Physical Enrichment Triggers Brain Plasticity and Influences Blood Plasma Circulating miRNA in Rainbow Trout (Oncorhynchus mykiss)

Early rearing of European seabass (Dicentrarchus labrax) with mild current enrichment modifies fish swimming behavior without altering their growth performance

The implementation of conditions that favor optimum swimming activity (e.g., suitable flow regimes), has been associated with enhanced growth and improved welfare in some farmed fish species. Despite the importance of European seabass in aquaculture, the potential beneficial effects of rearing flow conditions have not been sufficiently explored in this species. This study investigates how the application of fast (F, 0.01-0.20 m s-1) or slow (S, <0.01 m s-1) steady flows in rearing tanks for 75-77 days affected physiological and behavioral traits in seabass fingerlings. Growth performance, external and internal morphology, and several physiological variables, including hematocrit, plasma cortisol concentration, and osmotic and ionic balance were not affected by flow conditions during rearing. Also, behavioral tests implemented in groups or isolated individuals suggest that coping styles were not affected by the two tank-rearing conditions. On the contrary, the swimming behavior assessed in tests was modified by the flow condition experienced during rearing. Mean swimming speed, peak acceleration, swimming distance, angular velocity, and meander showed some variability across different tests and time, although consistently displaying higher values in seabass reared in the F condition, suggesting increased activity and more consistent swimming patterns in that group. However, the cumulative time in proximity between individuals measured in behavioral group tests suggested that group cohesion was variable, without displaying differences between F and S groups. These findings have particularly important implications for fish welfare and may suggest plasticity in the behavioral response to rearing conditions for this species, although not affecting the assignment of the individuals to the different coping styles. SUMMARY STATEMENT: We investigated how mild current enrichment applied during early rearing in seabass modifies physiological and behavioral responses involving swimming activity, exploring the potential associations to fish welfare.

Feeding predictability as a cognitive enrichment protects brain function and physiological status in rainbow trout: a multidisciplinary approach to assess fish welfare

Cognitive enrichment is a promising but understudied type of environmental enrichment that aims to stimulate the cognitive abilities of animals by providing them with more opportunities to interact with (namely, to predict events than can occur) and to control their environment. In a previous study, we highlighted that farmed rainbow trout can predict daily feedings after two weeks of conditioning, the highest conditioned response being elicited by the combination of both temporal and signalled predictability. In the present study, we tested the feeding predictability that elicited the highest conditioned response in rainbow trout (both temporal and signalled by bubbles, BUBBLE + TIME treatment) as a cognitive enrichment strategy to improve their welfare. We thus analysed the long-term effects of this feeding predictability condition as compared with an unpredictable feeding condition (RANDOM treatment) on the welfare of rainbow trout, including the markers in the modulation of brain function, through a multidisciplinary approach. To reveal the brain regulatory pathways and networks involved in the long-term effects of feeding predictability, we measured gene markers of cerebral activity and plasticity, neurotransmitter pathways and physiological status of fish (oxidative stress, inflammatory status, cell type and stress status). After almost three months under these predictability conditions of feeding, we found clear evidence of improved welfare in fish from BUBBLE + TIME treatment. Feeding predictability allowed for a food anticipatory activity and resulted in fewer aggressive behaviours, burst of accelerations, and jumps before mealtime. BUBBLE + TIME fish were also less active between meals, which is in line with the observed decreased expression of transcripts related to the dopaminergic system. BUBBLE + TIME fish tented to present fewer eroded dorsal fin and infections to the pathogen Flavobacterium psychrophilum. Decreased expression of most of the studied mRNA involved in oxidative stress and immune responses confirm these tendencies else suggesting a strong role of feeding predictability on fish health status and that RANDOM fish may have undergone chronic stress. Fish emotional reactivity while isolated in a novel-tank as measured by fear behaviour and plasma cortisol levels were similar between the two treatments, as well as fish weight and size. To conclude, signalled combined with temporal predictability of feeding appears to be a promising approach of cognitive enrichment to protect brain function via the physiological status of farmed rainbow trout in the long term.

Environmental enrichment improves cognitive flexibility in rainbow trout in a visual discrimination task: first insights

Research on fish cognition provides strong evidence that fish are endowed with high level cognitive skills. However, most studies on cognitive flexibility and generalization abilities, two key adaptive traits for captive animals, focused on model species, and farmed fish received too little attention. Environmental enrichment was shown to improve learning abilities in various fish species, but its influence on cognitive flexibility and generalization abilities is still unknown. We studied farmed rainbow trout (Oncorhynchus mykiss) as an aquaculture model to study how environmental enrichment impacts their cognitive abilities. Using an operant conditioning device, allowing the expression of a motivated choice, we measured fish cognitive flexibility with serial reversal learning tests, after a successful acquisition phase based on two colors discrimination (2-alternative forced choice, 2-AFC), and their ability to generalize a rewarded color to any shape. Eight fish were divided into two groups: Condition E (fish reared from fry stages under enriched conditions with plants, rocks and pipes for ~9 months); Condition B (standard barren conditions). Only one fish (condition E) failed in the habituation phase of the device and one fish (condition B) failed in the 2-AFC task. We showed that after a successful acquisition phase in which the fish correctly discriminated two colors, they all succeeded in four reversal learnings, supporting evidence for cognitive flexibility in rainbow trout. They were all successful in the generalization task. Interestingly, fish reared in an enriched environment performed better in the acquisition phase and in the reversal learning (as evidenced by fewer trials needed to reach the learning criterion), but not in the generalization task. We assume that color-based generalization may be a simpler cognitive process than discriminative learning and cognitive flexibility, and does not seem to be influenced by environmental conditions. Given the small number of individuals tested, our results may be considered as first insights into cognitive flexibility in farmed fish using an operant conditioning device, but they pave the way for future studies. We conclude that farming conditions should take into account the cognitive abilities of fish, in particular their cognitive flexibility, by allowing them to live in an enriched environment.