Biophysical Site-Suitability Summary Aquaculture, Southwest Madagascar
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Biophysical Site-Suitability Summary for Community Based Sea-cucumber Aquaculture, Southwest Madagascar ________________________________________________________________________________________________________________________________________________________________________________________________ Lead Author: Jessica Arnulla Contributing Authors: Alexander Tudhopea, A. Meriwether Wilsona, Bryne Ngwenyaa, Sebastian Hennigea, Amber Cartera, Kathryn Taylora a) University of Edinburgh, School of GeoSciences, Edinburgh UK This research was prepared for and funded by the Prince Albert II of Monaco Foundation, Project No. 2505: Community-Based Aquaculture as a Catalyst for Locally Managed Marine Areas: Developing a Scalable Framework for Economic and Environmental Sustainability Joint Research Project | University of Edinburgh and Blue Ventures Principal Investigator, Report Editor: A. M. Wilsona Report Date: April 2020
Abstract
Over the last few decades, unregulated and sustained overfishing of sea-cucumbers has led to dramatic
declines in world populations. In a bid to restore declining stock and develop a profitable industry, culture-
based aquaculture models are being developed in the Indo-Pacific region. One of the most valuable species
of these sea-cucumbers is Holothuria scabra (sandfish), which, once processed, sells for up to USD 1670 kg-
1
in Asian markets In southwest Madagascar, considerable progress has been made into developing a scalable
aquaculture model at sites across the Velondriake Locally Marine Managed Area (LMMA) with the help of
the NGO, Blue Ventures (BV). This report is part of a 2-year joint research project with BV and the University
of Edinburgh (UoE).
The research project overall focused on community-based sea-cucumber (or ‘sandfish’) aquaculture (CBA),
in southwest Madagascar. The project’s main aim was to study production, biophysical and social
considerations of an aquaculture farm (5 ha, 41, 900 sq.m. pens ) in Tampolove, Madagascar, with an
ambition to scale-up this model to other areas. During the course of this project, the Tampolove ‘model’ has
been replicated at another site in the region, Ambolimoke. This report focuses on investigating the
biophysical considerations needed to guide sea-cucumber aquaculture site selection in order to optimise
sustainable harvesting sales and wider benefits. This report integrates field work during 2018 and key points
from two associated MSc dissertations (Arnull, 2018 and Taylor, 2018), as well as further field work and
findings in 2019. The work in this report is organized around two primary research questions:
1) What are the key environmental parameters controlling sea-cucumber growth rates, in particular
water and sediment quality? For this, biophysical and biomass surveys were conducted, and
correlated against harvesting sales data, indicating how key biophysical factors influence sea
cucumber growth;
2) How do sea-cucumbers influence their surrounding habitat, in particular seagrass? This involved
controlled field experiments on the impact of varying densities of H.scabra on seagrass leaf growth
and seagrass biomass to determine the long-term impacts of sea cucumber farming on seagrass
distribution and health.
Results from sea-cucumber biomass surveys (RQ1) have been instrumental in refining the current model with
regards to stocking and harvesting. Results from biophysical surveys (RQ2) determined that seagrass cover
(correlated to water depth and sediment thickness) may be an important factor influencing sea cucumber
growth. Results from RQ2 indicated that H. scabra stocked at 300 g m2 had a statistically significant effect on
the growth of the important seagrass species T.hemprichii (p
1. Research Objectives
The overarching objective of this report is to assess the extent to which community-based aquaculture is
enabling a scalable framework for integrated economic and environmental sustainability within a locally
managed marine area. From a biophysical perspective, the purpose of study is three-fold:
1) To develop a suitability index for a site’s potential by improving understanding of the biophysical
requirements and challenges of sea-cucumber farming (Specific Goal 1; A5);
2) To gain a better understanding of the impacts that sea-cucumber aquaculture farming has on wider
ecosystem functioning (Specific Goal 2; A9) and;
3) To facilitate the development of a practitioner’s toolkit that enables and supports model up-scaling
and uptake in new areas, both nationally and internationally (Specific Goal 3; A11).
