Factors influencing recruitment of hatchery reared pearl oyster spat

Journal of Shellfisheries Research, Jan, 2005 by Pedro E. Saucedo, Horacio Bervera-Leon, Mario Monteforte, Paul C. Southgate, Pablo Monsalvo-Spencer

ABSTRACT We evaluated experimental collection of Pinctada mazatlanica spat on substrates of different textures and colors placed at different depths within settlement tanks. When larvae reached the pediveliger stage (day 25), black-colored spat collectors ("envelope" type) composed of the following materials were offered as settlement substrates: onion bag, mosquito net, fishing net, and 63% shade-cloth. Spat collectors measured 30 x 30 cm and were composed of an outer bag and inner substrate made of the same material. The influence of the color of spat collector material on recruitment was investigated using a second type of spat collector composed of onion bags as the outer bag and inner substrate ("bag" type). Bags were prepared in 3 different outer/inner (O/I) color combinations of green/green, green/black, and red/black (O/I). Both types of collectors were deployed at different depths within settlement tanks. Approximately 2.7% of the initial larval population survived, resulting in 35,583 spat. Of these, 71.3% recruited to bag type collectors, 21.9% to envelope type collectors, and 6.8% to the surfaces of the culture tanks. Spat recruitment was significantly affected by collector material (P < 0.01), with fishing net and shade-cloth yielding highest and lowest density of spat, respectively. Similarly, spat collector color combination significantly influenced spat collection (P < 0.01), with higher recruitment to red and green substrates. For both spat collector types, there was significantly greater (P < 0.01) recruitment of spat to collectors in the middle of the water column (60-90-cm depth). This study showed that both the type and color of material used for spat collectors, as well as depth of deployment, influenced recruitment of P. mazatlanica spat. These results will help fine-tune current hatchery techniques for this species.

KEY WORDS: pearl oyster, Pinctada mazatlanica, spat, recruitment, hatchery culture

INTRODUCTION

Settlement and metamorphosis in marine bivalves is a complex process influenced by physical and chemical characteristics of available substrates and environmental factors such as water temperature, light intensity, and currents (e.g., Crisp 1974, Gray 1974, Hadfield 1978). Under hatchery culture conditions, several natural and artificial materials have been investigated as substrates for collection of bivalve spat, including monofilament gillnet, PVC, glass, plastic, nylon rope, onion bag, asbestos natural shrubs and leaves, and bivalve cultch (e.g., Pearce & Bourget 1996). In general, concave or convex substrates seem to be more attractive for larval settlement than flat or even substrates; similarly, poorly-illuminated areas are preferred to clear or illuminated surfaces (Ajana 1979, Phelger & Cary 1983, Alagarswami et al. 1983, Rose & Baker 1994, Taylor et al. 1998, Doroudi & Southgate 2002).

Despite the high commercial value of pearl oysters (Fassler 1991), there have been few studies assessing settlement substrates for hatchery production of the 3 main pearl producing species, Pinctada fucata (Alagarswami et al. 1983, Alagarswami et al. 1987), P. margaritifera (Alagarswami et al. 1989, Southgate & Beer 1997) and P. maxima (Minaur 1969, Rose & Baker 1994, Taylor et al. 1998). These studies have generally attempted to define physical and chemical stimuli for larval settlement to optimize hatchery production protocols. This goal has been achieved for P. fucata in India (Alagarswami et al. 1987, Dharmaraj et al. 1991, Victor et al. 1995) and more recently in Australia for P. maxima (Rose 1994) and P. margaritifera (Southgate & Beer 1997, Doroudi & Southgate 2003). In these countries, hatchery-reared spat are now used for commercial pearl production.

In Baja California, Mexico, over-exploitation and depletion of natural beds of pearl oysters prompted research into cultivation of the 2 native species, P. mazatlanica (Hanley) and Pteria sterna (Gould). Because cultivation based on wild spatfall has been successful (Monteforte 1996), research into hatchery culture of these species has been scarce and discontinuous (McAnally & Valenzuela 1990, Serrano & Salinas 1993, Araya et al. 1995, Martinez-Fernandez et al. 2003) and reliable methods for large-scale hatchery culture have not yet been developed. On this basis, many aspects of larval behavior during settlement, metamorphosis, and recruitment are still unknown for these species, particularly P. mazatlanica. In an effort to generate information that will allow more reliable hatchery culture of this species, this study evaluated the influence of spat collector material and color and their depth on recruitment of hatchery-reared P. mazatlanica spat.

MATERIALS AND METHODS

Larval Rearing

Gametes were obtained from ripe P. mazadanica in July (summer) and larvae were reared according to methods described by Robles et al. (2000). Larvae were stocked at an initial density of 2 m[L.sup.-1] in duplicate 5,000-L, white, conical fiberglass tanks containing 1 -[micro]m filtered, gently aerated seawater (~37 [per thousand]). They were fed a 1:1:1 mixture (by cell number) of Isochrysis galbana, Chaetoceros muelleri, and C. calcitrans at total densities of 30 x [10.sup.3] cell m[L.sup.-1] from day 1-10 and 60 x [10.sup.3] cell m[L.sup.-1] from day 11 to settlement. Culture tanks were drained, washed, and filled with clean seawater every 3 days and water temperature was maintained at 26 [ or -] 1.5[degrees]C under a 12h:12h (light:dark) photoperiod.