Population parameters and exploitation rate of Indian mackerel (Rastrelliger kanagurta) on the Java's North Coast

Indian mackerel, Rastrelliger kanagurta, is one of the small pelagic fishes living in the coastal waters (neritic). Its distribution in Indonesia includes the whole coastal regions in the country. R. kanagurta is the leading commodity in Indonesia's commercial fisheries, and is one of the purse seine main catch. The present study assessed the population parameters and the exploitation rate of R. kanagurta on the northern coast of Java. The survey method was employed in this study. Fish samples were collected from four different locations, including Fish Auction Place (TPI) of Tegal Coastal Fishing Port (PPP), Pekalongan Archipelagic Fishing Port (PPNP), Juwana Pati Coastal Fishing Port (PPP), and Tasik Agung Rembang Coastal Fishing Port (PPP). The results showed that the R. kanagurta caught during the study had a total length ranging between 100 and 245 mm. The recruitment peak occurred from March to September. The length-weight relationship of the R. kanagurta was obtained following the equation W = 0.000004 · L3·19, with a positive allometric growth pattern. The R. kanagurta followed the von Bertalanffy growth equation, where the fish length at age t (Lt) was 289.47 (1-exp'°'45(t'0'147112)), with the growth performance index (ф) of 4.5764. The total mortality rate (Z) of R. kanagurta was 1.74 year-1, fishing mortality (F) was of 1.17 year-1 and natural mortality (M) was 0.56 year-1 (relatively low). The exploitation rate (e) was 0.67, exceeding the optimum level and indicating overfishing. It is assumed that in such cases the size of the caught fishes was dominated by immature fish.