Effects of dietary astaxanthin krill oil and high protein krill meal on the growth and survival of post larvae in hyper intensive nursery culture

New findings released in September 2020 showed enhanced growth performance of whiteleg shrimp during 51- and 42-day trials at the nursery stage, demonstrating the possibility of a shortened production cycle and reduction in shrimp mortality after transfer to grow-out ponds.

In shrimp farming, the post larval (PL) stage is an intermediate phase between the larval and juvenile stages. In a single-stage farming model, post larvae, usually at PL9-12, are transferred directly to the growout ponds; while in a 2 or 3-stage farming model with one or two nursery phases, the grow-out usually starts with shrimp with <3g body weight. The duration of the nursery phase is relatively short in the shrimp production cycle, and it is often characterised by an aggressive feeding program, which is typically high in frequency coupled with the use of nutrient-rich diets. The stocking density is quite dense at this stage, which stresses the shrimp significantly. Shrimp stocking density can range from 500-5,000 PL/m3 and end with juveniles between 300mg and 3g of body weight with a final yield of 1–3 kg/m3. Once shrimp complete the nursery phase, they are transferred to grow-out farms where they are grown to harvest sizes over a culture duration of 70 days or more.

After focussing on juvenile Litopenaeus vannamei and getting to know a lot on the effects of astaxanthin krill oil and krill meal on growth and survival in indoor and outdoor rearing systems under hyper saline rearing conditions, Dr Alberto J.P. Nunes, from the Instituto de Ciências do Mar, Brazil (Labomar) then turned his attention to the post larval stage. Below, he shares a first-hand perspective on the findings from a recent study on shrimp post larvae.

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In previous studies, the dietary supplementation of astaxanthin krill oil (AKO), a source of omega-3 polyunsaturated fatty acids (n-3 PUFAs) and the antioxidant astaxanthin, has been shown to improve the growth performance of juvenile whiteleg shrimp farmed under high salinity (Castro et al., 2018; Rufino et al., 2020).This study carried out in partnership with Aker BioMarine, determined how dietary graded levels of AKO and/or a high-protein krill meal (HPK) impact growth and survival of whiteleg shrimp during this critical stage of development.

What was your hypothesis when you set up this study?

Given the evidence on the impact of krill products on shrimp during the juvenile stage, we wanted to look at a new stage in the shrimp lifecycle, the post larval stage. Our goal was to evaluate whether dietary supplementation of krill oil and krill meal had a similar effect as with the juveniles, namely enhanced growth and survival. In addition, we also looked at osmotic and thermal stress resistance of post larvae reared under hyper-intensive nursery culture conditions.

How did you set up the study and what was unique in this setup?

We decided to study two different types of rearing systems: one outdoor and one indoor. We ran the study over 52 and 41 days, respectively, testing a range of dietary treatments on shrimp post larvae. Our aim was to mirror the dense conditions in commercial nursery systems by stocking tanks at a high density. We used PL10 (average body weight 3.6mg) from a local hatchery.

We designed the setup to simulate some culture practices as close as possible. The outdoor tanks were enclosed in a greenhouse using 70% dark shading net but were kept exposed to sunlight and rainfall. In the other rearing system, we had indoor tanks where there was 12 hours of artificial lighting. These conditions provided a greater control over environmental variables compared to the outdoor system. The other unique feature was the particle size of the feed pellets. We also used a counter to precisely determine the number of post larvae we stocked into the tanks.

What were the treatment diets fed to the post larvae? For the approximately 3,500 shrimp (stocking density of 2,370 PL/m3) in each of the outdoor tanks, we supplemented their diets with varying levels of AKO (Qrill™ AstaOmega Oil from Aker BioMarine), and compared them to a control diet lacking in krill-based

Dr Alberto J.P. Nunes is Professor at the Instituto de Ciências do Mar, Brazil (Labomar).

products. For the approximately 1,200 shrimp/tank (stocking density 2,371 PL/m3) in each of the indoor tanks, we supplemented the shrimp diet with varying levels of AKO along with HPK, testing their effectiveness as a substitute for fish oil and fish meal. These diets were compared to a krill-free control diet.

Post larvae were fed daily. In the outdoor system, shrimp were fed 20 times within 24-hour periods using an automatic feeder. In both studies, meals were adjusted daily for each rearing tank assuming an estimated daily drop in shrimp survival and daily increase in shrimp body weight across all diets. We also fed different particle sizes during the nursery period; in the outdoor tanks, four particle sizes were used, while six particle sizes were used in the indoor tanks. We observed the ability of the post larvae to capture the feed particles using a 1-L Becker glass container stocked with 20 post larvae.

