Download 2006 Thomson-Brooks Cole Chapter 19 Harvesting the Ocean`s

Chapter 19
Harvesting the Ocean’s
Resources
© 2006 Thomson-Brooks Cole
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.
© 2006 Thomson-Brooks Cole
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.
© 2006 Thomson-Brooks Cole
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.
© 2006 Thomson-Brooks Cole
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.
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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