Initial developments of an empirical MP for Indian Ocean skipjack tuna

and Bentley, 2013, Bentley and Adam, 2014a,b, 2015, 2016), reviewed and endorsed by the
Working Party on Tropical Tunas (WPTT), Working Party on Methods (WPM), and the Scientific
Committee (SC), the IOTC adopted Resolution 16/02 “On Harvest Control Rules for Skipjack
in the IOTC Area of Competence.” This described the harvest control rule (HCR) to be used
for setting a recommended catch for skipjack (SKJ) and stated that its first implementation
will be based upon the 2017 stock assessment agreed by the WPTT and then endorsed by SC.
Implementation of the HCR to give a recommended catch limit for 2018–2020 is described in
IOTC (2017a). The Resolution also requested a further review and possible modification of the
HCR to be conducted no later than 2021.
In 2018, the IOTC WPM noted that the SKJ HCR is not a fully specified Management Procedure
(MP), since the underlying data required and assessment methodology are not defined (IOTC,
2018b). Hence the WPM suggested that a review and potential revision required under Resolution
16/02 be conducted with the aim of determining a fully specified MP for SKJ. This was noted
by the SC in 2018 and provides the motivation and basis for the current work (IOTC, 2018a).
A fully specified MP is one that can be simulation tested, including at the least specification of the
data inputs, a decision algorithm, and management outputs. Testing requires an accompanying
operating model (OM) to describe dynamics of the resource under management and the generation
of observational data for iterated application of the MP forward in time. The current report
describes development of an empirical MP, which is tested using the OM developed by Edwards
(2020b). Previous work towards a model-based MP has shown that a simple biomass dynamic
model, on which the MP could be based, may not provide information on the stock depletion
and it was noted by the WPM that an empirical MP may be more useful for SKJ (IOTC, 2021).
The current report describes initial developments of an empirical MP based on CPUE inputs
from the Maldivian Pole and Line (PL) and European Purse Seine Log-School (PSLS) fisheries.