The effect of density on sex differentiation, sexual dimorphism, stress, and related gene expression in yellow perch

Abstract

A 180-day experiment was conducted to evaluate the effects of density on sex differentiation, sexual dimorphism, cortisol level, and stress related gene expression. Yellow perch, Perca flavescens, with initial mean body weight of 0.03 ± 0.001 g were reared in three different stocking densities: 1, 2, and 4 fish/L, termed as low (LD), moderate (MD), and high (HD) density, respectively, in a flow-through tank system. Results showed no significant differences in sex ratio in all density groups compared to normal population 1:1, and sexual size dimorphism (SSD) appeared when male and female were as small as the mean size reaching 11.5 cm and 12.3 cm in total length (TL) or 13.2g and 16.9g in body weight (BW), respectively. This female-biased sexual growth dimorphism was more pronounced in LD, although it was observed across all density groups. A significantly higher condition factor (K) of females than males in the LD group, and significantly higher R values of LD and MD than HD with the length/weight (L/W) linear relationships in females, were observed. Parallelly, fish reared in LD showed significantly higher mean body weight than those in the MD and HD groups, but there were no significant differences between the MD and HD. Similar results were also observed in all the other parameters of weight gain, specific growth rate (SGR), condition factor (K), and survival. These findings suggested that high density not only affected growth itself, but also affected SSD, growth trajectory or body shape, and general wellbeing in fish, especially in females. There were no significant differences in gonadosomatic index (GSI) and viscerosomatic index (VSI) among all the density groups; however, the hepatosomatic index (HSI) of LD was significantly higher than MD and HD, suggesting high density affected liver reserves or functions. Physiologically, plasma cortisol level was significantly highest in the LD among all groups, followed by MD, and lowest in HD. At the molecular level, the expression of the 70-kDa heat shock protein (Hsp70), glutathione peroxidase (GPx), and superoxide dismutase (SOD) genes involved in cellular stress were significantly upregulated in the HD group. The most significantly downregulated expression of these genes was consistently observed in the MD when compared to the LD and HD groups. In conclusion, increasing density induced chronic stress in yellow perch without affecting sex differentiation, but negatively affected expression of stress-related genes and mobilization of liver reserve, resulting in poorer wellbeing and reduced SSD, growth, and survival.

Understanding how high stocking densities and concurrent limited oxygen availability drive social cohesion and adaptive features in regulatory growth, antioxidant defense and lipid metabolism in farmed gilthead sea bream (Sparus aurata)

“…Plasma/serum cortisol is currently the most widely used stress biomarker in fish ( Sadoul and Geffroy, 2019 ; Noble et al, 2020 ), though its reliability is limited by a number of drawbacks, including among others: i) exhaustion or adaptive HPI-axis habituation to chronic stress, ii) dramatic increase in circulating cortisol levels by sampling itself, and iii) high biological variability within and among species that hampers a clear conclusion of stress condition. This makes sense because the underlying hypothesis for the fitted low plasma cortisol levels with the increase of stocking density is that acute stress sampling can become minimized in fish coming from a HD environment ( van de Nieuwegiessen et al, 2009 ; Santos et al, 2010 ; Othman et al, 2022 ). Furthermore, it was previously established that the regulation of gilthead sea bream plasma cortisol levels is largely dependent of the type and intensity of stressor ( Bermejo-Nogales et al, 2014 ).…”

“…Frontiers in Physiology frontiersin.org plasma cortisol levels with the increase of stocking density is that acute stress sampling can become minimized in fish coming from a HD environment (van de Nieuwegiessen et al, 2009;Santos et al, 2010;Othman et al, 2022). Furthermore, it was previously established that the regulation of gilthead sea bream plasma cortisol levels is largely dependent of the type and intensity of stressor (Bermejo-Nogales et al, 2014).…”