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NSEI sablefish stock assessment
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Effect of discard mortality on ABC and adjusting the abundance estimate #20

Open jysullivan opened 5 years ago

jysullivan commented 5 years ago

  1. How does the overall ABC look without using this discard rate? E.g., what is the effect on the abundance estimate?

  2. With the estimation of discards is the adjustment to the ABC necessary?

jysullivan commented 5 years ago

I've made some changes that were motivated by the method I used to estimate the F50. I had changed the equation to included discard mortality (dm) in the estimation of F50, and I think that was wrong. I think it should be based solely on landed catch, because using dm results in a higher F50 because you have to fish harder on the population in order to kill the same biomass of fish. This caused the model with no dm to have a lower quota, which is obviously not what we were going for.

  1. The yield per recruit function (YPR) no longer includes discard mortality (dm). This reverts the code back to how we did it last year and in previous years. You can see this on line number 462 in ypr.r in the function called SBf().
  2. Added a FLAG_DISCARD in the ypr.r script to test the impact of including discard mortality and created a comparison output summary table in output/2019_summary_table_NO_DM.csv.
  3. Based on a conversation with @apolson8 he'd rather have the discard mortality in lb reported in the memo and then deducted during the management decrements process. For that reason I'm going to take it out of the ABC calculation and report it in the summary table.
jysullivan commented 5 years ago

Results without discard mortality image

Results with discard mortality (recommended) image

jysullivan commented 5 years ago

This figure shows the impact on the ABC of adjusting the abundance estimate using the 15th percentile method in the context of recent ABCs. Both of these scenarios estimate discard mortality. Not accounting for discard mortality results in greater increases, regardless of how we treat the abundance estimate:

image

apolson8 commented 5 years ago

I think the use of the non-adjusted ABC is much simpler and removes the subjective portion of the model that the public and fleet most likely would not understand except that we are trying to account for uncertainty. We know discards are occurring from the low value of small fish in the fishery and by taking an estimate of discards from the model (16%) and applying this on the management decrements in determining the AHO is a lot more transparent and adds another level of conservatism that will help us stabilize the fishery.

jysullivan commented 5 years ago

I agree, @apolson8.

I think using status quo methods for abundance estimation and harvest policy (i.e. estimation of F_50 and calculation of the ABC), estimating discards, and accounting for them in the decrements process is the most transparent way to go.

jysullivan commented 5 years ago

Where I landed after dealing with new tags from NOAA (https://github.com/commfish/seak_sablefish/issues/29): image