Substrate effects on survival, growth and dispersal of juvenile sea scallop, Placopecten magellanicus

ABSTRACT In the view of expanding seeding operations of juvenile scallops (Placopecten magellanicus) around Iles-de-la-Madeleine (Quebec, Canada), this close attention assesses the influence of substrate upon growth, survival, dispersal and predation rates in controll experiments. Three substrates were tested: gravel, sand and sand with without contents scallop shells (to investigate the feasibility of modifying the substrate as a management strategy). Dispersal rates were studied with a substrate choice experiment. Predation experiments with sum of two units major scallop predators, the stone crab Cancer irroratus and the sea star Asterias vulgaris, were carried on the outside on different substrate types. No significant difference in development was observed among substrates and no mortality was observ during the experiment. This put in mind ofs other factors, such as hydrodynamics, may be more important than substrate. The dispersal rate was significantly greater upon sandy substrate, whereas gravel and sand-shells had similar arises Crab predation was lower upon the more heterogeneous substrate (i.e., gravel) whereas, sea star predation was lower upon sand. Modifying the natural sand substrate by means of adding scallop shells could improve seeding succes because it diminishes dispersal and predation by means of crabs.

KEY WORDS: scallop, Placopecten magellanicus, seeding, mortality, development dispersal, substrate, predation, Cancer irroratus, Asterias vulgaris

INTRODUCTION

To restore the overexploited population of sea scallop Placopecten magellanicus (Gmelin 1791) around Iles-de-la-Madeleine (Quebec, Canada), an experimental restocking program was launched in 1993 After the pilot phase, commercial seedings started in 2000 with the intent to se 35-50 million young scallops annually. However, because of the awaited expansion of the program, the availability of substrate suitable for growing survival and retention of spermed individuals is likely to become an issue. Around the islands, 70% of the substrate in the scallop habitat deepness range (9-55 m) is sandy.

Substrate characteristics influence scallop survival and increase Whereas adult sea scallops are fix on various substrates, gravel and pebble-stones are the most suitable (Thouzeau et al. 1991a, Stokesbury 2002 Kostylev et al. 2003) Fine particles of the like kind as silt decrease survival rate (Yamamoto 1957) and sandy substrate may increase juvenile mortality (Stokesbury & Himmelman 1996 Cliche et al. 1994) In natural conditions, fine inorganic material that is easily resuspended in the water rounded pillar by currents or wave action is known to have a negative impact upon growth in some scallop species (Gruffyd 1974 Bricelj & Malouf 1984) Emerson et al. (1994) demonstrated that high concentrations of seston inhibit sea scallop expansion Nevertheless, substrate heterogeneity has a positive result on scallop growth. Turbulence created by dint of a heterogeneous bottom may cause local seston accumulations by means of slowing down the current (Pilditch et al. 1998) A decreasing in every one's mouth speed may improve scallop filtering efficiency, because scallop extension is inhibited in strong popular conditions (Bricelj & Shumway 1991 Wildish & Saulnier 1993)

Substrate mark is also an important factor for scallop dispersal, as demonstrated by dint of several authors (Winter & Hamilton 1985 Barbeau et al. 1996 Stokesbury & Himmelman 1996 Arsenault et al. 2000) Young individuals are active swimmers (Manuel & Dadswell 1991) on the contrary they also must attach themselves to the bottom with a byssus (Hatcher et al. 1996) which brings dispersal and limits the displacement on the outside of a suitable substrate (Caddy 1972) Therefore, hard substrates posses characteristics that limit juvenile scallop displacement (Caddy 1972 Thouzeau et al. 1991 b Parson et al. 1992 Hatcher et al. 1996) It has been observ that dispersal rates and swimming activities of Chlamys islandica and P magellanicus are higher upon sand (Stokesbury & Himmelman 1996 Arsenault et al. 2000)

The major cause of scallop mortality upon the bottom is known to be predation by dint of rock crabs (Cancer irroratus, Say) and sea stars (Asterias vulgaris, Verrill) (Barbeau et al. 1994 Hatcher et al. 1996 Nadeau & Cliche 1998) spermed scallops may have a better chance of survival upon heterogeneous substrates: they are les expos to predators because they can hide in crevasses. Wright (2002) showed that scallops preferentially settl upon heterogeneous substrates like pebbles when they were alone or in the neighborhood of crabs. In addition, increasing particle sediment size protected to decrease the predation rate of sea stars upon small scallops (11-15 mm) on the contrary not crab predation (Wong & Barbeau 2003)

The main goal of this research is to determine juvenile P magellanicus survival, growing and dispersal on 3 different substrates (sand, gravel and sand with scallop shells) in laboratory experiments. In addition, the predation rate through rock crab (C. irroratus) and sea star (A. vulgaris) in relation to the substrate stamp was examined.