Advances in Food Technology and Nutritional Sciences

Open journal

ISSN 2377-8350

Influence of Body Size on Blood Hemogram in Rainbow Trout Oncorhynchus Mykiss (Walbaum, 1792)

Francesco Fazio*, Concetta Saoca, Irene Vazzana and Giuseppe Piccione

Francesco Fazio, DVM, PhD

Associate Professor, Department of Veterinary Sciences, Polo Universitario Annunziata, University of Messina, 98168 Messina, Italy Tel. +39 0903503516; Fax: +39 0903503975; E-mail:


Global fish profigureduction has notably increased in the last five decades and intensive farming has contributed significantly towards the development of aquaculture. Rainbow trout is a major farmed species in Italy and the second most common to be farmed in Europe after the Atlantic salmon.1 Together with commercial globalization, increasing trout farming consequentially leads to several fish diseases, thus rendering it necessary to prevent and cure related conditions while increasing the demand for veterinary drugs to treat the aquatic species. The evaluation of blood parameters assists in the culture of fish by facilitating the early detection of diseases and identification of sub-lethal conditions affecting the productivity.2 Various literary references report about the blood parameters and their variations observed in fish living in different habitats.3-5 In a previous study conducted to establish the reference range for haematological and biochemical values on Clarias gariepinus, a positive correlation was observed between the biometric and blood indices.4 Variations with respect to the normal values of blood parameters exist in fish and can be attributed to some internal and external factors such as physiological homeostasis and environmental factors.6 Body size alongside water quality and temperature, may contribute to the variability in the haematological parameters that are difficult to interpret. Considering that the changes in the blood parameters depend on the biometric parameters and the physicochemical conditions of the water, the aim of this study was to evaluate the influence of body size on some hematological parameters in the farmed trout.


A total of 50 Rainbow trout (Oncorhyncus mykiss) in excellent health status and supplied by a commercial farm in Palazzolo Acreide (Siracusa, Italy) were investigated in this study. All the fish were subjected to a natural day/night cycle (11L/13D) and were fed with a commercial dry food (crude protein 46%; crude fat 20%; ash 10%; fiber 1.5%) distributed twice daily for 7 days a week. The fish being studied were considered healthy on the basis of an external examination of any sign of abnormalities or clinical infestation. In the farm, the physical and chemical characteristics of water were recorded. Temperature, salinity, pH and dissolved oxygen (DO) were measured using a handheld multiparameter instrument (model YSI 556 MPS – Ohio, USA) and these values were recorded as has been shown in Table 1. After being captured, the fish were anaesthetized prior to blood sampling using 2-phenoxyethanol at a concentration of 200 mg L1 and were immediately individually weighted using a balance (Kern 440-49 N, Germany) and their total length was measured using an ictiometer (Scubla SNC, 600 mm, Italy) in order to obtain two groups of different body size: Group S1 (n=25) with weight (300-500 g) and length (25-32 cm); Group S2 (n=25) with weight (510-700 g) and length (33-38 cm). Blood samples were collected between 8:00 a.m. and 10:00 a.m. and were obtained by puncturing the caudal veins using microtubes (Miniplast 0.6 mL; LP Italiana Spa, Milano) containing ethylenediaminetetraacetic acid (EDTA) (ratio 1.26 mg/0.6 mL) as the anticoagulant agent for the recording of hematological parameters. All the samples were analyzed immediately (T0 ) by an automatic method using the blood cell counter HeCoVet C (SEAC, Florence, Italy) previously validated and used in this and the other species using a manual and an automatic system.7,8 The procedure of analysis was performed to determine the following parameters: white blood cell (WBC) count, red blood cell (RBC) count, hemoglobin concentration (Hb), hematocrit (Hct) value, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) concentration, mean corpuscular hemoglobin concentration (MCHC) and thrombocyte count (TC).

Table 1: Water quality values (Mean±DS) of farmed Rainbow trout Oncorhynchus mykiss (Walbaum, 1792).

Water Parameters

Oncorhynchus mykiss

Temperature (°C)


Dissolved Oxygen (mg/L)




All haematological analysis were performed in triplicate by the same operator with a same instrument under the same conditions, and in a short period of time”.

All the experimental procedures were carried out in accordance with the ethical considerations presented by the European legislation concerning the protection of animals used for scientific purposes (European Directive 2010/63). The recorded data was normally distributed (p>0.05; Kolmogorov–Smirnov test).

Statistical Analysis

The influence of body size on hematological parameters was determined by performing an unpaired t-test for two different groups. p values <0.05 were considered as statistically significant. Table 2 shows the descriptive statistics for haematological parameters obtained in the two different size groups of farmed Rainbow trout Oncorhynchus mykiss (Walbaum, 1792) showing a statistically significant difference.

