Download Food Webs

Marine Fisheries Food Webs
From: http://oceanworld.tamu.edu/resources/oceanography-book/marinefoodwebs.htm
Food Webs
Since the discovery of the astronomical number of microbes in the ocean, we now recognize two
important, overlapping food webs in the ocean.
1. The microbial food web discussed in that chapter. This web dominates the carbon, nitrogen, and
other nutrient cycles of the earth system. (Microbes—bacteria, viruses.)
2. The marine fisheries food web discussed here.
The two webs are coupled in many ways that are not yet understood. Microbes cycle nutrients, they
produce other nutrients such as vitamins needed by primary producers discussed here, and they infect,
sicken, and kill many organisms in the marine fisheries food web. We are not the only large animals
that get viral and bacterial diseases.
Step 1: Phytoplankton: Primary Producers of the Marine Fisheries Web
The sunlit upper layers of the ocean, called the surface zone, are home to vast numbers of single-cell
marine primary producers called phytoplankton. The term algae is another catch-all term for primary
producers with chlorophyll that formerly included many unrelated organisms, excluding land plants.
Phytoplankton are primary producers because they use energy from the sun to convert CO 2 and
nutrients into carbohydrates and other molecules used by life. Together, they account for about 95% of
the primary productivity in the ocean and about half of all primary productivity on earth.
Phytoplankton are most common in cooler, mid-latitude zones with sufficient nutrients, especially
nitrogen. Thus they are common in the north Atlantic and Pacific, and along coasts. They are much
less common in the central regions of the ocean and in the southern hemisphere.
Step 2: Zooplankton
The phytoplankton are eaten by the smallest floating animals, the zooplankton . They range in size
from single-celled organisms to larger multi-celled organisms. Small zooplankton are eaten by larger
zooplankton. Zooplankton include
1. Single-celled animals such as ciliates or amoeboids that never grow large.
2. Copepods .
3. Shrimp .
4. Larval (baby) forms of barnacles, mollusks, fishes, and jellyfish , all of which grow to be much
larger animals.
Example: Copepods
The copepods are a type of zooplankton. They are a class of crustaceans with
over 7,500 species, most of which are marine. Copepods are small (only a few
species over 1 mm) and extremely abundant, often dominating the plankton
community. They form a link in the food web between the primary-producing
phytoplankton and the plankton-feeding fish like Atlantic herring. Almost all
fish found in temperate and polar waters rely at some point in their life cycle
on copepods and other shrimp-like zooplankton (krill) as a food source.
Picture: Paraeuchaeta norvegica, a carnivorous copepod commonly found in fjords and North Atlantic
waters.
Step 3: Small Predators
Zooplankton are eaten by small predators:
1. Shrimp and krill .
2. Immature stages of larger animals such as jellyfish and fish.
3. Small fish such as sardines , menhaden , and herring .
Example: Clupeus harengus (Atlantic herring) is a small bait fish. It schools in coastal waters. It feeds on
small planktonic copepods in the first year, thereafter mainly on copepods. Adults are about 30-35 cm
in length, and they live about 20 years. They are eaten by many species of birds, fish, and marine
mammals.
Step 4: Top Predators
At the top of the marine food web are the large predators:
1. Jellyfish and cephalopods (squid and octopus).
2. Large fish such as sharks, tuna, and mackerel.
3. Marine mammals including seals , walruses, dolphins , and some species of whales (some eat fish,
others eat zooplankton directly).
4. Birds such as pelicans , albatross , penguins.
5. People, the dominant top predator.
Example: Thunnus alalunga (Albacore) is large, fast-swimming fish. Their average weight is about 9-20
kg. They are thought to become sexually mature when they are 5-6 years old and about a meter long.
They have a maximum lifespan of 8 years. They are well adapted to swim fast, and they prey on many
species of fish.
Food Chains and Food Webs
Phytoplankton, small zooplankton, larger zooplankton such as jellyfish, larger animals including bait
fish and squid , and top predators such as tuna, all interact in a marine food web . Each species eats and
is eaten by several other species. Big fish eat little fish; that’s how the food cycle works. Of course,
there’s more to it than that. The interactions in a food web are far more complex than the interactions
in a food chain. Furthermore, the branching structure of food webs leads to fewer top predators
compared with the numbers of top predators in a food chain. Food chains are much rarer than food
webs in marine ecosystems.
Over Fishing Changes Food Webs
Cod used to be abundant throughout the Atlantic Ocean, but they have been severely overfished.
Their numbers are especially low on the East Coast of Canada, even though the fishery there was closed
more than a decade ago. Scientists have found that the removal of cod and other large fish changed the
entire structure of the food web from top to bottom:
1. The population of small fishes and large invertebrates, including northern snow crab and
northern shrimp increased markedly.
