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Chapter 16
Testing Your Comprehension
1
Approximately 71% of Earth’s surface is covered by ocean waters containing, on average, about
3.5% salt. Water temperature declines with depth, and density increases slightly at lower temperatures
and higher salinities. Therefore, deep water tends to be colder, saltier, and denser than the surface water.
2
Ocean currents are driven by the prevailing wind currents at the surface, by gradients in water
temperature, by gravity, and by the Coriolis effect. Surface currents move horizontally in large
circulation patterns. Vertical currents (upwellings and downwellings) slowly mix the deep waters with
the surface waters, affecting the distribution of nutrients and primary productivity.
3
Biologically productive areas are concentrated in areas of upwelling, in the shallower waters
along continental margins, and at hydrothermal vents of the deep mid-ocean ridges.
4
Along the coasts there are kelp forests that shelter invertebrates, smaller fishes, seals, and top
carnivores such as great white sharks. Coral reef communities, which include zooxanthellae, anemones,
sponges, hydroids, tubeworms, molluscs, flatworms, starfish, urchins, and thousands of fish species, are
among the most diverse and productive ecosystems on Earth. Intertidal ecosystems include rocky and
sandy beaches, salt marshes, estuaries, and mangrove forests, which serve to buffer the land from the
effects of storm surges and act as nursery areas for many marine organisms of economic importance,
such as shrimp.
5
Coral reefs absorb wave energy and protect shorelines from damage, as well as providing
essential habitat for many species. Increased water temperatures from global climate change, turbidity,
nutrient influx (as from agricultural fertilizers in runoff), and toxic pollutants can all damage coral reef
communities. Salt marshes and mangrove forests are often drained and converted to residential,
commercial, recreational, or agricultural uses.
6
Examples include government policy regulating the shipping industry to cut down on oil spills;
volunteer beach cleanups to pick up plastic trash and other non-biodegradable debris that can choke or
injure organisms that ingest or become entangled in it; and policy and approaches to reduce overuse and
runoff of excess nutrients that cause eutrophication, as with the Gulf of Mexico’s dead zone (Chapter 7).
7
Overfishing can remove the larger and fully mature fish faster than they are replaced by the
population, thereby resulting in a decline in catch size and quality, and a decrease in the fish population
because the death and export of individuals exceeds birth and import. Some fishing techniques (bottom
trawling, for instance) physically damage or destroy certain marine ecosystems. The collapse of North
Atlantic cod fisheries is a prime example of overexploitation through trawling damage and direct fishing
pressure.
8
Myers and Worm concluded that the oceans today contain only one-tenth of the large-bodied
animals they once did, and that the loss (from industrialized fishing) happened so quickly in most places
that scientists never knew the original abundance of these animals.
9
Commercial driftnetting catches and kills (by drowning) marine mammals and turtles, as well as
many non-target fish species that die from exposure to air on ships’ decks. Similar by-catch problems
exist with longline fishing, which hooks unwanted species as well as those desired, and even catches
and kills marine birds. Bottom-trawling disturbs the seafloor and reefs, destroying habitat inhabited by
many species.
10
Nearly all marine protected areas allow fishing or other extractive activities, whereas marine
reserves do not permit such activities. Such marine reserves can serve as production areas for fish larvae
that then disperse outside the reserve and stock other parts of the ocean.
Interpreting Graphs and Data
1
Before the management plan, swordfish biomass was declining fairly rapidly. Beginning
immediately after the plan, biomass rebounded. The opposite trends are apparent for fishing mortality: it
rose before the plan and decreased after the plan. Overall, there is an inverse correlation between fishing
mortality and biomass of the stock.
2
If trends continue, the swordfish stock should continue to increase.
3
The establishment of marine reserves (i.e., protected habitat) is vital for many species, although
this is not always the case with large open-ocean fish such as swordfish unless the reserve is very large.
For a species hunted for its meat like the swordfish, consumer seafood preferences may make at least as
much difference; if people show concern for the species’ decline and reduce their consumption of
swordfish, these purchasing choices will drive down the price of the fish and fishers will have less
economic incentive to fish for them.
Calculating Ecological Footprints
Annual consumption
Consumer group
North America
China
World
(21.6 kg per
(27.7 kg per
(16.2 kg per
capita)
capita)
capita)
21.6 kg
27.7 kg
16.2 kg
You
Answers will
Answers will vary
Answers will vary
Your Class
vary
Answers will
Answers will vary
Answers will vary
Your State
vary
6.48 × 109 kg
8.31 × 109 kg
4.86 × 109 kg
The United States
1.38 × 1011 kg
1.77 × 1011 kg
1.04 × 1011 kg
The World
1
North American versus world fish consumption: 21.6 kg/16.2 kg = 1.33. North American vs.
world ecological footprints: 3-nation average of 6.6 hectares/2.2 hectares = 3.00. With regards to fish
consumption, North America is not as far above the world average as compared to our ecological
footprint as a whole. This may be because of a greater popularity of seafood in many other cuisines
around the world. We are, however, still 33% above the world average.
2
China’s large population already has an ecological footprint that exceeds the land area of its
country. In order to feed that population, they must either import food from other countries, or harvest
food from a common area that is part of no country (i.e., Earth’s oceans).
3
Answers will vary, but globally, fisheries are already suffering from overexploitation. The total
human impact on those fisheries is the product of our population size and our per capita consumption
rates, so if both are increasing, their product will increase even more quickly. The ecological
consequences of such overexploitation may not be reversible, and numerous marine species could be
driven to extinction.