Effects of Hatchery Rearing on Brain Structure, Aggression and Stress Hormones Levels of Mahseer (Tor putitora)

Abstract

Captive bred and reared fish are often used for conservation and replenishment of natural stock, but it is assumed that these fish are fraught with danger and shows low survival in a natural environment, may be due to behavior deficiencies. Therefore this study was designed to screen the possibl e influences of captive rearing environment on the growth of brains, hypothalamic - pituitary-interrena l (HPI) axis and aggression of mahseer Tor putitora. Eco-morphological and physio logica l approaches were used to investigate the possible impact. Adu lt hatchery reared Tor pulilora, average we ight 250-350 gm were purchased from Mahseer Hatchery Hattian, Attock whereas wi ld counterparts were captured from River Poonch and River Harro. They were kept for 15 days in circular tank under flow through system for acclimatization. After acclimatization, both wi ld and hatchery reared fishes were captured with dip net, immediately anesthetized and blood samples were co llected from the caudal vein for the plasma cortisol and testosterone levels. After noting length, the brain of al l these fish was dissected out for ecomorphologica l study. The area of the who le brain and the dorsally visible substructures (olfactory bulbs, OB; telencephalon, TE; optic tectum, OT and cerebellum, CE) were measured on the digital images using Image J 1.41 . For comparing the stress response, both wild and captive reared mahseer wh ich were he ld in circu lar tanks under flow through system were subjected to stress by co ntinuous chasing for about 5 min. Experiment was conducted in replicate and water samples were co llected before exposure to stress and at different interval of post stress. The results indicated the significantly (P<O.OOO I) small brain to body size ratio and reduced optic tectum in captive reared mahseer as compared to their w ild counterpart. The cerebellum and telencephalon of captive reared mahseer were also comparatively reduced but statistically comparable to wild ll1ahseer. The plasma cortiso l leve l of wild mahseer after mild handling stress was also significantly (P<O.OOO 1) higher in comparison to the status 111 captive reared mahseer. Furthermore, both populations showed considerable differences 111 pre-stress and post-stress water borne cortisol levels. The values were significantly higher in wild population as compared to captive reared ll1ahseer. Wild mahseer showed typical stress response with rapid increase in cO li iso l and quick recovery (within 4 hrs) after exposure to stress whereas captive reared ll1ahseer showed atyp ical stress response with gradual but continuous increase in cortisol even after the cessation of sampling (6 1m). However, significantly (P < 0.05) higher plasma testosterone levels was observed in captive reared mahseer compared to wild counterpart