Harvesting Populations - Resource management

1. Harvesting Populations: Applied Problems

Goal:  Harvest populations without destroying resource

2. Harvested populations: Yield or crop (# or wt. per time)

            Forestry

Agriculture

Fisheries (Aquaculture)

Wildlife Management

3. Modeling stocks

2 Factors decrease stock wt. (natural & fishing mortality)

2 Factors increase stock wt. (growth & recruitment age-specific)

4. Stability at any level of population density:

           Recruitment + growth = natural losses + fisheries yield

5. Critical Questions:

What level population stabilization > yield?

Graham 1935 – Sigmoid-curve theory

Maximum yield at S3

Maximum yield < K

6. Fisheries Models & Vital Statistics Population Growth

            Most Fisheries models do not have vital statistic data:

2 Approaches determining “optimum yield”

Logistic models

Dynamic pool models

7. Logistic models – Graham’s sigmoid-curve theory classic ex.

            Ecological Assumptions:

No time lags

Age structure has no effect on rate of population increase

Catchability remains constant all densities fish

8. Idealized model- Exploited population respond fishing

9. Logistic Model – crude & simple requiring little data

Useful populations steady state absence fishing similar pop. @ yr.

Example Peruvian anchovy upwelling, nutrient-rich waters

Short-lived fish spawn 1yr rarely live >3yrs

Enter fisheries ~5mths age

10. How sustainable is it to maintain yield at maximum levels?

            Problem with assumption of equilibrium populations

            Need to account for “bad” years El Nino in this case.

11. Dynamic Pool models – Slightly > Biologically explicit

Include estimates growth, recruitment, mortality

Example North Sea Plaice before WWII

12. Are number of recruits a function of stock size?

            > scatter in the data (?)

13. Why do some year classes fail?

Environmental factors influencing early life history stages may be critical.

Still a lot of variance in data & more information different spp. needed!

Fisheries biologist been more focused on statistics than biology-changing.

14.  Recruitment models now incorporate mechanisms survival

            Example: English Sole coast of Oregon survive age 4

15.  Laboratory Studies on Harvesting Theory

Useful analyzing basic principles of harvesting theory

Exploitation, population size, yield experimented Guppies

16.  Guppy experiments: 4 Principles of Exploitation

17.  Is Optimal Yield the End-Goal in Fisheries & other economic harvests?

Game fish, trophy hunting, variable stocks different resilience

Economics of the fishery – cost-benefit analysis

Max economic rent = revenue – cost totals

18.  Socioeconomic & Political pressures to Overexploit populations

            -Ludwig’s ratchet cause collapse of fishery

            -Most of worlds fisheries are overexploited beyond sustainable

19.  Problems of applying “optimum yield” as limits fisheries

Example: King Crab Fisheries, Cod Fisheries, Whaling

20. Management Strategies that reduce risk of total collapse fishery:

Redirect management to not try to achieve max. sustained yield

Rather max. average yield over yield over long period time

Minimize risk of collapse or extinction

Impose a threshold or “escapement” level where no harvesting

Reserve concept - recruitment

21.  Principles of Effective Resource Management

Include humans as part of system

Act before scientific consensus is achieved

Rely scientists recognize problems, not remedy them

Distrust claims of sustainable resource use

Confront uncertainty

22. Summary:

Management past based on maximum sustainable yield

Most models assume constant population equilibrium state

Fail in practice prevent overexploitation & collapse

Economics & politics complicate management valuable resources

Harvesting subject to the “ratchet effect”

Presently management based in politics not science

Because uncertainty populations need take risk aversive strategy

We need to start managing for tomorrow’s populations!