Day 2 Session 2 Biological reference points - Supplementary

Spawning Potential Ratio

Spawning Potential Ratio What is the “spawning potential ratio”? Spawning potential ratio (SPR) is also referred to as the spawning potential per recruit. SPR is a measure of the impact that fishing has on the ability of each recruit (i.e. the average recruit) to contribute to spawning. One definition for spawning potential ratio (SPR) is: the number of eggs that could be produced by an average recruit over its lifetime when the stock is fished… … divided by…. the number of eggs that could be produced by an average recruit over its lifetime when the stock is unfished. In other words, SPR compares the spawning ability (or reproductive capacity) of a stock in the fished condition to the stock's spawning ability (or reproductive capacity) in the unfished condition. (Refs: http://afrf.org/primer3/, and SC5-ME-WP-01)

Spawning Potential Ratio What is the “spawning potential ratio”? The expected lifetime reproductive potential of an average recruit is often defined as the spawning stock biomass per recruit (SSBR). This is believed to correlate closely to the potential for growth of a population. Subsequently, you will often see SPR expressed as the ratio of the fished to unfished spawning stock biomass per recruit: Spawning Potential Ratio (SPR) = (SSBR fished/SSBR unfished) Ultimately, SPR provides “a basis for evaluating the condition of the spawning stock and the establishment of criteria for managing the spawning potential of the stock” (Goodyear 1989). (http://www.gbrmpa.gov.au/__data/assets/pdf_file/0005/2597/rp33_vol1_yield_estimates.pdf)

Spawning Potential Ratio How is SPR used as a reference point? In general, a specific level of SPR is set as the reference point. For example: SPR = 0.4 What does this mean? It means the reference level of SPR is 40% of the unfished stocks SPR (which is often stated as SPR0). In other words, the reference point is reached when the SPR has been reduced by 60% due to fishing**. The level at which it is set depends on whether it is being used as a target or limit reference point. **get checked.

Spawning Potential Ratio How is SPR used as a reference point? It can also be expressed as the fishing mortality rate associated with that level of depletion in the SPR, e.g.: F40%SPR This is translated as “the fishing mortality rate that reduces the spawning potential to 40% of the unfished level” You can think of this in a similar manner to how you explain Fmsy (the fishing mortality rate that will result in the maximum sustainable yield) **get checked.

Spawning Potential Ratio How is SPR used as a reference point? And in a similar manner to Fmsy indicators, we can use the level of F to define “overfishing” Hence under the above example, overfishing is occuring if: Fc>F40%SPR **get checked.

Spawning potential ratio Example only: Limit Reference Point (TRP) 0.8 1.0 Target Reference Point (TRP) Fcurrent/F40%SPR Time

Spawning Potential Ratio What are the potential pros and cons of using SPR based reference points? SPR based reference points are not dependent on the stock recruitment relationship being known. This means that the uncertainty inherent in MSY based reference points due to the high uncertainty over “steepness”, is not an issue that limits the use of SPR. SPR are, however, like MSY reference points, very sensitive to changes in gear selectivity, hence having appropriate information and data available for accurate estimates of selectivity is important for the calculation of SPR based reference points Other…… **get checked.

Spawning Potential Ratio How do you determine what the appropriate reference level is for SPR? ***Need to talk to Nick, Joel etc…

Spawning Potential Ratio Is SPR used elsewhere? Yes, for example: SPR is widely used in U.S. fisheries managed under the Magnuson Fishery Conservation and Management Act, usually in the context of a percentage of the unfished spawning stock per recruit (SSBR). Its implementation is intended to be risk-aversive through selection of minimum acceptable levels above which stocks maintain acceptable productivity. (http://fishweb.ifas.ufl.edu/Allenlab/courses/Goodyear%201993.pdf)

Spawning Potential Ratio What data and information is required to calculate SPR? SPR reference point calculations are reliant on estimates of selectivity, maturity and natural mortality being available: Selectivity: this will determine which components of the stock are being most impacted by the gear (along with fishing effort and catchability), and this information combines with estimates of: Maturity at age: which defines the average productivity of fish in each age class (which should increase as they get older) Natural mortality: will act upon the unfished population and the fished population and is needed to help estimate numbers at age:

