Size at sexual maturity of the flower teatfish Holothuria ( Microthele ) sp . in the Seychelles

The intensive harvesting of sea cucumbers in the Seychelles over the last decade has raised concerns about the sustainability of this resource. The flower teatfish Holothuria (Microthele) sp. (locally called pentard) is one of the main target species, representing about two-thirds of annual sea cucumber catches. The biology of this species is largely unknown. To inform fishery management, the reproductive biology of the species was studied on the Mahé Plateau and the Amirantes Plateau during the 2018 northwest monsoon season. A macroscopic analysis of the gonads, and a description of the tubules were performed on 63 sea cucumbers. We observed that the population at the Amirantes Plateau was smaller in size than those from the Mahé plateau, although that difference was not statistically significant. Female tubules were wider and shorter than male tubules, showing there is sexual dimorphism in this species. The size at sexual maturity of flower teatfish was estimated at 30.3 cm (95% confidence interval: 28.5–32.3 cm) through logistical regression. Based on this result, a minimal harvest size of 31 cm may be recommended for this species as a conservative measure in the Seychelles. Figure 1. Flower teatfish – Holothuria (Microthele) sp. (locally called pentard) – from the Seychelles. Introduction Holothurians have been fished in the Seychelle Islands of the southwest Indian Ocean for hundreds of years (Aumeeruddy and Conand 2008). They are mostly collected by divers using scuba gear at depths of around 15 to 45 m. They are further processed and dried and exported to Asian markets as beche-de-mer. Globally, holothurian resources have increasingly been overexploited since the 1990s, due to the rising demand by the Chinese market. Up until the end of the 20th century, sea cucumber harvesting in the Seychelles was an open access fishery, without any management control due to the low economic value of the fishery. However, the Seychelles Fishing Authority (SFA), the governing body responsible for fisheries management, introduced a national management plan in 2005 to guide the management of the fishery as a response to the increase in fishing pressure and annual catches (Conand and Muthiga 2007). The teatfish group within the genus Holothuria (white teatfish H. fuscogilva, black teatfish H. nobilis, and 1 Seychelles Fishing Authority (SFA), PO Box 449, Victoria, Mahé, Seychelles 2 UMR ENTROPIE IRD-Université La Réunion-CNRS, Paris, France 3 Consultant for the SEACUSEY project 4 UMR ENTROPIE c/o IH.SM, University of Toliara, BP 141 Toliara, Madagascar * Corresponding author: cahuzac.salome@gmail.com flower teatfish H. (Microthele) sp. (locally known as pentard, Fig. 1) has been heavily targeted in the Seychelles in the 2000s because they are among the most valuable species on the world market (Purcell et al. 2012, 2017, 2018; Diassanayake and Gunnar 2010). With approximately 150,000–300,000 individuals harvested per year, the flower teatfish has been the main commercial species targeted in the Seychelles since 2006, followed by prickly redfish 20 SPC Beche-de-mer Information Bulletin #39 – March 2019 Figure 2. A fisher making a dorsal cut on a freshly harvested flower teatfish in order to remove the viscera. Thelenota ananas (15,000–72,000 individuals per year) and white teatfish (37,000–127,000 individuals per year) (Aumeerudy and Conand 2008; Léopold and Govinden 2018). The increasing demand for these species, driven by their increasing commercial value, has progressively led to their potential overexploitation as observed by the declining trend in catches over the last five years. The flower teatfish has been observed in the Seychelles, Comoros, Tanzania, Madagascar, Sri Lanka and the Maldives (Purcell et al. 2012). However, to date, that species has not yet been formally described, although this work is ongoing (G. Paulay, Curator, Florida Museum of Natural History, pers. comm.). Information on the ecology of flower teatfish is poor, which makes stock assessment difficult and the design of suitable management or conservation measures uncertain. To address this gap, this study aimed to improve our knowledge of the reproductive behavior of flower teatfish. Specifically, we conducted a biological survey to determine its size at first sexual maturity. The present work was performed as part of a larger research programme whose overall purpose was to strengthen the sustainability of the sea cucumber fishery in the Seychelles through adaptive co-management (Léopold and Govinden 2018). Materials and methods Survey area and sampling The survey was carried out on board SFA’s research vessel at two sites: one close to Fregate Island on the Mahé Plateau and the other close to Marie-Louise Island on the Amirantes Plateau, in February and March 2018, respectively. According to fishers, flower teatfish spawns during the monsoon period (October–March), which is also the spawning period of the related species white teatfish. Eight fishers, working on two commercial fishing vessels, volunteered to participate in the survey and facilitate the collection of sea cucumbers. This served to also improve awareness of the research program among fishers. The fishers manually and opportunistically collected flower teatfish specimens while scuba diving at depths ranging between 25 m and 30 m depth at different locations in both survey sites. The following observations were made by the scientific team on the fishing boat and on all specimens immediately after capture. Total body length (TL) was measured from mouth to anus to the nearest 5 mm with a metric tape and weighed to the nearest 1 g using an electronic balance without an anti-rolling system, and this likely affected the accuracy of weight measurements. Fishers then made a cut on the dorsal side of each specimen (Fig. 2). The entire gonad was removed, weighed to the nearest 1 g, and stored in individual 50-mL tubes filled with 10% formalin. After removing the internal organs and excess coelomic fluid, the gutted body was weighed to the nearest 1 g. The sexual stage was determined using the procedure defined by Conand (1981). Firstly, the gonad index (GI) was calculated as follows: GI = Wgonad x 100 / Wtot where Wgonad = gonad weight (g) and Wtot = total weight (g). GI was expected to be high when gonads were mature. Secondly, a macroscopic description of the gonads was conducted at SFA’s laboratory. Although the sexes are separated in this species, there is no morphological sexual dimorphism (Ghobadyan et al. 2012). Each gonad sample was first placed in a Petri dish. The approximate longest tubule was measured with a measuring tape to the nearest 0.1 mm. The average width of the tubules was measured to the nearest 0.1 mm on digital pictures using a trinocular microscope (x 1.8 zoom) using the Motic Image Plus 2.0 software. Then the tubules were cut using a scalpel. Their content was placed within a concave blade to determine the sex and maturity stage based on the description of maturity stages by Conand (1981), whereby five maturity stages were established according to morphological parameters of the gonad tubules: immature (I), resting (II), growing (III), mature (IV) and post-spawning (V). Holothurians were defined as mature between stages III and V. For females, digital pictures were used to measure the diameter of approximately 10 oocytes to the nearest 0.1 mm using the same procedure as described above. 21 SPC Beche-de-mer Information Bulletin #39 – March 2019