Growth related gene expression
Fish growth was affected by internal and external factors such as temperature, nutritional requirement, photoperiod,
etc.
(Moriyama et al. 2000). The physiological responses were processed and controlled by the brain-hypothalamus and pituitary gland in fish. The GH/IGF-1 axis is important in regulating fish physiology
(Long et al. 2019), such as cell proliferation and differentiation, protein synthesis and tissue maintenance
(Patel et al. 2005). The MSTN is a member of the Transforming Growth Factor Beta (TGF-β) cytokine superfamily, which negatively regulates muscle growth by inhibiting the muscle satellite cell differentiation during early development
(Artaza et al. 2005) and adult
(Welle et al., 2007). Picha et al., (2014) reported a positive correlation between somatic growth and IGF-1, which acts as a biomarker of fish growth. In this study, the relative gene expression of IGF-1, GH, MSTN, HSP70, HSP90A and GST were expressed in fold changes by considering lower stocking density (T1 group) as one-fold. Relative growth gene expression is shown in Fig 1 and stress gene expression in Fig 2. The study found a clear trend with a significant difference in gene expression of different stocking densities reared fish. In muscle, relative growth gene expression of GH/IGF-1 and MSTN were downregulated and upregulated, respectively, with an increase in stocking density. Relative stress gene expression of HSP70, HSP90A and GST were upregulated in muscle with an increase in stocking density. Relative GH/IGF-1 (1.094 ±0.012 folds and 1.075±0.023folds) and MSTN (2.183±0.011 folds) expressions were significantly higher in T1 and T5, respectively, groups reared fish. The GH and IGF-1 expressions were significantly lower in T5 (0.105±0.003 folds and 0.506±0.003 folds), respectively, in reared fish.
On the other hand, MSTN expression was significantly lower (1.004±0.001 folds) in the T1 group. The lower growth performance in the T5 group was accompanied by the changes in the expression of GH/IGF-1 and MSTN in muscle and stress created by the higher stocking density. In fine flounder (
Paralichthys adspersus), stocking density directly affected the growth and down-regulated the GH/IGF system
(Mendez et al. 2018). Another possible explanation for decreased growth performance in the high stocking density groups of Asian seabass might be the less feed intake under crowding stress, which upregulated the fish muscle transcripts of MSTN. There is a negative relationship between growth and stocking density, which could be due to energy imbalance and less nutrient digestion and absorption of fish during crowding stress. In Amur sturgeon (
Acipenser schrenckii Brandt), the GH/IGF-1 expression in the muscle was down-regulated with increasing stocking density (5.5 kg/m
3 to 11.0 kg/m
3), but stocking density had not influenced the IGF-2 expression
(Ren et al. 2018); IGF-1 and MSTN were significantly downregulated and upregulated with high stocking density (12 to 44 kg.m
3), respectively, in rainbow trout (
Oncorhynchus mykiss). Similarly, in
Eleginops maclovinus, GH/IGF-1 expression was significantly reduced in high stocking density (24 kg/m
3) due to increased biomass and crowding stress
(Oyarzun et al., 2020).
Heat shock protein is also known as stress proteins, a highly conserved family of cellular proteins that act as molecular partners in all organisms and play an important role in fish stress
(Qiang et al., 2015; Zahedi et al., 2019). Altered expression of HSP 70 andHSP90 in muscle tissue is a common biomarker as the elevated level is correlated with an elevated level of energy requirement and demotes the growth of Asian seabass same kind of results were observed in the Atlantic salmon and rainbow trout
(Bower and Johnston, 2010; Galt et al., 2018). GST belongs to the primarily soluble enzymes that have important functions in detoxification and antioxidation caused by the action of ROS
(OBrien et al., 2000). Relative expression of HSP70, HSP90A and GST was significantly higher in T5 groups reared fish. The HSP70 (1.038±0.140 folds), HSP90A (1.017±0.094 folds) and GST (1.001±0.025 folds) expressions were significantly lower in T1 reared fish. Increased HSP90 expression in muscle tissue in response to stress is hypothesized to protect muscle proteins from degradation and possibly promote protein synthesis or recycling
(Naito et al., 2000; Goto et al., 2003). Herein, crowding increased HSP90 in white muscle in cutthroat trout, brook trout and Atlantic salmon induced HSP90 elevation observed in other teleost tissues
(Vijayan et al. 2003). The GST mRNA level in the muscle was significantly downregulated with increasing the stocking density and similar kinds of results were recorded in turbot
(Chan 1995) and Chinese sturgeon (
Acipenser Sinensis)
(Long et al., 2019). The study found significantly elevated relative gene expression of GH/IGF-1 in the T1 group and this could be due to optimal nutrient digestion and absorption with a sufficient level of protein-sparing effect in fish to balance the energy required to overcome crowding stress.
Heamatologicl parameters
Haematological parameters are general indicators of fish health
(NRC, 1993). Haematological characteristic is an important tool that can effectively monitor physiological and pathological changes in fishes. Normal ranges for various blood parameters in fish have been established by different researchers in various conditions, including normal fish physiology, stress and pathological condition
(Kumar et al., 2017). Van Rijn (1996) revealed that the glucose level in serum might improve with the elevation of stress. The haemato-biochemical response in Asian Seabass towards different stocking densities is shown in Table 2. The present study results revealed that increasing stocking density creates chronic stress in the animal and decreases growth. Blood glucose levels might be affected by the high stocking density, capture and acute stress factors
(Luo et al., 2013). It acts as a transient indicator of the stress level; when stress is continuous, the blood glucose level falls to a pre-existing level. A previous study of
Pagrus pagrus (61.1±1.35mg/dl)
(Rotllant et al., 1997) and rainbow trout (5.1±0.37 mmol/L)
(Galt et al., 2018) suggested that chronic high-density stress has a limited effect on serum glucose levels in fish. In the present study, at a lower stocking density (70 fishes /m
3), glucose level was found to be 58.6±0.22 mg/dl, which increased with increasing stocking density (134.63±0.25 mg/dl).
Fotedar (2016) also reported similar results in Asian seabass, where the glucose level at low stocking density was (4.3±0.45 mmol) and at higher stocking density 9.8±0.03 mmol/L, which almost changed the metabolic activity of the fish and decreased the growth rate. Serum total protein level is also used as an indicator of fish health
(Tahmasebi-Kohyani et al., 2012). Higher-level Hb (10.4±0.05 mg/dl) and RBC (4.17 x 106±0.02 microliter) values were observed at higher stocking density (210 fish/m
3) could be attributed to the increasing the haemoconcentration, which was similar to the study of
Rotllant et al., (1997) in
Pagrus pagrus. The MCH and MCHC play a significant role in diagnosing anaemia in most animals and are known to indicate the erythrocyte status and oxygen-carrying capacity of the blood in fish
(Houston, 1997). An increase in the number of cells could relate to the increase in the respiratory demand in the higher stocking density
(Shen et al., 1991; Zhou et al., 2008).