The bone lander after recovering it on the back deck of the ship. The attached whale bones will be removed and analyzed by Dr. Craig Smith and colleagues to see what animals, big and small, have made these bones their home.

While the deep sea below the euphotic zone (the top 100 or so meters, or 300 ft, where light penetrates and primary production from algae occurs) was long thought as a vast oceanic desert where few organisms (even microbes) could survive, research in the last century starting with the Challenger Expedition between 1872-1876 has shown a rich diversity of marine life specialized to face the harsh conditions of high pressure, cold temperature, and complete darkness. One of these, the annelid worm Osedax, has developed the ability to feed on complex hydrocarbons in whale bones, using bacterial endosymbionts (bacteria living inside the worms) to break down the compounds inside the bones into a usable form of energy.

Our ship, the Palmer, breaking through sea ice in the Weddell Sea.

As luck or fate would have it, the sea ice around Antarctica seems to be unusually persistent this year, reaching far beyond its usual summer extent, which makes moving forward a slow going process. For those of you living in cold regions of the United States and the world, you might be used to seeing your lakes and rivers freeze and thaw as the seasons progress. Sea ice around the Antarctic goes through much of the same cycle, building during the winter (between April and September in the Southern Hemisphere) and melting during the summer. The extent of ice any given year is related to weather as well as global climate, and has been shown to decrease around the Antarctic Peninsula over the past 60 years.

Sea ice extent in September (austral winter) of 2009 as measured by satellite. Black corresponds to land, blue to open water, and the other colors to sea ice. The approximate location of the Larsen B ice shelf, our target, is indicated by a white circle. Notice the band of purple surrounding that location, indicating persistent sea ice.

Sea ice extent in December (summer) of 2009 as measured by satellite.

Here’s another view of the same data. In this version, grey corresponds to land, blue to open water, and white to sea ice. The approximate location of the Larsen B ice shelf, our target, is indicated by a black circle.

Sea ice extent in December (summer) of 2009 as measured by satellite.

Because the Antarctic serves as home to a rich assemblage of species, including fish, seals, sea birds, whales, and penguins, you can imagine that life doesn’t simply stop in the cold polar winter… it adapts. Algae, which you may think as growing only in bodies of water such as lakes, oceans, and rivers, can also grow on the underside and inside of sea ice. If you look at the picture of our ship’s track you’ll notice a surprising brown color to the normally white or bluish ice. This color is due to ice algae, which due to their adaptation to low light conditions thrive both in the summer and winter. Ice algae may play an important role in starting the phytoplankton blooms that are common in the ocean as the ice retreats in the spring. Because it grows in such great abundance it also provides an important source of food to higher trophic levels, include the krill that whales love to eat. So in a way, what happens on the often hidden underside of ice can have a great impact on the bigger Antarctic animals we all know and love!

Cracks in the sea ice expose algae growing underneath and inside the ice.

The three types of killer whales. From R. Pitman, P. Ensor, J. Cetacean Res Manage 5(2):2003.

Ross Sea killer whales appear in the McMurdo Sound area and the southern Ross Sea, in early December and ply various fast ice edges (ice attached to the land), which as the season progresses recede further and further south toward the continent. They also apparently forage under or along the edge of the Ross Ice Shelf by Cape Crozier on the other side of Ross Island. These whales feed on fish that live under the fast ice and as the ice recedes the whales are able to exploit more and more feeding territory. Sightings of these whales, from land, helicopters and ships have been carried out through the years, most recently from the Cape Crozier and Cape Royds penguin colonies on Ross Island, where it has been noticed that the presence of whales (including minke whales) follows a shift in the diet of the penguins.

Killer whales foraging in a sea ice crack.

In 2005 the ratio of C to B killer whales was 50-1, but over the next few years it steadily dropped to 16-1 by 2008. As the observed numbers of B whales (seal eaters) did not change during this time, the altered ratio was due to the decrease in Ross Sea killer whales. During the years of these observations another important series of events was taking place.

