Effect of different crude protein and lipid levels on growth and feed conversion in Yellowtail Kingfish

Yellowtail kingfish (Seriola lalandi) is a highly valued marine species known for its rapid growth and high market price. Predominantly farmed in sea cages in regions such as Australia, New Zealand, Japan, and Mexico, this species is now being explored for recirculating aquaculture systems in Europe and the USA. Given the fluctuating costs of raw materials, *optimizing dietary protein and lipid levels is crucial for improving growth performance and feed efficiency. This study, led by Sandeep Sharma, examines the effects of different crude protein and lipid levels on the growth, feed conversion, and body composition of large yellowtail kingfish.*

The nutritional trial was conducted at the National Institute of Water and Atmospheric Research (NIWA) in New Zealand. Yellowtail kingfish with an initial weight of approximately 810 grams were fed experimental diets for eight weeks under controlled conditions (22-23°C, 14:10 light-dark cycle). Seven experimental diets with varying levels of crude protein and lipid were tested using a Doehlert design.

Fish were fed ad libitum twice daily, and various parameters were measured, including weight, length, feces collection, and visceral and liver composition. Final whole-body composition analyses were conducted on starved fish. 

Results

Growth Performance 

Despite the variations in dietary protein and lipid levels, no significant differences were observed in the specific growth rate (SGR) of the fish. The fish approximately doubled their body weight, reaching around 1900 grams by the end of the trial. The SGR values ranged from 1.42 to 1.62, demonstrating robust growth across all dietary treatments.

Feed Conversion Ratio (FCR) 

The feed conversion ratio (FCR) showed significant variation in response to the dietary treatments. FCR generally decreased with increasing crude lipid levels, suggesting more efficient feed utilization. Two-way ANOVA revealed significant effects of both crude protein and crude lipid on FCR (p<0.0001),indicating that protein partially served as an energy source in the diet.

Protein Efficiency Ratio (PER) 

The protein efficiency ratio (PER) was significantly influenced by both crude protein (p<0.007) and crude lipid levels (p<0.0001). PER decreased with higher protein and lower lipid levels in the diet, highlighting the importance of balancing these nutrients for optimal feed efficiency.

Hepatosomatic Index (HSI) 

The hepatosomatic index (HSI) showed significant differences due to dietary protein and lipid levels (Figure 4). Higher dietary lipid levels led to increased HSI, reflecting greater liver lipid deposition. However, the viscerosomatic index (VSI) did not show significant differences, indicating consistent visceral fat deposition across treatments.

The study provides valuable insights into the dietary requirements of yellowtail kingfish.  

This paper identifies specific dietary compositions that enhance growth performance and feed utilization, contributing to more sustainable and cost-effective aquaculture practices. These insights are essential for the continued development and optimization of feeds for yellowtail kingfish, particularly in recirculating aquaculture systems.