Development of the mussel aquaculture lease site model MUSMOD ©: a field program to calibrate model formulations

Abstract

Field observations were used along with mathematical modelling to develop model formulations to represent mussel ( Mytilus edulis) growth at several shallow subtidal study sites along the Maine coast, USA. In order to match predicted growth from the model MUSMOD © (D.E. Campbell, C.R. Newell, 1997. MUSMOD ©: A mussel production model for use on bottom culture lease sites. J. Exp. Mar. Biol. Ecol., in press.) with observed growth rates in the field, modifications were made in the model with respect to: (a) factors affecting the supply of food to the mussel feeding zone (vertical mixing, settling, resuspension and particle depletion); (b) short-term variability in mussel feeding and respiration over a tidal cycle; (c) the effects of seasonal variations in food quality on assimilation, scope for growth, and observed growth. Surface and bottom water samples taken from 1989 to 1991 revealed daily fluctuations in SPM, POM, chlorophyll a, particulate carbon and nitrogen, and phytoplankton carbon which were similar to annual ranges. Maine waters are typified by a spring diatom bloom, followed by a rise in detritus in early summer. Successful representation of mussel growth was obtained when food concentration was expressed as phytoplankton and detrital carbon, and assimilation was modelled as a function of food type (phytoplankton or detritus), and detritus quality (as percent of maximum annual N/C ratio of the detritus). By expressing particle depletion as a percent reduction from conditions at the edge of the lease site, scope for growth was modelled over an annual period. Observations with a time-lapse benthic video monitor (TLBVM) and flow-through physiological chambers demonstrated tidal variations in filtration rate, respiration rate and shell gape in subtidal mussels, with periods of valve closure correlated with low particle concentrations. These observations support the hypothesis that mussel energy gain is maximized during fluctuating food availability by the control of pumping rate via the shell gape response. This may explain why field observations of filtration rate are sometimes lower than maximum published values, and allows the mussels to maintain a slightly positive scope for growth by reducing respiratory losses when food particles are limiting. The settling flux of phytoplankton and detritus, while poorly characterized in coastal New England waters, provides over 30% of the estimated food supply to subtidal mussel cultures and may explain, in part, the patterns of shell gape observed in this study.