USE OF IMTA HARVESTED SEAWEED, Ulva lactuca, IN A RAS HETEROTROPHIC SHRIMP BIOFLOC SYSTEM  

During shrimp biofloc trials conducted as a component of the HBOI land based Integrated Multi-trophic aquaculture (IMTA) system it was noted that the shrimp (Penaeus vannamei) fed on small pieces of Ulva latuca contained in the solids generated by the system.  Therefore eight week trials were conducted to determine its potential as a partial replacement for or supplement to pelleted diets. However, the anticipated benefits of adding freshly harvested U. lactuca to shrimp reared in clear water recirculating aquaculture systems (RAS) did not result in increased growth; rather growth rate decreased as pelleted feed was decreased, despite the addition of seaweed.  It was further noted that shrimp fed a 50:50 pelleted feed to seaweed diet did not consume all the U. lactuca.  In order to determine whether the size of the seaweed pieces offered had an impact on results, experiments were designed to determine whether further processing harvested seaweed to create smaller pieces would result in increased consumption of seaweed, potentially increasing growth. As shrimp in the IMTA system are cultured in a heterotrophic system, experiments were conducted utilizing biofloc rather than clear water RAS technology.     

A series of three ten-week trials have or will be conducted. Trial one has been completed, trial two is currently underway and a third trial is anticipated. For each trial twenty shrimp will be stocked into 80 L fiberglass tanks, six replicates per treatment group, in a randomized block design.  Water treatment consists of a sand filter, bead biofilter and UV sterilization. Salinity in the system will be maintained between 30-35 ppt and temperature between 26-30°C with a 12:12 light:dark cycle. Water quality parameters will be measured three times per week. In the first trial shrimp (~ 1 g), were placed in one of the following four groups: 100% pelleted (32% protein) control diet, and one of three 75:25 pelleted to fresh seaweed treatment diets in which U. lactuca was variously processed producing large (cut), medium (grated) or small (ground) seaweed pieces.  Trial two is similar to trial one except that the three treatment diets consist of a 50:50 pelleted to seaweed diet.  The design of trial three will be refined and dependent on results obtained in trials one and two.

Results for trial one showed no difference in growth or survival between treatment groups.  Survival ranged from 90-100%, final growth at harvest was 15.2-16.1 g, weight gain was 1.43-1.52/wk and % growth ranged from 1293-1673%. These results suggest that although the percent of pelleted feed may be cut 25% in a heterotrophic shrimp system, and replaced by U. lactuca, the size of seaweed pieces fed does not impact growth.  

Survival, weekly weight gain, final weight and FCR will be reported for all trials in an effort to determine the best way to process and utilize freshly harvested seaweed from the IMTA system to feed the biofloc shrimp component in the IMTA system.