This research is carried out through three inter-related activities (A5, A9, A11, described below) which
together contribute to following specific project goals:
SG 1. Refine and expand sustainable business and production models for community-based sea
cucumber (Holothuria scabra) and aquaculture production systems.
SG 2. Evaluate wider social, economic, environmental impacts and potential of aquaculture models to
improve LMMA effectiveness.
SG 3. Develop a community-based sea cucumber aquaculture practitioner's toolkit to support and
facilitate successful model uptake in new areas.
SG 4. Document conclusions in reports, publications, toolkits and methodologies; identify preliminary
priority sites for scale up of model, and capacity building for sustainability.
Work began on the three key project activities (A5, A9, and A11) in 2018, which resulted in completion of
two MSc dissertations (Specific Goal 14; A16) and has since been continued throughout 2019 building on the
2018 preliminary findings.
In this regard, it is convenient to split the biophysical aspect of the project into two main research areas. In
the first instance, this refers to the work associated with A5 and A11: to investigate the environmental
parameters controlling sea cucumber growth rates, which links with work carried out by MSc student K.
Taylor in 2018 (hereon in referred to as RQ1). It was hoped that the findings would help contribute to
discussions on model refinement and replication at other sites, optimal pen location and size, monitoring
requirements, further training and research, and a guide on biophysical site-suitability. This information to
help inform future sea cucumber aquaculture practitioners about the most suitable habitat for sea cucumber
farming, thus also contributing to a practitioner’s Toolkit. [Klückow, 2020. A Sandfish Farming Manual]
In the second instance, this refers to the work associated with A9: improving understanding of the
implications that sea-cucumber aquaculture has on wider ecosystem functioning. This work is closely related
to the field work carried out by J. Arnull in 2018, which provided a baseline assessment of the impact sea-
cucumber farming has in relation to the distribution and status of seagrass habitats, sediment composition
and carbon storage capacities through environmental mapping surveys of seagrass extent and diversity as
well as biophysical measurements of sediment characteristics throughout the farmed area. It also involved
in-situ experiments designed to ascertain whether farmed H.scabra has a net overall positive or negative
impact on seagrass growth/productivity (hereon in referred to as RQ2).
Figures a – e on the next page, illustrate the regional site context, aquaculture farm, KMZ file, and sample
data.
3Figure a) 2018-2019 Sea-cucumber aquaculture locations
Figure b) Velondriake Locally Managed Marine Area (LMMA)
Figure c) 2018-19 Tampolove Farm and Pens
Figure d) 2019 Google Earth Interactive Data / Tampolove Pens (screenshot KMZ file)
Figure e) Sample of clickable data (J. Arnull)
42. Research Approach
RQ1| 2018 à 2019
In 2018, work began looking at understanding the influence of temperature and water level fluctuations on
juvenile sea cucumber growth. This involved the construction of 12 experimental pens at four different sites
around and within the aquaculture farm (3 replicates per site). Each pen was stocked with 40 juvenile
H.scabra, whose weights/growth rates were monitored over the course of a month through biomass
weighing (n=3) during spring low tides. Each site was subject to differing ranges of water depth and seagrass
cover and water and sediment temperature was recorded. Although no statistically significant relationships
emerged between water depth and temperature, the results of the experiment indicated that exposure to
extreme low tides and temperature fluctuations of 8-10˚C may encourage the acclimatisation of juvenile
H.scabra and in some instances enhance growth rates. The results demonstrated that one of the
experimental sites located outside of the farming area (a site that experiences complete exposure during
spring low tides and previously thought to be inappropriate for the rearing of H.scabra), could be a potential
area for farm expansion. Whilst the study did not provide guidance as to the environmental conditions best
suited for rearing adult H.scabra, it did help bridge a gap in understanding of some growth boundaries
required for juvenile rearing.
