Interactive Effects of Dietary Starch Level and Ingredient Grinding Size on Growth, Intestinal Health and Liver Condition of Juvenile Giant Grouper (Epinephelus lanceolatus)

Abstract

Giant grouper (Epinephelus lanceolatus) is a highly valued aquaculture species known for its rapid growth. Traditionally reared using ‘trash fish’ as a nutrient source, production of giant grouper has outpaced the nutritional knowledge required for optimal production. Equally important is an appropriate application of processing aids that facilitate feed physical properties while also supporting growth and health. In extruded aquafeed, starch provides energy while enhancing durability and expansion, promoting floating properties important for feed management. Grinding raw materials is a process central to feed manufacture, facilitating feed processing. However, the interaction of particle size after grinding and fish production performance characteristics is understudied. To address this, the present study aimed to evaluate the effects of varying dietary wheat starch levels and ingredient grinding on growth, intestinal health and liver condition in juvenile giant grouper. Seven isonitrogenous (~55% crude protein) and isocaloric (~19 MJ/kg) diets were formulated, containing low (9%), medium (12%) and high (15%) wheat starch, with ingredients ground to either 1 or 0.5 mm with an additional very high-starch diet (15.9%) formulated exclusively at the 0.5 mm grinding size and not tested at 1 mm. After a 6-week trial, elevation of dietary starch levels above the minimum tested led to increasing inflammation in the posterior intestine, evidenced by a thicker lamina propria and a higher prevalence of eosinophilic granular cells characteristic of intestinal inflammation. Fish fed diets made from ingredients milled to 0.5 mm exhibited lower growth rates and enlarged livers due to elevated glycogen deposition in hepatocytes. This study does not support the practice of elevating starch levels above 9% of the feed, nor does it support grinding of raw materials to 0.5 mm. These results provide valuable insights for formulating nutritionally balanced feeds using wheat starches, supporting the sustainable development of giant grouper aquaculture.