PETER ROSS - KID'S CORNER

What can seals and whales tell us about our oceans?

Do you get excited when you see a seal, a sea lion, a dolphin or a whale? You're not alone! Most of us are in awe when we see these incredible creatures in the wild. They are at home in the ocean, and they may even wonder why we humans don't spend more time swimming like they do!

Marine mammals include the pinnipeds (Pinnipedia) such as the seals, sea lions and the walrus, the cetaceans (Cetacea) such as the porpoises, dolphins and whales, the manatees and dugongs (Sirenia), some of the mustelids (family Mustelidae) such as sea otters, and the polar bear. These animals are collectively grouped together as 'marine mammals' because they live in, or depend on, the world's oceans.

Marine mammals are therefore a diverse group of animals, ranging from the small harbour seal (70 - 130 kg) that is common in the coastal waters of the northern hemisphere to the blue whale (180,000 kg). Some, like the dugongs and manatees, eat plants. Others, like the northern elephant seals, eat fish and squid. Some, like many populations of killer whales, may even specialize in eating marine mammals! Some, like the Bowhead whale, live in very cold polar environments. Others, like the Mediterranean monk seal, live in warm sub-tropical areas.

Because marine mammals depend on the ocean for their food, changes in the amount or in the quality of their food can be a problem. Such changes might leave them hungry, weak or sick. And while Mother Nature does have a role to play in keeping populations of wild animals fit (natural selection 'weeds out' the weak individuals leaving the stronger ones to survive and reproduce), we might start to worry about the health of the ocean if marine mammals start having serious problems.

Can you think of reasons for changes in the amount (abundance) or quality of food for marine mammals? How big a role do you think humans might play in these changes? One can imagine some natural changes because of nutrient cycling in the ocean (less nutrients means less phytoplankton, less zooplankton, less fish and so on). One can also imagine that human activities might affect the abundance of food because of fishing, pollution, noise, and habitat destruction. Hunting, fishing and pollution can also directly harm or kill marine mammals.

Pollution is something that we are concerned with. Toxic chemicals (pesticides and industrial chemicals), oil, and metals all can affect the health of marine mammal habitat. These compounds can either cause a decrease in the quantity or quality of food (e.g. fish) by affecting the health of these prey, or by contaminating these prey. Marine mammals might therefore have either less food, or food that has low or moderate levels of harmful chemicals.

By studying marine mammals, we might be able to get information about the health of the ocean. Imagine that big changes were happening in the ocean, but we were so busy studying little parts of ocean systems such that we didn't see the 'big picture' anymore. What if we could study marine mammals to see if they were being exposed to chemicals that accumulated through the ocean food chain? What if we could study marine mammals to see if they had less food to eat? Both of these approaches represent useful scientific strategies that provide us with an overview of the health of the ocean or the coastal environment. Often situated at the top of the food chain, marine mammals can give us a glimpse into the state of the ocean and help us to understand Mother Nature's processes in a world that is very different from ours.

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