Many organisms exhibit social behaviors and are part of some scheme of social structure. Because of this, social isolation can act as a stressor and influence the way the brain and body behave. While stress is typically thought to reduce appetite and inhibit feed intake, prolonged (chronic) isolation can result in increased feed intake and frequency of binge eating. This may be useful when utilizing lower quality and less palatable diets, such as those based on plant meals. Changes in production and release of signaling molecules, namely cortisol, dopamine, and serotonin, may help to understand the effects of social isolation on feed intake and feed utilization. The objective of this study was to evaluate the effects of social isolation on feed intake, stress response, neurochemical signaling, and intestinal health of juvenile zebrafish fed a high-inclusion soybean meal (SBM)-based diet.
At 20 days post-fertilization (dpf), zebrafish were randomly assigned to chronic isolation (1 fish per 1.5L tank) or social housing (6 fish per 9.0L tank). Fish were allowed to acclimate to treatment tanks for 15 days while being fed a commercial diet to apparent satiation. At 35 dpf, zebrafish were switched to a high inclusion SBM diet and fed to apparent satiation for 16 days. Water samples for future cortisol analysis were taken before separation into treatment groups (20 dpf), 24 hours after separation (21 dpf), after acclimation period (35 dpf), 24 hours after SBM introduction (36 dpf), and at experimental termination (51 dpf). Brain and gut samples for future analysis of dopamine and serotonin related genes and histology were taken to assess levels of stress and intestinal inflammation, respectively.
At experimental termination, the mean weight and weight gain were not significantly different between treatment groups (t17 = -1.4035, p > 0.05). The mean total body length was also not significantly different between treatment groups (t17 = -1.4219, p > 0.05). Feed intake (t17 = 4.7258, p = 0.00019) and feed conversion ratio (t17 = 4.2046, p = 0.00059) were significantly higher in chronically isolated fish compared to those in social housing.
These results show that chronic isolation did not negatively affect growth parameters of juvenile zebrafish and suggest that isolation may be useful in promoting feed intake of less-palatable diets such as soybean meal. However, increased feed intake in that group did not translate to better growth, resulting in poor feed conversion ratios. Analysis of cortisol release, gene expression, and intestinal morphology will be presented during oral presentation and may provide insight into the associations between chronic isolation, stress, increased feed intake, and gut health in fish.