Fisheries
We wish to know:
• What is the importance of fish in the diet of
humans?
• What are the important marine resources, and are
they harvested sustainably?
• What is the sustainable yield of the oceans?
• What possible solutions might allow humans to
more sensibly obtain food from the seas?
Fish Stocks and Fish Harvests
What are the important marine resources?
• Demersal fish: bottom-living fish such as cod and
haddock
• Pelagic fish: water-column fishes from herring to
tuna
• Crustaceans: bottom-dwelling (crabs, lobsters)
and swimming (krill, shrimp) invertebrates
• Molluscs and Cephalopods (squid and octopus)
• Marine mammals: whales, walruses
What is the Importance
of Fish?
• Marine sources provide about 20% of the animal protein
eaten by humans.
• 60% of fish consumption is by the developing world.
• In Asia, about 1 billion people rely on fish as their primary
source of protein.
• Estimates suggest that seafood production from wild fish
stocks will be insufficient to meet growing U.S. and global
demand for seafood products in the next century.
• The fishing enterprise employs some 200 million people
worldwide
Principles and Terms
• Stock -- a harvestable species or population
• Harvest rate -- fraction or amount of stock
harvested per year
• Production rate -- sum of growth in weight
of individual fish plus the addition of
biomass from new recruits, minus loss in
biomass to natural mortality
Stock Management
• Stocks are assessed by:
– biological sampling
– annual catch statistics
– catch per unit effort statistics
• Global harvest history:
– catch has climbed fairly steadily since 1950s
– now ~ 100 million metric tons/year
– harvest per capita has grown little
Global Harvest History
Total catch
(million
metric tons)
appears to
have leveled
off before
1990.
Per capita
catch has
been level
since about
1970.
Evidence of Over-Exploitation
• Declines in catch per unit effort
• Declines in total catch
• Numerous case studies of collapse of
fisheries
– sardines of California (“Cannery Row”)
– Peruvian anchovy
– George’s Bank
New Methods, New Targets,
and Over-Capacity
• Numerous statistics point to over-capacity
• drift-fishing is a spectacular example of
more efficient fishing methods
• many previously unfished stocks are new
targets (“we are eating bait”)
• collapse of Peruvian anchovy, which in
boom years was largest new fishery in the
world
Illustration by Chris Van Dusen
Peruvian Anchovy
Anchovies (anchovetta) increased in 1986 as ocean
water cooled, but declined again after 1994 and
dropped sharply during the 1998 En nino. Shifts in
fish distributions affect national economies.
Climate Variability and Marine Fisheries
Episodes of ocean
warming seem to
favor catches of
Pacific Basin
sardines.
Note collapse of
California sardines
and Peruvian
anchovies may have
involved a
combination of overfishing and climate
fluctuations
Georges Bank
This rich fishing ground
is famous for its demersal
fish, especially cod.
Georges Bank is an oval-shaped shoal
(shallow ocean areas, 240 km long by
120 km wide, that lies at the
southwestern end of the Grand Banks,
which extend from Newfoundland to
southern New England. Located 120
km off the coast, it is larger than the
state of Massachusetts.
http://sciencebulletins.amnh.org/biobulletin/biobulletin/story1249.html
Maximum Sustained Yield of
the World’s Oceans
• A theoretical estimate puts estimated annual
production at 240 million metric tons
– estimated annual harvest is half: 100-120 mmt
– current annual harvest is about 100 mmt
• Estimate is obtained by dividing ocean into three
zones: open ocean, coastal areas, upwelling areas
• Estimate is based on three values:
– primary (plant) production
– food chain length
– food chain efficiency
Food Chains and Efficiency
• Two lectures from GC-I may be useful
– Lecture on primary production explains how
primary production is estimated in marine
waters
– Lecture on food chains discusses ecological
efficiency and loss of energy with each trophic
transfer (“10% rule”)
While the open ocean
comprises most of the
area of the aquatic
world, it contributes
proportionately less to
total productivity,
because production
per unit area is low
(note log scale). Even
so, most of the world’s
aquatic primary
production occurs in
the open oceans
The open ocean is less productive than
coastal zones and upwelling zones.
Estimated Production of
Harvestable Fish
open
coastal
upwelling
ocean
waters
areas
nutrient concentr.
low
intermediate
high
primary production low
intermediate
high
Food chain length
long
intermediate
short
ecological efficiency lower --------------------> higher
fish production
negligible
~ half
~half
of total
of total
A theoretical estimate, based on this logic, puts the total fish
production of the world’s oceans at 200 m tonnes
A Paradox: “Fishing Down”
Reduces Yields
• Pauly, Dalsgaard and colleagues analyzed diet of 220 key
species to assign each species of catch to a trophic level
(Science 279:860, 6 Feb 1998)
• from 1950 to 1994, catch has gradually shfted from longlived, high-trophic-level fish (eg, cod, haddock) to lowtrophic-level fish and invertebrates such as anchovy and
krill). (“We are eating bait”)
• paradoxically, catches stagnated or declined, as
competitors (eg, jellyfish) fill the void. “If things go
unchecked, we might end up with a marine junkyard
dominated by plankton”
Solutions
• Aquaculture
– Fish farms now account for more than 1/8th of the
world’s catch
• Downsize the existing fishing fleet
– likely that a reduction of 30-50% will be required
• Reduce the subsidies
– annual cost of fishing  80 billion, revenues  75
billion
• Forge international agreements on fish catch limits
• Establish Marine Reserves
SUMMARY
• Harvest of fishery resources is an important source of
animal protein for many, as well as a source of jobs and
income
• The global fish harvest, now ~ 100 million tonnes, is near
its upper limit
• Meanwhile many fisheries have experienced over-harvest
and collapse (Peruvian anchovy, George’s Bank)
• Over-capacity and improved technology threaten further
damage, including “fishing down” the food chain
• solutions include greater reliance on aquaculture, fleet
downsizing via regulation and reduced subsidies,
establishment of marine reserves, and effective
international agreements