. El campo canadiense-naturalista. 2004 Notas 265 Figura 1. Abr May Figura 1. El porcentaje de horas de sol que Bay Mejillón camas son completamente sumergido en la costa de la Columbia Británica. Tiempo de inmersión de mejillones fue estimado por calculat- ing el número de horas que la marea estaba por encima de 3,7 m, a.c.d, utilizando tablas de marea de Canadá (1999), durante las horas de luz de día, entre el amanecer y el atardecer. pueden ser más vulnerables a los patos de buceo tales como Surf Scoter y Barrow Goldeneye como los patos no necesita gastar energía bucear para alcanzar los mejillones. Sin embargo, Smeathers y Vincent (1979) encontró que los mejillones expuestos a AI

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. The Canadian field-naturalist. 2004 Notes 265 FIGURE 1. Apr May Figure 1. The percent of daylight hours that Bay Mussel beds are completely submerged in coastal British Columbia. Mussel submergence time was estimated by calculat- ing the number of hours the tide was above 3.7 m, a.c.d, using Canadian tide charts (1999), during day- light hours, between sunrise and sunset. may be more vulnerable to diving ducks such as Surf Scoter and Barrow's Goldeneye as the ducks do not need to expend energy diving to reach the mussels. However, Smeathers and Vincent (1979) found that mussels exposed to air have twice the tensile strength of those submerged. If the exposed mussels have un- dergone some degree of desiccation, they may require more energy to remove than submerged ones, and may therefore, not be as profitable as first postulated. Alternatively, juvenile and sub-adult Surf Scoters and Barrow's Goldeneyes may be forced to feed on exposed mussels owing to the reduced mussel abundance and distribution from over-winter predation. The lower por- tion of the distribution of Bay Mussels is determined by biological factors, primarily predation from the Ochre Sea Star {Pisaster ochraceus) (Seed and Sucha- nek 1992; Quayle 1978) and sea ducks (Lacroix 2001). Through the winter, the combination of sea star and sea duck predation may eliminate the lower distribution of mussels, hence reducing their overall abundance. It is therefore plausible that the ducks observed were forced to forage on the less profitable prey (i.e., the ex- posed mussels) because there are few or no submerged mussels in the lower portion of their distribution. Acknowledgments We thank R. W. Butler and A. J. Erskine for making valuable suggestions to the manuscript. D. L. Lacroix received funding from a postgraduate NSERC schol- arship and from the Canadian Wildlife Service Geor- gia Basin Initiative. Literature Cited BeUrose, F. C. 1980. Ducks, geese and swans of North Amer- ica. Stackpole Books, Harrisbu