Aquaculture America 2020

February 9 - 12, 2020

Honolulu, Hawaii

ESTABLISHING A BETTER COMBINATION OF ALGAL DIETS TO IMPROVE THE SURVIVAL RATE OF MEGALOPAE IN THE LARVAL REARING OF MANGROVE CRAB Scylla serrata (Forsskal, 1755)

Miguel A. Delos Santos 1,2
2 Scottish Oceans Institute
East Sands , University of St Andrews
St Andrews , Fife , KY16 8LB
United Kingdom
 
 
1 Cooperative Research and Extension
Palau Community College
P.O. Box 9, Koror, Palau 96940
 
2 Scottish Oceans Institute
East Sands, University of St Andrews
St Andrews, Fife, KY16 8LB
United Kingdom
 
 

A series of  larval rearing trials using 100L capacity tanks were conducted to determine the effect of using different combinations of background algae  on the survival rate of the mangrove crab megalopae.  Six  algal products that include d frozen algae pastes (Reed Mariculture ) such as the Nannochloropsis (Nanno 3600), Tetraselmis (Tetraselmis 3600), Isochrysis (Isochrysis 1800), Thalassiosira (TW 1200),  were used in addition to Spirulina powder and live algae Nannochloropsis and Chaetocerus muelleri (CM).  Treatment combinations included: t reatment A (Control): Without algae;  treatment B: Nanno , Tetra,TW and Spirulina powder at ratio of 8:2:2:1 grams per ton; treatment C: Nanno, Tetra, TW, Spirulina + live CM ; t reatment D; live Nannochloropsis alone; t reatment E: live Nannochloropsis + live CM; t reatment F: Nanno, Tetra, TW and Spirulina powder at ratio of 8:2:2:1 grams per ton (without Oxytetracycline) . There were 6 replicates of each treatment. T he experimental tanks were setup inside 4m x 8m rectangular concrete tanks filled with 0.5m deep water  which served as water bath equipped with a heater with thermocontroller and submersible pump to allow water to recirculate . The water temperature in the water bath was set at 29°C and  data loggers were placed inside and outside of the experimental tanks to monitor the temperature fluctuation throughout the experiment. Newly hatched zoeae were stocked at the density of 80 per litre. Rotifers were added immediately after stocking at the density of 5 per ml.  N ewly hatched Artemia were added on day 3 at the density of 0.2 per ml and  gradually increased to 5 Ar temia per ml by day 10 . Survival rate of the larvae was determined by counting the total number of larvae at 5, 10,  and 15 days after hatching and at the end of culture period, when the zoeae  had  completely molted to  the megalopa stage.  Samples of the larvae and the live feed organisms (rotifer and Artemia) were collected, flash frozen with liquid nitrogen and stored in the -80 C freezer for future determination of lipid profile .  Results for each treatment were compared statistically using ANOVA.  Initial results revealed that at day 15, the larvae in tanks fed with  live and  frozen algae mix with the addition of live diatom Chaetoceros mulleri have a higher survival rate tha n  those reared without background algae. Some of the larvae in  these tanks had already started molting  into megalopae. The larval cultures reared without using oxytetracycline,  started to collapse  at day 10.  At day 17, some of the larvae in tanks fed with live algae (Nannochloropsis and Chaetoceros ) were still  at the late zoeae stage, while many dead zoeae and megalopae were observed. This could be an indication that the larvae in these tanks have been affected by Moulting D eath S yndrome.  M ost of the larvae in the tanks fed with frozen algae mix had already moulted to  the  megalopa stage.  Even though  neither  dead zoeae nor megalopae  were observed during sampling , the survival rate  still  dropped to less than 40%.  When the culture period in all treatments was extended to 19 days the survival rate continued to drop significantly.  This suggests that the  larval  mortality  was  caused mainly by  the megalopae's cannibalis tic behaviour. The highest production of megalopae occured  at d ay 17. G iven the same environmental and dietary conditions, it is recommended that the timing  for the  termination  in larval rearing of mangrove crabs is at day 17.