Size structure, reproduction, and growth of skipjack tuna (Katsuwonus pelamis) caught by the pole-and-line fleet in the southwest Atlantic

Skipjack tuna (Katsuwonus pelamis) is one of the world's main fisheries resources. In Brazil, it is the most abundant tuna and sustains an important pole-and-line fishery in the southwest Atlantic. We systematically monitored landings of the pole-and-line fleet from January 2014 to May 2016 on fishing ports in Niteroi (RJ) to evaluate the size structure of the catches, identify species reproductive patterns and to estimate growth, recruitment and mortality parameters on the southeastern Brazilian coast. Captured specimens of K. pelamis (n = 5650) ranged from 36.5 to 84.7 cm (fork length-FL, mean = 52.0 cm +/- 7.0) and from 0.9 to 15.3 kg (total weight-TW, mean = 3.1 kg +/- 1.6). There was a modal progression of sizes throughout the austral seasons, with smallest individuals entering in the spring, and larger individuals in the fall. The sex ratio was 1:1 (M:F), and reproductive indices indicated a period of greater reproductive activity between the spring and early summer, with spawning peaks in January. The length at maturity was estimated at 45.6 cm FL. The estimated von Bertalanffy growth parameters were L-infinity = 90.1 cm; k = 0.24 year(-1); t(o) = -0.54. Recruitment into the fishery occurs between 2 and 3 years of age. The total mortality, fishing mortality, natural mortality and exploitation rate were estimated at 1.42 year(-1), 0.95 year(-1), 0.47 year-1 and 0.67, respectively. Our results demonstrate changes in population parameters of skipjack tuna captured in southeastern Brazil. The reduction in the size structure of the fisheries, anticipation of length of first sexual maturation, small increase in the growth rate and the 30% increase in the exploitation rate indicate that, after 30 years of intense fishing activity, the Western Atlantic stock is under a high fishing pressure and must be monitored closely.