First published in Landings, July, 2013.
Each season Maine lobstermen are setting new records for the pounds of lobster landed in the state. Last year a total of 126 million pounds were landed, an all-time record. These large volumes have caused people to think about new ways to deal with the abundant supply and limited demand through better marketing. But what effects does a high population have on the lobsters themselves?
That question led Noah Oppenheim and Rick Wahle to their 2009 study of lobster predation. “Rick had hours of footage of tethered lobsters in the ocean during the day from his study in 1992 that showed fish preying on lobsters. We wanted to see what animals were preying on lobsters both during the day and at night,” Oppenheim explained. “We tethered a small lobster and deployed it in shallow water with a video camera and infrared light and recorded.” What they saw surprised them. The most common predators during the day weren’t fish, as they were in Wahle’s previous experiments, but green crabs. And even more surprising was what was preying on the small lobsters at night – other lobsters.
“Cannibalism in lobsters is common in captivity and holding tanks, but it’s never been seen in the wild,” said Wahle. “We haven’t really seen shells in lobster’s guts to indicate that they prey upon each other. Sometimes there are small pieces of shell, but you have to remember that they eat their own molt.”
Wahle explained that lobster cannibalism does make sense, though. As the number of large fish that typically prey upon lobsters declines, the lobster population continues to grow. “Lobsters are outgrowing their predators much faster now so they have a higher survival rate,” he explained. “The overall abundance of predators (fish) hasn’t changed as much as people think. The small and juvenile fish are still there, it’s the large fish that have declined.”
With less pressure from predators, juvenile lobsters have a better chance of surviving. Unless they run into a bigger, hungry lobster, that is. “Lobsters have to compete with each other now for resources like food and shelter,” Wahle said. Because there are so many lobsters competing for food, when lobsters come across something that is edible, they eat it. “Cannibalism is common typically when populations are booming and there is over-crowding,” as seen in other animal populations, Wahle explained.
“With higher populations, lobsters have a higher encounter rate,” Oppenheim said. “Predation is higher now than it was 20 years ago.” His working hypothesis is that the increase in cannibalism is a direct result of the explosion of the lobster population. “The major question now is, is this strong enough to decrease lobster populations long-term? That is still a complete unknown.”
Oppenheim and Wahle plan to take the next step this summer to test if cannibalism is a direct result of population density. “We want to see if anything changes when we tether small lobsters in different depths and different areas of the coast,” said Oppenheim. The 2009 study took place in mid-coast Maine in less than 20 feet of water. Both Wahle and Oppenheim expect that at greater depths they will see different predators on film. “Green crabs were the main predator during the day,” Oppenheim said. “But they are restricted to the upper tidal zone, so we don’t expect to see them preying on a tethered lobster in deeper waters.”
The specific locations along the coast at which the small lobsters are tethered may also make a difference in the type of predators seen, according to Wahle. Daytime video footage from southern New England, for example, shows no indication of cannibalism in lobsters. “It may be because there is a greater diversity of fish in that area,” he said. “Also shell disease is more common in southern New England so they haven’t seen the boom in lobster population like mid-coast and Downeast Maine has.” If this summer’s video footage shows lobsters preying on each other, that would be a strong indication that cannibalism in lobsters is due to population density.Category: Science