INTER-AMERICAN TROPICAL TUNA COMMISSION - Comisión ...

C. SKIPJACK TUNAAn age-structured catch-at-length analysis (A-SCALA) has been used to assess skipjack tuna in the eastern Pacific Ocean (EPO).The methods of analysis are described in IATTC Bulletin, Vol. 22, No. 5, and readers are referred to that report for technical details.This method was used most recently for skipjack tuna in 2004 (IATTC Stock Assessment Report 5; available on the IATTC website), and included data up to and including 2003. More recently, data- and model-based indicators have been used to evaluate thestatus of the stock.The catches used in the assessment are presented in Figure C-1.Yield-per-recruit analysis indicates that maximum yields are achieved with infinite fishing mortality because the critical weight(weight at which the gain to the total weight of a cohort due to growth is equal to the weight loss to that cohort due to naturalmortality) is less than the average weight at recruitment to the fishery. However, this result is uncertain because of uncertainties inthe estimates of natural mortality and growth.The results of an analysis described in IATTC Stock Assessment Report 7, in which an index of relative abundance was developedfrom the ratio of skipjack to bigeye tuna in the floating-object fishery, were consistent with previous assessments, and suggest thatthere is no management concern for skipjack tuna, apart from the associated catch of bigeye in floating-object sets.Eight data- and model-based indicators are shown in Figure C-2. The standardized effort, which is a measure of exploitation rate, iscalculated as the sum of the effort, in days fished, for the floating-object (OBJ) and unassociated (NOA) fisheries. The floatingobjecteffort is standardized to be equivalent to the unassociated effort by multiplying the floating-object effort by the ratio of theaverage floating-object CPUE to the average unassociated CPUE. The purse-seine catch has been increasing since 1985, and iscurrently above the upper reference level. Except for a large peak in 1999, the floating-object CPUE has generally fluctuated aroundan average level since 1990. The unassociated CPUE has been higher than average since about 2003 and was at its highest level in2008. The standardized effort indicator, which is a measure of exploitation rate has been increasing since about 1991, but declined inrecent years. The average weight of skipjack has been declining since 2000, and the 2008 average weight is approaching the lowerreference level. The biomass, recruitment, and exploitation rate have been increasing over the past 20 years.The main concern with the skipjack stock is the constantly increasing exploitation rate. However, the data- and model-basedindicators have yet to detect any adverse consequence of this increase. The average weight is near its lower reference level, whichcan be a consequence of overexploitation, but it can also be caused by recent recruitments being stronger than past recruitments.77