Effect of handling and crowding on the susceptibility of Atlantic salmon (Salmo salar L.) to Lepeophtheirus salmonis (Krøyer) copepodids

Abstract

Lepeophtheirus salmonis is an ectoparasite causing economic concerns in Atlantic salmon farming. Salmon lice infestation management methods can be stressful and impact fish welfare. This work investigated the stress effect on the attachment of L. salmonis copepodids to Atlantic salmon through two approaches: (a) handling by netting and air exposure (acute stress), and (b) crowding with restricted surface access in a tank (chronic stress). In the first experiment, we compared the number of attached L. salmonis and cortisol levels between a group of handled salmon and a control group. In the second experiment, a group of densely packed salmon was compared to a control group based on the number of attached copepodids, cortisol levels and neutrophil:lymphocyte ratios. Handled salmon showed significantly higher plasma cortisol levels (p < .001) and more attached copepodids (p = .01) than control salmon. Conversely, the cortisol level and number of attached copepodids were not significantly different between the densely packed and control salmon (p > .05). The neutrophil:lymphocyte ratio was significantly higher (p = .0014) in the densely packed salmon than in the control salmon. Handling salmon increased their risk of infestation by L. salmonis. This has implications for reinfestation rates following delousing treatments in commercial salmon aquaculture, which often involve crowding and handling salmon.

Experimental Handling Challenges Result in Minor Changes in the Phagocytic Capacity and Transcriptome of Head-Kidney Cells of the Salmonid Fish Coregonus maraena

“…The resistance to pathogens is apparently weakened in teleost fish following a disruptive stimulus, as has been concluded by other authors ( 50 – 52 ). Similar conclusions were drawn from studies that investigated the relationship between handling stress and parasite infestation ( 26 ).…”

“…These studies have provided an extensive list of a wide variety of parameters that change after typical handling procedures, such as chasing and netting, exposure to air, and transfer to (confinement) tanks ( Table 1 ). The investigated parameters include body scores ( 15 ), concentrations of osmolytes and metabolites (e.g., hormones, carbohydrates, lactate, nucleotides, free fatty acids, and sialic acids) ( 16 – 21 ), blood cell counts ( 16 , 22 ), enzyme activities ( 23 – 25 ), behavioral characteristics ( 23 ), susceptibility to parasites ( 26 ), and a long and diverse list of tissue-specific transcripts recorded via qPCR ( 27 – 29 ) or transcriptomic approaches ( 30 – 34 ).…”

Blood Will Tell: What Hematological Analyses Can Reveal About Fish Welfare

“…Differential blood counts have been used to assess the effects of acute and chronic stress events, such as heavy metal exposure in common carp (Cyprinus carpio) (133), organochloride herbicide exposure in African catfish (Clarias gairepinus) (134), or crowding in Atlantic salmon (Salmo salar) (135). Nevertheless, conducting a differential blood count in fish is not very common in clinical laboratory diagnostics, often because reference values are missing.…”

Reduced Infestation Levels of Lepeophtheirus salmonis in Atlantic Salmon (Salmo salar) following Immersion Exposure to Probiotic Aliivibrio spp.

“…High fish density is a known stressor, and stress is regarded as an important factor that may increase host susceptibility to infection [17]. Still, fish density at the end of trial 2 was 34.5 kg/m 3 three weeks after the second lice count, lower than what is shown to cause negative effects when other parameters are within recommended levels [54,55]. In addition, trial 1 density was 36.2 kg/m 3 at the lice count and trial 3 density was 26.8 kg/m 3 one week after the lice count, indicating that all trials likely had similar densities throughout the lice challenge period, far below density limits for post-smolt suggested by Calabrese et al Growth is one of the main parameters affected by chronic stress [14].…”