About the study

The study, titled “Effect of dietary graded levels of astaxanthin krill oil and high protein krill meal on the growth performance and resistance of post-larval Litopenaeus vannamei under hyper-intensive nursery culture”, was authored by Alberto Nunes, Artur Soares, Hassan Sabry-Neto and Lena Burri. It was published in the Aquaculture Nutrition Journal. Aquacult Nutr. 2020; 00:1– 15. https://doi.org/10.1111/anu.13187

EFFECT OF DIETARY GRADED LEVELS OF ASTAXANTHIN KRILL OIL ON THE GROWTH PERFORMANCE AND RESISTANCE OF POST-LARVAL Litopenaeus vannamei UNDER HYPER-INTENSIVE QRILL NURSERY CULTURE AstaOmega Oil (AKO) DIETS

CONTROL: Control (18% salmon meal, 2% salmon oil)

TRIAL 1 1, 3 and 5% AKO (replacing soybean oil, 18% salmon meal, 2% salmon oil) RESULTS TRIAL 1 P < 0.0001

STUDY DETAILS

Fortaleza

AKO

TRIAL 2 3,5 and 7% AKO in combination with 8% QRILL High Protein meal

Increased shrimp body weight after 51 (trial #1) of rearing in a nursery system

Shrimp were raised in outdoor tanks and fed diets with 1, 3, and 5% AKO Cumulative shrimp survival after sudden exposure to osmotic and thermal stress for 0, 10, 20 and 30 min POST-LARVAE 2.5 MG (PL10)

GROWING METHODS Two- or three-stage culture system combining hyper-intensive nursery systems with semi-intensive or intensive pond cultures AVANTAGES Full compensatory growth after pond stocking Improved shrimp survival and greater size uniformity at harvest CHALLENGES Higher stress to animals because of the increased stocking density, organic water loads and handling, therefore a need for supplements

CONCLUSIONS

INCREASED STRESS RESISTANCE WITH 1% AKO

INCREASED SHRIMP BODY WEIGHT

INCREASED SURVIVAL WITH 1% AKO

RESULTS TRIAL 2 P < 0.0001

Increased shrimp body weight after 42 days (trial #2) of rearing in a nursery system

Shrimp were raised in an indoor tanks and fed diets containing 3, 5 and 7% AKO in combination with 8% QHP

Source: Alberto Nunes, Artur Soares, Hassan Sabry-Neto, Lena Burri - Aquaculture Nutrition (2020) (water salinity decreased from 39 to 0 g/L ) (temperature decreased from 28 to 19oC)

Was there a final body weight difference between shrimp reared indoors or outdoors?

In the outdoor tanks, given the more varying environmental conditions, we saw that 50g krill oil/ kg feed was an effective amount to enhance the body weight. However, in more controlled conditions indoors, just 30g krill oil/kg feed combined with 80g krill meal/ kg feed, was sufficient in increasing final body weight.

You mentioned the high stress, dense conditions in this stage of shrimp development. Did the krill oil or krill meal have any impact on shrimp survival?

AKO supplementation had a significant effect on survival when shrimp were subjected to a severe drop in both temperature and salinity. We saw clear evidence that by including a minimum of 10g of AKO/kg feed, shrimp can withstand a sudden and acute drop in water salinity and temperature.

What is it about krill that promotes growth and survival?

The enhanced growth and increased survival under stress that we witnessed during this study may be due to the increased amount of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and astaxanthin that Antarctic krill provides. These nutrients give the post larvae a much-needed boost at this critical developmental stage.

Our diets have also exceeded the recommended quantitative levels of n-3 highly unsaturated fatty acids (HUFA) for the optimum growth of the L. vannamei juvenile (0.4–4.7g body weight), which has been determined to be 5g/kg of the diet as reported by González-Félix et al. (2002). In both the outdoor and indoor culture systems, shrimp weight was maximised with a total dietary HUFA (sum of arachidonic acid (ARA), EPA and DHA) content of 8.3 and 7.0g/kg, respectively.

What is your recommendation for shrimp farmers after the completion of this study?

Our team concluded that krill oil, especially when combined with krill meal, can deliver significant advantages to shrimp in the post larval stage. The enhanced growth performance during nursery culture, serves to shorten the overall production cycle, saving farmers both time and money; their overall yields are better given the high survival, despite the high density and extreme conditions.

We also recommend further work to explore whether the acquired beneficial effects during the nursery phase can be sustained at the later stages of shrimp growth.

How did the shrimp post larvae respond to the test diets?

The indoor-reared shrimp showed better survival overall, which I believe may be due to the greater environmental control. Regardless of rearing systems, all post larvae receiving astaxanthin krill oil showed significant improvements in body weight by the end of the experiment.

We also noticed that body weight started to differentiate between dietary treatments early during the nursery phase, that is after 10 days of stocking in both the outdoor and indoor tanks.