Characterization of the cortisol response following an acute challenge with lipopolysaccharide in yellow perch and the influence of rearing density

Two experiments were performed to characterize the corticosteroid response of yellow perch Perca flavescens following an intraperitoneal injection of lipopolysaccharide (LPS) and determine if sustained differences in rearing density alter this response. In the first experiment, yellow perch were injected with LPS (3 mg kg⁻¹), saline, or handled without receiving any injection. Concentrations of cortisol in plasma were elevated in all groups relative to non‐disturbed fish at 1·5 and 3 h after handling but by 6 h after injection the mean concentration of cortisol in plasma from LPS‐injected yellow perch were three to five times higher than fish before injection and significantly larger than groups of fish not treated with LPS. In the second test, yellow perch were held at different rearing densities (9 v . 18-19 kg m⁻³) for 3, 7 and 14 days before injection with LPS (3 mg kg⁻¹). The cortisol response of yellow perch following LPS injection of fish held for 14 days at high density was significantly lower than that of fish held at the low density for the same duration. Additionally a trend of a decreased cortisol response to LPS injection as duration of holding increased was observed among fish held at high density relative to fish held at low density. These data illustrate that the corticosteroid response of yellow perch following LPS injection is distinct from handling alone and that the magnitude of the response is impacted by rearing density.