Effect of Rearing Environment on HypothalamicPituitary-Interrenal (HPI) Axis of Mahseer (Tor putitora)

Abstract

Release of hatchery reared fish in the natural environment is one of the most important strategies to replenish the natural stock of endangered fish species mahseer (Tor putitora). Most of the reintroduction projects are not providing the desired results, since captive-reared fish may not possess the well-adapted behavioural skills and physiological responses required for survival in the natural environment. Early rearing environment affect the stress sensitivity and shaping the behaviour of fish, while enrichment in the rearing environment, improve the behaviour and physiological responses of fish by modulating HPI axis and dopaminergic and serotonergic systems. Here an attempt has been made to investigate the impact of rearing environment on the physiological stress response and behaviour of endangered fish Tor putitora. First, the physiological status of wild caught and captive reared fish was compared by adopting invasive and non-invasive methodology and measured the pre-stress and post stress levels of plasma and water-borne cortisol, blood glucose, whole brain serotonergic activity (5HIAA/5HT ratio), dopaminergic activity (DOPAC/DA and HVA /DA ratios) and Norepinephrine (NE) levels. Life skill activities like exploratory, predatory and anti-predatory behaviour of both populations under laboratory condition were also studied. The captive reared mahseer displayed a typical stress response i.e., low activation of HPI axis and brain monoamenergic (serotonergic and dopaminergic) system and delay recovery period as compared to wild counterpart. Moreover, the wild fish display significantly (p ˂ 0. 05) more exploratory, predatory and anti-predatory behaviour in comparison to the captive reared fish. However, captive mahseer appeared bolder. In the second part of the study, to test the impact of the enriched rearing environment on hypothalamic–pituitary–interrenal (HPI) axis, brain monoamines system and life skills of fish were devised three different rearing environments (barren, semi-natural and physically enriched) that differed in their levels of complexity and heterogeneity and reared mahseer 15 days old hatchlings up to advanced fry stage in these environments. Similar non-invasive and invasive techniques as used for comparison of wild caught and captive reared fish were followed for evaluation of pre and post stress levels of whole-body and water borne cortisol, blood glucose and brain monoamines as well as the behaviour of fry from three different rearing environments. The pre-stress basal cortisol, blood glucose and brain monoamines serotonergic, dopaminergic and central norepinephrine (NE) levels were higher in fry reared in barren and semi-natural environment indicating the impact on rearing environment on the stress sensitivity fish. Furthermore, exposure of acute physical stress induced increase in whole-body cortisol, blood glucose, whole-brain serotonergic and dopaminergic activity and Norepinephrine (NE) levels in all three different rearing groups up to 0.75 hr. Although, the peak levels of all stress parameters were observed at 0.75 hrs post stress, but there was a significant difference in the increasing trend and magnitude of cortisol, blood glucose and brain monoamines at peak levels among three rearing groups of mahseer previously reared in different rearing environments. At 0.25 hrs, rapid stress response was observed in semi-natural reared mahseer, compared with barren and physically-enriched reared fish. After 0.75 hrs post stress, the levels of cortisol, blood glucose, brain-monoamines (ratios of 5HIAA/5HT, DOPAC/DA and HVA /DA) and NE in all rearing groups showed a steady decreasing trend and recovered to its basal level after several hours of stress. The recovery time of wholebody cortisol, blood glucose and whole-brain monoamines were significantly higher in barren reared mahseer compared with the other two groups. Moreover, it was also observed that the increased structural complexity during early life significantly affect various behavioural characteristics of the fish. Exploratory, predatory and antipredatory behaviours were significantly (p ˂ 0.05) more pronounced in fry reared in physically enriched and semi-natural environment than barren environment. The results of the present study specify the role of rearing environment in shaping the stress response and life skill activities of fish and suggest an improvement in the hatchery rearing environment to reduce the physiological and behavioural variations among wild and captive reared counterpart. We further illustrate that the increased structural complexity, i.e., physical enrichment in the early rearing environment significantly modulates various physiological stress coping mechanisms and life skill behaviour of mahseer. These outcomes have important implications for a possible way of improving the outcomes of restocking programs of endangered fish species by modifying conventional hatchery-rearing environments.