Fisheries Reference Points Single and multispecies

1
Fisheries Reference Points(Single- and
multi-species)

• Fish 458, Lecture 23

2
(Fisheries) Reference Points

• Quantities that provide guidelines for targets
  and limits on harvesting
• (are usually fishing mortality rates or
  biomasses).

3
Traditional Reference Points(BMSY / MSY)

• BMSY the biomass at which Maximum Sustainable
  Yield, MSY, is achieved.

MSY
BMSY
4
Traditional Reference Points (BMSY / MSY)

• Calculating MSY and BMSY given a biomass dynamics
  model
• MSY is defined as

5
Yield-Per-Recruit Reference Points
Fmax
F0.1
F0.1 is defined by
6
Spawner Biomass-per-Recruit Reference Points
Scaled to 100 for F0
Typical choices include 35, 40
F40
7
Stock-Recruitment Relationship Reference Points
Fhigh
Fmed
Flow
8
Combining Yield-per-Recruit and
Stock-Recruitment-I

• Spawner biomass, S, Spawner biomass per recruit
  multiplied by recruitment, R
• Substitute into a Beverton-Holt stock recruitment
  relationship

or
9
Combining Yield-per-Recruit and
Stock-Recruitment-II

• To calculate a yield vs. spawner biomass plot
• Set the exploitation rate to 0
• Calculate the yield-per-recruit and the spawner
  biomass-per-recruit as a function of exploitation
  rate
• Compute recruitment given spawner
  biomass-per-recruit and the stock-recruitment
  relationship.
• Multiply yield-per-recruit and spawner
  biomass-per-recruit by recruitment to calculate
  yield and spawner biomass.
• Increase the exploitation rate and repeat steps
  2-4.

10
Combining Yield-per-Recruit and
Stock-Recruitment-III
MSY
F0.1
BMSY
11
Combining Yield-per-Recruit and
Stock-Recruitment-IV
How sensitive is the yield curve to steepness
Steepness 0.5 / 0.8 / 0.95
BMSY
12
Other Biomass Reference Levels

• 20 B0 when selecting policies consider the
  probability of dropping below 20 B0 (a level
  one does not go below).
• An example Accept no policy that has a greater
  than 10 probability of dropping below 20 B0
  over a 20-year projection period.
• Problems with approaches based on a fixed
  proportion of B0 arbitrary, too cautious for
  some species, not cautious enough for other
  species.

13
Multi-species Reference Points
14
Multispecies Yield-Per-Recruit-I

• Multispecies yield-per-recruit extends
  single-species yield-per-recruit by linking the
  fishing mortality rates for each species

15
Multispecies Yield-Per-Recruit-II

• Notes
• The species are independent, except through the
  impact of fishing.
• We have adopted a continuous formulation here
  rather a discrete formulation why?

16
An Example of Multispecies Yield-per-Recruit

• Pink ling
• Slow growing (?0.14), long lived (M0.15yr-1),
  domed shaped vulnerability pattern.
• Spotted warehou
• Fast growing (?0.3), medium lived (M0.15yr-1),
  asymptotic vulnerability ogive.

17
The yield-per-Recruit Curve
18
The Spawner Biomass-per-Recruit Curve
19
Multispecies Yield-per-recruit(Advantages and
Disadvantages)

• Disadvantages
• It is often very difficult to estimate the
  catchability coefficients.
• A value-per-recruit analysis may be more
  appropriate but that requires specifying the
  relative size of the recruitment of each species.
• Discarding is ignored this is, however, often
  an important aspect of technological
  interactions.
• No account is taken of the impact of reduction of
  spawner biomass on recruitment.

20
Multispecies Yield-per-recruit(Advantages and
Disadvantages)

• Advantages
• It allow us to predict the overall consequences
  of changes in fishing effort in terms of yield-
  and spawner biomass-per-recruit.
• The approach can be extended to handle discarding
  and the stock-recruitment relationship.

21
Readings

• Clark (1991).
• Hilborn and Walters (1992) Chapter 14.
• Pikitch (1987).
• Quinn and Deriso (1999) Chapter 11.
• Sissenwine and Shepherd (1987)