The sea cucumber holothuria arguinensis, as a new species for aquaculture

Resumen

Sea cucumbers have an important economic value due to high demand from the Asiatic markets, as they are considered as delicacy and functional food, and used in traditional Chinese cuisine and medicine. The heavy market demand, uncontrolled exploitation and inadequate fisheries management, have led to many sea cucumber stocks becoming heavily over-fished and driven to catch new target species around world. Nowadays, the fishery pressure is also focused on species from the NE Atlantic Ocean and Mediterranean Sea. As fisheries expanded around world, sea cucumber aquaculture started to be developed in China in the 80s to supply bêche-de-mer (dry sea cucumber) to the Asiatic market, and as a way to reduce the fishery pressure on wild populations and to allow restocking for enhancement of depleted populations. Traditionally, sea cucumbers have been farmed in earthen ponds and in sea reaching systems, although sea cucumbers have been also integrated in polyculture systems with other species. Sea cucumber aquaculture in Europe has an important potential for its development, incorporating high values species to this growing sector and including sea cucumbers in integrated multi-trophic aquaculture. Holothuria arguinensis is a sea cucumber species distributed in the North Eastern Atlantic (from Peniche (Portugal) to Mauritania, including the Canary Island, (Spain)), and recently colonizing the transitional waters in the Alborán Sea (SW Mediterranean Sea). This species has been illegally harvested in Portugal and Spain due mainly to the high prices, 70 – 350 €/kg dry weight, that can reach depending on product quality. H. arguinensis has an adequate nutritional value, similar than other commercial sea cucumber species, being suitable for human consumption. Therefore, H. arguinensis could be a right species for aquaculture development. The aim of this Thesis was to assess the different subjects needed to settle the baseline for H. arguinensis aquaculture. Broodstock maintenance in tanks was assessed and artificial diet based on seagrass debris was established to ensure its growth during spawning season. Induced spawning, and larvae and juveniles rearing protocols were also assessed. In addition, larvae growth and survival were evaluated using different microalgae diets. The culture stocking density and critical biomass for H. arguinensis were determined, and its habitat preferences in the coastal lagoon Ria Formosa assessed to establish preferential areas to allocate grow-out systems. As last aim, H. arguinensis monoculture was compared with different combinations of polyculture systems with the green macroalgae Ulva lactuca and the purple sea urchin Paracentrotus lividus. Additionally, the feasibility to use sea bream (Sparus aurata) biodeposits as food source for H. arguinensis was preliminary assessed. H. arguinensis broodstock can be maintained in tank-based conditions with sediment at the bottom, increasing their weight (Specific Growth Rate (SGR) = 0.2 %/d) by feeding on the supplied sediment (mean feeding rate: 27.88 g/ind day) and showing high values of absorption efficiency (80%) in terms of organic matter. H. arguinensis is highly influenced by seawater temperature and salinity, which might induce itse entrance in a hibernation period. During this time, H. arguinensis reduces its feeding, movement, absorption efficiency and growth, once the seawater temperature drops under 19ºC. During broodstock maintenance, artificial feed made of seagrass debris (Zostera noltii) can be used to ensure growth during this period (SGR = 0.09 ± 0.06 %/d, Absolute Growth Rate (AGR) = 0.11 ± 0.07 g/d). However, seagrass debris of Cymodocea nodosa seems to be an unsuitable food source for H. arguinensis, since it showed negative growth. Independently of the seagrass species and the proportion used in the diets, any important change on H. arguinensis nutritional values was registered. H. arguinensis can be induced to spawn by thermal stimulation during the summer months along its reproductive period (June-October). The embryonic and larval development were followed and described. H. arguinensis, as most aspidochirote holothurians, exhibited the classic five larval stages (early, mid and late auricularia, doliolaria and pentactula), reaching the juveniles stage after 18 days. Low mortality was registered during the pelagic larval stage, however high mortality (85-95%) was recorded at doliolaria and juvenile stages during the first year. This survival has been improved in the followed years, increasing the juveniles´ production. Feeding diets based on single microalgae (Chaetoceros calcitrans, Isochrisys galbana and Tetraselmi chuii) were compared to combined microalgae diets (i: C. calcitrans and T. chuii (C:T), pre-stablished feeding diet, and ii: C. calcitrans, T. chuii and I. galbana (C:T:I)) to improve growth and survival of H. arguinensis. The larvae fed with the combined microalgae diet of T:C:I showed higher survival and larger larval and stomach sizes than the ones fed with the single microalgae diets and the pre-established microalgae diet (C:I). The stocking density is one of the main parameters affecting sea cucumbers´ growth, feeding and survival. Juveniles of H. arguinensis showed significant higher growth (SGR = 1.25 ± 0.08 %/d, AGR = 0.94 ± 0.05 g/d and %WC = 104.66 ± 9.98 %) at the stocking density of 5 individuals/m2 (1 ind./0.2 m2), decreasing significantly as the stocking density increased. The critic biomass (471.65 g/m2) was reached in four weeks. Additionally, H. arguinensis habitat preferences were assessed in the Natural Park Ria Formosa coastal lagoon (South Portugal). H. arguinensis was preferentially distributed along the lower intertidal zone, linked to Z. noltii meadows on muddy and sandy bottoms, where the denser population and the largest individuals were found. This habitat selection by H. arguinensis could be linked to the larger number of emersion hours and exposition to high temperature and UV-irradiance of the upper intertidal zone. Therefore, areas located in the lower intertidal zone with Z. noltii meadows on sand-muddy bottoms should be selected to allocate sea pens to grow-out H. arguinensis. H. arguinensis monoculture growth, productivity and economic benefits were compared with three polyculture systems where the green macroalgae Ulva lactuca and the purple sea urchin Paracentrotus lividus were combined. H. arguinensis co-cultured with U. lactuca and addition of artificial feed (dry powder of U. lactuca) showed the highest growth, productivity and economic benefits for both cultured species. The three polyculture systems produced larger economic benefits than the monoculture systems, therefore these systems could be implemented to improve productivity, in terms of biomass and profits. Additionally, H. arguinensis feasibility to use the sea bream Sparus aurata biodeposits as food source was assessed to perform further research of co-culture. H. arguinensis under this condition showed low feeding rate, negative absorption efficiency and growth, which might be highly influenced by the low seawater temperature during the experiment performed in winter. However, H. arguinensis could be co-cultivated with sea bream, since in previous experiment showed high feeding rate and absorption efficiency, but further research should be done during spring-summer months. Based on the results obtained from this Thesis, it can be confirmed that H. arguinensis is a highly suitable sea cucumber species for aquaculture and integrated multi-trophic aquaculture. This Thesis has addressed many different and important aspects needed to develop sea cucumber aquaculture, focused on H. arguinensis, being the first complete study dedicated to a North-Eastern and Mediterranean Sea cucumber species. In this way, this Thesis has established the baselines for H. arguinensis´ aquaculture, and further research will improve its aquaculture production.