Physiological Response of Atlantic Salmon (Salmo salar) to Long-Term Exposure to an Anesthetic Obtained from Heterosigma akashiwo

Abstract

Despite the invaluable role of anesthetics as a tool for ensuring animal welfare in stressful situations, there is currently a lack of anesthetic drugs that meet the requirements of intensive aquaculture. In response to the growing interest in anesthetic substances of natural origin, this study evaluated the physiological and health impact of an anesthetic based on an extract of the microalga Heterosigma akashiwo on juvenile salmon (Salmo salar) exposed for a period of 72 h. To simulate a condition closer to reality where fish are subjected to stimuli (e.g., transport), the animals were exposed to 50 mg L^-1 of algal extract and to physical stress. Functional, physiological, and histological parameters were evaluated in blood and tissues at different sampling periods (0, 24, and 72 h). There was no mortality and the induction and recovery times observed were within the established criteria for anesthetic efficacy. The anesthetic extract did not induce any side effects, such as stress or metabolic damage, indicating that this extract is a viable option for supporting fish welfare during deleterious events. This study provides information to support that the anesthetic extract tested, derived from H. akashiwo, is a promising candidate drug for operations requiring sedation (e.g., Salmonid transport).

Keywords: Atlantic salmon (Salmo salar); natural anesthetic; physiological response; welfare practices.

Figure 1

(A) Plasma glucose; (B) Triglycerides, and (C) Total cholesterol levels (mg dL⁻¹) of juvenile Atlantic salmon before (0 h) and during 24 h, 48 h and 72 h exposure to the anesthetic (50 mg L⁻¹) without stress (EXT50), or with handling stress (EXT50-S). Bars indicate mean ± SD (n = 6). Lines indicate differences between the groups under 2-way ANOVA and Tukey’s HSD test (p < 0.05).

Figure 2

(A) Blood hemoglobin concentration (g dL⁻¹) and (B) hematocrit (%) of juvenile Atlantic salmon before (0 h) and during 24 h, 48 h and 72 h exposure to the anesthetic (50 mg L⁻¹) without stress (EXT50), or with handling stress (EXT50-S). Bars indicate mean ± SD (n = 6). No significant differences were observed between groups under 2-way ANOVA (p > 0.05).

Figure 3

Plasma aspartate (A) and alanine (B) transaminase activities (U L⁻¹) in juvenile Atlantic salmon before (0 h) and during 24 h, 48 h and 72 h exposure to the anesthetic (50 mg L⁻¹) without stress (EXT50), or with handling stress (EXT50-S). Bars indicate mean ± SD (n = 6). No significant differences were observed between groups under 2-way ANOVA (p > 0.05).

Figure 4

Plasma cortisol levels (ng mL⁻¹) in juvenile Atlantic salmon before (0 h) and during 24 h, 48 h and 72 h exposure to the anesthetic (50 mg L⁻¹) without stress (EXT50), or with handling stress (EXT50-S). Bars indicate mean ± SD (n = 3). No significant differences were observed between groups under 2-way ANOVA (p > 0.05).

Figure 5

Histological score representing the histological changes occurring in the gills of juvenile Atlantic salmon after 72 h exposure to the anesthetic (50 mg L⁻¹) without stress, or with handling stress (+S). Bars indicate mean ± SD of the difference in scores compared with the initial condition before exposure (0 h) (n = 3). No bars indicate score 0 (i.e., no histological changes), and no error lines indicate same score in all measured individuals. No significant differences were observed between groups (Kruskal-Wallis, p > 0.05).

Figure 6

Histological score representing histological changes to the liver and intestine in juvenile Atlantic salmon after 72 h exposure to the anesthetic (50 mg L⁻¹) without stress, or with handling stress (+S). Bars indicate mean ± SD of the difference in scores compared with the initial condition before exposure (0 h) (n = 3). No bars indicate score 0 (i.e., no histological changes), and no error lines indicate same score in all measured individuals. No significant differences were observed between groups (Kruskal-Wallis, p > 0.05).

Figure 7

Activity of the enzymes responsive to oxidative stress superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase (GR) and the concentration of tiobarbituric acid reactive species (TBARS) in muscle, liver, and gills of juvenile Atlantic salmon before (0 h) and during 24 h, 48 h and 72 h exposure to the anesthetic extract (50 mg L⁻¹) without stress (EXT50), or with handling stress (EXT50-S). Bars indicate mean ± SD (n = 6). Lines indicate differences between groups under 2-way ANOVA, Tukey’s HSD test (p < 0.05).

Figure 8

Heat map of overall modulation in mean scores over time within the various categories of the parameters analyzed in blood, muscle, liver and gill of juvenile Atlantic salmon exposed for 72 h to a microalga-derived anesthetic extract (50 mg L⁻¹).

Figure 9

Experimental design of the in vivo trial for assessment of physiological and stress effects in juvenile salmon (Salmo salar) exposed for a prolonged period to a microalgal (Heterosigma akashiwo) extract (50 mg L⁻¹) with anesthetic properties. Asterisk (*) indicates applied extract, and sign plus (+) indicates applied stress.