Suspension culture of the great scallop Pecten maximus in Galicia, NW Spain—intermediate primary culture of hatchery produced spat

Journal of Shellfisheries Research, Jan, 2005 by Angeles Louro, Gyda Christophersen, Thorolf Magnesen, Guillermo Roman

ABSTRACT A technique for growing small P. maximus spat in suspension culture from rafts is described. Hatchery produced spat of initial size 3.0- and 4.3-mm shell height were transferred to a sea-based primary nursery system in May. Scallop spat (3.0 mm) grew to 16.8 [ or -] 3.0 mm during 85 days. Survival was 70.0 [ or -] 8.1%, but decreased due to presence of predators in 33.4% of the sampling units, to between 0 and 37.4%. Important predators were the crabs Atelecyclus undecimlineatus, Liocarcinus arcuatus, and Necora puber and the starfish Asterias rubens. A significant negative correlation was found between Asterias rubens arm length and survival of scallops. Initial shell height affected growth and survival significantly whereas stocking density (50-400 spat [quarter.sup.-1] and initial coverage of 1% to 17%) had significant effect on growth. Spat of 4.3 mm initial size were significantly bigger than the spat of 3.0 mm after 34 and 57 days, whereas survival was 96.4 [ or -] 4.4% versus 71.4 [ or -] 12.8% after 37 days. No mortality occurred during the second sampling period, but growth was negatively correlated to increased density of spat. The results give important information regarding development of economic production of scallops and showed promising opportunities for optimizing stocking density in intermediate primary culture. The choice of production methods in the next production step (secondary nursery stage) can be based on the results obtained.

KEY WORDS: Pecten maximus, great scallop, spat, intermediate primary culture, stocking density, mesh size, predation, Galicia

INTRODUCTION

The great scallop Pecten maximus is a highly appreciated and sought after species in Europe. The species has been heavily fished and the natural resources are generally depleted (Dao et al. 1999), which has brought about great interest in developing scallop aquaculture. The availability of spat is a key factor to successful scallop farming worldwide. Obtaining P. maximus spat from wild has been shown unreliable in Europe with the exception of a few sites in Ireland and Scotland (Dao et al. 1999). Commercial hatchery production of P. maximus was developed in France in the 1980s and later established in United Kingdom, Ireland, and Norway (Dao et al. 1999). Also in Galicia, Spain, the landings have decreased dramatically during the last years, and attempts to catch wild spat on collectors have not been satisfactory (Roman et al. 1987, Ramonell et al. 1990). Thus, to develop a commercially great scallop culture in Galicia, spat must be obtained from hatchery production or by import from another area.

Culture technique and handling practice will change according to production step and scallop size. The spat production techniques will likewise differ depending on the seed source, either from hatchery or natural collection. An experimental hatchery system has been developed at the Centro Oceanografico de A Coruna (COAC) in Galicia, Spain for producing scallop spat. Adult broodstock were brought into the hatchery for conditioning and spawning and the larvae reared in larval tanks before transfer to a settlement system for growth to a size of approximately 2 mm (Roman 1991). Further spat growth takes place in a nursery or intermediate culture system. According to Bourne & Hodgson (1991) the nursery stage are divided in 2, primary and secondary nursery. The primary nursery stage in Galicia has two growth phases. The first takes place in the hatchery and lasts from metamorphosis until a size of 2-4 mm, when the spat are transferred to the sea. The second phase is in suspension culture in the sea until roughly 20 mm shell height. The secondary nursery stage comprises juvenile growth from 20-60 mm shell height and final culture is to the commercial size of 100-mm SL.

The use of a primary nursery system is to ensure maximum growth and survival, but high mortalities of spat transferred from the hatchery to the sea are experienced (Bourne & Hodgson 1991, Grecian et al. 2000, Christophersen & Magnesen 2001). Both environmental conditions and husbandry practice affect growth and survival of scallops in suspension culture in the sea. Performance during early scallop spat stages are clearly affected by temperature, salinity, and diet (Paul 1980a, Paul 1980b, O. Foighil et al. 1990, Martinez et al. 1992, Heasman et al. 1994, Martinez et al. 1995, Heasman et al. 1996, Lu & Blake 1996, Laing & Psimopoulous 1998, O'Connor & Heasman 1998, Parrish et al. 1999, Robert & Nicolas 2000, Laing 2000, Christophersen & Magnesen 2001, Nicolas & Robert 2001, Laing 2002, Christophersen & Lie 2003, Christophersen & Strand 2003, Milke et al. 2004, Rupp & Parsons 2004). The effect of initial spat size, stocking density, type of equipment, fouling, and predation of spat in primary nursery are less documented. Parsons & Dadswell (1994) evaluated different gear types for intermediate culture of Placopecten magellanicus and also found an inverse relationship between growth and stocking density of 10-15 mm spat (Parsons & Dadswell 1992). Grecian et al. (2000) on the other hand did not find significant effects of density of smaller sized spat (1.5-3 mm) of the same species but an effect of gear type on growth. Initial spat size of P. magellanicus and P. maximus at deployment is important, as is the timing of deployment (Grecian et al. 2000, Christophersen & Magnesen 2001, Grecian et al. 2003). Although kept in a protected environment during suspension culture, the spat are exposed to predators and fouling organisms, because planktonic larvae can settle and colonize inside the equipment. Due to the small size of the newly transferred spat a wide range of predator species may feed on them (Dao et al. 1994), and, in particular, the presence of starfish may cause heavy losses (Levy et al. 1998, Nadeau & Cliche 2003).