2. The population of large plant-eating zooplankton (> 2
mm) decreased markedly.
3. Phytoplankton increased markedly.
4. Seal populations are increasing exponentially.
5. The economic value of the crab and shrimp fisheries now
exceeds the earlier value of the cod fishery.
6. Actions to restore the cod fishery have failed despite a
nearly complete shutdown of cod fishing.
7. Cod stocks in other areas north of 44 degrees North have
also failed to recover, while cod stocks in areas south of
44 degrees North have started to recover.
Picture: The cascading effect of the collapse of cod and other
large predatory fishes on the ecosystem on the coast of Canada
during the late 1980s and early 1990s. The size of the spheres
represents the relative abundance of the corresponding
organisms. The arrows depict the inferred top-down effects.
The changes in marine ecosystems due to overfishing is often called fishing down the marine food web.
As top predators are removed by fishing, fishers target smaller fish lower in the food web, reducing their
numbers.
Fishing down the marine food web. After the large fish at the top of the food web are fished out,
fisheries go after smaller fish and invertebrates at lower levels in the food web while their trawling
(dragging nets along the bottom) destroys animals and plants on the sea floor. Time increases toward
the right along the blue arrow.
A scientist studied historical photographs spanning more than five decades that she collected from
Florida. The study showed a drastic decline of so-called "trophy fish" from Key West. Notice how the
fish got smaller in each picture!
Photographs showing trophy fish caught on Key West charter boats a) 1957, b) early 1980s, and c)
2007.
Marine Protected Areas
Marine protected areas are places where human activity/interference is strictly limited. The rules vary,
but usually they put restrictions on oil/gas mining, tourism, and other activities. In some MPA’s,
people are not allowed to fish in these areas (called a “no-take” zone). As of 2010, the world hosted
more than 6,800 MPAs, encompassing 1.17% of the world's oceans. MPAs have also been recognized as
an effective tool to maintain localized fish populations. The general concept is to create an area where
the local populations of sea creatures can thrive and create a localized over-population. When this over
population occurs, the extra creatures will expand into the surrounding areas, known as spillover. This,
in turn, helps support the local fisheries and the areas surrounding the MPA, while maintaining a
healthy population. Such uses of the MPA have been seen in many areas around the world, but most
commonly in very densely population areas like coral reefs.
Marine Protected Areas are an especially important management tool for coral reef systems worldwide.
Over the past two centuries, coral reef systems have been in great decline due to overfishing and
pollution. Currently, 30% of the world’s reefs are already severely damaged, and approximately 60% of
the reefs will be lost by 2030 if actions are not taken to recover and protect them. Coral Reef Systems
are also facing extinction due to changes in the chemistry of the ocean and the rising ocean
temperatures, both caused by global climate change. Marine Protected Areas are currently the best tool
for managing the impacts to coral reefs, with “No Take Zones” acting as the most effective form of
management. Though Marine Protected Areas and No-Take Zones cannot stop the effects of global
climate change, these management tools can protect the coral reefs from human damage and
exploitation, allowing the reefs and the species they house to build resilience to recover to the changes
brought by global climate change more effectively and rapidly.
Name: _____________________________________ Date: __________ Period: _____ ASSIGN
Questions—Marine Food Webs and Overfishing
1. Underline things you already knew. Circle things you didn’t know (at least 5 for each. Don’t say you
know it all!).
2. What is a microbe? _______________________________________________________
3. How do microbes relate to the ocean (marine) food web?
4. Phytoplankton are producers/consumers (circle one). Where are they most common?
5. Are zooplankton plants or animals? ______________________________________
6. What are copepods? Why are they important?
7. Give 3 examples of small predators that eat zooplankton.
a. _____________________________
b. _____________________________
c. _____________________________
8. Give 3 examples of top predators.
a. _____________________________
b. _____________________________
c. _____________________________
9. How do the animals listed in the article interact in a marine food web?
10. Which is more complex, food chains or food webs? ______________________________
11. Explain why removing cod and/or other large fish from the ecosystem on the East Coast of Canada
changed the entire ecosystem. (HINT: use your notes!)
12. What is “fishing down the marine food web?”
13. What is trawling and why is it a problem? (bottom of page 3)
14. Look at the food web to the right. Translate the food web into a
paragraph on a separate sheet of paper
15. What is a Marine Protected Area (MPA)?
16. What happens to the fish population in an MPA?
17. Currently, what % of the world’s coral reefs are damaged?
18. What is the most effective way to protect coral reefs?
19. On a separate sheet of paper, draw a food web based on the
organisms in steps 1-4 in the article. You should include at least 8
organisms.