Investigating the ecological role of farmed Holothurian scabra on seagrass, sediment and blue carbon dynamics in southwest Madagascar1 ___________________________________________________________________________________________________________________ Lead Author: Jessica Arnulla Contributing Authors: Alexander Tudhopea, A. Meriwether Wilsona a) University of Edinburgh, School of GeoSciences, Edinburgh UK Experimental pens in seagrass habitat near Tampolove (Photo: J. Arnull) 1] This report is an Executive Summary of the MSc Dissertation: Arnull, Jessica, 2018. “Investigating the ecological role of farmed Holothurian scabra on seagrass, sediment and blue carbon dynamics in southwest Madagascar, Master of Science Dissertation for an MSc in Marine Systems and Policies, University of University of Edinburgh, 75 pp This research was funded through 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 Executive Summary Report Date: March 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 rapidly developed in the Indo-Pacific region. One of the most valuable species of these aquaculture species is Holothuria scabra (sandfish), which sells for up to US$1670kg- 1. Coined “vacuum cleaners” of the seafloor, holothurians play a particularly important role in benthic ecosystem functioning by remineralising nutrients and organic matter through feeding and excretion, as well as contributing to enhanced sediment oxygen levels through bioturbating activities, all of which influence nutrient recycling, benthic productivity and meiofauna population dynamics. Seagrass is the main habitat for these animals and these plants are considered to underpin a particularly important coastal ecosystem, through their ability to sequester large amounts of organic carbon within their sediment. However, current understanding of the ecological role holothurians play in wider ecosystem functioning is negligible. Through controlled field experiments, this study investigated the impacts of varying densities of H.scabra on aspects of seagrass growth rate and ecosystem status at an aquaculture farm in southwest Madagascar. A key result is that the growth-rate of seagrass (measured as leaf linear extension rate) was significantly (p <0.0001) higher in the presence of H. scabra than in the control sites with no H. scabra. This apparent effect of H scabra on seagrass growth rate was seen even at moderate stocking densities of ~200g H. scabra/m2 , where seagrass leaves grew ~60-70% faster than in the control treatment. Increasing from 200g/m2 stocking density up to 800g/m2 had a smaller effect on seagrass growth rate. These results support the hypothesis that seagrass ecosystems benefit from holothurian aquaculture. Although there were no significant relationships 2 between Corg stocks and H.scabra densities (R =0.445; p=0.555), the data did indicate an indirect effect 2 through seagrass biomass, which was a significant indicator of Corg stocks (R =0.981; p=0.019). Calculations of the change in areal seagrass extent before and after the introduction of the aquaculture farm indicated that the removal of holothurians might reduce seagrass health and productivity 1. Research Rationale Holothurians are benthic organisms belonging to the phylum Echinodermata that have been harvested for centuries as a luxury food item in Asian food markets. One of the most valuable of these species is Holothuria scabra (Jaeger, 1833) or ‘sandfish’, which inhabit shallow coastal waters such as seagrass beds in the Indo-Pacific (Mercier et al., 2000; Purcell et al., 2012b; Juinio-Meñez et al., 2016). Typically selling for US$153-251kg -1 (2016) in Hong King or Guangzhou – the primary trading hubs before being distributed to markets in mainland China – H.scabra fisheries have become an important source of income for artisanal fishermen across the Indo-Pacific (Purcell et al., 2018; Wolkenhauer et al., 2010; Rasolofonirina, 2007). However, concerns have been raised over the status of H. scabra populations due to the severe exploitation of natural stocks (Mercier et al., 1999). In some cases, populations have fallen so dramatically that fisheries governance and regulatory measures alone have been incapable of restoring populations (Friedman et al., 2011; Purcell et al., 2012). The management of sea cucumbers has consequently become a worldwide concern, and, like many other fisheries, there has been a surge of research into the development of the aquaculture sector (Eriksson et al., 2012). Besides supplying markets, aquaculture provides more