the barge
relocated back to Omakiwi Cove on 29 April. Upon relocation the new system was tested and
refined (e.g., altering water pressure and angle of spray bars) and was found to successfully
separate fine sediment (silt, mud, sand) from larger material (exotic Caulerpa, shell fragments).
A secondary vessel was employed to transport one-tonne bags of dredged material ashore,
while the primary barge was able to continue dredging.

(A)
(B)
(C)

Figure 1. A new trommel system was installed prior to commencing Phase II of the suction dredging trial. (A) The
trommels separate fine sediment from exotic Caulerpa. (B) Sediment is returned to the seafloor. (C) Exotic Caulerpa is
retained onboard in dewatering bags. Photos: NRC.

3

Deposition of dredge spoil
The separation of dredged material requires sediment to be returned to the seafloor via ‘stinger
pipes’. At shallow depths these pipes fold, preventing sediment deposition. A mechanical
method of adjusting the pipe length was suggested by JBL, in addition to integrating a
positioning system to enable precise deposition of dredge spoil into previously dredged areas.
Modifications of this system are ongoing.

Installation of moorings
Dredge activity in Phase II was impacted by adverse weather conditions. On several occasions
the barge was unable to remain in Omakiwi Cove, which necessitated moving it to a secure
mooring in Waipiro Bay (south of Omakiwi, within the Controlled Area Notice zone). Moving the
barge out of Omakiwi required a biosecurity inspection of the hull by NRC divers (Fig. 2A, B) and
full decontamination of all dredging equipment (Fig. 2C) to ensure adherence with biosecurity
conditions and procedures (see Davidson et al. (2024) Appendix 1). Hull inspections revealed
exotic Caulerpa fragments entangled in biofouling, which were removed by hand before the
vessel was given clearance to move. This time-consuming process will be circumvented by
application for, and subsequent installation of, two temporary moorings in Omakiwi. The barge
will then be able to remain safely in Omakiwi during periods of adverse weather.

(A)
(B)
(C)

Figure 2. Inspection and decontamination procedures required to enable the barge to move from Omakiwi Cove in the
event of bad weather. (A) A diver inspects the hull of the barge. (B) Exotic Caulerpa fragments were found entangled in
biofouling on the hull. (C) All dredge equipment was decontaminated prior to moving the barge. Photos: NRC.

Equipment repairs
Toward the end of Phase I, a 700mm length of reinforcing bar was dredged up and caused
damage to the dredge head and pump (9 April). The dredging equipment was still operational,
albeit at a reduced capacity. Divers from NRC undertook several search dives, using a metal
detector, to locate and remove any remaining rebar. Replacement brushes and shafts were later
fitted onto the dredge head and subsequent testing of the new equipment was undertaken (22 -
26 May).
4

Data reporting
During Phase I, GPS data was not readily available to reliably map or measure the area targeted
by the suction dredge. Instead, dredged areas were mapped by divers using surface markers to
signal the location of dredged patches. During Phase II, JBL were able to provide daily GeoTIFF
files taken from the dredge. The GeoTIFF format embeds geospatial metadata into image files so
that they can be used in GIS applications such as the Caulerpa Infestation Dashboard. The
GeoTIFF files are processed in ArcGis so the area dredged can be calculated and displayed.

2.2. Phase II dredging activity
Dredging
During the initial part of Phase II (29 April to 16 June), 21 days of dredging were undertaken,
resulting in a total area dredged of 11,856 m2 and 226 one-tonne bags offloaded (Fig. 3; Table 1).
Dredging activity during this time can be divided into three distinct phases. From 29 April to 2
May, the new trommel system was tested. From 3 May to 20 May the dredge operated in Zone
DRG with a primary focus of connecting and expanding the test patches conducted during
Phase I (Fig. 3). This style of dredging somewhat constrained dredge operations as frequent
lifting and repositioning of the dredge head was required to work around the areas already
dredged. From 27 May onwards, the dredge was fully operational in Zone B. During this time,
using the days for which data is available (11 days; Table 1), the average rate of removal was
1,013 m2 per day, or 154.85 m2 per hour.

Offloading
The offloading procedure in Phase II was made significantly more efficient via the use of a
supplementary barge to ferry one-tonne bags ashore, meaning the primary dredging barge was
able to continue working. The supplementary barge was grounded at high water and, once the
tide receded, the one-tonne bags were craned off and transported to the on-land disposal site
on small tipper trucks. This process took approximately two hours each time and was carried
out on 10 May (50 bags), 1 June (68 bags), and 11 June (58 bags). The disposal site was
inspected by NRC compliance officers (6 May) and was found to be in good condition.

