Modelling current speed and carrying capacity in long-line blue mussel (Mytilus edulis) farms Jan Aure,Tore Strohmeier & Òivind Strand Institute of Marine Research, Bergen, Norway Correspondence: T Strohmeier, Institute of Marine Research, PO Box1870, Nordnes, NO-5817 Bergen, Norway. E-mail:Tore.Strohmeier@imr.no Abstract The development of the mussel ( Mytilus edulis ) farm- ing industry in Norway is based on suspended long- line culture, and large areas of the coast are poten- tially suitable for farming. Norwegian fjords and coastal waters are regarded as oligotrophic environ- ments in comparison with sites where most studies on mussel feeding on natural seston have been car- ried out. High mussel culture densities in oligo- trophic water may cause seston depletion, resulting in low growth or tissue wasting due to reduced feed- ing and negative net energy balance. In this paper we present a carrying capacity model based on rate con- ditional processes, balanced against £ushing and with emphasis on £ow reduction as a function of farm design. The model is based on assumptions that friction forces are a function of geometric shape of the channel made up by the suspended mussel ropes as vertical boundaries and it quanti¢es carrying ca- pacity according to information of farm length, space between long lines, seston concentration and back- ground current speed and the relative importance of these factors. Estimates of how stocking density in mussel farming can be optimized in relation to the food supply (i.e. carrying capacity) are crucial to pro- duction management decisions, and the model may provide predictors for decisions regarding new site selection or expansion of existing operations. Keywords: carrying capacity, current speed, farm design, model, mussel culture, Mytilus edulis , seston Introduction The growth of suspension-feeding bivalves is largely controlled by food availability (Winter 1978; Bayne & Newell 1983; Soniat & Ray 1985; Berg & Newell 1986), which in turn is a¡ected by seston concentration, composition and seston transport rate (Incze & Lutz 1980; Frechette, Butman & Geyer1989; Blanco, Zepa- ta & Morono 1996). Food availability is also linked with phytoplankton dynamics (Rosenberg & Loo 1983; Smaal & Stralen 1991), and large volumes of mussels are typically farmed in areas with a high concentration of phytoplankton (Dame & Prins 1998; Pitcher & Calder 1998; Fuentes, Gregorio, Gira Ø ldez & Molares 2000; Figueiras, Labarta & Ferna Ł ndez Reiriz 2002) and resuspended organic matter. In many tem- perate coastal areas, limitations in nutrient availabil- ity may cause extended periods of relatively low concentrations of phytoplankton. After the algal bloom in early spring, the concentration of chloro- phyll a (Chl a ) along the Norwegian coast may be as low as o1^2 mgL 1 (Erga 1989; Frette, Erga, Hamre, Aure & Stamnes 2004), due to nutrient limitation (Paasche & Erga 1988). Coastal waters and fjords are relatively deep, and the resuspension of organic ma- terial available to mussels in suspension culture is probably insigni¢cant. Hence, Norwegian fjords and coastal waters are regarded as low-seston environ- ments compared with the sites at which most studies on mussel feeding on natural seston have been car- ried out (Grant, Emerson & Cranford 1997; Dame & Prins 1998; Pitcher & Calder 1998; Cranford & Hill 1999; Figueiras et al . 2002; Hawkins, Duarte, Fang, Pascoe, Zhang, Zhang & Zhu 2002). The development of the mussel ( Mytilus edulis ) farming industry in Norway is based on the technology and methods of suspended long-line culture, and large sheltered coastal areas are potentially suitable for farming. However, the anticipated expansion and export vo- lumes have not been realized, in part because of low meat content, probably related to overcrowded stocks Aquaculture Research, 2007, 38, 304^312 doi: 10.1111/j.1365-2109.2007.01669.x r 2007 The Authors 304 Journal Compilation r 2007 Blackwell Publishing Ltd