Surplus production and sustainable yield

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Surplus production and sustainable yield
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The key issues in conservation and management

• Sustainable exploitation how many
• Magnusson Stevens Act
• Risk of extinction and concerns of low abundance
  too few
• Endangered species act, Marine Mammal Protection
  Act
• Pest control too many
• Sustainable ecosystems

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Thomas Huxley 1825-1895
Best known as Darwins Bulldog an aggressive
defender of the theory of evolution. Engaged in
the famous debates with Bishop Wilberforce over
Darwins theories. Believed that marine
fisheries were inexhaustible
"that the cod fishery, the herring fishery, the
pilchard fishery, the mackerel fishery, and
probably all the great sea-fisheries, are
inexhaustible that is to say that nothing we do
seriously affects the number of fish. And any
attempt to regulate these fisheries seems
consequently to be useless."
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Sir Ray Lankester 1847-1929
Disagreed with Huxley based primarily on basic
biological understanding.
It is a mistake to suppose that the whole ocean
is practically one vast store-house, and that the
place of the fish removed on the particular
fishing-ground is immediately taken by some of
the grand total of fish, which are so numerous in
comparison with mans depredations as to make his
operations in this respect insignificant.
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Johan Hjort1869-1948
Norwegian scientist, premier fisheries scientist
of his era. Leader of the first scientific
fisheries organization ICES (International
Council for the Exploration of the Seas). An
advocate of scientific approach to fisheries.
The early skeptic of the effectiveness of cod
hatcheries. The godfather of fish population
dynamics. Laid the groundwork for calculation of
maximum sustainable yield.
To make my ideas clearer, I will proceed to draw
a comparison between this Fishery Research and
the science of Vital Statistics 1. Birth Rate
2. Age-distribution 3. Migration.
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Basic population dynamics
The result of this is known as exponential growth
shown for Grey Whales below
Assume that each individual gives birth to a
fixed number of offspring over the course of a
generation and that the birth rate is greater
than the death rate
r is often called the intrinsic rate of
increase and differs greatly depending on life
history
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Thomas Malthus 1766-1834
Predicted that populations would grow until they
run out of resources. Strongly influenced
Charles Darwin in the concept of the struggle
for existence. The basic premise of Paul
Ehrlichs Population Bomb.
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Logistic Growth
Populations cannot grow indefinitely, they must
run out of resources food, space etc.
The point where the population stops growing is
called carrying capacity or commonly K.
R0.2 K1,365
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More on Logistic Growth
Population this year
Population next year
Fraction of potential increase that is realized
Potential increase

x

Logistic and exponential growth can also be
expressed in terms of total biomass instead of
numbers
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Surplus Production
Surplus production is how much the population
would increase in the absence of harvesting And
conversely how much harvest could be taken and
maintain the population where it is
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Sustainable yield

• When population is high (near k) there is little
  sustainable yield (too much competition)
• When population is low there is little
  sustainable yield (too few individuals to
  reproduce)

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Maximum sustained yield

• Is realized somewhere in the middle of abundances
  (usually 20-40 of k)

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But all population sizes produce sustainable yield

• What should be our goal in fisheries management?
• Maximum tons harvested
• Maximum economic yield
• Maximum employment

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Reference points

• MSY maximum sustained yield
• Is it constant?
• BMSY biomass that produces MSY
• FMSY harvest rate that produces MSY
• Most countries have adopted MSY as a target or
  limit.

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Types of reference points

• Targets where you want to be
• MSY used to be a target
• Limits points you dont want to go beyond
• 25 of virgin stock size is a typical limit
  defining overfishing
• BMSY is now often consider the limit

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James Wilson 194? -
A skeptic that fishing affects abundance the
only evidence offered for the existence of this
relationship fishing affecting abundance sic of
which we are aware comes from the recovery of
fisheries stocks as a result of the cessation of
fishing during the two world wars."
Unless one wants to be deliberately deaf, dumb
and blind in the name of theory, it is hard to
walk away from the information provided by
fishermen and biologists and not be profoundly
skeptical of the simple biology that tends to
characterize fisheries economics and management.
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Why larger stock sizes are better

• Economically more efficient
• Less impact on other species
• More stable to environmental changes
• But if we find ourselves below MSY is it worth
  rebuilding?

