Features Fish Nutrition Research
Feed for Thought: From sensitive picky eaters to laid-back gluttons

Shrimp production increased a lot over the past several years. As of 2022, shrimp is the topmost aquacultured species, with close to six million tons produced. Increasing culture areas, improving aeration, supplying shrimp with nutritionally adequate feed, among other factors, contributed to the increased production. However, farmers still face a lot of challenges and obstacles that they have little control over. 

Shrimp are “sloppy” eaters that tend to waste a lot of their feed while “chewing.” A lot of the feed offered to them is simply wasted by the formation of fine particles and nutrient leaching. That worsens the feed conversion ratio and can deteriorate water quality. Additionally, shrimp are sensitive to salinity changes, especially in the early life stages. Salinity of the ponds and culture areas can fluctuate for a variety of reasons, mainly the change of tides and rainfall. Experiencing salinity changes like this can be lethal for the shrimp. Therefore, it is important to further improve shrimp production by getting the animals to “finish their meal” and “unwind.”

The idea
Shrimp are continuous benthic feeders. They feed at the bottom of the water column and have small digestive tracts that don’t allow them to consume feed in big quantities. Consequently, studying their feeding behavior can prove difficult, but it is not impossible. One recent technology allows scientists to “eavesdrop” on the shrimp as they feed and form a better understanding of their feeding behavior. The idea is to drop hydrophones – underwater microphones – in the center of a glass aquaria and listen to the animals “clicking” their mandibles and breaking down the feed pellets. The benefit of eavesdropping lies in getting all of the information needed without disturbing the animals. We refer to this procedure as Passive Acoustic Monitoring, or simply PAM.

Improving shrimp production does not stop at knowing how they feed. The two main goals of current research are using nutritious yet affordable ingredients for feed formulation and raising the animals to be healthy and stress-free. Nutritionists are trying to incorporate a lot of terrestrial animal protein sources and by-products into aquaculture feed to make it more cost-effective. One such source of protein is poultry meal. 

Poultry meal is a by-product of one of the biggest sectors in the food industry, comprised of the inedible poultry by-products that are seldom consumed by humans, mainly feet and heads, among other organs. With proper processing, poultry meal can be used as an alternative to the traditional protein source used in aquaculture i.e. fishmeal. Poultry meal has a similar nutritional profile to fishmeal, with the exception of some amino acids. However, shrimp seem to be averse to consuming poultry meal-based feed. It seems that incorporating poultry meal in the feed decreases its palatability and shrimp are no longer attracted to it. Consequently, it would be interesting to see if supplementing shrimp feed with an attractive element would incentivize the animals to consume poultry meal-based feed.

Our team of researchers from Auburn University decided to evaluate the aforementioned hypothesis. Additionally, limited data present in the literature suggest that some attractive elements of synthetic or natural origins could potentially improve shrimp tolerance to stress, particularly the one related to salinity. Accordingly, the research team designed a three part experiment to assess the use of a feed effector with attractive properties on Pacific whiteleg shrimp feeding behavior, feed consumption, growth, and tolerance to salinity stress.

Figure 1: Average survival of shrimp offered feed containing increasing levels of the feed effector. Error bars represent standard error of the mean, and different superscripts indicate significant differences (p<0.05).

The approach
We started the study by producing three sets of three experimental diets each, allowing for a total of nine diets. We used poultry meal as main source of animal protein in the first set, whereas the remaining sets of diets had fishmeal as the protein source. The main difference between the second and the third set was the level of fishmeal included, which was either medium (at 6%) or high (12%). Each set of diets had a control diet with no feed effector, and two other diets with feed effector supplemented at either 0.1 or 0.2%. The main reason for our choice of animal proteins used was to compare poultry meal to an attractive source of protein, and to eliminate the effect of diet matrix on the possible results.

After the diets were made, we assessed the shrimp’s feeding behavior and how much feed they’d consume. For that purpose, we relied on PAM to record shrimp “clicks” when offered a known quantity of feed in each aquarium, over a period of 30 minutes, at the end of which we would collect any leftover feed and dry it to determine the feed consumption.

