Temporal and spatial variation in food availability and meat ratio in a longline mussel farm (Mytilus edulis) T. Strohmeier a, ⁎ , A. Duinker b , Ø. Strand a , J. Aure a a Institute of Marine Research, PO Box 1870 Nordnes, 5817 Bergen, Norway b National Institute of Nutrition and Seafood Research (NIFES), PO Box 2029 Nordnes, 5817 Bergen, Norway Received 27 November 2006; received in revised form 25 January 2008; accepted 28 January 2008 Abstract The influence of temporal and spatial variation in food availability on mussel meat ratio and biomass was studied in a longline mussel farm (100 m wide and 250 m long, Mytilus edulis) during an eight-month period. Current velocity and phytoplankton concentration were measured and mean mussel biomass, density, wet weight and meat ratio were determined. The longline farm aligned the current direction lengthwise through the farm and reduced the current speed and flow to approximately one half to one third of reference station. The mean fluorescence depletion in the centre of the farm was 11% and the phytoplankton concentration (cells L - 1 ) was 20 to 91% less in the centre of the farm compared to the reference station. The mean meat ratio increased 1.8 times through the spring phytoplankton bloom. The mean meat ratio (%) and biomass (kg) were spatially variable through the farm with low values in the centre and increasing values towards the edges of the farm. This variation in meat ratio and biomass was observed at all natural phytoplankton concentrations and attributed to spatial variation in food availability through the farm. © 2008 Elsevier B.V. All rights reserved. Keywords: Bivalve aquaculture; Current velocity; Food availability; Meat content; Mussel; Mytilus edulis; Seston depletion 1. Introduction The development of the mussel (Mytilus edulis) farming industry in Norway is based on the technology and methods of suspended longline culture and large sheltered coastal areas are potentially suitable for farming. However, the anticipated expansion and export volumes have not been realized, in part because of low meat ratio, probably related to overcrowded stocks and lack of husbandry knowledge. The growth of suspension-feeding bivalves is largely con- trolled by food availability (Winter, 1978; Bayne and Newell, 1983; Soniat and Ray, 1985; Berg and Newell, 1986), which in turn is affected by seston concentration, composition and trans- port rate (Incze and Lutz, 1980; Frechette et al., 1989; Blanco et al., 1996). Food availability is often coupled to phytoplankton dynamics (Rosenberg and Loo 1983; Smaal and van Stralen, 1991) and large volumes of mussels are typically farmed in areas with a high concentration of phytoplankton. Examples of high chlorophyll a (Chl a) concentrations are 4–12 mg m - 3 in Ria de Arousa (Figueiras et al., 2002), 8 mg m - 3 in Benguela Bay (Pitcher and Calder, 1998), 7.5 mg m - 3 in Oosterschelde, 4–22 mg m - 3 in Marennes–Oléron Bay (Dame and Prins, 1998) and 6.9 mg m - 3 in Chesapeake Bay (Dame and Prins, 1998). Several of these farming sites are shallow bays with high tidal amplitude leading to resuspension of organic material and an additional increase in food availability. In comparison, farming sites along the western coast of Norway are considerably deeper and resuspension of organic material available to mussels in suspension culture is likely to be insignificant since phytoplankton constitutes the major component of the seston in western Norwegian fjords (Erga, 1989a,b; Erga et al., 2005). The biomass of phytoplankton along the Norwegian coast follows a seasonal pattern with a period of algal blooms in late winter/early spring, late spring/ Available online at www.sciencedirect.com Aquaculture 276 (2008) 83 – 90 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. Tel.: +47 55236897; fax: +47 55238531. E-mail address: tore.strohmeier@imr.no (T. Strohmeier). 0044-8486/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2008.01.043