Berkeleyan
 A sea otter makes a meal of abalone off Pt. Lobos. Despite concerns expressed by free-diving abalone harvesters, the otter's range off the central California coast may not expand into the sportfishing waters of Mendocino County, due to the high esteem in which they are held by great white sharks. (Photo (c) 1999 by Don Baccus, dhogaza@pacifier.com) |
Along our coast, kelp, otter, and abalone have co-evolved
The otter's menu preferences have kept kelp from developing defensive toxins. And though the marine mammal's taste for abalone has long been blamed for declining stocks, human impacts are more severe, research suggests
| 17 November 2005
Abalone divers, who typically despise the California sea otter because of its voracious appetite for the largest and tastiest of the shellfish, may actually have otters to thank for the dinner-plate size of the prized delicacy.
Worldwide, abalone tend to be small, in the range of 2 to 4 inches across, according to evolutionary biologist David Lindberg, professor and chair of integrative biology and a former abalone diver himself. Along the California and Pacific Northwest coasts, however, abalone have grown much bigger, culminating in the 12-inch-diameter red abalone, Haliotis gigantea, avidly sought by divers and once the center of a thriving commercial fishery.
In the last century, however, a surge in the sea otter population killed off the commercial abalone industry along the central and southern California coast and made it hard for sport divers to find any abalone, let alone large, 12-inch ones. Many divers, fishermen, and even wildlife biologists worry that the otter is driving the abalone to extinction.
A new study of the interaction between abalone and California's coastal kelp forests, however, suggests that the sea otter played a key role in driving up the size of the abalone. Unlike kelp and algae in tropical waters, kelp in cold waters like those along the California coast have not developed toxic chemicals to ward off voracious grazers like sea urchins and snails. As a result, abalone, which live off the drifting, dead kelp so abundant along the Pacific Coast, thrive and grow huge on the highly nutritious food.
 A 40-mm tropical abalone, Haliotis glabra, is dwarfed inside the shell of the world's biggest, the cold-water Pacific species Haliotis rufescens, the prized red abalone. (David Lindberg photo) |
"We realized with this study that most abalone are small, averaging two inches across, and live on coral reefs in the tropical Indopacific," Lindberg says. "So why does the world's largest abalone live in the same place as this major predator, the sea otter? We think the abundance of kelp, the ability of abalone to stay hidden in crevices, and the predation of otters set up an ideal system to ratchet up the size of abalone."
Lindberg has always been puzzled by claims that the otters are driving the abalone to extinction, since it is known that sea otters and abalone have shared coastal waters for the past 5 million years. Humans are the newcomers: Middens (refuse piles) along the coast show that, historically, the California Indians also loved abalone and were the first to deplete the abalone population along segments of the coast.
"Since we can demonstrate that the large size increase took place in the presence of otters, the only hypothesis left as to the cause of the demise of a lot of abalone stock is human overfishing and probably destruction by humans of habitat," Lindberg says. "Getting an evolutionary grasp on a system gives us power in understanding its ecology and physiology, even if you are talking about management of wildlife stocks."
Lindberg and a longtime colleague, James Estes, a research scientist and marine ecologist with the U.S. Geological Survey (USGS) who works out of UC Santa Cruz's Long Marine Laboratory in Monterey, had often talked about the interactions along the coast among sea otters, abalone, and the abalone's principal food, kelp. Lindberg, a malacologist and former director of Berkeley's Museum of Paleontology, specializes in the evolution of mollusks. Estes, who has studied otters for some 35 years, has had a longstanding interest in the history of the kelp forests off the California coast. The two teamed up with former Berkeley postdoctoral fellow Charlie Wray, a molecular geneticist, to look at the evolution of abalone size and how abalone may have affected the evolution of kelp. (Estes, Lindberg, and Wray published their findings in the current issue of the journal Paleobiology.)
Estes earlier had suggested that, as the earth began to cool 45 million years ago, kelp moved out of their tropical homes into cooler waters in temperate regions of the Northern and Southern hemispheres. The abalone, which originated about the time the dinosaurs died out 65 million years ago, moved into the same temperate areas around the same time as the kelp. Perhaps because the kelp left behind many of the grazers that keep tropical kelp and macroalgae down, the temperate, cold-water kelp never developed the toxic chemicals that are typical of tropical kelp, and abalone were able to grow large on the abundant food.
"In tropical systems, abalone are little bitty things," says Estes. "But wherever one looks around the world where there are kelp-forest systems in cold oceans, that's where the bigger abalone occur. One of our major conclusions is that this has happened relatively recently in evolutionary time, starting around 5 million years ago."
Of eight abalone species now living along the Pacific Coast from Baja California to British Columbia, six - the red, black, pink, white, green, and flat - are all seven inches across or larger. Several species off the Japanese coast and in Antarctic waters also are significantly bigger than tropical species, which have remained small.
Interestingly, tropical kelp and algae have developed such toxic tissues that abalone no longer feed on them, but instead have turned nocturnal and feed on blue-green algae and diatoms that form a scum on reef surfaces. Over time, kelp from the north has even returned to southern waters, yet abalone still avoid it as if it contained the same toxins as the original tropical kelp.
To both Lindberg and Estes, though, the California sea otter stood out as an intriguing member of the kelp/abalone community. Fossil evidence shows that otters have long lived along the coast and eaten the biggest and best of the abalone. Otters today take stones and pound on the underwater abalone until the shell margins break, then pry the abalone off the rocks with their paws. Or they cave in the shells with a rock and come back later to peel the weakened abalone off the rocks. While in some situations predators can drive prey to become larger to better fight off attacks, in the case of abalone, getting bigger just made them more attractive as a food source.
"If size is not a refuge, why would you get big, especially where you have a convenient predator taking you out at the same time?" Lindberg asks.
A system out of balance
Hence the researchers' speculation that abalone grew large, in part, because of otters. The system lost its balance in the 1700s, when Russians first began slaughtering otters and fur seals for their pelts. By the time otter and seal harvesting was outlawed in the early 1900s, the otter was thought to be extinct. Abalone populations rose, and they moved into more- open spots.
"We got a taste for abalone in the absence of otters, and now that the otter has come back, we're blaming it as a threat to the abalone," says Lindberg.
A remnant population of otters off Big Sur eventually grew to populate the entire central California coast, though it has yet to move south in numbers to the Channel Islands, or north beyond San Francisco. Divers fear that the California abalone sport fishery is doomed if otters do move toward the Mendocino coast.
"If otters spread into Northern California, which is entirely a sport fishery now, a recreational fishery, a free-diving activity - you can't exploit them using scuba, just breath-hold free diving - it will be pretty much over," Estes says.
The good news is that sharks may be keeping them from spreading that far north, Estes says. He notes, too, that killer whales seem to have decimated otter populations along Alaska's Aleutian chain.
Lindberg says that the abalone/otter conflict is one of several unforeseen consequences of the Marine Mammal Protection Act, which was signed in 1972 and has led to a rapid rise in sea-mammal populations off the California coast. Seabird nesting areas are being impacted by a burgeoning sea- elephant population, while sharks drawn by the mammals increasingly are attacking humans.
"We make management decisions all the time without an evolutionary perspective on how the system assembled over time," Lindberg says. "An evolutionary perspective is critical for understanding ecological context."