effective management of fisheries whilst also restoring sandfish populations (Juinio-Meñez et al., 2017; Battaglene et al., 1999; Eriksson et al., 2011). Despite the socio-economic benefits of sea-cucumber aquaculture, there has been a considerable lack of research addressing the basic biology and ecology of hatchery-bred H. scabra juveniles (Purcell 2010, Hamel et al., 2001; Mercier et al., 1999; Wolkenhauer et al., 2010). This has subsequently caused problems for the development of aquaculture sites and restocking programmes, as is the case in Tampolove. Recent studies have identified substantial ecological impacts when benthic marine organisms are removed from a particular coastal environment (Solan et al., 2004), and given the importance of sea cucumbers in recycling and remineralising nutrients and organic matter through feeding, excretion and bioturbation activities, it is reasonable to assume that both the removal and the introduction of these animals – especially at high 2 densities – could generate not only considerable ecological impacts on the surrounding coastal ecosystem, but also to the communities depending upon them (Thomson, 2017; Lee et al., 2018). Of particular interest, is the relationship that H.scabra have with seagrass. Typically found in a protected lagoon or bay, a favourable habitat for H. scabra are seagrass beds, as larvae and juveniles rely on seagrass for their settling cues and early life stages by providing protection from predation and a suitable substrate to grow on (Mercier et al., 2000a; Hamel et al., 2001; Hair et al., 2016; Purcell, 2012). Whilst seagrass habitats provide clear benefits for H.scabra, there may be evidence to suggest seagrass also benefit from the presence of holothurians. H.scabra in particular is very important in this process, because it is one of the rare species that exhibit a regular burying cycle, continuously moving in and out of sub-surface sediments, meaning they could possibly have a greater impact on sediment displacement and bioturbation than other holothurians that solely interact with surface sediment (Wolkenhauer et al., 2010). In the southwest region of Madagascar, sea cucumber or ‘zanga’ fishing is actively pursued by coastal communities and forms an integral aspect to the livelihoods of the Vezo people who inhabit this region (Robinson and Pascal, 2009). Over the past decade, the region has become a hotspot for the emergence of sea cucumber aquaculture projects (Robinson and Pascal, 2009). Novel partnerships between local communities, private sector stakeholders and overseas non-governmental organisations (NGOs) have facilitated a platform that is pioneering towards a form of community-led aquaculture whereby hatchery- reared juvenile H.scabra are raised and farmed by the local community in sea-ranching pens (Robinson and Pascal, 2009). Ease of harvesting, low processing costs and high profit makes the business highly attractive to small-scale fisheries (Dissanayake and Stefansson, 2012; Purcell et al., 2013) and 200km north of Toliara in the village of Tampolove, British based marine conservation group Blue Ventures, have been making considerable progress in developing an innovative sandfish aquaculture livelihood model. Tampolove is small Vezo fishing village situated in the ‘Bay of Assassins’ and the protected waters of the Velondriake Locally Marine Managed Area (LMMA). Since 2009, in partnership with Blue Ventures, the Velondriake association have been working with the village to help establish a viable community-led sea cucumber aquaculture business that not only repopulates the local marine area with juvenile zanga, but also allows a steady monthly income through the sale of mature adults, thus simultaneously helping to alleviate poverty within the community whilst also reducing fishing pressures exerted onto the region’s fragile marine resources (Parker, 2017). 1.1 Scope of study If this positive symbiosis does exist between sandfish and seagrass, it is reasonable to assume that a greater density of holothurians could be positively correlated to greater seagrass growth, as suggested by Rougier (2013). Seagrass ecosystems are particularly poignant in this regard as they are considered to be amongst the productive and diverse on earth, supporting not only incredible biodiversity but also many of the world’s poorest communities who depend on their services to sustain livelihoods (Lee et al., 2018). They provide key ecological functions to coastal ecosystems including sediment stabilisation, coastline