Combining Weight-Length Relationships and condition factors to estimate the population structure for Skipjack tuna in the Western and Central Pacific Ocean

The arguments between Weight-Length Relationship (WLR) and Condition Factor (K) have been lasted since the day they occurred. This paper described WLRs and Ks of Skipjack tuna (Katsuwonus pelamis) samples in Purse Seine fisheries from three cruises (August-September cruise (A-S) in 2009, November-December cruise (N-D) in 2012, and June-July cruise (J-J) in 2013) in the Central and Western Pacific Ocean (CWPO). The results showed that fork length of more than 70% of specimen was below 60 cm (76% in A-S, 87% in N-D, and 73% in J-J). b values of WLRs in class of fork length > 60cm were below 3 significantly (P = 0.062), while b values when fork length < 60 cm were > 3 significantly (P = 0.028). Moreover, K values in different fork length classes for each cruises had one turning point: 60-65cm for J-J, 60-65cm for N-D, and 55-60cm for A-S, and K values were still significantly larger than those of fork length < 40cm (P = 0.06). However, b values at larger fishes were significantly smaller than those of fork length <40cm. We suggest to combine WLRs and K values at different growth phases for evaluating population structure for skipjack tuna.