| You've been reading this month about food chains and food webs. |
| Today we'll discuss these in relation to seafood. |
| How many of you like seafood? |
| Mmm, most of you. |
| So do I. |
| In fact, I'm on a seafood diet. |
| I see food, I eat it. |
| Haha, get it? OK. |
| But seriously folks... |
| Today we're going to talk about trophic relationships in marine food chains and webs. |
| Who can remind us what a trophic relationship is? |
| Yes, Ms. Li. |
| It's what an organism eats, and uh, the things that eat that organism. |
| Very good. |
| Trophic relationships describe the relationship between producers and consumers, |
| so they help us diagram food chains and food webs. |
| Now, in marine ecosystems, like other ecosystems, |
| food is needed for matter - growth and reproduction, |
| and for energy - metabolic processes within the body. |
| Also like other ecosystems, |
| marine ecosystems have producers, consumers and decomposers. |
| The primary producers are autotrophic plankton. |
| Auto trophic means these plankton can synthesize their own food. |
| Autotrophs are consumed by heterotrophic organisms. |
| "Hetero" means other; |
| so in this case, heterotrophic means organisms that can't synthesize their own food. |
| They must rely on autotrophs for food energy. |
| The primary consumers in marine food chains are the plant eaters - herbivores, |
| and the secondary consumers are both the meat eaters - carnivores - and predators |
| that eat both meat and plants - omnivores. |
| The decomposers are heterotrophic bacteria, |
| which get energy from body wastes and dead tissue, |
| thus cycling it back to the producers. |
| A simple marine food chain, then, might look like this: |
| the top predator, |
| trophic level number 4, is a herring. |
| Herring fish eat level 3, carnivorous zooplankton. |
| The carnivorous zooplankton eat trophic level 2, herbivorous zooplankton. |
| And herbivorous zooplankton eat level number 1, phytoplankton, which is a type of autotroph. |
| In marine food chains, energy transfer is not very efficient. |
| Phytoplankton utilize only about one percent of the energy available from the sun. |
| Between 70 and 90 percent of the energy made by producers or eaten by heterotrophs |
| is used in their bodies or expelled as waste. |
| This leaves only 10 to 30 percent that is retained in the body's biomass |
| and available for consumers at the next highest trophic level. |
| Thus, the biomass at each trophic level is controlled by the efficiency of the energy transfer. |
| At the lowest trophic level, |
| animals will generally have high biomass, |
| and there will be lots of small producers. |
| At the highest trophic level, |
| animals will generally have low biomass, |
| and there will be only a few large animals. |
| Now, let's expand our simple food chain into a food web. |
| In this web, a herring is no longer a trophic-4 predator. |
| There is a bigger fish, um... a tuna, that eats the herring. |
| But there's even a bigger animal that eats the tuna. And that is...? |
| Us! |
| Yes. And there's something else from the seas that will even eat us. |
| Sharks! |
| Correct. |
| A food web is more complex than a food chain. |
| And then there are gigantic animals, like whale sharks and baleen whales, |
| that are herbivores and only eat plankton. |
| But let's focus for a few moments on us. |
| What are the implications of trophic levels for the fish we eat? |
| Well, looking at the fish harvest worldwide, |
| 88% of the fish we catch are fish with fins. |
| 8% are shellfish, |
| and 4% are crustaceans. |
| Fish caught in the open ocean, such as tuna, |
| are high-level predators on an inefficient food chain. |
| Fish caught in coastal waters, such as cod, herring and haddock, |
| are at the top end of shorter, more efficient food chains. |
| This is because there is a high level of phytoplankton, |
| so consumers expend less energy catching food. |
| These fish, then, provide more energy and better nutrition for us. |
| In "upswelling" areas, off the west coasts of America and Africa, |
| the fish are even healthier. |
| Here they are small, very efficient food chains, |
| and the fish are small, fast-growing, eat lots of phytoplankton and travel in dense schools. |
| Two examples of such fish are anchovies and sardines. |