Stand-downs
The barge is unable to operate in wind above 30-35 knots from the west, southwest, or
northwest. Consequently, several stand-down days / periods of days have been required during
Phase II as a result of adverse weather conditions (Table 1). In some instances, the barge had to
be re-located offsite to shelter and wait for the weather to pass, resulting in multiple days of no
dredging.
5

Figure 3. Area dredged by the mechanical suction dredge (indicated in grey) during the initial part of Phase II (29 April to 13 June). Dredging activity in Zone B is coloured according to each
days’ dredging, with the darkest shade of grey indicating the earliest dredging activity during that day and the lightest shade indicating later dredging. The different shades in Zone DRG
represent different days of dredging.
6

Table 1. Mechanical suction dredging undertaken during the initial part of Phase II (29 April to 16 June). Dredging activity
is divided between two distinct phases; connecting test patches in Zone DGR, and full operational dredging in Zone B.
Days on which no dredging occurred are omitted. Note – days that NRC did not receive an update from JBL have been
left blank.

Date
Area dredged (m2)
Quantity dredged (bags) [1]
Time dredged (h:m)
Dredge rate (m2/hour)
Zone DGR
3 May

4 May

16
2:00

5 May
No dredging - Sunday
6 May

7 May

8 May
705
30
6:30
108
9 May

9
6:10

10 May

15
5:50

11 May

7:00

12 May
No dredging - Sunday
13 May

18
7:15

14 May

21
7:15

15 May
No dredging - weather
16 May
No dredging - weather
17 May
No dredging - weather
18 May
No dredging - weekend
19 May
No dredging - weekend
20 May

21 May
No dredging - hull inspection prior to relocating for weather
22 May
No dredging - weather, maintenance
23 May
No dredging - maintenance
24 May
No dredging - maintenance
25 May
No dredging - maintenance
26 May
Not fully operational - testing new shafts and brushes
Zone B
27 May
600

7:00
86
28 May
1,110
14
8:50
126
29 May
No dredging - weather
30 May

31 May

1 June
775
7
4:40
166
2 June
No dredging - Sunday
3 June

4 June
1,360
7
6:40
204
5 June
1,350
8
5:25
249
6 June
1,046
9
6:30
161
7 June
1,240
11
9:30
131
8 June
No dredging
9 June
No dredging
10 June
1,300
21
6:00
217
11 June
1,302
18
4:24
296
12 June
792
11
7:50
101
13 June
276
11
4:40
57
14 June
No dredging

15 June
No dredging

16 June
No dredging

[1] The weight of one full bag was found to be 1.4 tonne, however this does not necessarily equate to 1.4 tonnes of exotic
Caulerpa. Other dredged material is present within the bags (e.g. large shell fragments), in addition to water weight.
7

(A)
(B)
(C)
(D)

Figure 4. Area dredged by the mechanical suction dredge over three days during the initial part of Phase II. (A) 27 May. (B)
28 May. (C) 31 May. (D) Three days combined; 27 to 31 May.

3. Diver-operated suction dredging
During Phase I, remnant exotic Caulerpa was observed in dredged areas. This resulted from the
bulldozing effect of the dredge head creating mounds of exotic Caulerpa on the edges of
dredged areas, and patches of unattached exotic Caulerpa moving into dredged areas. This was
identified as a significant challenge to be addressed during Phase II (Davidson et al. 2024).
During Phase II, Commercial Dive Specialists (CDS) were contracted to undertake diver-
operated suction dredging to support the mechanical suction dredge operations by targeting
patches of exotic Caulerpa that had either been missed by the mechanical dredge or had moved
into the dredged area afterwards (Fig. 5A). The diver-operated suction dredge feeds exotic
Caulerpa into a filter cage on the surface (Fig. 5B). Large material (exotic Caulerpa, shell
fragments) is retained in the cage, while smaller material (sand, silt) filters out and returns to the
seabed. When the cage is filled to a quarter of its capacity it is taken ashore and the material is
disposed on land.

8

(A)
(B)

Figure 5. Diver-operated suction dredging was trialled during Phase II to address patches of remnant exotic Caulerpa. (A)
A drone photo illustrates patches of exotic Caulerpa left within a dredged area. (B) The filter cage used by Commercial
Dive Specialists Ltd. Photos: MES and CDS.