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The New Zealand snapper 1 fishery

• Estimated to be at 50 of BMSY producing 92 of
  MSY (1996-7 assessment)
• To rebuild to BMSY would have required a 40
  catch reduction for 20 years,
• in order to achieve an 8 catch increase after 20
  years
• Would you accept a 40 pay cut if economists said
  they thought it would result in an 8 increase 20
  years later?

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What is overfishing

• Growth overfishing
• Fish dont have a chance to grow very large
• Recruitment overfishing
• The spawning stock is reduced enough that
  significantly fewer fish are born and survive to
  spawn and be caught

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The great fishing experiment

• During WWI most of the North Sea was mined and
  not fished
• At the end of the war there were more large fish

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Results of Great Fishing Experiment
Clear evidence that the catch rate of fish was
higher, and therefore that fishing affected
abundance
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Some comments
With the end of the war and the clearing of the
mine fields, there was a rush to the fishing
grounds. They were found to be teeming with fish
of good marketable size Men with little of no
experience bought trawlers at absurd prices and
all with one accord set to work to loot the
replenished fishing grounds. Henry Maurice
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Was there recruitment overfishing?
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W.F. Thompson

• there is no way of knowing the strain a species
  will stand save by submitting it to one 1922
• proof that seeks to modify the ways of commerce
  or of sport must be overwhelming 1919

Dean of UW College of Fisheries Founded UW Alaska
Salmon Program and FRI Director Halibut
Commission Director International Pacific Salmon
Fisheries Commission
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Martin David Burkenroad

• Studied starfish in New England and found long
  term cycles in abundance
• Believed that many changes in fish abundance were
  due to environmental changes and the evidence
  that reduced catches had let to rebuilding were
  inconclusive
• Disagreed with Thompson about the causes of the
  rebuilding of halibut abundance

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Thompson - Burkenroad Debate

• Thompson - changes in halibut abundance caused by
  fishing affecting recruitment.
• Burkenroad - changes in halibut recruitment due
  to environment.
• At this point the question was what determines
  recruitment.

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The key question

• Does a species so saturate its environment with
  eggs that all available opportunities (food,
  habitat etc) are occupied, and the number of eggs
  is largely irrelevant?
• This implies very strong density dependence,
  additional eggs dont survive.

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Michael Graham (1943)English fisheries scientist

• as the fishing power increases the stock falls,
  but the yield at first rises. Later it ceases
  to rise , and that creates the main problem of
  fishing.
• After a certain point the total yield of a
  fishery fails to increase any more, whatever the
  fishermen do. This is the key to the history of
  fishing, all over the world.

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W.E. Ricker (1908-2001)

• Stock and recruitment 1954 most cited paper
• More citations than any other fisheries scientist
  (double anyone else)
• Synthesis of biological components
• Review of existing data
• Mathematical models
• Exploration of behavior from models

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Key 1950s events

• Ricker 54 paper
• Beverton-Holt 57 book
• Schaefer 54 57 papers
• Ricker 58 book

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The revolution had occurred

• Knowledge was codified into books
• Normal science was identified (plot surplus
  production or stock-recruit relationships)
• Prescriptions were given - stock goes down,
  reduce fishing pressure
• This science formed the basis for the next 50
  years of fisheries management advice

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The alternative paradigm

• Environmental influences - Burkenroad
• David Cushings book climate and fisheries
• The anomaly - California Sardine

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Research trawl Gulf of Alaska in 1960s (left)
and 1980s (right). Shrimp and crabs were almost
totally replaced by groundfish
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The Thompson-Burkenroad debate continues today

• But by far the majority of scientists now
  recognize that fishing and environment are both
  interacting to affect abundance
• The problem is how to determine the relative
  importance of each

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Key points and terms

• MSY, growth overfishing, recruitment overfishing
• Populations cannot grow without limits
• The simple theory does not include
• Changes in environment
• Other elements of ecosystem
• The ecology of fishermen and managers
• A comment about fishermen and fishers

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Study Guide people and terms to be familiar with
Exponential Growth Logistic Growth Carrying
Capacity Intrinsic Rate of Increase Surplus
production Sustainable yield Maximum sustainable
yield MSY Biomass that produces maximum
sustainable yield (BMSY) Reference points Limit
reference points Target reference
points FMSY Overfishing Growth overfishing Recruit
ment overfishing the great fishing experiment
Thomas Huxley Ray Lankester Johan Hjort Thomas
Malthus WF Thompson Michael Graham WE Ricker MD
Burkenroad