The Philippine Agricultural Scientist Vol. 91 No. 4 (December 2008) 439
Gonad Index and Growth of Cultured of T. gratilla M.A. Juinio-Meñez et al.
THE PHILIPPINE AGRICULTURAL SCIENTIST ISSN 0031-7454
Vol. 91 No. 4, 439-449 December 2008
Effect of Type of Feed, Stocking Density and Grow-out site on Gonad Index, Growth and Survivorship of Cultured Sea Urchin (Tripneustes gratilla)
Marie Antonette Juinio-Meñez
1,*, Helen Grace P. Bangi
1,2and Maria Celia D. Malay
1,31Marine Science Institute, College of Science, University of the Philippines, Diliman, Quezon City 1101, Philippines
2College of Aquatic Sciences and Applied Technology, Mariano Marcos State University, Currimao Campus, Currimao, 2903, Ilocos Norte, Philippines
3Department of Zoology and Florida Museum of Natural History, University of Florida
*Author for correspondence; e-mail: meneza@upmsi.ph
Grow-out culture of sea urchins in sea cages was developed as a means to enhance the recovery of depleted natural populations and to provide a supplemental source of income for fisherfolk. To improve reproductive output and commercial value of hatchery-produced sea urchins, the effects of food type, stocking density and grow-out site on test diameter (TD) growth, survivorship and gonad index (GI) were determined. Sea urchins fed with Sargassum spp. had higher TD growth rates, GI and better gonad color than those fed with seagrass (Thalassia hemprichii). No significant differences were found in the average growth rates of Sargassum-fed sea urchins reared at low and high stocking density. However, survivorship and average GI were significantly higher in lower stocking density cages (200–250 sea urchins per m2). Reduction of density in the cages before harvest increased GI significantly. Survivorship in the sea cages was high in all grow-out sites except during the second experiment where mass mortalities occurred due to low salinities. Grow-out site and period had significant effects on GI but not on TD growth. Possible effects of environmental and temporal factors, and implications of the results on the resource management and culture of sea urchins are discussed.
Key Words: feeding, gonad index, grow-out culture, stocking density, Tripneustes gratilla
Abbreviations: GI – gonad index, HSD – high stocking density, HINP – Hundred Islands National Park, LSD – low stocking density, TD – test diameter
INTRODUCTION
The world production of sea urchins reached its peak in 1995 with an estimated total production of 120,306 tonnes fresh weight (Andrew et al. 2002). World production de- clined thereafter to 17,664 tonnes in 2005 (Food and Agri- culture Organization 2007). Major fishing areas have been depleted or have reached their maximum, while potential new fishing grounds are limited. The high demand for sea urchin roe, particularly in the Japanese market, cannot be met by the declining sea urchin fisheries production. As in many other places in the world, sea urchin fisheries production in the Philippines has declined. Based on avail- able export volume data, the estimated sea urchin produc- tion in the country peaked at 60,545 kg in 1997 and steadily decreased to its lowest volume in 2001 with 3,173 kg. The export volume increased in subsequent years but was only 7,978 kg in 2006. Moreover, there was also a shift in the
type of sea urchin product being exported. From 1994 to 1999, sea urchin paste comprised the bulk of export volume but thereafter, sea urchin roe in frozen, dried, or salted form became the major exported product (Bureau of Agricultural Statistics n.d.).
Tripneustes gratilla is the most important commercial sea urchin species in the Philippines. For example, the col- lapse of the multimillion-peso fishery of this species in Bolinao resulted in substantial losses in terms of livelihood among many fisher families who depend on the fishery for their primary source of livelihood, as well as local traders and exporters (Talaue-McManus and Kesner 1995; Juinio- Meñez et al. 1998). Overharvesting has depleted spawning stocks in Bolinao and other areas in northwestern Luzon.
Because these populations are likely tightly linked by lar- val exchange (Juinio-Meñez and Villanoy 1994), overexploitation may have severely decreased the resil- ience of natural populations to withstand fishing pressure
