Individual differences in cortisol levels and behaviour of Senegalese sole ( Solea senegalensis) juveniles: Evidence for coping styles

Abstract

Individual variation in stress physiology and behaviour has been previously reported in several fish species. As seen in other vertebrates, existence of stress coping styles seems to be reflected by the presence of individual variation. Aggressive behaviour, amongst others, is one of the most commonly used parameters to characterize coping styles. However, not all fish species exhibit aggressive behaviour, such as the flatfish Senegalese sole, Solea senegalensis (Kaup, 1858). Therefore, the goal of this study was to determine the magnitude of individual variation in behavioural parameters other than aggression (feeding motivation and activity during stress) as well as in growth and stress response in Senegalese sole. The relationship between these variables was investigated to determine whether they could be used as indicators of coping styles. Thirty-six juvenile fish (9.9 ± 2.2 g) were individually housed for 73 days. Feeding motivation, measured as the time (in s) taken by each fish to react to feed, was determined on days 10, 17, 24 and 31. Blood samples for plasma cortisol were collected on days 51 and 71 for determination of undisturbed and stress levels, respectively. The stress test consisted of holding each fish individually in a net, outside the water, for 3 min. Duration of escape attempts, i.e. the time taken by each fish to stop struggling (in an attempt to escape) in the net, was quantified. The results showed a pronounced individual variation in both control (CV = 54%) and acute stress (CV = 71%) cortisol levels. Senegalese sole also exhibited high coefficient of variation in the behavioural parameters: 75% in feeding latency and 96% in duration of escape attempts. Growth (RGR = 1.17 ± 0.38) showed to be the parameter with lower variation of only 32% and was not correlated with any of the measured parameters. A significant correlation between undisturbed cortisol levels and duration of escape attempts was found. Undisturbed cortisol levels (8.08 ± 4.36 ng/ml) were negatively correlated with duration of escape attempts ( P = 0.009, r s = −0.503). Correlations between plasma cortisol levels after stress (398.45 ± 282.67 ng/ml) and the behavioural parameters were not found. The observed individual variation in behaviour and stress physiology as well as their relationship suggests the existence of coping styles in Senegalese sole where proactive individuals exhibit shorter feeding latency, higher duration of escape attempts and lower undisturbed cortisol levels than passive individuals.