Assessment of reproductive maturation of southern bluefin tuna ('Thunnus maccoyii') in captivity

Abstract

Reproductive maturation of wild caught female and male southern bluefin tuna ('Thunnus maccoyii') (SBT) held under captive conditions at two locations were examined: (1) production SBT ('n'=132) held in sea cages located offshore of Port Lincoln, South Australia; and (2) broodstock SBT ('n'=24) held in an onshore breeding facility at Arno Bay, South Australia. With the exception of one large female specimen sampled from the sea cages offshore of Port Lincoln, there was no overlap in size range (fork length), and very little overlap in the range of whole body weight of the fish sampled at the two locations (production SBT 17 to 101 kg and 98 to 155 cm; broodstock SBT 97 to 152 kg and 160 to 182 cm). Production SBT were assumed to be aged 1 to at least 7 years old and broodstock SBT were aged between 8 and 12 years old. The production and broodstock SBT were generally heavier for a given length compared to that predicted for wild SBT and the broodstock SBT were younger in age for a given fork length compared to that determined for wild SBT. Therefore, predictions from wild stocks may not be a reliable indication of the age of captive reared fish or their capacity to reproduce in captivity. Gonadal histology showed that female and male production SBT were mostly sexually immature. Female and male broodstock SBT were sexually mature. Sexually mature female SBT were observed from 101 kg in body weight and 155 cm in fork length, while male SBT were sexually mature at a smaller size (51 kg body weight and 128 cm fork length) compared to female SBT. Gonadosomatic and gonad index generally increased with increasing stages of maturation for female and male SBT; however, there was no relationship between general condition index, fat-body index or hepatosomatic index and stages of maturation. Furthermore, the study showed that SBT sourced from the wild and held in the captive environment for up to 9 years can achieve sexual maturity. The production SBT found at advanced stages of maturation indicates that future broodstock can be sourced directly from the Great Australian Bight without the need for long term holding.