Age, growth and preliminary estimates of maturity of bigeye tuna, Thunnus obesus, in the Australian region

Biological parameters such as age, growth and age (or size) at maturity are vital for accurate stock assessments and management plans to ensure that fisheries develop sustainably. Despite this, very few validated age studies have been conducted for large tropical pelagic species within the Australian region. Age and growth parameters were estimated for bigeye tuna, Thunnus obesus (Lowe, 1839), sampled from longline fisheries in the Australian region using validated techniques based on counts of annual increments. Poor increment clarity reduced the number of otoliths included in the final analysis to only 50% of the 3200 selected for reading (39-178-cm fork length). Microincrement analysis confirmed the position of the first two annual increments in these otoliths. A maximum age of 16 years was obtained, but over 80% of fish in the Australian catch were < 5 years old. Growth is most rapid in the first few years of life and asymptotic length is reached at about age 9 to 10 years. The von Bertalanffy growth parameters were estimated at L-infinity = 169.09, k = 0.238, and t(O) = -1.706 for the south-west Pacific Ocean and L-infinity = 178.41, k = 0.176, and t(O) = -2.500 for the eastern Indian Ocean. These parameters were significantly different, suggesting that there is little mixing between populations in the Pacific and Indian Oceans. Length at 50% maturity for females sampled in northern Queensland was estimated to be 102.4-cm fork length.