Spawning Potential Ratio UNEXPLOITED POPULATION EXPLOITED POPULATION Age 1 (0 eggs/fish ) Age 2 (50 eggs/fish ) Age 3 (100 eggs/fish ) SPR = (200/5)/(300/5) = 67% 0 eggs 0 eggs 150 eggs 100 eggs 200 eggs 100 eggs

Depletion based reference points

Depletion based reference points What are depletion based reference points? Depletion based reference points are generally biomass based reference points that define a reference level of depletion of either the total or, more commonly, the spawning biomass: e.g. 30% of total biomass (B30%); 20% of spawning stock biomass (SSB30%) ……..with the level specified dependant upon whether it is intended to be used as a limit or target reference point. In general they are used as limit reference points. The level of depletion is calculated as: Bcurr/B0 or SSBcurr/SSB0 ….the ratio of the current or recent average biomass (Bc) to the initial (starting) biomass of the population

Depletion based reference points What are depletion based reference points? Depletion based reference points are generally biomass based reference points that define a reference level of depletion of either the total or, more commonly, the spawning biomass: e.g. 30% of total biomass (B30%); 20% of spawning stock biomass (SSB30%) ……..with the level specified dependant upon whether it is intended to be used as a limit or target reference point. In general they are used as limit reference points. The level of depletion is calculated as: Bcurr/B0 or SSBcurr/SSB0 ….the ratio of the current or recent average biomass (Bc) to the initial (starting) biomass of the population An advantage of depletion-based reference points that use the candidate indicators (SSBcurr/SSB0), is that they are relatively stable from assessment to assessment (despite changes in those assessments), and in many of the tuna and tuna-like stock assessments have provided the least variation in the range of results across a range of steepness values used (e.g. Kolody et al, 2010, Anon, 2010).

Depletion based reference points What are the potential advantages of depletion based reference points? Depletion based reference points might typically be preferred when….. An advantage of depletion-based reference points that use the candidate indicators (SSBcurr/SSB0), is that they are relatively stable from assessment to assessment (despite changes in those assessments), and in many of the tuna and tuna-like stock assessments have provided the least variation in the range of results across a range of steepness values used (e.g. Kolody et al, 2010, Anon, 2010).

Depletion based reference points What are depletion based reference points? For candidate indicators and the limit reference points itself, it has been noted in the tuna stock assessments in other RFMOs (as above) that use of SSB is preferable over total biomass (TB), because the most recent recruitment estimates are least reliable, whereas for the estimates of SSB, there are more data in the stock assessment model to estimate these and they, therefore, tend to be estimated more reliably. Depletion levels can be calculated from stock assessment model output (e.g. the Mulitfan-CL output produced from the SPC assessments). They require an estimate of the initial or reference time SSB and last year, or years, of the stock assessment SSB. The stock assessments themselves require a considerable amount of data (e.g. Harley et al 2010).……

Depletion based reference points What are depletion based reference points? ME: the level of depletion at which recruitment becomes impacted is still no clearer for depletion or SPR based indicators than it is for MSY based indicators Essentially the SPR and depletion based estimates are not pretending that that point is known and instead rely on observations across fish stocks with varying life history characteristics and productivity to provide a reference level which perhaps “experience” in other fisheries suggests would be an appropriate (low risk) limit in the current fishery The concern can be of course that the limits are too precautionary and could potentially lead to foregoing potential yield. …..talk to Nick and Adam and Joel about this.

Depletion based reference points Tuna assessments and depletion estimates Depletion estimates are in fact already included in the WCPO tuna assessment papers (along with a suite of MSY and other reference points)

Depletion based reference points What are depletion based reference points? Its important not to confuse depletion based estimates which use the unfished starting biomass as the comparison point with estimates which use the predicted biomass at each time point if the fishery had not occurred (i.e. fishing mortality had been zero)

Depletion based reference points What are depletion based reference points? The reference points which are based on predicted biomass in the absence of fishing can provide quite different levels of depletion, particularly if there are trends in recruitment over time (e.g. BET 2010): 0.32 v 0.17………can you think of why they are so different?