458 Fisheries Reference Points (Single- and multi-species) Fish 458, Lecture 23

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

458 Traditional Reference Points (B MSY / MSY) B MSY – the biomass at which Maximum Sustainable Yield, MSY, is achieved. B MSY MSY

458 Traditional Reference Points (B MSY / MSY) Calculating MSY and B MSY given a biomass dynamics model: MSY is defined as:

458 Yield-Per-Recruit Reference Points F max F 0.1 is defined by : F 0.1

458 Spawner Biomass-per-Recruit Reference Points Scaled to 100% for F=0 F 40% Typical choices include: 35%, 40%

458 Stock-Recruitment Relationship Reference Points F high F low F med

458 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

458 Combining Yield-per-Recruit and Stock-Recruitment-II To calculate a yield vs. spawner biomass plot: 1. Set the exploitation rate to 0 2. Calculate the yield-per-recruit and the spawner biomass- per-recruit as a function of exploitation rate 3. Compute recruitment given spawner biomass-per-recruit and the stock-recruitment relationship. 4. Multiply yield-per-recruit and spawner biomass-per-recruit by recruitment to calculate yield and spawner biomass. 5. Increase the exploitation rate and repeat steps 2-4.

458 Combining Yield-per-Recruit and Stock-Recruitment-III B MSY MSY F 0.1

458 Combining Yield-per-Recruit and Stock-Recruitment-IV Steepness = 0.5 / 0.8 / 0.95 How sensitive is the yield curve to steepness B MSY

458 Other Biomass Reference Levels 20% B 0 – when selecting policies consider the probability of dropping below 20% B 0 (a “level one does not go below”). An example: Accept no policy that has a greater than 10% probability of dropping below 20% B 0 over a 20-year projection period. Problems with approaches based on a fixed proportion of B 0 : arbitrary, too cautious for some species, not cautious enough for other species.

458 Multi-species Reference Points

458 Multispecies Yield-Per-Recruit-I Multispecies yield-per-recruit extends single- species yield-per-recruit by linking the fishing mortality rates for each species:

458 Multispecies Yield-Per-Recruit-II Notes: 1.The species are independent, except through the impact of fishing. 2.We have adopted a continuous formulation here rather a discrete formulation – why?

458 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.

458 The yield-per-Recruit Curve

458 The Spawner Biomass-per-Recruit Curve

458 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.

458 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.

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