Positive effects of bubbles as a feeding predictor on behaviour of farmed rainbow trout.

Occupational enrichment emerges as a promising strategy for improving the welfare of farmed animals. This form of enrichment aims to stimulate cognitive abilities of animals by providing them with more opportunities to interact with and control their environment. Predictability of salient daily events, and in particular predictability of feeding, is currently one of the most studied occupational enrichment strategies and can take several forms. In fish, while temporal predictability of feeding has been widely investigated, signalled predictability (based on a signal, such as light or sound) has received little attention. Depending on the type of predictability used and the ecology of the species, the effects on fish welfare often differ. The present study aimed to determine which feeding predictability would be most appropriate for rainbow trout, the main continental farmed fish in Europe, and what the consequences might be for their welfare. We tested four feeding predictability conditions: temporal (based on time of day), signalled (based on bubble diffusion), temporal + signalled (based on time and bubble diffusion), and unpredictable (random feeding times). Behavioural and zootechnical outcomes recorded were swimming activity, aggressive behaviours, burst of accelerations, and jumps, emotional reactivity, and growth. Our results showed that rainbow trout can predict daily feedings relying on time and/or bubbles as predictors as early as two weeks of conditioning, as evidenced by their increased swimming activity before feeding or during feed omission tests, which allowed to reinforce their conditioned response. Temporal predictability alone resulted in an increase in pre-feeding aggressive behaviours, burst of accelerations, and jumps, suggesting that the use of time as the sole predictor of feedings in husbandry practices may be detrimental to fish welfare. Signalled predictability with bubbles alone resulted in fewer pre-feeding agonistic behaviours, burst of accelerations, and jumps than in the temporal predictability condition. The combination of temporal and signalled predictability elicited the highest conditioned response and the level of pre-feeding aggression behaviours, burst of accelerations and jumps tended to be lower than for temporal predictability alone. Interestingly, fish swimming activity during bubble diffusion also revealed that bubbles were highly attractive regardless of the condition. Rainbow trout growth and emotional reactivity were not affected by the predictability condition. We conclude, therefore, that the use of bubbles as a feeding predictor could represent an interesting approach to improve rainbow trout welfare in farms, by acting as both an occupational and physical enrichment.

Sources of variation of DNA methylation in rainbow trout: combined effects of temperature and genetic background.

Phenotypic plasticity is a key component of the ability of organisms to respond to changing environmental conditions. In this study, we aimed to study the establishment of DNA methylation marks in response to an environmental stress in rainbow trout and to assess whether these marks depend on the genetic background. The environmental stress chosen here was temperature, a known induction factor of epigenetic marks in fish. To disentangle the role of epigenetic mechanisms such as DNA methylation in generating phenotypic variations, nine rainbow trout isogenic lines with no genetic variability within a line were used. For each line, half of the eggs were incubated at standard temperature (11°C) and the other half at high temperature (16°C), from eyed-stage to hatching. In order to gain a first insight into the establishment of DNA methylation marks in response to an early temperature regime (control 11°C vs. heated 16°C), we have studied the expression of 8 dnmt3 (DNA methyltransferase) genes, potentially involved in de novo methylation, and analysed global DNA methylation in the different rainbow trout isogenic lines using LUMA (LUminometric Methylation Assay). Finally, finer investigation of genome-wide methylation patterns was performed using EpiRADseq, a reduced-representation library approach based on the ddRADseq (Double Digest Restriction Associated DNA) protocol, for six rainbow trout isogenic lines. We have demonstrated that thermal history during embryonic development alters patterns of DNA methylation, but to a greater or lesser extent depending on the genetic background.