Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aquaculture Improving scallop (Pecten maximus and Placopecten magellanicus) spat production by initial larvae size and hydrodynamic cues used in nursery system Réjean Tremblay a,* , Gyda Christophersen b,c , Jean-Bruno Nadalini a , Iften Redjah a , Thorolf Magnesen b , Sissel Andersen d a Institut des Sciences de la mer de Rimouski (ISMER), Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, QC G5L 3A1, Canada b Department of Biology, University of Bergen, P.O. Box 7803, NO-5020, Bergen, Norway c Møreforsking Ålesund AS, P.O. Box 5075, NO-6021, Ålesund, Norway d Institute of Marine Research, Austevoll Research Station, Sauganeset 16, NO-5392, Storebø, Norway ABSTRACT There are several factors affecting scallops during the metamorphosis process that could explain the relatively low post-larvae yield observed in hatcheries. Competent bivalve larvae respond to different settlement cues to undergo metamorphosis and without adequate cues, larvae delay their metamorphosis. The objective of this study is to improve the settlement ratio of the two scallop species, Placopecten magellanicus and Pecten maximus by physical cues associated with hydrodynamic conditions, stocking density in settlement units and larval size at time of transfer to settling units. For each treatment, physiological condition was determined by fatty acid analysis to determine the energetic reserves and structural lipids. We observed similar results for the two important commercial pectinid species and validate the hypothesis on the positive effect of increased flow rate and larval size after transfer to settlement systems on settlement success. Increasing flow rate also affects positively the physiological condition of settled post-larvae by a higher accumulation of total fatty acids in neutral lipid fractions. Furthermore, no effect of larval stocking density until 90 larvae cm -2 in the downwelling sieves was observed. To our knowledge this study is the first to characterize the effect of seawater flow rate on settlement success of different pectinid species cultured under similar conditions. The experiments were performed in a close to commercial scale and thus are relevant to industry situations. 1. Introduction Worldwide aquaculture scallop production has increased from around 0.5 million tonnes in the mid-seventies to 2.6 million tonnes in 2013–2015 (FAO, Online information, http://www.fao.org/fishery/ statistics/global-production/en), and in the North Atlantic Ocean the sea scallop (Placopecten magellanicus) and the great scallop (Pecten maximus) are the main scallop species commercially exploited. The total production of these two species reached a maximum of 0.37 million tonnes in 2004, but was reduced to 0.25 million tonnes in 2015 ac- cording to FAO. Spat supply is still one of the major limiting factors for the development of these aquaculture industries, and the collection of spat from nature has been unreliable or highly variable (Dao et al., 1999; Cyr et al., 2007; Shumway and Parson, 2016), thus stimulating intensive hatchery production (Bergh and Strand, 2001; Andersen et al., 2011). Moreover, hatchery production of scallop spat has until recent years been characterized by variable yields (Karney, 1991; Couturier et al., 1995; Robert and Gérard, 1999; Andersen et al., 2011). After several years of optimization, increased survival and lower variability in larval production have been achieved in P. maximus and P. ma- gellanicus hatcheries (Pernet and Tremblay, 2004; Pernet et al., 2005; Magnesen et al., 2006; Tremblay et al., 2007; Magnesen and Jacobsen, 2012). However, only around 20% of the competent larvae normally result in viable metamorphosed and settled post-larvae (Andersen et al., 2011, 2013). According to Galley et al. (2017), the difficulties in the culture of P. maximus are associated to the transition from larval to juvenile stage, showing low development synchronicity and variable survival. To achieve successful hatchery development of P. maximus and P. magellanicus, settlement and metamorphosis rates need to be in- creased and stabilized. There are several factors affecting scallops during the metamor- phosis process that could explain the relatively low post-larvae yield observed in hatcheries. Competent bivalve larvae respond to different settlement cues to undergo metamorphosis and without adequate cues, larvae delay their metamorphosis (Pechenik, 1990; Martel et al., 2014; Lagarde et al., 2018). In scallops, some chemical cues have been de- monstrated to increase settlement (Yvin et al., 1985; Chevolot et al., 1991; Mesías-Gansbiller et al., 2008; Galley et al., 2017), but generally https://doi.org/10.1016/j.aquaculture.2019.734650 Received 7 May 2019; Received in revised form 11 September 2019; Accepted 28 October 2019 * Corresponding author. E-mail address: rejean_tremblay@uqar.ca (R. Tremblay). Aquaculture xxx (xxxx) xxxx Available online 02 November 2019 0044-8486/ © 2019 Elsevier B.V. All rights reserved. Please cite this article as: Réjean Tremblay, et al., Aquaculture, https://doi.org/10.1016/j.aquaculture.2019.734650