Upon further field work in 2019, the site had been running for over a year, providing us with harvest data for
each commercial pen over the past 18 months. Preliminary analysis carried out on the harvesting sales data
indicated that some pens had nearly four times the harvest yield than others. As such, the aim of the work
carried out during 2019 was to determine which biophysical conditions were causing the large variations in
productivity, allowing us to define a biophysical framework for future projects, in the following context:
1. In order to achieve sufficient growth to reach a marketable size (>400g), H.scabra farming pens must
be stocked at or below their critical biomass value (CBV), beyond which individuals will not grow. This is
largely determined by the sediment’s carrying capacity, which naturally varies from place to place. As such,
knowledge of a site’s CBV can drastically influence the success and survival of an aquaculture venture, due
to the susceptibility of the cucumbers to stunted growth as a result of overstocking, which can also influence
disease outbreak (Plotieau et al., 2013). Biomass monitoring during monthly harvests allow BV technician
staff to calculate the optimal stocking density that will produce the highest yields. In 2019, stocking density
for the farm was 300g/m2. With this in mind, the first activity was to determine total scabra biomass for the
6 most and 6 least productive pens (n=12) and to establish the total number of animals at a marketable size
(>400g). Time and logistical constraints meant only 2 pens – the second highest (T10) and second lowest
(T38) productive pens – were actually surveyed. Results from this were intended to shed light on the large
variation in between pen productivity. (Project Activities 5, 11)
2. The second activity was to complete an extensive biophysical characterization of all of the
commercial pens. This included: estimating % seagrass coverage; sediment sampling for subsequent analysis
(which involved % Corg, grain size and % carbonate); and obtaining measurements for sediment thickness,
seabed and rockbed elevation. A principal component analysis was then used to determine which
environmental variable(s) best explain the variation in harvesting sales. Multivariate regression analyses
were also carried out on the data. It was hoped that the results from this could then be used to help define
boundaries (or ranges) for certain biophysical variables that future practitioners could use as a rough guide
when choosing a suitable aquaculture site. (Project Activity 9)
3. In addition to the fieldwork carried out at Tampolove, biophysical data was also collected at other
new and developing sites across the Velondriake region: Ambolimoke, Lamboara and Antsatsamoroy. These
3 sites are still very much in the development phase and all are located in distinctive geographical and
biophysical settings.
• Ambolimoke is currently a replicate of Tampolove, with 41 commercial pens built and stocked with
H.scabra. However the site has only been running for two months and has not yet had a harvest.
5• Lamboara and Antsatsamoroy each have 4 experimental pens constructed and stocked with some
sea cucumbers. Growth is being monitored regularly. Currently, it would seem that the geographical
setting of Lamboara would not feasibly produce a productive aquaculture farm due to the physical
location of the site (i.e. located in a turbulent and shallow inlet). Initial growth results from
Antsatsamoroy look more promising.
The biophysical surveys undertaken at Tampolove were partially replicated at these three sites in order to
get a first-order understanding of site characteristics. % Seagrass cover and sediment samples were obtained
for 22 pens in Ambolimoke (later analysed for %Corg, grain size and %carbonate) and sediment samples were
also taken from Lamboara and Antsatsamoroy (again analysed for %Corg, grain size and %carbonate content).
Once productivity data is available it is hoped we will be able to correlate environmental variables to sea
cucumber growth rates, which will help develop a better understanding of suitability.
RQ2| 2018 à 2019
Globally, little research has been carried out on the effect bioturbators (animals which mix sediment during
their activities) have on coastal ecosystems, although more research is being carried out to help understand
the impact that coastal bioturbation could have on sediment Corg, (De Boer, 2009; Thomson, 2017). Whilst
sandfish cannot be classified unconditionally as a ‘bioturbator’, the diel burying cycle that sandfish undertake
can be assumed to emulate this role to a certain extent. These burying activities are thought to increase
primary production by irrigating and oxygenating deeper sediment layers, thus providing belowground
seagrass organs with more oxygenated sediment. Moreover, it is thought that sea cucumbers may play an
important role in the recycling of nutrients in the water column thus providing increased potential for growth.
