Social and Environmental Impacts of an Exploited Fishery: Holothurians in Marovo Lagoon, Solomon Islands

Abstract

Holothurians or sea cucumbers, commercially known as beche-de-mer when referring to the processed form, have been harvested and traded for more than 100 years in the Indo-Pacific region. Overfishing has caused marked declines in holothurian populations throughout the region, including Marovo Lagoon, Western Province, Solomon Islands. The contribution of beche-de-mer by the Western Province to the annual total beche-de-mer production of the Solomon Islands declined from 58% in 1989 (total beche-de-mer production 87,095 kg dried weight) to 17% in 2003 (total beche-de-mer production 173,633 kg dried weight). This prompted the current investigation of their biomass and diversity in Marovo Lagoon, as well as an assessment of changes to critical ecosystem services and the prospects for population recovery following a fisheries closure. The re-opening of the fishery in 2007 resulted in a decline in high-value species from 437 ind ha-1 to 208 ind ha-1 over a five-month period following the resumption of fishing, while low-value species slightly increased from 677 ind ha-1 to 781 ind ha-1, over the same period based on diurnal and nocturnal surveys. During the day the following low-value species (Holothuria atra, H. coluber, H. edulis, H. avomaculata, Hong payfish and Pearsonothuria graeffei) and high-value species (Actinopyga lecanora, Bohadschia vitiensis, Stichopus herrmanni, S. horrens and Thelenota ananas) were observed throughout the entire study. The decline in species diversity and abundance raised concerns about deleterious changes in sediment nutrient dynamics following the removal of an important grazing and bioturbating guild. Anecdotal reports of changes in carbonate sand colour from white to brown signaled a possible ecosystem-wide increase in benthic microalgae (BMA) and prompted in-situ enclosure experiments with S. herrmanni in this heavily exploited tropical lagoon setting. This study was further supported by field experiments in Queensland, Australia. These experiments were repeated with H. atra in an unexploited subtropical reef lagoon (Lady Musgrave Lagoon, Great Barrier Reef, Australia). Both experiments measured the effect of holothurian grazing and burrowing on BMA and carbohydrate in the sediment. In addition to this, aquarium experiments were conducted with H. scabra in Moreton Bay, Australia to investigate the effect of burrowing holothurians on BMA and carbohydrate in the sediment. Both field and aquarium experiments indicated that the grazing and burrowing of holothurians in general reduced the BMA biomass and carbohydrate concentration in the sediment. At higher densities holothurian grazing and burrowing may reduce the BMA community and carbohydrates, which is an important component of nutrient recycling. The preservation of holothurian populations is thus essential in balancing the BMA, and nutrients in the sediment-water interface. The reduction in holothurian abundance and diversity could also affect the livelihoods of the Marovo people. Approximately 80% of the Solomon Islands population depends on marine resources for direct subsistence or economic needs; fisheries management thus, has direct implications for local livelihoods. This thesis used a multidisciplinary and socio-ecological approach to investigate the relationships between holothurian fishing, current management approaches and household economics in Marovo Lagoon. Overall, holothurian harvesting accounted for 21% of total household income. Interviews and dive surveys revealed a reduction in holothurians due to the inconsistent national management approach, resulting in a `boom and bust' cycle. Continued observation of the recovery, post closure, and any subsequent harvests in Marovo Lagoon would thus be beneficial in understanding the population dynamics and may contribute towards a sustainable harvest plan for holothurians in the future. The findings from this thesis could provide critical insight into the applicability of an Ecosystem-Based Management approach built on management actions that incorporate biophysical and social processes affecting the holothurian population and fishery. This study suggests that future research should focus on monitoring of holothurians via both diurnal and nocturnal surveys, to track the potential recovery and provide data towards developing a rural management framework. The future of the holothurian fishery is imperative to Solomon Islanders; hence, the research findings from this thesis will provide much needed knowledge on a local and national level for protecting this environmentally and socially valuable fishery.n