Download Marine Fisheries 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.
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.
Date ______________ pg. ____
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.
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.
pg. _____ Date_______________________
Questions—Marine Food Webs and Overfishing
1. Annotate the article.
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. What is a Marine Protected Area (MPA)?
15. What happens to the fish population in an MPA?
16. Currently, what % of the world’s coral reefs are damaged?
17. What is the most effective way to protect coral reefs?
18. 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.