As of yet, no studies address the role that burying holothurians could have on seagrass ecosystems,
particularly in the context of an aquaculture farm, where densities tend to be higher than usual. Seagrass
ecosystems are particularly poignant in this regard as they are considered to be amongst the most productive
and diverse on earth, supporting not only incredible biodiversity but also supporting many of the world’s
poorest communities who depend on their ecosystem services to sustain their livelihoods (Lee et al., 2018).
The purpose of the work carried out in 2018 for the MSc dissertation (Arnull, 2018) was to ascertain whether
bioturbating activities carried out by farmed H.scabra has a net overall positive or negative impact on
seagrass, and in particular, for blue carbon stored beneath them. In order to establish this potential
relationship between sandfish and seagrass, an experiment was designed wherein seagrass growth was
measured in relation to different densities of H.scabra. In this experiment, growth was only measured in the
leaf component of Thalassia hemprichii by stapling the base of the leaves and left to grow after the
introduction of H.scabra. After a two-week period, shoots were harvested and distance between the base of
the leaf and the location of the staple was measured, allowing us to calculate growth rate in cm shoot-1 day-
1
and in g DW m2 day-1 (after determining the dry weight of 1cm). Four treatments (3 replicates per treatment)
were applied to 12x1 m2 experimental pens: control pens (C), whereby H.scabra were excluded; medium
density pens (M), with a total H.scabra biomass of 200 g m2; high-density pens (H), with a total H.scabra
biomass of 400 g m2; and over-capacity pens (OC), with a total H.scabra biomass of 800 g m2. The design of
these experimental pens was grounded in the understanding of how critical biomass value relates to pen
stocking density, which at the time of the experiments was 460g m2.
To understand what impact a potentially more productive seagrass ecosystem could have on ‘blue carbon’
potential, sediment cores were extracted from inside and outside of the farmed area and analysed for total
organic carbon content (TOC). This was done in order to establish any substantial differences in carbon
storage potential between an undisturbed environment and one dominated by sandfish. These carbon
estimates were then used to approximate the current sedimentary Corg stock and net carbon sequestration
rates for the farm. It was hoped that these results could be used to gauge the plausibility of the farm allowing
Tampolove to offset carbon emissions, touching on the concept of carbon neutrality.
6Based off 2018 results, it seemed that H.scabra were making the seagrass more productive, with initial data analysis suggesting that in pens where H.scabra were excluded, seagrass growth was reduced by ~60-90% depending on the stocking density of the cucumbers (at a statistically significant level of p
Seagrass biomass sampling was undertaken in order to give insight as to what the impact 18-months of
continuous H.scabra presence may have had on the seagrass, thus providing us with a better understanding
of the long-term impacts sea cucumber farming may have on the above and below-ground structure of the
seagrass meadow. Two areas with a contrasting species composition were chosen next to sites B
(T.hemprichii and H.uninervis, hereon in referred to as Th-Hu) and C (C.serrulata and H.uninervis hereon in
referred to as Cs-Hu. H.uninervis was the less dominate species for both locations. Samples were taken inside
(n=6) and outside (n=6) of the farmed pens for each area. Locations for excavations were chosen randomly
within the two areas but each had a similar AGB coverage (~%).
