How can natural ingredients support Atlantic Salmon in coping with handling stress?

Atlantic salmon (Salmo salar) farming industry frequently faces the challenge of managing sea lice infestations, which necessitates mechanical treatment procedures that can stress the fish. Handling stress from crowding and delousing can negatively impact fish appetite and growth, leading to economic losses. This study, led by Jinni Gu, evaluates the efficacy of a feed product supplemented with natural plant-based ingredients rich in phytobiotics in mitigating stress-induced growth loss in Atlantic salmon.

The trial was conducted over nine months (September 2022 – May 2023) in sea cages in Norway. The study aimed to mimic the conditions of mechanical sea lice treatments, involving four handling events. Each event consisted of crowding the fish for 1.5 hours, followed by netting, anesthetizing, individual weighing, and manual delousing. The test feed was pulse-fed to the salmon two weeks before and during the week of handling events. Three groups were included in the study: 

1. Unstressed Control (Unstressed): Fed a standard grower diet without handling stress. 

2. Stressed Control (Stressed-C): Fed a standard grower diet with handling stress. 

3. Stressed Test (Stressed-T): Fed a test diet supplemented with natural plant-based ingredients, with handling stress. 

Results

Stress Biomarkers

The handling events significantly increased stress biomarkers such as glucose, lactate, calcium in serum, and cortisol deposited in scales in regardless of  feeding groups (Figure 1) confirming the fish had been suffering stress caused by the handling.

\ Feed Intake and Appetite Recovery

The Stressed-T group recovered their appetite more quickly after handling events compared to the Stressed-C group. This effect became more pronounced after the second handling event, indicating that the test diet helped mitigate the negative impact of stress on feeding behavior (Figure 2).Growth Performance 

The thermal growth coefficient (TGC) was significantly higher in the Stressed-T group during the entire stressed period, particularly after the third handling event. The final body weight of fish in the Stressed-T group was 11,3% higher than the Stressed-C group. This suggests that the test diet not only helped maintain growth during stress but also improved overall growth performance.

Fillet Quality and Pigmentation 

The fish fed the test diet (Stressed-T) showed significantly better fillet quality with higher Salmofan scores, indicating improved pigmentation compared to the Stressed-C group (p<0.05). Additionally, there was less downgrading of fillets due to gaping and inelasticity in the Stressed-T group.

The study demonstrates that a diet supplemented with natural plant-based ingredients can effectively mitigate the negative impacts of handling stress in Atlantic salmon. The faster appetite recovery, enhanced growth performance, and improved fillet quality observed in the Stressed-T group highlight the potential benefits of such dietary interventions. can significantly improve their ability to cope with handling stress. This dietary intervention leads to faster recovery of appetite, better growth performance, and improved fillet quality. Such solutions can be applied to other fish species, providing a practical approach to managing stress and enhancing performance in aquaculture.