Timing of mussel mortality and predator activity in sheltered bays of the Gulf of Maine, USA

Abstract

The timing of mortality in the mussel Mytilus edulis and the activities of its predators were monitored on the mid and low shore at three similar sites in sheltered bays. Experimental clumps containing an average of 15 mussels were attached to the rocky shore for 11–57 days during three summers. The arrival of predators, deaths due to predators and to other causes were recorded on successive visits to each clump. Failure time analysis was used to analyze the effects of site, intertidal height and years on: (1) the time of the first arrival of the predatory snail Nucella lapillus, (2) the time to the commencement of drilling, and (3) the time to the first mussel death due to Nucella. Attacks by crabs were rare, and no attacks by seastars were recorded. Analyses showed striking differences among sites, years, and height. At the site most affected by predators, Nucella arrived at clumps in less than 4 days, commenced drilling within 4 days, and killed the first mussel within eight days in all three years. At the least affected site, arrival and attacks by predators were so rare that the estimated median time of the first death was greater than 7 weeks. Rates of arrival, attacks, and mortality tended to be greater on the mid shore than the low shore. Analyses suggest mortality rates are very variable among seemingly similar sites and among years. Data on abundance of Nucella showed site-to-site variation, but no correlation with mussel mortality.