Photo 1. Arnull sampling seagrass blades (Photo by G. Cripps)
Figure f. Seagrass species
Photo 2. Seagrass in aquaculture pen ((Photo by G. Cripps)
83. Main 2019 Findings RQ1 a) Key results The biomass surveys revealed two important findings. Firstly, the number of animals in T38 (low productivity) was nearly 3 times the number found in T10 (high productivity). Secondly, T10 had a greater proportion of animals over 400g, whereas the majority of animals found in T38 were less than 400g. It became clear that the low productivity pen was vastly overstocked (likely as a result of the biophysical conditions). This stocking density issue meant that few animals were reaching the marketable size and instead were plateauing at around 350g. As such, very few animals were being removed from the pens, but the pens were still being stocked every month with the same number of juveniles. This stocking density issue was therefore not only stunting growth, but also reducing monthly income for the community members. Results from the principal component analysis indicated that the main environmental controls related to water depth, grain size and seagrass cover, suggesting that in order to improve sea cucumber growth, a site should have a consistent water level over 5cm during spring low tides, a smaller grain size and presence of seagrass. Due to limits in the dataset, developing a mixed-effects model was beyond the capabilities of this study, due to the tendency for the model to become ‘over-fit’. Despite this, we were able to develop a break- point regression model, which explained about 35% of the variation of average harvesting sales. Whilst this model does not serve to predict average monthly harvests, it did suggest that intermediate levels of seagrass cover seem to lead the lowest levels of harvest, while low or high levels of seagrass lead to higher harvests. Reasons for this may be attributed to differences between seagrass species, which are likely influenced by sediment thickness and seabed elevation. b) Implications Based on the findings from the biomass surveys, BV made the decision to cease juvenile stocking for the entire farm until the establishment of a more efficient model. This was to remove any bias and unfairness that may arise for those members of the community owning the less productive pens, as members owning high productivity pens would continue to increase their earnings, whilst the former would lose out. One way in which to make the stocking programme more efficient would be to replace the correct number of animals that have been harvested, thus ensuring an equal number of animals at any one given time at the correct stocking density. The findings from this have also enabled BV to reflect on the current governance system to ensure all farmers get an equal share of earnings by either altering the way in which pens are allocated to members of the community (for example, pen allocation through clan ownership) or in the farm design – i.e. move from an enclosed pen structure, to an open, cyclical style of farming – which would put the community at the heart, as opposed to enabling some families to do better than others through individual pen ownership. The results indicate that water depth and sediment grain size are two of the most important environmental variables influencing sea cucumber growth. Therefore, these two variables should be carefully assessed before embarking on developing a new site. Having said this, there will undoubtedly be a number of other external factors that could occur at other sites that will not occur at Tampolove but should be considered. RQ2 a) Short-term impacts GB1 results indicated no statistically significant differences in growth rates between pens within and between sites meaning we could rule of the possibility of other environmental parameters such as sediment thickness or seabed elevation as having a significant effect on growth rates. GB2 results indicated that H.scabra stocked at 300g/m2 had a statistically significant effect on Thalassia hemprichii growth (p
dependent effect, but should be taken in the context of knowing the experiment did not account for below
ground growth, which is an important aspect of ‘whole plant’ growth. Root and rhizome biomass and
production (belowground biomass) constitute a significant proportion of total seagrass productivity; ideally
the experiment would have incorporated this aspect of growth through rhizome marking/plastochrone
interval.
b) Long-term impacts
Results from the biomass sampling show no clear statistical relationship between seagrass biomass inside
and outside of the farming area. They did suggest however that the long-term implications of sea-cucumber
presence on seagrass biomass are species dependent. For C.serrulata, results indicated that the presence of
H.scabra might have increased belowground biomass, whereas for T.hemprichii, the presence of H.scabra
may have increased aboveground biomass. Assuming the activities carried out by H.scabra are leading to an
increase in energy availability (be it through increased oxygen or nutrient availability for example), the
contrasting pattern of results is likely due to functional differences between the two species, thus explaining
why the allocation of additional energy is occurring in opposite ways. However, the results must not be over-
interpreted, as inconsistency in sampling makes comparisons of AGB: BGB ratios between the two areas
difficult and variability in the data makes it hard to draw solid conclusions. Despite this, overall, results
indicate that 18 months of continuous sea cucumber farming has a positive influence on seagrass biomass.
