Whole egg and alevin changes of cortisol and interrenal tissue differences in rainbow trout Oncorhynchus mykiss exposed to different stocking densities during early development

Abstract

Cortisol level changes in response to stocking density in the early stages of rainbow trout were measured. Eggs were exposed to low, normal, and high (2.55, 5.10 and 7.65 eggs cm(-2)) densities during the incubation period. Cortisol of maternal origin was found in pre-fertilized eggs (5.09 ± 0.12 ng g(-1)) of rainbow trout. In newly fertilized eggs, resting Cortisol levels (3.68 ± 0.14 ng g(-1)) decreased to 0.58 ± 0.08, 0.60 ± 0.12, and 0.57 ± 0.16 ng g(-1) at low, normal and high densities by day 10 (organo-genesis), respectively. Resting Cortisol levels remained constant until the eyed stage (day 18). Then, Cortisol showed an increase at hatching to 1.16 ± 0.11, 1.20 ± 0.12, and 1.21 ± 0.14 ng g(-1) at low, normal, and high densities, respectively. The pattern of change in Cortisol level was similar in all three densities. Interrenal cells were observed in 1-day old alevins at all three densities. Hematopoietic tissue, renal tubules and nucleated red blood cells were clarified through the head part of kidney. Higher numbers and larger interrenal cells were observed at high-density groups. Chronic density stress test conducted on embryonic stages of rainbow trout revealed no differences in Cortisol levels, but had an effect on the abundance and size of the interrenal cells. Densities were equaled after hatching (200 alevins per replicate) to investigate the different densities of eggs on stress indices in rainbow trout alevins. An acute stress (air exposure of eggs for five minutes) was applied in three treatments two weeks after hatching, and samples were taken at 0, 1, 3, 6, and 24 hps (hours post stress). Cortisol content increased under low density in 1 hps and reached from 5.21 ± 0.13 ng g(-1) to 6.01 ± 0.18 ng g(-1) (P < 0.05). Cortisol levels increased under normal density in 1 hps from 6.03 ± 0.28 ng g(-1) to 10.84 ± 0.18 ng g(-1) (P < 0.05). In high density also Cortisol increased from 6.83 ± 0.23 ng g(-1) to 8.86 ± 0.26 ng g(-1) (P < 0.05). At 3 hps;, Cortisol level was returned to basal level under low (P > 0.05) and normal (P > 0.05) densities, but significantly decreased (P < 0.05) under high density. Results revealed that the Cortisol biosynthesis was observed in rainbow trout between eyeing to the hatching stage. An increase in the density of eggs until 7.65 egg cm(-2) impaired Cortisol secretion and feedback system in alevins. However, more studies are needed to identify the exact time for Cortisol synthesis ability from eyeing to hatching in this species.