Theme F: Enhanced knowledge on populations and ecosystems from releases of cultured animals
46. GROWTH, SURVIVAL AND REPRODUCTION OF SANDFISH HOLOTHURIA SCABRA RELEASED IN A PILOT SEA RANCH IN THE PHILIPPINES
Marie Antonette Juinio-Meñez, Ronald Dionnie Olavides, Marie Antonette Paña, Julissah Evangelio, Christine Mae Edullantes, Glycinea de Peralta
Marine Science Institute, University of the Philippines, Diliman Quezon City, Philippines 1101
meneza@upmsi.ph
Communal sandfish sea ranching was piloted in the Philippines to evaluate its viability as a source of supplemental income for small fishermen and help rebuild the spawning population of Holothuria scabra which one of the high-value sea cucumber species. Multiple batches (n= 10) of cultured sandfish (> 3 g) were released in a 5-hectare pilot sea cucumber sea ranch located in the seagrass area in Victory, Bolinao Pangasinan over a 16-month period starting from December 2007. All juveniles (n= 16,117) were tagged with flourochrome dyes and released in a 2,200 m-2 core release zone at the center of the sea ranch. Sandfish were sampled every 3-4 months over 26 months using belt transects to determine densities in different zones and size structure (length and weight), to estimate growth and survival. At the same time, the growth and survival of a single batch of juveniles released in three circular 100 m-2 pens ( n = 200 ind pen-1) in
the sea ranch were monitored by retrieving and measuring all surviving animals in each pen during each monitoring period.
Released juveniles were found only within the core release zone after four months, however, by the seventh month, many juveniles have moved to the outer reserve area, over 150 m away from release area. During this time, the maximum estimated density and biomass in the sea ranch were only 0.06 ind m-2 and 5.4 g m-2 respectively. Over the next months, density decreased progressively from the core release zone to the peripheral areas including the adjacent area outside the sea ranch. Distribution was very heterogeneous with densities consistently lowest in the southern portion of the sea ranch which has coarser substrate and lower sediment stability. These validate previous studies indicating active substrate or microhabitat selection by sandfish.
The estimated survival rate in the sea ranch varied widely during each sampling period (14- 44%) in part confounded by the timing of the juvenile releases and the low retrieval rates of smaller animals (< 40 g). Modal weight progression was fastest during the first seven months reaching a modal size of 160-199 g. The growth curves based on the estimated von Bertalanffy growth frequency (VBGF) over a 19-month period is shown in Figure 1. In the pens, average survival rate was 29 % (± 3) over the same period. Average total weight increment was also fastest during the first seven months (~25 g month -1) and leveled off at an average size of about 200 g after 10 months (Fig. 1). Total biomass ranged from 84–200 g m-2 in the pens suggesting that growth may have in part been affected by density dependent factors. Sharp decreases in the modal sizes after typhoons indicate that sandfish are sensitive to decreases in salinity and strong wave action. Recovery was fast after a storm in May 2009 which deposited a lot of suspended fine sediment and organic matter into the sea ranch. However, longer-term changes in the quality of sediment and grazing area after consecutive storms and protracted heavy rainfall in September-October 2009, resulted in a progressive decrease in the estimated total biomass of about 65% and 15% in estimated abundance within 5 months.
Notably, 31% of the sampled sandfish have reached size at sexual maturity (> 200 g) after only 7 months. Local managers observed sandfish in the sea ranch exhibiting the typical spawning posture and behavior on several occasions and during different times of the day. The estimated density of sexually mature animals in the sea ranch was 100 ind ha-1 by the seventh month, and reached up to 590 ind ha-1after a year and half. These demonstrate that a critical spawning population can be established in a suitable and well-managed sea ranch area in less than a year. Moreover, synchronized mass spawning at midday was documented in 23 February 2010 wherein 19.7% of the sandfish observed released gametes. In addition, spicule analysis revealed wild recruits (i.e. without flourochrome stains) in the sea ranch. Whether larval settlement and benthic recruitment are enhanced by the high density of adults in the sea ranch remains to be investigated.
Results of this study provide valuable insights in optimizing the ecological impacts of sandfish sea ranching. Multiple small batch releases and periodic selective harvests of animals > 320 g (minimum export grade size) and regular monitoring in community-managed sea ranch are good management practices. These promote genetic diversity conservation, maintenance of effective spawning biomass, increased fertilization success and larval supply that can contribute to the replenishment of the wild population. The pilot communal sea ranch has become a demonstration and learning site for effective area management within an integrated fishery management framework to restore depleted stocks and provide incentives for community participation in resources management.
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