c) Implications
Whilst placing an economic value on the ecosystem services that this area of seagrass may provide was
beyond the scope of this present study (largely due to lack of data on these tropical species), for Tampolove,
it is evident that the benefits of managing a sea cucumber aquaculture farm go beyond direct monetary
earnings. Not only can the farm offer an alternative sustainable livelihood opportunity and injection of
income for local communities, but it can also offer indirect additional ecosystem services that are created
through seagrass habitat protection/support. Whilst historically seagrass habitats have been destroyed to
make way for aquaculture projects, in this instance, the seagrass is ‘critical’ for the success of the project. As
the site operates under a ‘no-take-zone’ policy, not only is the farm enabling the sea-cucumber aquaculture
farm to operate, it is simultaneously protecting, conserving and even ‘boosting’ the seagrass habitat in which
the site is based, thus increasing the potential for seagrass ecosystem services for the community. These
findings may help justify the worth of other sea cucumber aquaculture farms within the LMMA.
For Tampolove, this conserved area of seagrass could feasibly help the recovery of fish stocks and enhance
biodiversity, both of which could be interesting to monitor over the course of the next few years. Likewise,
for an area prone to tropical storms, the seagrass beds could help reduce the impacts by attenuating storm
energy and whilst the true impact of the sandfish aquaculture on sediment Corg is undetermined, carbon
stored within seagrass biomass will undoubtedly be contributing to some form of carbon storage.
4. Link to Interactive Google Earth KMZ File
Using harvesting sales data and data collected from biophysical surveys conducted at the site over August
2019, an accessible interactive Google Earth kmz.file has been developed for future aquaculture projects.
Users from around the world can access, load and open this file into (free-to-use) Google Earth, which we
decided would be more manageable than other mapping software. All is needed is a laptop/computer and
Google Earth Pro downloaded. On opening the link, users will be taken to the Tampolove site in Google Earth,
where they will get a feel for the farm setting through Google Earth functions and drone imagery. GPS data
delineates the farm structure and pen outline, allowing users to ‘visit’ each pen and view the following
information:
• An image of the sediment/seagrass;
• Typical monthly harvest data (i.e. pen productivity) and 9 month total average (which is the number
of months the site has data for);
• Sediment characteristics (sediment thickness, seabed and rock bed elevation, median grain size, %
Corg, % carbonate);
• % Seagrass cover.
104. Conclusions
RQ1 | The work carried out during this study has helped to grow our understanding of how best to optimise
a sea-cucumber aquaculture model thus hopefully enabling future projects with improved guidance through
a biophysical suitability index and development of a practitioner’s toolkit/guide. It is hoped that this research
has highlighted the importance of understanding the basic biophysical parameters of a site; how this can
influence stocking density and ultimately regulate sea-cucumber productivity and harvesting successes.
RQ2 |The findings from this research suggest that sandfish stocked at certain density could promote the
growth of some species of seagrass. This could have positive implications on the overall seagrass ecosystem
functioning by way of: promoting increased carbon sequestration; sediment stabilisation; coastline
protection against flooding and weather events; nutrient cycling; fisheries support; and biodiversity
enhancement (Hemminga and Duarte, 2000; Orth et al., 2006; Belsche et al., 2017). For vulnerable
communities such as Tampolove, any means of protecting and conserving important coastal environments
is paramount.
In summary this study highlighted the ecological benefits that sea-cucumber aquaculture can have on the
surrounding environment and thus serves as justification for upscaling and expansion of future sites. For
vulnerable coastal communities whose livelihoods require change in order to succeed and thrive in an ever-
shifting world, an opportunity such as sea-cucumber aquaculture would undoubtedly provide considerable
ecological and social benefits as well as scope for sustainable and self-determined livelihoods.
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