As of June 16, approximately 775.1 m2 had been dredged via diver-operated suction dredging,
with approximately 420 kg of dredged material removed (values exclude unknown metrics from
11 June; Table 2). Diver operations were able to cover, on average, 155.02 m2 per day, or 33.05
m2 per hour, however this was highly variable. The operation rate was influenced by the density
of exotic Caulerpa; in areas where it is sparse, a larger area was able to be covered, however
divers frequently encountered dense patches which slowed the dredging operations. On one
occasion the pump mechanism was blocked entirely by a large patch. Subsequently, any
patches encountered that were considered too large for the diver-operated dredge were marked
for removal at a later date, either by the mechanical dredge or benthic matting.

Table 2. Diver-operated suction dredging activity undertaken during the initial part of Phase I .

Date
Area dredged (m2)
Approx. quantity
Time dredged
Dredge rate
Dredge rate
dredged (kg)
(h:m)
(kg/hour)
(m2/hour)
10 June
Testing pump, not fully operational
11 June
Unknown
Unknown
4:40
Unknown
Unknown
12 June
50.6
120
4:49
24.92
10.51
13 June
110
30
5:09
5.83
21.36
14 June
51.5
120
3:09
38.1
16.35
15 June
196
75
5:00
15
39.2
16 June
367
75
4:43
15.89
77.82

9

Figure 6. Area dredged by the diver-operated suction dredge (indicated in blue) during the initial part of Phase I (29 April
to 16 June). In Zone DRG, the black and dark grey shaded areas indicate dredging activity undertaken during Phase I, and
the white shaded areas indicate dredging undertaken in Phase II. In Zone B, dredging activity is coloured according to
each days’ dredging, with the darkest shade of grey indicating the earliest dredging activity and the lightest shade
indicating later dredging activity.

4. Monitoring
4.1. Turbidity

Following the modification of the dredging process there was potential for increased turbidity
resulting from the return of sediment to the seafloor instead of collecting and disposing it on
land. Compliance Officers from NRC conducted a site visit on 6 May to assess adherence of the
new dredging system to conditions of the permit issued to discharge dredge spoil to the seabed.
The evaluation revealed full compliance; a sediment plume was discernible only <20 m from the
dredge operation, and monitoring by Secchi disk at 150 m from the dredge operation was also
compliant. A later turbidity survey undertaken by Maxwell Ecological Services Ltd. on 6 June
indicated a more significant sediment plume than had been observed previously (Fig. 7),
however this was still within the 300 m compliance radius (Fig. 7 insert). It is thought the
increased turbidity resulted from problems with the stinger pipes that deliver sediment back to
the seabed. Modifications are ongoing. See Davidson et al. (2024) for details of resource
consent conditions and compliance monitoring methods.
10

Figure 7. Sediment plume observed on 6 June. Insert: Turbidity monitoring results. Photo: MES.

4.2. Dredged material

During initial testing of the new trommel system, dredged material was sampled by Cawthron (3
May). The larger material collected by the trommel was found to consist of exotic Caulerpa and
broken shell fragments. The material destined to be returned to the seafloor was also sampled
to determine presence of viable exotic Caulerpa. A 500 L sample of this material was sieved to
1 mm and found to consist mostly of fine sediments (silt, sand). Eight small (<5 mm) exotic
Caulerpa fragments were found within this sieved material.

4.3. Percent cover of exotic Caulerpa

Divers from Cawthron conducted pre- and post-dredge monitoring within defined zones where
dredging operations were scheduled to be undertaken. This work commenced on 30 April with a
karakia, with the first phase of diving completed on 17 May 2024. Surveys were conducted in
10 x 2 m plots randomly distributed across six zones. In total, 56 pre-dredge plots and 17 post-
dredge plots were completed during the first phase of dive assessments. Within plots, census-
style (whole-plot) surveys measuring exotic Caulerpa cover were completed. Divers swum the
full length of the 10 m transect recording data for each 1 m2 grid cell either side of the transect
(i.e. 20 grid cells per plot). Quadrats (0.5 m x 0.5 m) were also used as a finer-scale assessment
of exotic Caulerpa cover within treatment plots. Five quadrats were haphazardly placed within
each 10 x 2 m plot (n = 365) and were assessed for percentage cover of surface (attached) and
free-floating exotic Caulerpa. Perspex cores (6 cm) were also used to gauge the depth of exotic
Caulerpa stolons and make substrate observations. See Davidson et al. (2024) for full
methodology.
11

4.4. Ecological monitoring

Divers collected epibiota and infauna data from within the 10 x 2 m transect plots discussed
above. For each quadrat, the presence of native seaweeds and epifauna abundance and
diversity were recorded. These data were recorded at a broad taxonomic level of functional
groups (crabs, other crustaceans, anemones, echinoderms, bivalves, other molluscs, sponges,
and polychaetes). Divers also collected five replicate 10 cm diameter cores (to a depth of 10
cm) from within each plot (n = 352). Core samples were sieved to 1 mm and infauna abundance
and diversity at a broad taxonomic level recorded. These methods provided an insight into
ecological communities present prior to dredging and were repeated after dredging had
occurred to quantify any ecological impacts on non-target species. See Davidson et al. (2024)
for full methodology.

