Dietary crude ulvan from Ulva intestinalis enhances growth, salinity stress tolerance, and immunity against Vibrio harveyi in early juvenile milkfish Chanos chanos

Abstract

<i>Ulva</i> green seaweeds are known for their rapid growth and limited commercial value, and hold significant potential as sources of bioactive compounds for aquaculture. Among these, the sulfated polysaccharide ulvan has shown promising growth-promoting and immunostimulatory properties. These properties may offer a natural solution to vibriosis caused by <i>Vibrio harveyi</i> that hinders the sustainability of milkfish (<i>Chanos chanos</i>) nursery production. To evaluate this potential, a 30-day feeding trial was conducted in a completely randomized design. Five dietary treatments in triplicate, including a control diet without ulvan and four diets supplemented with graded levels of ulvan (10, 50, 100, and 500 mg kg⁻¹) were evaluated on early juvenile <i>C. chanos</i> (14.62 mg mean weight). Diets with 10 and 50 mg kg⁻¹ ulvan significantly increased weight gain, specific growth rate, and protein efficiency ratio, while lowering the feed conversion ratio compared to the control group and higher ulvan doses (100 and 500 mg kg⁻¹). Fish fed as little as 10 mg kg⁻¹ ulvan also demonstrated enhanced salinity stress tolerance as evidenced by their higher cumulative stress index values. Further analysis of immune gene expression revealed that supplementation of 10 mg kg⁻¹ ulvan significantly improved immune response by upregulating <i>TLR5, IL6</i>, and <i>LEAP2</i> mRNA expression levels, resulting in higher survival rates following a <i>V. harveyi</i> bacterial challenge. Overall, these results indicate that adding 10 mg kg⁻¹ ulvan to the nursery <i>C. chanos</i> diet is practical and efficient to boost growth and immunity, offering a promising solution to enhance nursery resilience to <i>V. harveyi</i>.