Table 2: Statistical results for haematological parameters in farmed Rainbow trout Oncorhynchus mykiss (Walbaum, 1792). Means without the same alphabetical characters within the same parameters represent statistically significant differences (p<0.001).



(body size)

Mean±SD 95% confidence interval (CI)

2.5th-7.5th percentile range

WBC (x103/µL)

Group 1 (n=25)

19.89±0.74a 19.59± 20.20


Group 2 (n=25)

20.12±0.92a 19.74± 20.45


RBC (x106/µL)

Group 1 (n=25)

1.53±0.14a 1.47±1.59


Group 2 (n=25)

1.64±0.12b 1.59±1.69


Hb (mmol/L)

Group 1 (n=25)

1.64±0.15a 1.57±1.70


Group 2 (n=25)

1.73±0.11b 1.68±1.78


Hct (%)

Group 1 (n=25)

29.00±3.07a 27.73±30.27


Group 2 (n=25)

31.62±1.78b 30.88±32.36


MCV (fL)

Group 1 (n=25)

189.2±8.57a 185.70±192.70


Group 2 (n=25)

193.4±16.48a 186.6±200.20


MCH (pg/cel)

Group 1 (n=25) 10.76±1.34a 10.20±11.31 10.10±11.54

Group 2 (n=25)

10.60±0.96a 10.20±10.99


MCHC (%)

Group 1 (n=25)

5.70±0.82a 5.37± 6.04


Group 2 (n=25)

5.49±0.42a 5.32±5.66


TC (x103/μL)

Group 1 (n=25)

59.20±10.60a 54.83±63.57


Group 2 (n=25)

59.40±10.76a 54.96±63.84



The results of this study showed the influence of body size on some haematological parameters in farmed trout as reported in Table 2.

Statistical analysis (unpaired t-test) showed a significant effect of size (p<0.05) on some parameters (RBC, Hb and Hct). In particular, a significant increase in the RBC count, Hb concentration and Hct value was observed for the fish of Group 2 relative to that of Group 1. No statistically significant differences were observed in WBC, MCV, MCH, MCHC and TC.


Hematological parameters are closely linked with metabolism thus, these results could be attributed to the higher metabolic activity and high energy demand during growth. The increased metabolic demand during the growth of the trout could increase the volume of RBC, which could lead to a consequential increase in the Hct value. Some authors9 observed that the values of RBC count, Hb concentration and Hct value increased with an increase in the fish size. Moreover, in fish, the hematological parameters are closely associated with the metabolic activity related to size. Previously our researchers10 have shown that the values of RBC count and Hb concentration were higher in mullet compared to other species with the highest biometric parameters. It should be noted that the differences recorded in blood parameters between the fish of various sizes are genetically determined,11 even though these differences might be due to the higher metabolic rate of bigger fish compared to smaller ones.12 In a research conducted on Nile tilapia Oreochromis niloticus, there was an effect of fish size on all the parameters except for MCV.13 In a study on Mugil platanus, it was verified that the mean values for Hct, Hb, MCV, MCH and MCHC showed a slight tendency to increase as the fish continued to grow larger.14 In addition, larger individuals present higher mean values for MCV, MCH and MCHC for Dolphin fish Salminus maxillosus.15 WBCs, the cells responsible for the body’s defence system, are considered an important parameter to evaluate the health status of the fish; the recorded values of WBC count are indicative of their immune responses and determine the ability of the fish to fight infections. According to some authors,16,17 the WBC levels depend on various factors such as water temperature, environmental stress, fish age and sex. Species with higher counts of WBC were able to fight infections more effectively than other species. Among the defense cells of fish, the thrombocytes have a phagocytic capacity besides their functions in coagulation. Our findings did not suggest the influence of body size on WBC and TC, but it is probably on account of a small number of fish being studied.


In conclusion, our study suggests that the determination of hematological parameters require a correct interpretation in relation to the body size. Body size is an important factor, not only from the commercial point of view, but also for the accurate determination of hematological parameters, that many veterinarians demand to effectively control and manage the health status of the aquaculture fish.


All authors have made substantial contributions to each step of the experimental procedure and article preparation. The idea for this study was conceived by Francesco Fazio and Giuseppe Piccione.

The experiment was designed by Francesco Fazio.

The experiment was performed by Concetta Saoca and Irene Vazzana

The data was analyzed by Francesco Fazio.

The article was written by Francesco Fazio and Concetta Saoca.


The authors declare that they have no conflicts of interest.

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