From the Russian River Times November 2010
Late each summer, great white sharks return from their Northeastern Pacific ‘Café’ to the same locations on the California Coast. One of the major destinations is a triangular area generally defined by Anno Nuevo to the south, the Farallon Islands to the West and the mouth of Tomales Bay, home to many elephant seals, sea lions and harbor seals; a major source of prey for the Great Whites. A recent paper from the Royal Society of Sciences shows how electronic tracking of tagged sharks is providing new insight into the behavior of this important predator.
Tracks of sharks moving between sites off the California Coast (Royal Society 2009) The thickness of the lines indicate the number of times sharks moved back and forth between locations, with the greatest number of patrols between REY (Point Reyes) and across TOM (mouth of Tomales Bay) A total of 179 sharks were tagged and the numbers in the circle represent the number of sharks with failed tags who were identified photographically. San Francisco Bay is located to the right of the image.
The illustration is from “Philopatry and migration of Pacific white sharks, Jorgenson et.al, 2009 Proceedings of the Royal Society,” published on-line where you can read the full explanation of the chart and see how the sharks move between the Café and the coast on-line. All web links are published at the end of the article, and given the recent fatal attack on a surfer on 22 October which occurred just north of Santa Barbara, readers may wish to read more in the Shark Research Committee information sheet on shark predation, included in the list of links.
Each winter, the sharks leave their coastal sites and swim thousands of kilometers to what the paper calls the ‘Shark Café’, a name selected by the authors to imply an area where foraging and breeding might explain why these predators make the long, energetically costly migrations from the coast to the ‘Café’ and back. It is located within the eastern boundary of the Pacific Gyre, in an area where the paper hypothesizes that ‘shoaling’ of the oxygen minimum layer concentrates pelagic (living in the open ocean) fish within a shallower layer of the ocean. And, as summer ends, the sharks return to the coast to the areas where there are the most pinnipeds (elephant seals, sea lions and harbor seals), for which the great whites are atop the food chain as the apex predator.
Do the Great Whites take a bite out of Park Service claims against the Drake’s Estero oyster farm?
The shark research came to light as part of the Russian River Times coverage of the attempts to shut down the historic oyster farm located within the Point Reyes National Seashore. The Park Service has based its claims of harm to the seal population on one of their own research papers which purport to show that disturbances by oyster workers are causing a decline in population. Nowhere in the papers does it mention predation as a significant factor in seal population. However, California Fish and Game lists Great Whites as “playing a crucial role in the marine ecosystem, helping to suppress pinniped populations. The only real threats white sharks face are from humans, as well as the occasional killer whale”.
Despite the fact that the NPS website for Point Reyes (listed below) references over 23 years of observational sightings and 11 years if decoy surveys, and notes that the white sharks seem to be feeding mostly on harbor seals and sea lions, none of this is mentioned in the NPS paper blaming a decline in harbor seals on the oyster operation. Apparently, none of the NPS and related papers on shark predation appear to have been presented to the National Academy of Sciences who were requested by Senator Feinstein, to review the NPS science on the estero, except for one paper on trends in pinniped population dynamics, (Sydeman/Allen 1999) which makes an oblique reference to an increase in predation of juvenile and subadult elephant seals by white sharks on the South Farallon Islands and that shark predation may also be limiting seal populations.
None of the specific papers on shark predation in the areas adjacent to the Estero were provided to the Marine Mammal Commission panel currently reviewing the oyster farm and the seal population, nor to the earlier half-million dollar NAS study that concluded that the NPS had misrepresented and misstated the impacts of the oyster farm, which they found to have no significant negative effects from the oyster farm, and that the seals in the estero should be treated as a single population. Other crucial data has also been withheld from researchers reviewing the estero, including two and one half years of automatic cameras.
Researcher says Great White Population may be Recovering
To understand the role the Great Whites play in the overall ecosystem, the Russian River Times talked with Dr. Chris Lowe, director of the Shark Lab at CSU Long Beach. He and his colleagues have just completing a study of California Great White population, to be published in a forthcoming paper. “You can’t do biological population studies without understanding the role of predators and the Great White is the apex predator at the very top of the ocean food chain”, says Dr. Lowe. Predators fulfill a vital role in the overall health of their prey. They weed out the unhealthy animals, making sure that the fittest survive and breed, and keep the system in balance. If there is an overabundance of pinnipeds because of a lack of their predators, they will deplete the fish stocks on which they feed, which, along with environmental changes caused by other events like the El Nino cycles, can lead to starvation.
