Indian Ocean Skipjack Purse Seine Catchability Trends Estimated from Bigeye and Yellowfin Assessments

Relative abundance indices derived from commercial catch per unit effort (CPUE) are the most important data inputs to most tuna stock assessments (along with total catch removals), but their use is critically dependent on the assumption that one can establish a time series that is related to abundance in a manner that is understood (usually proportional) and consistent over time. The 2017 IOTC skipjack (SKJ) assessment included purse seine log set (PSLS) CPUE, even though the standardization analysis did not identify any evidence that catchability had changed over time (i.e. nominal and standardized CPUE series were essentially the same). Recognizing that this was not consistent with the efficiency improvements expected through technological development, alternative catchability trends of 0 and 1% per year were imposed in the assessment with equal plausibility weighting. These were fairly arbitrary values, not supported by any quantitative analysis. This paper follows up on one of the 2017 WPTT suggestions - to estimate PS catchability trends from assessments for other species which have more reliable data.
The analysis involves adding PSLS CPUE series to the (2016) yellowfin and bigeye assessments (in a manner that is uninformative about the population dynamics) and calculating the PSLS catchability trends that would be consistent with these assessments. The estimates obtained from IOTC bigeye suggest a substantial (4.1% per year compounded annually) PSLS catchability increase. The yellowfin estimates suggest a fairly continuous catchability increase of 1.25% per year. The bigeye and yellowfin trends are similar over the past two decades, but the bigeye trend went through a rapid increase 20-25 years ago. Provided that the recent yellowfin and bigeye assessments are credible, this suggests that PSLS catchability trends are not consistent among species. Recommendations for future skipjack assessments require further consideration. In the absence of other information, this analysis suggests that minimum catchability trends of at least 1.25 % per year might be more defensible than 0-1%.
While this analysis is presented in the spirit of trying to improve the IOTC skipjack assessment, the fundamental point remains - relying on purse seine CPUE as a relative abundance index may be very misleading and is not recommended.