Changes in growth of juvenile southern bluefin tuna (Thunnus maccoyii): ananalysis of length-frequency data from the Australian fishery

Modal analysis is applied to historical length-frequency records of the Aus tralian southern bluefin tuna fishery, in order to quantify the variation i n mean length from year to year. In the South Australian fishery in the fir st half of March, the mean length has ranged between 54 cm and 64 cm for 1- year-old fish, 73 cm and 85 cm for 2-year-old fish, and 85 cm and 100 cm fo r 3-year-old fish. The mean lengths of 2-, 3- and 4-year-old fish, and the increment from age 1 to age 3, have increased substantially over the histor y of the fishery. This increase in growth is probably a response to a decli ne in the population due to heavy fishing. In many years in the Western Aus tralian fishery, two or more groups of 1-year-old fish were found: the mean lengths of these groups typically differed by 10 cm. Growth rates also var ied markedly according to the season of the year.