The effect of increasing stocking density on growth performance, cortisol concentration and heat shock protein gene expression in rainbow trout (Oncorhynchus mykiss)

Stocking density (SD) is considered as a chronic stressor in aquaculture. The aim of the present study was to compare the growth performance, cortisol response and the mRNA expression of heat shock protein70 (HSP70) gene in rainbow trout (Oncorhynchus mykiss) reared at two common SDs of Iranian fish farms in an open system. For this purpose, young all-female rainbow trout were studied at two SDs of 24 (medium SD or MSD) and 44 (high SD or HSD) Kg/m3 and loading density of 1 L/min/Kg of fish for 60 days. The obtained results showed a decrease of mean final weight in HSD groups by 6.2% as compared to MSD. However, no differences were observed between treatments in terms of fish length (total, fork, standard). Furthermore, relative fin lengths (pectoral, pelvic, anal, dorsal and caudal) were similar in the two categories after two months of treatment. On the other hand, body weight gain (%), daily growth rate, specific growth rate and thermal growth coefficient were decreased significantly in HSD in comparison to MSD sets. In contrast, no changes were observed in fish condition factor. High SD caused an increase of coefficient of variance of weight and feed conversion ratio in respect to MSD. Blood glucose and cortisol levels remained unchanged between treatments. At last, muscle HSP70 gene expression at mRNA level increased significantly in HSD versus MSD. In conclusion, our results suggested that increasing the SD in allowable ranges in trout fish farming systems with no oxygenation might cause a stress in fish indicated by change in some studied growth parameters as well as increased HSP70 mRNA expression. Therefore, SD increase should be accompanied by proper management schemes as well as nutritional approaches aimed to relieve the related induced stress and to strengthen the fish immune system.