Fisheries Reference Points (Single- and multi-species)

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 F=0 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 (M=0.15yr-1), domed shaped vulnerability pattern. • Spotted warehou • Fast growing (=0.3), medium lived (M=0.15yr-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)