Lecture 8:Introduction to Stock Assessment

WMAN 445:

Fish Management

Lecture Notes

Fisheries Management Approaches

1. Manage Fish Populations Directly

Stocking Harvest Regulations

Size limits Creel limits Technology limits Limited Entry Seasonal Limits “Special Reg” Areas

Fisheries Management Approaches

2. Manage the Associated Community

Food / Prey supplementation Control Predators / Parasites

Fisheries Management Approaches

3. Manage Habitats and Water Quality

Habitat Improvement Structures Treat pollution / acid precipitation Bank Stabilization Artificial Reefs Limit Activities that Destroy Habitat

(bottom trawls, Dredging)

Fisheries Management Approaches

4. Manage Activities in the Watershed

Best Management Practices for Agriculture and Forestry

Development Planning to minimize impacts to aquatic systems

Erosion Control Stormflow Control

Fisheries Management Approaches

Watershed Processes

Aquatic Habitat and

Water QualityAquatic

Communities

Targeted Fish Species

1

2

34

More Sustainable

Less Sustainable

Need for Fisheries Stock Assessments

All fish management approaches require quantitative information on the status of the fishery and factors that influence, have influenced, or are likely to influence that status.

Need for Fisheries Stock Assessments

Managers must know the answer to questions such as:

What is the size of the targeted fish population? What is the population’s age structure? What is the reproductive capacity of the population? How many fish can be harvested each year? Which sizes

should be left alone? Which habitat or water quality conditions influence the

population? Do interactions with other species affect the population? Which human activities other than fishing affect the fishery?

Some Important Definitions

1. SPECIES: a group of similar organisms that can freely interbreed (example: Coho Salmon)

2. POPULATION: a group of individuals of the same species that have a high probability of interacting with each other and interbreeding (example: Upper Snake River Coho Salmon)

3. STOCK: a harvested or managed unit of fish (may include more than one population or species) (example: Pacific Northwest Salmon off Oregon and Washington Coast)

And a Couple More…

4. SUBSPECIES: taxonomically distinct individuals that occupy non-overlapping geographical ranges, but share the essential characteristics of the species such as interbreeding (example: Yellowstone Cutthroat Trout, Southern Appalachian Brook Trout)

5. EVOLUTIONARILY SIGNIFICANT UNIT (ESU): a geographically isolated portion of the species population; has high level of genetic difference from other subpopulations of the species, but not recognized taxonomically as a separate subspecies or species (California Coho Salmon)

What is a Stock Assessment?

The use of statistical and mathematical calculations to make quantitative predictions about the response of fish populations to alternative management choices (usually alternative harvest levels).

What is a Stock Assessment?

Stock Assessment includes:• Estimate of current population size.• Estimate of current harvest rate.• Estimates of population size or harvest rates at

which the stock can be maintained over time.• Predictions of population dynamics under

various management scenarios.

Stock Assessment vsFisheries Management

Where does the data come from?

Commercial and / or Recreational Fishery Catches (Creel Surveys, “Landings”)

• Lots of cheap data• Biased

Where does the data come from?

Independent Fisheries Surveys (NOAA, Universities, State Agencies)

• Expensive data• Unbiased

Stock Assessment in Practice: What We Measure

Basic Population Parameters Fish Abundance Recruitment Fishing Mortality Natural Mortality Fishing Effort Surplus Production

Stock Assessment in Practice: What We Measure

Calculated Yield Functions Production Function Long-Term Potential Yield (LTPY) Current Potential Yield (CPY) Recent Average Yield (RAY) Stock Level Relative to LTPY Status of Utilization