Growth, mortality, food consumption, and exploitation status of bullet tuna Auxis rochei (Risso, 1810) caught in Prigi waters, Trenggalek, East Java

The bullet tuna (Auxis rochei) is the smallest tuna species and has high economic importance in Prigi waters, Trenggalek, East Java. This research aimed to identify growth and mortality rates and to evaluate the exploitation status of A. rochei. Assessment of growth parameters (L∞ and K), mortality coefficients (Z, M, and F), and exploitation ratio (E = F / Z) was conducted based on catches during December 2019-March 2020 for total samples of as many as 2203 fish. The samples had a fork length (FL) range between 18.0 and 28.0 cm and were analyzed using FAO-ICLARM Stock Assessment Tools (FiSAT) software. The results suggested that the length and weight relationship was negative allometric. The sex ratio was 1:1.53. Gonad maturity level (GML) was found in both male and female levels I, II, and III. While gonadosomatic index (GSI) value reached the highest average of 0.18%. The values of length at first maturity (Lm) and length at first capture (Lc) were 24.20 cm and 23.86 cm, respectively. Mollusca (plankton) was the dominant food, with an index preponderance (IP) value of 32.8%. The growth rate analysis obtained length infinity (L∞) = 29.90 cm FL; K = 0.46 per year and t₀ =-0.45 year. The values of total mortality (Z), natural mortality (M), and fishing mortality (F) were 3.8, 1.12, and 2.68, respectively. The exploitation rate (E) was 0.71, indicating that the fish were being overfished, and a reduction effort is required to protect the remaining stocks.