SPAWNING PERFORMANCE OF YELLOWFIN TUNA Thunnus albacares BROODSTOCK IN CONCRETE TANK AND FLOATING NET CAGE
The success of hatchery production of Yellowfin tuna depends on sustainable egg supply. Research has been conducted at the Institute for Mariculture Research and Development (IMRAD), Gondol, Bali under collaborative program among Indonesia, Japan and Australia.
Baby tunas were captured, transported and reared in holding facilities, fed with scads mackerel and squid at 2.5-10% of biomass per day, reduced the feed as fish grows bigger. During the rearing period, main water quality variables measured were dissolved oxygen and temperature.
Period of 2003 through 2010 broodstock reared in a land-based circular concrete tank with volume of 1,500 m3, semi-closed water circulation system and since 2013 other broodstock groups were reared in floating net cages (50 m diameter and 9 m depth of net, volume 20,000 m3) near offshore.
Yellowfin tuna spawned continuously in both systems. In circular concrete tank, first spawning occurred in October 2004 with broodstock size was about 9.1 kg of body weight and 82 cm fork length at two-year of age while in circular floating net cage, the first spawning occurred in January 2015 with estimated broodstock size was about 20.0 kg of body weight and 100 cm of fork length and two-year of age.
The highest productivity of broodstock reared in a land-based tank was in year 2005 and 2006 with spawning frequency of more than 100 times annually. Similar data were recorded in 2015 with spawning frequencies of more than 100 times. The spawning frequency could be higher as tuna eggs were found in floating net collector daily.
The disadvantages of broodstok reared in land based concrete tank, however, were mortality due to collision to tank wall, feed intake is less throughout the rearing period and natural spawning seemed impossible for a long in captivity. In contrast, much less mortality of Yellowfin tuna broodstock reared in floating net cage, feed uptake continuously increases and spawning is still occurred naturally as presented in the table below.