Impacts of crowding stress on aquatic animals and its mitigation through feed additives supplementation - a review

Continuous and sustainable aquaculture development is essential to provide affordable protein sources for the global population. Nevertheless, the intensification of fish farming at high stocking density led to chronic crowding stress that could impair the growth performance, immune system, and antioxidant capacity of an aquaculture species. Consequently, the prevalence of disease outbreaks increases, resulting in low aquaculture production. Various countries remain highly dependent on antibiotics and chemotherapies to overcome this problem. Recently, researchers have proposed and developed alternative solutions using environmentally-friendly feed additives, such as probiotics, phytobiotics, and synthetic immune stimulants, to mitigate crowding stress. The feed additives reportedly alleviate crowding stress, promote growth performance, enhance gut microbiota, immune system, and antioxidant capacity, and stimulate disease resistance, contributing to the overall improvement of the health status of aquaculture species and boosting aquaculture production. The application of feed additives in aquafeed has been investigated in various aquaculture species to explore their beneficial effects at optimum doses. The study outcomes illustrated that feed additives in aquafeed are promising in mitigating crowding stress among aquaculture species. Herein, this review paper discussed and summarised the impacts of crowding stress on the growth, immune system, and antioxidant capacity of aquatic animals. In addition, this review highlighted the potential of feed additives such as probiotics, phytobiotics, and synthetic immune stimulants in mitigating crowding stress in aquatic animals was highlighted in this review.