Afterwards, we moved to the second part of the experiment which was the growth evaluation. For this part, we used a clearwater recirculating aquaculture system comprised of identical glass aquaria connected to pumps and a reservoir tank. We stocked 15 shrimp of similar size per aquarium, randomly distributing the diets among the aquaria, which allowed for four replicate aquaria per diet. We offered the shrimp feed manually four times a day for a total of 36 days. During that period, we were assessing shrimp survival on a weekly basis, and adjusting the feed ration based on the number of shrimp available in the aquarium.

After 36 days, we terminated the second part and proceeded to stress evaluation. At the time, we did a final count of the shrimp in each aquarium, and group weighed the animals to obtain growth data. We then randomly selected five shrimp from each aquarium and induced an acute salinity stress by submerging them in a freshwater bath for one minute before returning them to their respective aquaria. After 24 hours, we went back and assessed their survival post-salinity stress.

The outcome
The results we obtained from this study were very propitious, especially in terms of shrimp feeding behavior and stress tolerance. Incorporating the feed effector in the feed seemed to increase the number of “clicks” emitted by the shrimp, especially when it came to poultry meal-based feed. The shrimp tended to find the poultry meal-based feed more palatable and attractive when the feed effector was supplemented. Their “clicks” also seemed to increase with the increased level of the feed effector. The results were similar for the set of diets containing high fishmeal, despite the ingredient being a palatability enhancer itself. The interesting yet unexplainable result was that observed for the medium fishmeal feed. For some reason, the number of “clicks” decreased with the increase in the level of feed effector in the feed. A similar trend was also observed in terms of feed consumption. The animals tended to consume more of the poultry meal-based feed and the high fishmeal feed when more feed effector was supplemented.

Results of the growth assessment, however, were not in accordance with the feeding behavior and feed consumption experiment. Shrimp offered the various feed grew similarly and exhibited similar feed conversion ratios and survival. That is not necessarily a bad thing, considering that the experiment was performed in a laboratory setting rather than a production setting such as a pond. But the most important observation we obtained was that the incorporation of the feed effector in the feed significantly improved shrimp survival following exposure to salinity stress.

The data collected opened our eyes to the benefits of supplementing shrimp feed with feed effectors that have attractive properties. Having that supplementation, we can confidently use poultry meal or animal by-products to produce cost effective nutritious aquaculture feed. That nutritional improvement does not jeopardize the feeding behavior of the animals or their growth. Also, shrimp tolerance to salinity-related stress increases. That can benefit shrimp production in a variety of aspects, mainly by decreasing production expenses as well as resisting stress caused by salinity fluctuations, which is an environmental factor that farmers have less control over.  

Suggested readings

• Full scientific article citation: Tabbara, M., Strebel, L., Peixoto, S., Soares, R., Morais, S., & Davis, D. A. (2024). Use of passive acoustic monitoring to evaluate the effects of a feed effector on feeding behavior, growth performance, and salinity stress tolerance of Litopenaeus vannamei. Aquaculture, 582, 740499. https://doi.org/10.1016/j.aquaculture.2023.740499
• Reis, J., Peixoto, S., Soares, R., Rhodes, M., Ching, C., & Davis, D. A. (2022). Passive acoustic monitoring as a tool to assess feed response and growth of shrimp in ponds and research systems. Aquaculture, 546, 737326. https://doi.org/10.1016/j.aquaculture.2021.737326
• Tabbara, M. (July/August 2023). The future of shrimp feeding in ponds: Eavesdropping on animals has unexpected benefits. Aquaculture North America, 14(4), 20-21.
• Zhu, T., Morais, S., Luo, J., Jin, M., Lu, Y., Le, Y., & Zhou, Q. (2019). Functional palatability enhancer improved growth, intestinal morphology, and hepatopancreas protease activity, replacing squid paste in white shrimp, Litopenaeus vannamei, diets. Journal of the World Aquaculture Society, 50(6), 1064–1077. https://doi.org/10.1111/jwas.12615