4.5. Delimitation surveys

A wider surveillance project was undertaken between 15 April and 16 May 2024 to monitor the
spatial extent of exotic Caulerpa beyond Omakiwi Cove. Twenty-five harbours and bays and 19
islands/island groups within the Northland region were surveyed by CDS, following a
standardised search method. An infestation was found in one location; the south side of
Poroporo Island (Fig. 8A, B). Benthic matting and chlorine tablets were applied to treat this
infestation (Fig. 8C, D). See Northland Regional Council (2024a, b) for methodology and results.

12

(A)

(B)
(C)
(D)

Figure 8. (A) Wider surveillance outside of Omākiwi Cove identified an infestation on the southern side of Poroporo
Island. Insert: Poroporo Island is situated in Te Rāwhiti Inlet, within the Controlled Area Notice zone. (B) Exotic Caulerpa
at Poroporo Island. (C) Infestations were treated with benthic mats and chlorine. (D) Mortality of exotic Caulerpa
following treatment. Photos: NRC.

5. Phase II observations

The following are interim observations only.

5.1. Operational capacity
There has been a notable increase in the operational capacity of the mechanical suction dredge
during Phase II, as evidenced by the area and quantity dredged per day. During Phase I, each
one to two minutes of dredging necessitated a stand-down period of approximately 45 minutes
13

to allow for the collection bag to dewater. Additionally, the disposal of this material required the
barge to be grounded for extended periods.
In Phase II, modifications such as the installation of trommels and the use of a support barge to
transport dredge material ashore have eliminated these requirements. Although operations
during the initial days of Phase II were somewhat constrained due to the style of dredging
(focused on connecting Phase I test patches), the later part of Phase II (from 27 May onward)
saw more uniform dredge sweeps, leading to increased operational capacity.
During this period, for the days where data is available (11 days; see Table 1), the average rate of
removal was 1,013 m² per day. To date, the total area dredged in Phase II is 11,856 m², a
significant improvement from Phase I, where the total area dredged was 1,998.8 m².
The desired aim of the trial was to achieve clearance of spatial scales of over a hectare per day,
and we acknowledge that we are still a long way off from achieving this outcome. To meet the
expectations of our communities and hapu partners, further advancements need to be applied
to improve speed and efficacy.

5.2. Mechanical dredge efficiency

While the operational capacity increased markedly in Phase II, a question remains regarding
optimal dredge efficiency (i.e. the percent of exotic Caulerpa removed from dredged areas); this
requires an appropriate balance between spatial coverage and quality of dredging. The results
of percent cover monitoring from Phase I indicated that areas of remnant exotic Caulerpa
persisted within dredges, and observations made by divers during Phase II indicate that this
issue persists. This is evidenced by the quantity of material removed by the diver-operated
suction dredge within an area that the mechanical suction dredge had already been through
(Table 2; Fig. 5). Following discussion with JBL it was decided that operations for the remainder
of the Phase II would be adjusted to focus on more thorough dredging (i.e. more Caulerpa
removed, but less area covered).

Additional to the speed of the dredge head over the seafloor, there are a number of other factors
that affect the efficiency of the mechanical dredge in removing exotic Caulerpa. For example,
the number of passes the dredge head makes over one area, the weather (wind and swell
conditions can make dredge operations challenging), and the operator of the dredge. In order for
the dredge to work successfully, the operator needs to consider and maintain control of
numerous factors. For example, depth of the cut, speed that the brushes turn, angle of the
leading edge of the dredge, etc. All these factors are constantly adjusted without the operator
being able to see the dredge.

14

5.3. Diver-operated dredge efficiency

It is evident that the operational capacity of diver-operated dredging is significantly less than the
mechanical suction dredge (c.f. Tables 1 and 2). However, because the diver-operated suction
dredging was conducted toward the end of the date range for this report, no observations have
yet been made regarding the comparative efficacy of diver-operated dredging for removing
remnant exotic Caulerpa. However, one key observation is that on several occasions the divers
encountered mats that were simply too large and / or thick to be removed with hand-held
dredges. For these situations the power of the mechanical dredge is required, and subsequently
the discussion around diver vs mechanical operations must consider exotic Caulerpa density as
well as spatial extent.

