Stock assessment for oceanic whitetip shark in the Western and Central Pacific Ocean

This is the second stock assessment for oceanic whitetip shark in the WCPO following that
of Rice and Harley (2012), and the first since CMM2011-04 became active in 2013, enacting a
no-retention measure for this species for WCPFC Members, Cooperating Non-Members and
Participating Territories (CCMs). This assessment for oceanic whitetip shark was performed
in the Stock Synthesis modelling framework (Methot Jr & Wetzel 2013), an integrated agestructured population model previously used to assess the status of sharks in the Pacific
Ocean and elsewhere. The population dynamics model was informed by three sources of data:
historical catches, time series of catch-per-unit-effort and length frequencies. We maintained
the four-fleet structure used in the previous stock assessment, splitting the longline fishery into
bycatch and target fleets, and the purse-seine fishery into fleets of associated and unassociated
sets.
A new development in this assessment was the inclusion of discard mortality (DM) scenarios
in the historical catches. This was a key step to account for the potential impacts of the noretention
measure for oceanic whitetip sharks. Three scenarios were used assuming 25%,
43.75% and 100% mortality on the discards, accounting for mortality at different stages of
the discarding process from the catch event and crew handling to post-release mortality. In
addition, results from two new WCPO growth studies predicted a much less productive
profile for the stock than what had been assumed previously. Because growth was a key
uncertainty in this assessment, we use two growth and fecundity assessment ‘profiles’ to reflect
the differences between growth studies, and ensure that conclusions about stock status were
robust to the uncertainty about life-history parameters...