Taurine supplementation could support protein substitution in fish feed

Supplementing sea bass diets with a higher level of taurine boosts fish growth and feed efficiency when using a low-fishmeal, high soy protein diet, researchers say.

An international team of researchers from the US and Europe explored the use of supplemental taurine in soy-based diets and the influence on growth and fillet quality in young European sea bass.

The present study aimed to evaluate the impact of dietary taurine supplementation in soybean-based diets on growth, feed intake, body and muscle composition and histology of intestine and liver of juvenile sea bass during the on-growing phase, as well as on the evaluation of fillet quality by means of texture and sensory analyses,” ​the researchers said.

The highest level of supplemental taurine, 1%, was found to improve fish growth performance and feed efficiency, they found. However, muscle and body proximate composition were similar for fish on each of the diets, said the team.

Taurine concentration in muscle increased as higher levels of supplemental taurine were added to the diet, they said. Fillet texture altered with increased taurine supplementation – fillet hardness and chewiness grew along with a slight drop in fillet adhesiveness and a tendency for reduced elasticity

Gut and liver tissue remained similar for fish regardless of diet, they said.

“Overall, the findings of the present study showed that 1.0% taurine supplementation in diets with high levels of soybean products might have a beneficial effect on growth performance and a pronounced effect on flesh quality of European sea bass."​

Reducing fishmeal in aqua feeds​

Fishmeal (FM) has been a primary protein in aqua feeds, but fish feed formulators are looking to reduce levels of fishmeal as the finite feed ingredient increases in price. The authors stressed that reducing industry reliance on marine ingredients is necessary to support the “sustainable expansion of the aquaculture industry.”

“The level of FM inclusion within compound diets for marine fin fish has steadily declined during the last years due to the incorporation of proteins derived from plants (PP) or microalgae and heterotrophic bacteria,” ​they said. “In [the] search for alternative, economically-viable protein sources for aqua feeds, many plant raw materials have been tested, for example, soybean meal, rapeseed meal, sunflower meal, lupin seed meal, pea seed meal, etc., with varying degree of success.”​

The increased use of plant proteins has been linked to reduced fish performance due to the inherent anti-nutritional factors in some plant materials, with nutrient digestibility and bioavailability compromised, they said.

Soy-based plant proteins have generated the most interest as they provide “consistent nutritional composition”​ along with a reasonable price and balanced amino acid profile, they said. “However, the use of soy protein in feeds developed for carnivorous fish represents several challenges, which include low methionine and cysteine content, lower protein digestibility, indigestible oligosaccharides, low phosphorus availability, anti-nutritional factors, poor palatability and undetectable levels of taurine,”​ they added.

A way to address those challenges is to supplement diets with functional feed additives and amino acids, the researchers said.

Why supplement feed with taurine?​

Taurine is a “non-proteinaceous beta-amino acid”.  ​It is synthesized from methionine via cysteine by a series of enzymatic reactions, a process that is considered as species- and developmental stage dependent, said the authors. 

Fishmeal and animal byproducts typically have high concentrations – about 0.5% to 1% – of taurine, however, it is typically not present in any quantity in plant proteins.

“Even when all essential amino acid requirements are met in plant-based diets for carnivorous fish, growth performance still is often reduced when compared to diets containing high FM levels,” ​they said. “Therefore, taurine supplementation may be required for plant-based diets.”​

Previous taurine related research focused on olive flounder, rainbow trout, cobia, yellowtail and red snapper showed improved feed efficiency and weight gain when diets were supplemented with the amino acid, they said. 

European sea bass is a commonly raised fish in Mediterranean aquaculture, the researchers said.  

Benefits from the amino acid were noted with sea bass fry on a high FM diet, they noted.

But little work has been done exploring taurine supplementation with low FM diets on fish growth performance during the growing phase, the researchers said. The influence of the feed additive on fillet quality has not been examined either.

Feeding trial details​

During the feeding trial, juvenile fish received one of five diets for 12 weeks, the researchers said.

The diets included a blended fishmeal (30%) and soybean meal (20%) positive control (C+) and a 25% fishmeal, 20% soybean meal and 12% soy protein concentrate negative control (C-), they said. Other diets contained 25% fishmeal (FM) with 20% soybean meal (SBM) and 12% soy protein concentrate (SPC) and three levels of crystalline taurine at 0.2%, 0.5% or 1% (T0.2, T0.5 and T1).

“All diets were iso-nitrogenous (44%), iso-lipidic (20%) and iso-energetic (22 MJ kg−1),” ​they added.

A selection of fish was gathered before the feeding trial to check whole-body composition, they said. During the trial, feed consumption was recorded daily.

All fish were weighed at the end of the feeding period and a selection was harvested to collect fillets, liver and intestines for analysis, the researchers said. Perivisceral fat and fish livers were weighed and fish were checked for dry matter, ash, crude fat and amino acid composition.

Another sample of fish was harvested for color and texture analysis, they said. A section of fish on the C+, C- and T1 diets was gathered for “sensory analysis.”​

Fillet color change was checked for lightness, redness and greenness, yellowness and blueness, they said. Texture was evaluated using a texture analyzer and descriptive sensory analysis was completed with a trained group of panelists examining fillet texture, color, taste and flavor intensity.

Specific growth rate, total feed intake, relative feed intake, daily growth index, thermal growth coefficient (TGC), the feed conversion ratio (FCR), the protein efficiency ratio (PER), lipidosomatic index (LSI), hepatosomatic index (HSI) and the protein productive value were calculated, they said.

Results ​

Overall, survival rates ranged between 94 and 99% during the feeding trial, the researchers said. Fish on the T1 diet presented a slightly higher average final body weight, weight gain, TGC and daily growth index compared to fish on other diets.

“The findings of the present study showed that juvenile European sea bass can be fed a diet with high levels of soy products that contain a mixture of soybean meal and soy protein concentrates up to 32%, without impeding fish performance either inducing alterations, morphological changes or inflammatory symptoms in the distal intestine,”​ they said.

Fish showed similar HSI and LSI results regardless of diet, they said. Parameters of feed use including FCR and PER were similar for all fish.

Whole-body proximate composition for the fish remained the same regardless of diet, but fillet taurine levels increased as more of the supplement was included in the diet, the researchers said. “No significant differences in the histological structure of the intestine were found among groups.”​

However, all fish showed some level of fatty infiltration in the liver, they said. Fish on the C- diet had the lowest redness values while those on the C+ had the highest and all other color variables were similar for all fish.

“The highest hardness and chewiness values were found in fish fed the T1.0 diet, whereas the lower ones in the C+ groups (P < 0.05), whereas the rest of dietary treatments showed intermediate values,” ​they said. Fish on the T1 diet had slightly lower fillet elasticity, taste intensity and fillet darkness compared to fish on the C+ and C- diets.

Source: Aquaculture​

Authors: Y. Kotzamanis, V. Kumar, T. Tsironi, K. Grigorakis, V. Ilia, I. Vatsos, A. Brezas, J. van Eys, E. Gisbert