The influence of rearing density as environmental stressor on cortisol response of Shortnose sturgeon (Acipenser brevirostrum)

Cortisol levels have been determined in cultured Shortnose sturgeons (Acipenser brevirostrum) in order to monitor stress related to the stocking density.Fish used in the experiments were initially reared at a density of 15 kg/m 3 .For the experiment fish were stocked at low, moderate, and high densities of 10 kg/m 3 , 20 kg/ m 3 , and 40 kg/m 3 , respectively.After 14 d, the medium and high rearing densities resulted in consistent elevations of cortisol levels (29.62 ± 11.36 ng/ml, n = 20; 17.16 ± 5.27 ng/ml, n = 19), while the low density group did not reflect significant changes in this parameter (3.56 ± 2.75 ng/ml n = 15).The regrouping after 14 d to a common density of 15 kg/m 3 led to a significant decrease of the cortisol levels for both moderate (6.16 ± 3.53 ng/ml) and high density (4.73 ± 3.19 ng/ml) groups while the group initially reared at low density revealed a slight -though not significantly -increase in cortisol concentration from 3.56 ± 2.75 to 11.54 ± 5.08 ng/ml (n = 15).It has been demonstrated for the first time that in sturgeon too, rearing density is an important environmental stressor.In addition, it was obvious that the conditions utilized in commercial rearing of Shortnose sturgeon are near to optimum, revealing the lowest cortisol levels measured.However, after re-establishing the initial stocking density a faster relaxation in cortisol levels was observed in the two groups transferred from higher to low densities.This suggests that conditioning of sturgeons to more stressful rearing densities induces an adaptive capacity for rapid relaxation after handling.