© 2006 thomson-brooks cole chapter 19 harvesting the ocean’s resources

© 2006 Thomson-Brooks Cole

Chapter 19

Harvesting the Ocean’s Resources


Key Concepts

• Fish and shellfish are renewable resources that must be properly managed to produce a sustainable yield.

• Increased demand for food from the sea has placed a great deal of pressure on natural fish and shellfish populations.


Key Concepts

• The advent of mechanized fleets and better fishing techniques, coupled with natural phenomena, has caused a decrease in the size of commercial fish catches.

• Overfishing has brought some fisheries to the brink of collapse.

• Techniques such as aquaculture have helped relieve fishing pressure on natural populations but not without new impacts on natural environments.


Key Concepts

• Large numbers of noncommercial animals are killed as a result of current, mechanized fishing techniques.

• Our limited knowledge of the basic biology of many commercial species hampers our ability to properly manage and conserve these resources.


Key Concepts

• The sea is an important source of minerals, including salt (NaCl) and manganese, and the sulfides of valuable metals such as gold and uranium.

• Fresh water for drinking and irrigation can be produced from seawater by removing the salt.

• The oceans contain energy reserves in the form of fossil fuels and methane hydrate.


Commercial Fishing

• There has been a dramatic increase in fish/shellfish taken from the sea in the past 50 years– increased demand resulted from increase in the

human population

• Recently, the world catch has not increased proportionately to the fishing effort

• Use of the catch has become less efficient as more is used for fish-meal products to feed livestock


Commercial Fishing

• Fisheries management– fish and shellfish are renewable resources

as long as animals who aren’t caught continue to reproduce and replace those that are caught

– the goal of fisheries management is to maintain these resources by enacting policies and setting catch limits that will prevent overfishing

– this is difficult when the basic biology of a commercial species is not well known


Commercial Fishing

• Fisheries management (continued)– monitoring fish populations

• determining population distribution and movement

– range is divided into stocks (separate populations)

– tagging—catching fishes and marking them with identification tags, used when they are re-caught

– unique molecular markers (DNA sequences) can be used to identify members of specific stocks


Commercial Fishing

• Fisheries management (continued)– monitoring fish populations (continued)

• determining population size and age structure– sampling experiments– landings—the catch made by fishing vessels– fishing effort—the number of boats fishing,

number of workers working, and number of hours they spend fishing


Commercial Fishing

• Fisheries management (continued)– monitoring fish populations (continued)

• fishery yield– potential yield—the number of pounds of fish or

shellfish that the stock can yield per year without being overexploited

– sustainable yield—the maximum yield that may be sustained over several years without stressing the population

• problems in managing diverse species– proper management of one species may conflict with

proper management of other species


Commercial Fishing

• Overfishing– occurs when fish are caught faster than

they reproduce and replace themselves– changes in genetic diversity

• harvesting larger specimens leaves only smaller ones to reproduce, exerting a selective pressure for smaller animals

• trawls—large nets that are dragged along the bottom, capturing virtually everything that enters

• overfishing reduces population size, hence reducing genetic diversity


Commercial Fishing

• Overfishing (continued)– changes in species diversity

• overfishing can reduce the number of species in an ecosystem

– changes in habitat• fishing activities can damage or destroy

habitat

– controlling overfishing• coastal zones

– exclusive economic zones (EEZs)

• developing new fisheries• consumer education


Commercial Fishing

• Other factors affecting marine fisheries– destruction of coastal habitats

• resulted in a loss of feeding, breeding and nursery grounds for commercial fishes

– wasteful and destructive fishing practices• incidental catch—non-commercial species

killed each year during commercial fishing (a.k.a. bycatch or “trash fish”)

• drift nets—large nets composed of sections called tans which are set in the evening and retrieved in the morning


Commercial Fishing

• Other factors (continued)– wasteful and destructive fishing practices

• trawling produces a large bycatch and damages benthic ecosystems

– shrimp trawlers catch and kill many sea turtles– use of turtle exclusion devices reduces turtle deaths

• inefficient use of the catch


Commercial Fishing

• Other factors (continued)– aquaculture—the use of agricultural

techniques to breed and raise marine organisms• monoculture—only 1 species is raised• polyculture—several species are raised

together• fish aquaculture• raft culture—juveniles of commercially

valuable molluscs are collected and attached to ropes suspended from rafts


Commercial Fishing

• Other factors (continued)– aquaculture (continued)

