Abstract
A popular hypothesis, that on the south coast of Nova Scotia the green sea urchin (Strongylocentrotus droebachiensis) has been released from control by predators, is critically reviewed. Predator control is given the functional definition of limiting sea urchin grazing sufficient to permit abundant growth of seaweed. Of the predators identified, the American lobster (Homarus americanus) is the least likely to have been important in sea urchin control in recent decades. Feeding rates, stomach contents, and field biomass are available for this species. A smaller amount of data available for the rock crab (Cancer irroratus) and benthic feeding fish points to the conclusion that they likewise cannot control urchin populations. Among other temperate sea urchin species the case is strongest for sea otter control of Strongylocentrotus sp. in some areas of the west coast of North America. The formation of sea urchin grazing fronts, a usual step in the destruction of seaweed beds, has been explained as a behavioral response to predators. This may in fact be merely a response to the location of food. Evidence for suspected enhancement of lobster production by seaweeds is equivocal, and comparative field data collected in and out of seaweed beds (e.g. lobster abundance, food, growth, and survival) have not been published.
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