Results

• Author: Diana González-Troncoso - Fernando González-Costas - Agurtzane Urtizberea - Ricardo Alpoim - Carmen Fernández - Thomas Brunel - José de Oliveira - Antonio Ávila - Alfonso Pérez - Panayiota Apostolaki - Santiago Cerviño - Esther Román - Dorleta García - Ane Iriondo - Eduardo Santos
• Pages: 27-47

5 RESULTS

Task 1

Objectives and achievements of Task 1

Task 1. Organise a workshop focusing on the current assessment model and

the uncertainty in the projections for cod in Division 3M.

In order to support a robust full assessment for cod in Div. 3M, there were 4 areas that

the workshop needed to address:

1.1. Revision of the currently available input data for the assessment with

the aim to improve the quality of the stock assessment. This Task was

successfully completed with t he work done during the workshop and

the benchmark meeting (Task 2). Some errors were found in the

input data that have been corrected.

1.2. Revision of the R code of the current assessment model. This code has

been fine-tuned in recent evaluations (NAFO SCR 15/033); however, an in-

depth review and debug of the code is necessary. Th e 2017 assessment

model code was revised and used in the assessment o f that year. A

new code was developed in Task 2 due to the change o f assessment

model in 2018.

1.3. Suggestion of potential alternatives to estimate natural mortality (M).

Different options were presented to estimate M, both by the

assessment models or externally taking into account the biological

characteristics. The option adopted at the benchmark m eeting was

to estimate M within the approved assessment model.

1.4. How to implement the uncertainty in the catch projections. This issue

was pointed out by the FC at its meeting in 2015 (NAFO/FC 15/23). With

the work contr ibuted by the Project, the way in which uncertainty is

incorporated into the proje ctions as well as the way to estimate the

risk of being below Blim or above Flim for a given TAC have been

improved.

Results of Task 1

The workshop was held in Vigo during March 21 st に 23rd 2017. The results of this Task

have been detailed in the workshop Report sent to the EASME on May 9th 2017

(Deliverable D1.1) and presented during the June 2017 SC meeting (NAFO SCS 17/07).

Workshop Conclusions:

The results of the workshop can be summarized in proposals submitted to the SC and

reviewed during the June 2017 meeting, and recommendations to be carried out

during the benchmark process.

EASME/EMFF/2016/008 Provision of Scientific Advice for fisheries beyond EU Waters

"Support to a robust model assessment, benchmark and development of a management strategy evaluation for cod in NAFO

Division 3M". Final Report.

Proposals submitted to the June 2017 SC meeting. These proposals are explained in detail in the NAFO

SCR 17/017.

1. The workshop proposed that the next assessment used th e final year values of the inputs

in the projections instead of the usual mean of the final three years, to reduce the impact

of the trend in biological parameters. It also recommended that acc ounting for uncertainty

must be studied in more depth before considering its implementation.

Marked decreasing trends in the values of the biological parameters cause problems in

projections, because the inputs used for the projected years usually are overe stimated,

leading to an overestimation of the projected SSB and the associated TAC for a given F.

Next year, the F associated to the proposed TAC must be greater than those assumed

the p revious year to catch the last year approved TAC. To reduce the impact of this

overestimate in th e projections results, the suggestion was to use the weights and

maturity ogives of the last year as inputs in the projections instead of the usual mean

of the last three years.

The inputs of the projections have no uncertainty incorporated except in the case of

the maturity ogive U         possible

changes in biological parameters from one year to the next was also analysed during

the workshop. This was done by examining the interannual changes observed in past

years and considering the correlation observed between these variables.

Conclusions of the June 2017 SC meeting: Due to the marked decreasing trend shown for

the biological parameters (mean weight in catch, mean weight in stock and maturity

ogive) in the last years, it was agreed to use the final year value as inputs for these

parameters in the projections. In the case of the selectivity (partial recruitment), the

         because there is no trend in the last years.

It was agreed that the use of uncertainty in the inputs of the short-term projection needs

further in-depth analysis, and therefore this inputs uncertainty was not used during this

meeting.

2. The workshop proposed a new method to estimate risk in the 3M cod projections. The new

method measures the risk associated with fishing a unique TAC instead of a distribution of

TACs as was done in the past.

This procedure was seen as a more reasonable one since the management is done

based on a single TAC, and therefore, it is more likely that managers are interested in

knowing the risk that fishing that single TAC creates. Projections as done in previous

years applied an F (e.g. Flim or 3/4Flim) to each of the 5000 iterations, resulting in a

distribution of TACs (5000 different TACs) for each projectio n scenario. T 

proposed method is to take the median of these 5000 TACs (this is the TAC that would

be used to provide advice if that projection scenario was t he basis for the advice) and

to apply this unique TAC in each of the 5000 iterations. This will result in 5000 different

Fs corresponding to catching the unique TAC in each of the 5000 iterations. The risk of

F being above Flim can then be directly estimated by comparing the resulting F with Flim

iteration by iteration.

Conclusions of the June 2017 SC meeting: The Scientific Council agreed to use the new

projections method and risk measure proposed in the workshop for all stocks that us e an

assessment with uncertainty.