INCREASING SURVIVAL, COMPETENCE AND POST SETTLEMENT SURVIVAL IN LARVAL Tripneustes gratilla THROUGH VARIOUS STOCKING DENSITIES

In Hawaii, the sea urchin  Tripneustes gratilla  has recently been aquacultured for use as a biological control agent to help eradicate dense, rapidly growing populations of invasive algae . While these urchins have been successfully reared in hatcheries, most research thus far has focused on adult gonad development/ production  rather than larval production.  Current aquaculture methods yield a post-settlement survival of less than 0.02% of initial larvae spawned at 46 days post fertilization (DPF). T his research aimed to increase larval survival, competency and post settlement survival by testing various larval stocking densities i n order to increase production yields to better utilize  T. gratilla  in biocontrol .

A  stocking density trial was conducted with  T. gratilla  larvae for 29 days .  Six stocking densities (1/mL, 2/mL, 4/m L, 8/mL and 16 /mL) were tested (n=5). Urchins were kept in 1.5 L conical bot tles with constant aeration,  24 hr light and  fed a mixed diet of Chaetoceros mulleri  and Tisochrysis lutea. Each bottle was sampled daily to monitor urchin health/survival, algal consumption, and water quality. At the end of the larval period, urchins were assessed for competence and stocked into two settlement trials to monitor post settlement survival: a small sc ale 7-day trial to track individual urchin development  and  a large scale 37-day trial to better compare post settlement survival to commercial hatchery production .

After 29 days, average larva l survival was highest  (43.26%) in the 2 /mL treatment but  was only sign ificantly  greater than those in the 8/mL and 16 /mL  treatments (p =0.045 and p =0.005 respectively,  Fig. 1).  In the small-scale settlement trial, survival in the 1/mL treatment  was significantly higher than other treatments (p<0.05, Table 1A ). In the large-scale settlement trial,  survival from initial larvae spawned was 1.11%  in the 1/mL treatment (Table 1B), a ten- fold increase than what is normally observed using commercial aquaculture techniques.  This research shows  that stocking of  T. gratilla larvae at densities higher than 6 urchins/mL can have detrimental effects on survival during the larval period and may have lasting effects into post settlement survival to the juvenile stage.