Dr. Lowe and colleagues studied fishing records back to 1936, documenting how the fishing industry has interacted with sharks. The record shows that responsible fisheries management is benefiting the Great Whites, as well as the targeted fisheries. Bycatch (fish accidentally caught when fishing for other stock) increased dramatically with the introduction of monofilament gill nets, used just offshore to harvest halibut, sea bass, white croakers and others. In the halibut fisheries, this highly indiscriminate method had a target species catch of only 11.2% and a discard rate of 67.8%. Fatalities included not only immature Great Whites, who feed immediately off-shore, but seabirds, and marine mammals, with California Fish and Game reporting that in some years, over a thousand pinnipeds were killed. Fortunately, the use of monofilament gillnet fishing has been banned in California state waters since ’94, as well as in nearly all other coastal states.
In addition to the gillnet ban, taking of Great White sharks has been banned in California waters since 1997. By comparing bycatch in offshore areas where gillnet fishing is still permitted, Dr. Lowe recorded an increase in the Great White shark bycatch, indicating a recovery of the shark population. In addition, the Monterey Bay Aquarium, which collaborates with a number of southern California fishermen to bring bycatch Great Whites (all immature) for tagging and release, shows that most of them are surviving. But how do they feed? Sharks go for the ‘fat content’, says Dr. Lowe. “Imagine there’s two tins of brownies….one has all the butterfat and sugar, and the others are lowfat. After one sample, you know which ones taste best. However sharks don’t have hands and do their tasting with a large bite. Immature Great Whites (~ 8-10’ long) will attack sea otters, but will spit them out, because the otter is a furry, muscular animal, with comparatively very little body fat. The reason sharks scavenge off dead whale carcasses is because of the high blubber content.”
This bite and spit approach is often fatal, even if the shark decides not to eat what it has just sampled. The Great White ‘ambushes’ its prey, by cruising along the bottom, apparently relying on the silhouette of a pinniped on the surface. Researchers suspect that this is why divers and surfers are vulnerable. The shark uses different techniques depending on the size of the prey. Its preferred prey is the automobile sized adult elephant seal, where it will lunge upwards, take the largest possible bite out of the hindquarters to disable the prey and wait for it to bleed to death before tearing off chunks and eating them on the bottom. Smaller pinnipeds, attacked on the surface, are pulled to the bottom till they stop struggling. Juvenile harbor seals are often eaten whole. Nature is not always gentle in the struggle to survive.
Dr. Lowe says that his studies show that responsible fisheries management can benefit the whole ecosystem, including the Great Whites, even though they are not a targeted fishery, and that it is really important to understand the entire food chain, especially the apex predators like the Great Whites. “ I nearly always end my talks by reminding people that we humans are the top of the food chain, and will bear the consequences of our actions. Toxic pollutants get concentrated as you move up the food chain. For example, artisanal (or traditional local) fisherman in Mexico, who fish for their own food, often eat the Great Whites that they catch, even though our studies showed that the mercury content is eight times the EPA recommended levels for a single monthly adult serving. Currently, little is known about the effects of these levels on the sharks, but we do know a lot of the mercury comes from human activities.” We are what we eat.
“Hitched to everything else in the Universe”
Studies like the Royal Society study of migrations, and the work of Dr. Cole and his colleagues on Great White population serve to show how important it is to understand the entire ecosystem, not only the sharks, but the pinnipeds and the salmon and other fish on which they feed as part of a greater, complex web of ocean life. A National Oceanic and Atmospheric Administration (NOAA) paper, Impact of sea lions and seals on Pacific Coast salmonids, shows areas where pinniped predation on salmon and steelhead is of concern, noting that harbor seals have been documented feeding on salmonids in the Russian River, and the steelhead runs that spawn in Papermill Creek, which feeds into Tomales Bay, are in danger of extinction. However, much remains to be learned of the interaction of these species. It is impossible to separate the health of one species from another, and John Muir put it best when he said, “When we try to pick out anything by itself, we find it hitched to everything else in the universe.”
Philopatry and migration of Pacific white sharks. http://rspb.royalsocietypublishing.org/content/early/2009/10/29/rspb.2009.1155.full
White Sharks at Point Reyes http://pointreyesscience.wordpress.com/2008/05/28/white-sharks-at-point-reyes/
Predatory behavior of Great Whites http://www.sharkresearchcommittee.com/predation.htm
NOAA Areas of concern for pinnipeds/California. (chapter from NOAA techmemo tm28) http://www.nwfsc.noaa.gov/publications/techmemos/tm28/areas.htm#ca