• shrimp farming• eco-friendly aquaculture• problems associated with aquaculture

– mangrove ecosystems are destroyed in Ecuador to make room for shrimp farms

– large numbers of fish must be caught to supply food for shrimp and salmon aquaculture, making these fish unavailable to their natural predators

– antibiotics and pesticides used in aquaculture become harmful runoff into coastal waters


Commercial Fishing

• Case studies– anchovies

• overfishing caused a reduction in fish size, so more fish had to be caught to meet demands

• a record catch in 1972 + ENSO dramatically reduced the catch in 1973

• quotas were instituted to protect the anchovies fishery

• since then, the anchovies catch has periodically been decreased by ENSO


Commercial Fishing

• Case studies (continued)– tuna

• purse seines—huge nets up to 1,100 m long and 180 m deep with bottoms that can be closed by pulling on a line

• purse seines exploit schooling behavior of tuna• dolphins follow tuna in and get caught and

killed• Marine Protection Act passed in 1972• backing down—procedure in which the skiff

draws the purse seine halfway toward the purse seiner; when the dolphins are at the edge, the boat backs up to let them escape


Commercial Fishing

• Case studies (continued)– salmon

• to maintain salmon fisheries, overfishing must be avoided and their spawning grounds preserved

• disruption of spawning grounds has made the spawning population quite small

• ocean ranching (sea ranching)—raising young fish and returning them to sea, where they develop into adults and increase the size of the population


Commercial Fishing

• Case studies (continued)– shellfish

• hard-hit by pollution that contaminates estuaries and near-shore waters

– toxic algal blooms render some shellfish poisonous

• the king crab fishery declined in the 1980s and is now regulated

– overfishing + lack of knowledge about king crab biology have been blamed for this decline


Salt and Water

• 30% of the salt supply comes from the sea; 70% from deposits left when ancient seas evaporated

• Extraction of salt from seawater– seawater is directed into shallow ponds

where it is concentrated, then evaporated– in cold regions, ice (which is nearly pure

water) is removed, leaving concentrated seawater which is heated to evaporate the remaining pure water


Salt and Water

• Desalination—process of removing salts from seawater (so it is potable)– process is energetically/financially

expensive– usually more expensive than obtaining

water from groundwater or surface sources

– used in Israel, Saudi Arabia, Morocco, Malta, Kuwait, Caribbean islands, parts of Texas and California


Mineral Resources

• Sulfides– formed when mineral-rich solutions from

fractures in rift valleys come into contact with colder seawater, and precipitate

– no technology exists for sampling/mining

• Manganese– used as a component of several alloys– nodules are found on the ocean floor– attempts to develop mining technology

were largely suspended in the 1980s


Sand and Gravel

• Most widespread seafloor mining operations extract sand and gravel for use in cement, concrete and artificial beaches

• Calcium carbonate deposits– lime, cement, calcium oxide for removing

magnesium from seawater, gravel

• Tin is extracted from sand in coastal regions of Southeast Asia


Sand and Gravel

• Uranium extracted from bottom sediments of the Black Sea

• Platinum extracted from coastal sands in the U.S., Australia, South Africa

• Mining sands/gravel can cause pollution and habitat destruction in the marine environment


Energy Sources: Coal, Oil, Natural Gas, and Methane Hydrate

• Coal– formed from prehistoric swamp plants– coal is mined from under the sea in Japan

• Oil and natural gas– represent 90% of the mineral value taken from

the sea– formed from remains of diatoms and other

microorganisms– oil is mined in the Persian Gulf, North Sea, Gulf of

Mexico, northern coast of Australia, southern coast of California, and around the Arctic ocean


Energy Sources: Coal, Oil, Natural Gas, and Methane Hydrate

• Methane hydrate– methane hydrate—ice crystals that trap

methane, and can be burned– world’s largest known fuel reserve– methane gas rapidly escapes from the

crystals when they are brought to the surface

– experiments indicate it may be possible to exploit this resource, but geologists and biologists have concerns

© 2006 thomson-brooks cole chapter 19 harvesting the ocean’s resources

Documents

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