Although commercial fishing of the Antarctic toothfish (sold as “Chilean sea bass”) in the Ross Sea began in 1996, it was expanded in 2004 from 9 to 22 fishing vessels; not surprisingly that same year the catch reached its allowed limit of 3500 tones. These boats target the largest adult toothfish, which is the same size those taken by the whales. Toothfish are a slow growing species which do not reach maturity until 16 years old. Many of these fish taken in the fishery were over 25 years old, some older.

Antarctic toothfish.

Since 2004, the commercial catch has remained steady year by year. Catch and release efforts of toothfish by scientists in McMurdo Sound remained steady from the years 1974 to 2000, but dropped 50% in 2001 a few years after the commercial fishing began and then to 4% in 2007, only 3 years after the peak commercial catches began. It would appear that the drop in Ross Sea killer whale numbers is related to the increase in the commercial fishing of the toothfish.

Are there any other animals that would be affected by the reduction in toothfish numbers?

Weddell seals also take toothfish as a primary food source and their numbers have not decreased in McMurdo Sound, though trends elsewhere along Victoria Land are unknown. Seals are able to dive deeper and stay under longer than the whales and therefore able to catch the fish which are safe from the whales. Seals therefore not only forage where the whales forage, but also in areas the whales can not reach, places covered with extensive fast ice where small cracks provide breathing holes. Seals also eat silverfish. It is thought that whales also eat silverfish but there are no confirmed sightings for this. The whales therefore may be more sensitive to changes in toothfish availability. If the toothfish industry continues to extract the current yearly numbers, it is predicted these creatures will decline more rapidly.

For more information about the Ross Sea, the toothfish industry and how it is affecting penguins, whales and seals go to The Last Ocean.

For some time it was thought that Weddell seals did not eat the toothfish and therefore would not be affected by the reduction of these fish in the ocean. The fishing industry has pushed to increase catch limits based on this assumption. We’re learning, though, that this is not true by indirect means.

One way researchers determine what an animal eats is by sorting through their scats (body waste). Indigestible parts pass through the body of seals and whales and can be identified. In the case of fish, the ear bones, or otoliths, are used to determine not only what species of fish are eaten, but how old and large they are. Toothfish otoliths have not been found in seal waste. But recently we’ve learned why.

Antarctic Toothfish ear bone (otolith).

As is the case with many discoveries chance plays a large part. While out on a diving expedition one researcher discovered the heads of many toothfish near a crack in the ice. The only predators in the area are seals, so these heads must be the remains of their meal. No wonder there are no otoliths in the seal waste, they don’t eat the heads! By observing seals in holes drilled through the ice for scuba access, it has been observed that seals remove the heads so this information was already known. But many people still doubt the implications of this or contend that it is a ‘local’ phenomenon. Finding these heads, in the company of seal holes, was another clear indication that this belief is wrong. Retrieving these heads would also mean that scientists could remove the ear bones (otoliths) and determine the age of the fish as well as where the fish grew up (one of the many mysteries about toothfish that remains unsolved).

The crack, where seals come to find toothfish hiding under the ice.

The helicopter landed us in this remote place on the McMurdo ice shelf.

So off we go. First a helicopter ride to the place where the fish heads were first found, and then a 10 km walk over and around the rough terrain along the crack in search of other evidence. All in all the remains of 30 fish were found, and 20 heads were brought back to the lab to extract the otoliths.

Antarctic toothfish heads, the remains of a Weddell seal feast.

Searching for Antarctic Toothfish heads on the McMurdo Ice Shelf crack.

Bagging Antarctic Toothfish heads.

As it turned out, most of the heads had become mummified, i.e. freeze-dried, and acidic action in the flesh during the process of decomposition in many cases dissolved the otoliths. There were just little ‘puffs’ of white stuff where the otoliths should have been. Skuas had eaten the otoliths in other of the heads. But, we did find otoliths in 6 heads, and these will be tested and analyzed in a lab in the US. Providing evidence to fishery biologists that toothfish are an important food source for seals will help the argument to limit the commercial catch.

Learn more about Antarctica toothfish and conserving the Ross Sea for all marine organisms by visiting The Last Ocean.