Effects of chronic heat stress on growth performance, liver histology, digestive enzyme activities, and expressions of HSP genes in different populations of Largemouth bass (Micropterus salmoides)

Heat stress has become a significant issue in Largemouth bass (Micropterus salmoides, LMB). Identifying populations with superior heat tolerance abilities contribute to the breeding of new varieties with stress resistance. In this study, three populations of Northern LMB, including "YouLu No.3 '' LMB (YL3), wild Northern LMB (NLMB) and Taiwan LMB (TW), as well as one Florida LMB population (M. floridanus, FLMB) were subjected to chronic heat stress. After being exposed to a temperature of 34 degrees C for 60 days, the FLMB group showed the highest final mean body weight (FBW), survival rate (SR), specific growth rate (SGR), and the lowest feed coefficient ratio (FCR) compared to the three Northern LMB groups. The lowest SR was observed in the TW group, while the lowest FBW and SGR, as well as the highest FCR were observed in the NLMB group. The FLMB group showed the lowest damage to the liver, with small focal infiltrations of inflammatory cells. However, the degree of liver damage was gradually worsened in the NLMB, YL3, and TW groups. The NLMB group had a small number of inflammatory cells, while the TW group had more lipid vacuoles and inflammatory cells. The FLMB group exhibited the highest activities of pepsin and lipase, whereas the NLMB group showed the lowest activity of amylase. Compared to the three Northern LMB groups, the FLMB group exhibited higher expressions of HSP60/ 70 and HSP90aa1/b1. Additionally, the YL3 group exhibited higher expressions of HSP60 and HSP90aa1/b1 compared to the NLMB and TW groups. In conclusion, our results indicated that the FLMB population had better chronic heat tolerance than the Northern LMB populations. Additionally, the YL3 population exhibited superior heat tolerance compared to the TW and NLMB populations. Our study provides valuable information for the future selective breeding of LMB varieties.