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A breaching Humpback whale.
Photo: Cynde Bierman

March 7, 2003
Whales and the Food Chain

Log Transcript

From the clear waters of the equatorial Indian Ocean to the cold, turbid waters of the Northern Atlantic, I have had the opportunity to study whales in two different environments. As an educator with Cape Ann Whale Watch in Gloucester, Massachusetts, I spent most of my time at sea watching baleen whales feed on large schools of fish, krill, and zooplankton. Here aboard the Odyssey, our focus is on studying the sperm whale, toothed whales that prey primarily on squid. At first glance, these two places appear as though they could not be more different. They literally lie on opposite sides of the planet at vastly different latitudes. However, similarities exist between these two areas.

The first and most obvious difference is in the depth between the ocean surrounding the Maldives in the central Indian Ocean, and that off the coast of Massachusetts in the North Atlantic. Here, fifteen miles offshore, the water can be 2000-4000 meters deep. Conversely, off Gloucester, we can find whales in water where the maximum depth is only 150 meters. Despite this disparity in depths, the bottom topography, or the bottom features, that makes both areas good places to find whales is largely the same.

Stellwagen Bank, for example, is a large underwater plateau with a depth of about 30 meters, while the surrounding sea floor drops to around 100 meters. It is this sudden change in depth that begins the food chain that attracts the great baleen whales.

It all begins with nutrients that sink to the bottom of the ocean. These may come from the decaying matter of an animal that has died – perhaps a whale, or a piece of wood that has been blown into the ocean. These nutrients eventually settle on the ocean floor. Once there, currents will begin to stir up the sediments and move the nutrients. Currents might be caused by the wind from a storm, the shift of the tides, which may be nearly 4 m around Gloucester, or temperature differences in the water. You may have noticed in a swimming pool how some parts of the water are warm while others are much colder. The same thing happens in the ocean. These patches of cold water are denser than the warmer water surrounding them, so they sink. As they sink, the water below is forced out of the way. In the case of Stellwagen, the water forced out along with suspended nutrient particles, moves along the sea floor until it reaches the bottom edge of the plateau. With nowhere to go but up, the water and nutrients are forced to the surface. This process is called upwelling. Using the nutrients, tiny plants and algae called phytoplankton grow and reproduce. You might be wondering why this does not occur at the bottom of the ocean. The nutrients act much like a vitamin pill would for us. It may help us to grow, but it doesn't provide the energy we need to live. Unlike us, plants and algae can make their own food by a process known as photosynthesis (using light from the sun). Even at the relatively shallow depths (by ocean standards) of 100m, much of the sun's light does not reach the bottom. So, upwelling becomes a very important step in creating the food web.

Odyssey science intern, John Cannon.
Photo: Chris Johnson

The waters of the North Atlantic have a green color because of the great abundance of phytoplankton. Grazing on this phytoplankton are almost microscopic animals called zooplankton. In turn, small schooling fish such as herring and sand launce feed on the zooplankton. In their huge numbers, these schooling fish provide enough food to support humpback, fin and minke whales—all of whom use long baleen plates made out of keratin to collect the fish in their enormous mouths.

In the Maldives, eight thousand, five hundred miles from Stellwagen Bank, the food chain begins in much the same way. The nutrients are moved upwards in the water column by currents, although in this case, they are more apt to be caused by marked differences in the water temperature than by tides and winds. The ever-changing tides, caused by the gravitational pull of the moon, are hardly noticeable here because we are so close to the equator. And the wind will not have as great an effect because of the great depths. Still the current is strong enough to cause upwelling of nutrients from the sea floor and start a plankton bloom. The steep drop-offs, where shallow water quickly becomes deeper, act in a similar way to the edge of Stellwagen Bank, forcing the water and nutrients upwards. The plankton is again fed on by zooplankton. Small fish and other animals consume zooplankton. Some deep dwelling animals like squid prey on these small fish. The waters in this part of the Indian Ocean are deep enough to support great numbers of squid, which happens to be a favorite prey of sperm whales.

Although the oceanic sperm whale may differ from the more coastal baleen whales, they have one important thing in common: a dependence on food. Regardless of the species, understanding their food source and where it can be found gives us insight into where the whales will be. And whether it is the sea surrounding Massachusetts or the Maldives, life beings with the upwelling of nutrients from the ocean floor.


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