5.4. Costing information

The set-up fees for the Omakiwi Suction Dredging Trial, including the design and build of sand
extractors/trommels, piping manifolds, and amendments and repair costs incurred during the
Phase I trials, are estimated to be up to $600,000. Final costing of this build will be included in
the final report upon all contractors submitting final invoices. It must be acknowledged that the
time constraints placed on the accelerated trial did increase costs. Understandably, this is
expected in a response scenario; however, future builds would not be expected to incur the
same costs. The estimated daily dredge rate, which includes the barge, dredging equipment,
pump hire, hoses, fuel, Morooka hire, operator labor, shoreside excavator, and project manager,
is approximately $20,000 per day. Stand-down rates, which cover non-suitable conditions or
mechanical downtime and include costs for barge, dredging equipment, Morooka hire, and
other associated hire rates, are estimated to range between $10,000 and $12,000 per day. Given
the time of year and the weather conditions, unfortunately, this project has incurred
considerable stand-down days.
The Suction Dredging Trial experienced a mix of operational and non-operational days. Dredging
operations during this interim period are estimated to have occurred on 18 days. The trial faced
weather-related stand-downs on an estimated seven days, while mechanical or preparation-
related activities accounted for approximately 13 days of non-dredging operations, such as
equipment preparation, unloading, and decontamination. The trial also involved a significant
mobilization phase from April 13 to April 30, with continuous efforts to prepare for Stage 2 of the
operations. This phase included various setup activities, testing of screening plants, and the
mobilization of equipment from Opua to Omakiwi. Please note, accurate total stand-down days,
operational days, and stand-down days will be presented in the final report upon receiving final
logs from our dredging contractor.

15

5.5. Ongoing challenges

Stinger pipes

Deposition of dredge spoil remains challenging. If the pipes are too long, they fold up upon
themselves and sediment deposition is prevented. If they are too short, water pressure pushes
the pipes away from the seafloor, resulting in a sediment plume and widespread deposition of
sediment. Several factors (tide, swell, barge repositioning) mean that the optimal depth for the
pipes changes throughout the day. JBL are continuing to work on a solution.

Buried exotic Caulerpa

Divers reported finding extensive areas of exotic Caulerpa buried under sediment. This
presumably occurs as a result of the issues with the stinger pipes, discussed above.

Visibility

The sediment discharged by the barge is very fine, and subsequently causes poor visibility for
dive operations within approximately 100 meters of the barge (depending on how the stinger
pipes are operating). This can prevent divers from conducting ecological monitoring in areas
recently dredged during the same day unless dredging ceases.

Data management

The provision of daily GeoTIFF files has been a significant improvement to reporting capabilities
during Phase II, however access to this information remains variable. The data is not necessarily
transferred by the dredge operator at the end of each day or, depending on personnel, may not
be transferred at all. No automated data transfer system is available / possible.

Weather

Phase II activities have been impacted by adverse weather conditions on a number of
occasions, which has resulted in several full stand-down days, and days when dredging was
only able to occur for part of the day or at a reduced capacity. This has implications for the utility
of the dredge, particularly at times of year when adverse weather is a more common
occurrence.

Acknowledgements

16

Northland Regional Council and Cawthron Institute would like to acknowledge the ongoing
support, leadership, and involvement of Ngāti Kuta and Patukeha hapū throughout the Caulerpa
response in Te Rāwhiti, Bay of Islands, in addition to the following partners: Bay of Plenty
Regional Council, Commercial Dive Specialists, the Department of Conservation, Discovery
Marine Limited, Ethos Environmental, Johnson Bros Ltd., Marine and Environmental Field
Services, Maxwell Ecological Services, the Ministry of Primary Industries, the National Institute
of Water and Atmospheric Research, and Wai Knot.

References

Davidson I, Fletcher L, Richards D, Bennett-Jones L, Nevill-Jackson M, Smith A, Floerl L,
Gammon M, Leonard K. 2024. Omakiwi Cove exotic Caulerpa removal. Phase I: method
development for mechanical suction dredging. Nelson: Cawthron Institute. Cawthron Report
4040. Prepared for Northland Regional Council and Biosecurity New Zealand.

Northland Regional Council (2024a) Wider surveillance for exotic Caulerpa in Northland.
Prepared for Biosecurity New Zealand.

Northland Regional Council (2024b) Perimeter management and treatment report. Prepared for
Biosecurity New Zealand.

17