Introduction to Stock Assessment and Management

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Introduction to Stock Assessment and Management
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Definition of Stock Assessment
Stock assessment is the quantitative prediction
of the reaction of fish populations to
alternative management actions. - paraphrased
from H W, p. 3
3
Definition of Fisheries Management
Fisheries Management is the application of
regulations and/or enhancement in a fishery in
order to sustain both fish stocks and the fishing
industry. - paraphrased from H W, p. 3

• Good stock assessment
• summarizes biological information
• not all information of equal value
• assists managers in choosing among
• alternative management actions

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Stages in the Development of a Fishery
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Some Examples of Fisheries Development

• Atlantic
• Cod
• Pacific
• herring
• Lake Winnipeg
• sauger
• walleye
• whitefish

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Northern Cod Fishery
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Trends in Cod Abundance
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Trends in Cod Catch Rates
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Trends in Pacific Herring
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Pacific Herring
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Manitoban Fisheries
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Catch Trends in Lake Winnipeg
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Objectives of Fisheries Management
What/Who benefits from good management?

• Objectives
• Biological
• Economic
• Recreational
• Social

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Biological Objectives
Sustainable Harvest (i.e. avoid local extinction)
other considerations

• MSY
• fish highly variable
• single MSY unrealistic
• arbitrary safety margin
• e.g. F0.1
• maybe too little

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Economic Objectives

• Maximize net profits (no pun)
• a.k.a. resource rent
• value minus cost

Note only variable costs shown
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Social Objectives

• may focus on subjective issues
• self image of fishers
• cultural preservation
• may be economically inefficient

• Attempt to measure as
• number of jobs
• settlement patterns
• income distribution
• among individuals
• among communities

An uneasy mix of traditional attitudes and
technological innovation
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Recreational Objectives

• What is important to fishers?
• number of fish caught
• size of fish caught (trophies)
• time spent fishing

Managers may attempt to maximize one of these.
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Decisions and Risk
Do you prefer A) 50 B) 100 if I flip
heads C) either A or B
Classification of Strategies
Preference Type A Risk
Averse B Risk Prone C Risk Neutral
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Which Strategy Would You Choose?
A50_at_100 B100_at_50 Ceither

• You owe 25 on your boat
• You owe 75 on your boat
• You owe nothing on your boat
• You owe nothing on your boat but have a fuel
  and food bill of 35
• You owe 150 on your boat

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Discounting and Resource Management
What should you do to maximize profits?
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Does Discounting Make Sense?

• What is the value of letting one more fish breed?
• without discounting, it is infinite when
  summed over all time

• Discounting Captures
• alternative investments (opportunity costs)
• uncertainty about the future

• Discounting Ignores
• non-monetary or unquantified objectives

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Multiple Objectives
So how can a manager use this?