Early larval shrimp stages rely on microalgae as their main feed. Cultivating these microalgae can be a real burden for individual shrimp hatcheries that focus only on the reproduction and culture of the shrimp itself . Although this is the common practise, ensuring the availability of a constant and large supply of live microalgae is not without challenges. Microalgae cultivation requires constant maintenance, is often space-demanding and requires a considerable investment. Timing of the start and harvest of the culture of utmost importance . Over time, live microalgae cultures may also show variation in their nutrient profile or crash due to biological reasons or contaminating grazers.
Fortunately, advances in cultivating, processing and storing techniques offer promising alternatives . Freeze-dried (FD) microalgae are a reliable and external source that is available all-year-round. The long shelf-life allows an easy distribution from a central production site. The freeze-drying process preserves the biochemical composition of the microalgae and allows for single cell dispersion upon rehydrating the biomass . Today, only a handful of companies specializes in the production of processed microalgae from several species. Despite these advances, the use of processed algae-based products in shrimp aquaculture is still limited a nd instead focusses on the production and enrichment of live feed (e.g. rotifers and copepods ) or inclusion as ingredient in formulated feed. Few publications mention the use of alternative processed algae-products for shrimp larvae.
In the current study, we investigated the potential of the FD form of a selection of microalgae species commonly used in shrimp hatcheries and that have a relevant nutritional profile, as feed for larval penaeid shrimp (Litopenaeus vannamei).
Materials and methods
L. vannamei nauplii ( stage 1) were obtained from Imaqua BVBA (Lochristi, Belgium) and stocked in 100-L cylindroconical poly-ethylene tanks filled with 90L of filtered natural seawater, at a density of 140 larvae L-1 . Water temperature was set at 29°C, and the light-dark cycle at 12:12 hours. The larval tanks had a 200 µm mesh sieve at their outlet, and were further connected to a filter unit consisting of a protein skimmer and a biofilter, allowing to operate the tanks in batch and later-on in recirculation (RAS) . All tanks were connected to the filter units once the larvae reached the mysis 1 stage. Water renewal rate was set at 300% of the tank volume per day. A standard feeding protocol was applied, providing microalgae at around 100 000 cells ml-1 for the zoea stages , and 100 000 towards 50 000 cells ml-1 for the mysis stages. FD microalgae were prepared through a series of blending and hydrating steps before administration to the tanks. Artemia instar I n auplii were introduced from the late zoea 3 stage onwards.
A series of experiments was conducted, wherein the following f actors were investigated: microalgae species, micro algae form (live vs FD), rearing system (batch vs RAS) and feed inclusion rate (100% live, 100% FD or 50% of both). Depending on the experimental setup, treatments were performed in triplicate or quadruplicate. Trials were terminated when all treatments reached the postlarval stage (10-12 DPH). Evaluation criteria included survival to the postlarval stage, developmental stage index, fecal production , body fouling and water quality (TAN). All microalgae were supplied by Provion ( Hemiksem, Belgium) , including live Chaetoceros muelleri , Thalassiosira pseudonana , Isochrysis sp. Tahitian strain and Tetraselmis chuii , and their FD form, known under the product names ChaetoPrime, ThalaPrime P, IsoPrime and TetraPrime C, respectively.
High survival to the postlarval stage was observed for shrimp larvae fed freeze-dried microalgae: 50.0 ± 8.8 % and 61.5 ± 7.6 % for FD C. muelleri and T. pseudonana, respectively. The highest survival, 70.3%, was observed under FD T. pseudonana. Both diets were supplemented with FD T. chuii for its antimicrobial effect. These survival rates were similar to those of larvae fed live microalgae.
We also observed a substantial body fouling of the shrimp larvae during the zoea stages , more specifically on all antennae and the setae of the maxillipeds and the telson. The most severe fouling was observed on zoe a 1 and 2 stages , and consisted mainly of small algae clumps . No more body fouling was observed from mysis 1 onwards. The fouling did not prevent the larvae from metamorphosing to the next developmental stage, as shed molts covered with micro algae clumps were observed. Visual observation showed that some heavily fouled larvae experienced a lower mobility compared to those free of fouling. Additionally, larvae fed FD micro algae also experienced a 1-day delay in development compared to larvae fed live algae. Whether this is the cause of the body fouling has not been be validated yet.
Conclusion, challenges and opportunities
Live microalgae can be substituted 100% by their freeze-dried counterparts in larval shrimp diets, without affecting survival, albeit with some side-effects including body fouling and a delay in developmental stage. Current and future studies are further investigating the cause of these observations and how further advances in microalgae processing and preparing could mitigate these side-effects.
The BlueMarine³.Com project is funded by the Flemish government through Flanders Innovation and Entrepreneurship (VLAIO) and is facilitated by the Blue Cluster program.