Predicting wave dislodgment of mussels: Variation in attachment strength with body size, habitat, and season

Breaking waves impose large hydrodynamic forces which may dislodge mussels and other organisms living on exposed rocky shores. We examined the effect of variation in attachment strength with size, habitat and season on the predicted probability of wave dislodgment of mussels Mytilus trossulus and M. edulis on an exposed shore on the Atlantic coast of Nova Scotia, Canada. Mussels on this shore are exposed to maximum significant wave heights of at least 7 to 9 m each winter and to maximal water velocities of at least 12 m s^-1. We used Denny's (1995) mechanistic approach to predict probabilities of dislodgment from measures of attachment strength of mussels. Predicted probabilities of dislodgment indicated that mussels of 10 to 25 mm shell length are vulnerable to dislodgment by water velocities of >7 m s^-1. As a result of variation in dislodgment forces, probabilities of dislodgment did not differ consistently between tidepools and emergent rock, or with mussel size. Attachment strength increased from July to February as mussels were exposed to stronger wave action, reducing the probability of dislodgment by a given water velocity. This study indicates that knowledge of patterns of change in attachment strength are necessary for prediction of probabilities of dislodgment of mussels.