COMPARATIVE GROWTH AND SURVIVAL STUDIES OF YELLOWFIN Thunnus albacares AND PACIFIC BLUEFIN Thunnus orientalis TUNA LARVAE  

Comparative feeding studies of yellowfin (Thunnus albacares) (YFT) and Pacific bluefin (Thunnus orientalis) (PBT) tuna larvae were conducted between 2011 and 2014 at the Inter-American Tropical Tuna Commission's Achotines Laboratory in Panama and the Kinki University Fishery Laboratory in Japan as part of a 5-year research project of the SATREPS Program (Science and Technology Research Partnership for Sustainable Development) funded by JICA (Japan International Cooperation Agency) and JST (Japan Science and Technology Agency).  

One of our objectives of these studies was to compare the survival and growth potential between the two species after exposure to a gradient of relatively low and high mean daily food levels (170-, 318-, 505-, 2022-, and 3752 prey^.L^-1) of uniformly small prey (< 200 µm prey width) during the first 10 days of feeding and after exposure to a two-day delay in feeding of optimal prey levels.  Experiments were also conducted in 2013 and 2014 to estimate starvation duration of PBT and YFT larvae reared at water temperatures between 24°C and 29°C.  These experiments were conducted from the time of first feeding until the time when 100% mortality occurred.

Prey availability has a large influence on vital rates (i.e. growth and mortality) of tuna larvae, especially during the first week of feeding when larvae may encounter suboptimal feeding conditions.  Although PBT are temperate to subtropical and YFT are tropical to subtropical in their adult life histories, the early life stages of both species require warm-water (> 24°C) ecosystems as nursery grounds, thus providing a common background for comparative growth studies.  PBT larvae may be vulnerable to adverse feeding conditions given the spatio-temporal limitations of PBT spawning compared with that of YFT.  

Our experimental results indicate that PBT larvae hatch and initiate feeding at slightly larger sizes than YFT.  PBT larvae, given their larger size and greater endogenous energy reserves, exhibit greater resistance to starvation at first-feeding (9-26 hrs longer, depending on temperature) compared to YFT.  However, larger size confers no apparent advantage to PBT larvae in growth or survival when small microzooplankton prey are the prevalent forage.  YFT larvae exhibit greater growth potential and higher survival than PBT larvae when foraging on small prey.  The greater size of PBT larvae may confer feeding and growth advantages when foraging on larger prey and this hypothesis was experimentally investigated in 2015.