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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.