684 



Fishery Bulletin 88(4). 1990 



Figure 9 



Linear regression of Mytilus i-dutis shell growth rate 

 on scope for growth for combined data from Avila 

 Beach (•) and Platform Holly (O) with a time lag of 

 3 weeks, y = 0.22x + 4.96, r =0.75. 



Discussion 



Spatial variation in phytoplankton biomass may affect 

 the growth and nutritional condition of filter-feeding 

 species along the California coast (larval fish Engraulix 

 niordax, Lasker and Smith 1977, O'Connell 1980; cope- 

 pod Calanus pacificus, Willason et al. 1986; anomuran 

 crab Emerita analnga, Dugan and Wenner 1985). We 

 found that mussel gr'owth increased with chlorophyll a 

 concentration, except during the fall and winter months 

 at Avila. The low growth rates at Avila during fall and 

 winter, despite chlorophyll a concentrations exceeding 

 4 fig/L, may reflect a leveling-off or decline in particle 

 ingestion rates associated with high seston concentra- 

 tions (Foster-Smith 1975, Widdows et al. 1979). 



Temporal patterns of growth were similar for mus- 

 sels at Avila and Holly (Fig. 6). These data suggested 

 that ingestion rate was not appreciably higher for- mus- 

 sels at Avila, despite the higher POC concentrations 

 there (generally >600 fig/L). In addition, both the high 

 background POC concentration (623 ^^glL, Fig. 5) and 

 the lack of correlation between chlorophyll a and POM 

 concentrations at Avila indicated the presence of a high 

 concenti'ation of nonphytoplankton particulates (e.g., 

 bacteria, microzooplankton, detritus) which may not 

 support rapid mussel gi'owth. MytUus edulis has shown 

 poor growth when supplied only with nonphytoplank- 

 ton food sources in laboratory experiments (Winter 

 1974, Williams 1981). 



The lack of a correlation between growth rate and 

 water temperature for grouped data from both loca- 

 tions is consistent with the view that water tempera- 

 ture is not an important factor influencing mussel 

 growth in California waters (Page and Hubbard 1987). 



The lack of a stock or genotypic effect on mussel 

 growth rate in our transplant experiments was not 

 sur'prising, as the distance between the study sites was 

 only about 120 km. Mytilus edulis has a planktonic 

 larval stage of about 3 weeks, and typical current 

 velocities of 0.5 km/hour (Chambers Group 1986) would 

 permit larvae to drift as much as 250 km. Mytilus 

 edulis also has the potential to delay metamorphosis 

 and to exist as a pediveliger in the plankton for several 

 days (Bayne 1964), facilitating genetic exchange be- 

 tween spatially separated populations. The transplant 

 experiments also indicated that postsettlement selec- 

 tion (Koehn and Hilbish 1987), which might result in 

 differences in growth rate between locations, was not 

 an important factor in this study. 



Our field growth-rate data generally conform to 

 predictions from the theoretical SFG analysis. This 

 analysis suggested that mussels at Avila and Holly 

 absorbed similar amounts of POM and had similar SFG 

 because seston concentrations were high at Avila (>4 

 mg/L) and mussels at this location had a higher rate 

 of pseudofeces production and lower absorption effi- 

 ciency than mussels at Holly. Shell growth rate corre- 

 lated with theoretical SFG after incorporation of a time 

 lag. SFG is a measure of the energy instantaneously 

 available for the gi'owth of soft tissue and shell. In small 

 mussels, shell growth rate is correlated with soft tissue 

 growth (Nielsen 1985). The time lag likely reflects the 

 time required for metabolic conversion of absorbed 

 energy and nutrients into the shell. 



The relationships between potential food availabil- 

 ity and mussel growth at Avila and Holly agree with 

 gener'al predictions developed pr'imarily tVom physio- 

 logical studies of A/, edulis in British estuarine envi- 

 ronments where the energy available for growth was 

 1'egT.ilated by the food quality, reflected by the percent 

 POM, rather than by quantity of the seston when 

 seston concentrations exceeded 4-5 mg/L (Bayne and 

 Widdows 1978, Widdows et al. 1979). The growth rate 

 of mussels in California coastal waters, with seston 

 concentrations comparable to those at Avila, is thus 

 likely limited by the quality rather than the quantity 

 of the seston. 



Acl<nowledgments 



We thank J. Dugan, D. Huhhard, A. Wenner, J. 

 O'Brien, and two anonymous reviewers for comments 

 on the manuscript. This wor'k was partially supporter! 

 hy a grant from the Atlantic Richfield Company. 



